The purpose of this review is to integrate what is currently known about the role of brain-derived neurotrophic factor (BDNF) in the pathophysiology of mood disorders including major depressive disorder (MDD) and bipolar disorder (BD). We reviewed the pre-clinical and clinical papers demonstrating that BDNF plays a role in the pathophysiology of mood disorders and in the mechanism of action of therapeutic agents. Pre-clinical studies suggest that the expression of BDNF might be a downstream target of antidepressant treatments and mood stabilizers such as lithium and valproate, and that BDNF exerts antidepressant activity in animal models of depression. Furthermore, BDNF protects against stress-induced neuronal damage, and it might affect neurogenesis in the hippocampus, which is thought to be involved in the pathogenesis of mood disorders. Clinical studies have demonstrated that serum levels of BDNF in drug-naive patients with MDD are significantly decreased as compared with normal controls, and that BDNF might be an important agent for therapeutic recovery from MDD. Moreover, recent findings from family-based association studies have suggested that the BDNF gene is a potential risk locus for the development of BD. These findings suggest that BDNF plays a critical role in the pathophysiology of mood disorders and in the activity of therapeutic agents in patients with mood disorders. New agents capable of enhancing BDNF levels may lead aid the development of novel therapeutic drugs for patients with mood disorders.
Damage to the central nervous system (CNS) leads to cellular changes not only in the affected neurons but also in adjacent glial cells and endothelia, and frequently, to a recruitment of cells of the immune system. These cellular changes form a graded response which is a consistent feature in almost all forms of brain pathology. It appears to reflect an evolutionarily conserved program which plays an important role in the protection against infectious pathogens and the repair of the injured nervous system. Moreover, recent work in mice that are genetically deficient for different cytokines (MCSF, IL1, IL6, TNFalpha, TGFbeta1) has begun to shed light on the molecular signals that regulate this cellular response. Here we will review this work and the insights it provides about the biological function of the neuroglial activation in the injured brain.
A genetic etiology to schizophrenia was recognized a century ago by E. Kraepelin [Ein Lehrbuch fur studirende und aerzte, Vol II. Leipzig, Verlagvon, Barth (1899)], yet no clear inherited pattern or mechanism has been established. In the last decade, a new wave of molecular genetic studies of families with schizophrenia has yielded unconvincing evidence for the involvement of multiple putative loci. The task of the next century will be to use genomics to define the true nature of deviant brain growth and development throughout the lifetime of an individual that could result in the perceptual disturbances characterized as schizophrenia.
The central goals of this manuscript are (1) to better characterize what appears to be the most parsimonious account of schizophrenic long-term memory impairment in the neuropsychological literature: a contextual binding deficit rooted in the medial temporal lobes; (2) to link this deficit to concrete abnormalities at the level of the hippocampus; and (3) to suggest that this deficit could lead to the functional impairment experienced by schizophrenia patients in their daily lives. As far as long-term memory is concerned in schizophrenia, there seems to be a general agreement to conclude that explicit mechanisms are disturbed compared to relatively spared implicit mechanisms. More precisely, both subsystems of explicit memory (i.e., episodic and semantic) appear to be dysfunctional in this patient population. Errors during the encoding processes could be responsible for this dysfunction even if retrieval per se is not totally spared. Recently, a number of studies have suggested that impairments in conscious recollection and contextual binding are closely linked to episodic memory deficit. Since the hippocampal formation is considered to be the central element in the neural support for contextual binding and episodic memory, we have conducted an extensive review of the literature concerning the hippocampal formation in schizophrenia. Emerging evidence from varying disciplines confirm the coherence of the different anomalies reported concurrently at the neuroanatomical, neurodevelopmental, biochemical, and genetic levels. It seems highly probable that the synaptic disorganization in the hippocampus concerns the regions crucial for encoding and contextual binding memory processes. The consequences of these deficits could result in schizophrenia patients experiencing major difficulties when facing usual events which have not been encoded with their proper context.
The black reaction allowed Golgi to describe with amazing detail the morphology of glial cells as well as their proximal location and intimate connections with neurons and blood vessels. Based on this location, Golgi hypothesized that glial cells were functional units in the nervous system and were not merely a structural support medium. Relatively recent advances have confirmed the importance of glial cells in nervous system function and disease. The occurrence of gliosis is considered the hallmark of damaged tissue. Gliosis can differentially influence disease development and it is a prevailing characteristic of temporal lobe epilepsy. Its presence in the epileptic hippocampi might contribute to hyperexcitability, the development of aberrant neurogenic changes and inflammatory processes related to seizures. Considering the accumulating data regarding the pathological role of glial cells in epilepsy, novel therapeutic approaches that target glial cells are being explored. Such therapeutic approaches directed to glial cells present a novel perspective for the management of refractory pathologies.
The organization of lateral septal connections has been re-examined with respect to its newly defined subdivisions, using anterograde (PHAL) and retrograde (fluorogold) axonal tracer methods. The results confirm that progressively more ventral transverse bands in the hippocampus (defined by the orientation of the trisynaptic circuit) innervate progressively more ventral, transversely oriented sheets in the lateral septum. In addition, hippocampal field CA3 projects selectively to the caudal part of the lateral septal nucleus, which occupies topologically lateral regions of the transverse sheets, whereas field CA1 and the subiculum project selectively to the rostral and ventral parts of the lateral septal nucleus, which occupy topologically medial regions of the transverse sheets. Finally, the evidence suggests that progressively more ventral hippocampal bands innervate progressively thicker lateral septal sheets. In contrast, ascending inputs to the lateral septum appear to define at least 20 vertically oriented bands or subdivisions arranged orthogonal to the hippocampal input (Risold, P.Y. and Swanson, L.W., Chemoarchitecture of the rat lateral septal nucleus, Brain Res. Rev., 24 (1997) 91-113). Hypothalamic nuclei forming parts of behavior-specific subsystems share bidirectional connections with specific subdivisions of the lateral septal nucleus (especially the rostral part), suggesting that specific domains in the hippocampus may influence specific hypothalamic behavioral systems. In contrast, the caudal part of the lateral septal nucleus projects to the lateral hypothalamus and to the supramammillary nucleus, which projects back to the hippocampus and receives its major inputs from brainstem cell groups thought to regulate behavioral state. The neural system mediating defensive behavior shows these features rather clearly, and what is known about its organization is discussed in some detail.
Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.
The circumventricular organs are small sized structures lining the cavity of the third ventricle (neurohypophysis, vascular organ of the lamina terminalis, subfornical organ, pineal gland and subcommissural organ) and of the fourth ventricle (area postrema). Their particular location in relation to the ventricular cavities is to be noted: the subfornical organ, the subcommissural organ and the area postrema are situated at the confluence between ventricles while the neurohypophysis, the vascular organ of the lamina terminalis and the pineal gland line ventricular recesses. The main object of this work is to study the specific characteristics of the vascular architecture of these organs: their capillaries have a wall devoid of blood-brain barrier, as opposed to central capillaries. This particular arrangement allows direct exchange between the blood and the nervous tissue of these organs. This work is based on a unique set of histological preparations from 12 species of mammals and 5 species of birds, and is taking the form of an atlas.
A wealth of animal data implicates the amygdala in aspects of emotional processing. In recent years, functional neuroimaging and neuropsychological studies have begun to refine our understanding of the functions of the amygdala in humans. This literature offers insights into the types of stimuli that engage the amygdala and the functional consequences that result from this engagement. Specific conclusions and hypotheses include: (1) the amygdala activates during exposure to aversive stimuli from multiple sensory modalities; (2) the amygdala responds to positively valenced stimuli, but these responses are less consistent than those induced by aversive stimuli; (3) amygdala responses are modulated by the arousal level, hedonic strength or current motivational value of stimuli; (4) amygdala responses are subject to rapid habituation; (5) the temporal characteristics of amygdala responses vary across stimulus categories and subject populations; (6) emotionally valenced stimuli need not reach conscious awareness to engage amygdala processing; (7) conscious hedonic appraisals do not require amygdala activation; (8) activation of the amygdala is associated with modulation of motor readiness, autonomic functions, and cognitive processes including attention and memory; (9) amygdala activations do not conform to traditional models of the lateralization of emotion; and (10) the extent and laterality of amygdala activations are related to factors including psychiatric status, gender and personality. The strengths and weakness of these hypotheses and conclusions are discussed with reference to the animal literature.
The epsilon4 allele of apolipoprotein E (ApoE) is a well-established risk factor for late onset Alzheimer's disease (AD). This knowledge has generated interest in the role of ApoE variants in normal cognition. Varying degrees of cognitive dysfunction have been described in non-demented individuals with one or two epsilon4 alleles leading to suggestions that the gene plays a role in normal cognition or helps calibrate the aging process. In this paper, these hypotheses are critically evaluated. It is argued that ApoE variants play no role in cognitive development. Given the differential neurocognitive sequelae of normal aging and AD, we also suggest that accelerated aging is unlikely to account for the pattern of deficits observed in non-demented epsilon4 allele carriers. We conclude that the neuropsychological dysfunction reported in non-demented epsilon4 carriers is most likely to be the result of incipient AD.
In the adult mammalian central nervous system lost nerve cells are not replaced and there is no regeneration of injured axons in white matter. Together, these two facts mean that there are no spontaneous reparative mechanisms in operation. Instead, the adult central nervous system copes with the risks of injuries and diseases by protective encapsulation in bone, by a multitude of neuroprotective mechanisms, and finally by the fact that many important functions are represented by a much larger number of neurons than minimally needed. The long life expectancy of a human being nevertheless means that the risk that the central nervous system is affected by disease, injury or other forms of insults for which it cannot fully compensate is relatively high. Experimentally, two strategies are being pursued in order to develop ways of minimizing various forms of CNS damage, namely neuroprotective and reparative strategies. Here we present a possible reparative intervention applicable to spinal cord injury based on multiple white-to-gray matter peripheral nerve bridge grafts and work based on the specific role of Nurr1 for dopamine neuron development, suggesting that development of ligands to transcription factor might be a new inroad to neuroprotective treatments in Parkinson's disease.
Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.
Advances in measurement techniques have enabled the extracellular concentration of dopamine to be monitored inside striatal structures during transient electrical stimulation of the medial forebrain bundle. The observed concentration changes can be accounted for by a mathematical model as a function of the frequency employed and the stimulus duration. Overflow curves can be described by 3 kinetic parameters: the concentration of dopamine released per stimulus pulse, and the Km and Vmax of uptake. In terms of this model, the kinetics of overflow during stimulation is found to be identical in the nucleus accumbens and caudate nucleus with the exception that the Vmax for uptake is lower in the former region. Maximal uptake is also found to be lower in animals with partial lesions of dopamine neurons. Measured concentrations vary with stimulation frequency from 10 to 60 Hz in a manner that can be predicted by the model. Competitive uptake inhibitors have their primary effect on overflow in the limit of low stimulus frequencies. In contrast, D2 antagonists, which increase the concentration of dopamine released per stimulus pulse, have a moderate effect in low and high frequency ranges, but cause a significant maximal increase in extracellular dopamine concentrations at a mid-range frequency. Both calculated response and experimental findings indicate that in the caudate nucleus, the upper frequency for observable uptake inhibition and the characteristic maximum frequency for the receptor-mediated response occur at higher values than in the nucleus accumbens. The model appears to be useful for predicting dopamine extracellular concentrations over a wide range of conditions, and its predictions may be valid when extended to more physiological situations.
Serial sections of human thalami, cut in the 3 standard planes, were stained in alternating series for Nissl substance, myelin, cytochrome oxidase and acetylcholinesterase. Nissl and acetylcholinesterase-stained sections revealed a parcellation of the nuclei that could be correlated with that used in the macaque monkey thalamus. Human nuclei were accordingly re-named using the monkey nomenclature. Apart from differences of size, the nuclei of the human and monkey thalamus are remarkably similar. In the human ventral nuclear complex there is a very clear histochemical distinction between nuclei which, on the basis of comparison with the monkey, probably form the pallidal, cerebellar and lemniscal relays to premotor, motor and somatic sensory cortex, respectively. In the human somatic sensory relay nucleus there is a further clear cytoarchitectonic distinction between components that are probably equivalent to the relays for deep and cutaneous receptors in the equivalent monkey nucleus.
We will describe the identity and function of two unexpected estrogen binding proteins from rat brain cell membranes in search for the putative membrane estrogen receptor (mER). An E-6-BSA column retained a distinctive 37-kDa protein that showed 100% homology with glyceraldehyde-3-phosphate dehydrogenase (GAPDH). A P-3-BSA column also retained the same protein but with less affinity. E-6-BSA bound to GAPDH with an IC50 of 50 nM, whereas the IC50 for P-3-BSA was about 500 nM. A dose of 10 nM 17beta-estradiol stimulated the catalysis of GAPDH, whereas progesterone at 100 nM inhibited it. Other steroids were ineffective. We examined if GAPDH activity would change during the rat estrous cycle, and what would be the effect of ovariectomy and estrogen treatment. The hippocampus and cerebellum were collected and GAPDH catalysis in both cytosolic and plasmalemmal-microsomal fractions was tested. The highest activity was found in Proestrus morning and the lowest in Estrus in both fractions. After ovariectomy (3 weeks) the hippocampus membrane fraction showed significantly reduced activity compared to that of Diestrus. An injection of estradiol in ovariectomized rats (10 microg/rat, s.c.) increased GAPDH activity in the hippocampus membrane fractions close to 60% from that of ovariectomized oil-treated controls 24 h after treatment maintaining similar levels by 48 h. No changes were detected in the preparations from the cerebellum of the same rats. The other protein retained by E-BSA columns was a 55-kDa protein identified as beta-tubulin. Two other proteins were also co-purified from the rat hippocampus: a 37-kDa (GAPDH) and a 45-kDa (actin). A purified brain tubulin (Cytoskeleton) was also retained with high affinity by the E-6-BSA, but with less affinity by an E-17-BSA column and not retained by either BSA, P-3-BSA or C-21-BSA columns. E-6-[125I]BSA bound with high affinity to tubulin (1 microg) and 17beta-estradiol completely displaced the binding at 10(-7) M. 17alpha-estradiol was ineffective and neither progesterone, corticosterone, DES nor 2-methoxyestradiol (2-ME) was able to displace the ligand. The T-3-[125I]BSA also bound to tubulin. But it seems to interact with another binding site, because colchicine at 10(-5) M completely eliminated the binding of T-3-[125 I]BSA to tubulin but did not displace the E-6-BSA site. Taxol competed off both ligands but only by 50%. None of the two ligands bound actin. These novel findings add new information to be considered in the intracellular actions of estradiol, particularly in the remodeling and functions of the cytoskeleton.
Progressive tumor growth depends on angiogenesis to sustain metabolic needs of tumor cells, thus providing a potential target for cancer therapy. Malignant gliomas have retained their dismal prognosis despite aggressive multimodal conventional therapeutic approaches, illustrating the need for novel therapeutic strategies. Gliomas are a suitable tumor type for probing angiogenesis inhibition as their proliferation is characterized by a prominent proliferative vascular component. In the present review, we discuss the current status and future directions of angiogenesis inhibition in gliomas. We focus on recently developed approaches inducing an antiangiogenic response such as targeted gene delivery, protein tyrosine kinase inhibitors and encapsulated producer cells. Although several of these modalities have shown promising results on their own, the true potential of these novel approaches lies in their combined use with radiotherapy or 'metronomically scheduled' chemotherapy. A combined approach potentially counteracts the selective pressure on hypoxia-resistant malignant tumor cells, circumvents endothelial resistance induced by local cytoprotective responses and enhances the delivery of cytotoxic agents by normalizing vascular physiology. Surrogate markers of angiogenesis currently under study may provide accurate assessment of response in individual patients. Future research on endothelial markers expressed on tumor-associated vasculature as well as endothelial responses to cytotoxic treatment will provide new avenues for molecularly targeted therapy in malignant gliomas.
Increases in attentional effort are defined as the motivated activation of attentional systems in response to detrimental challenges on attentional performance, such as the presentation of distractors, prolonged time-on-task, changing target stimulus characteristics and stimulus presentation parameters, circadian phase shifts, stress or sickness. Increases in attentional effort are motivated by the expected performance outcome; in the absence of such motivation, attentional performance continues to decline or may cease altogether. The beneficial effects of increased attentional effort are due in part to the activation of top-down mechanisms that act to optimize input detection and processing, thereby stabilizing or recovering attentional performance in response to challenges. Following a description of the psychological construct "attentional effort", evidence is reviewed indicating that increases in the activity of cortical cholinergic inputs represent a major component of the neuronal circuitry mediating increases in attentional effort. A neuronal model describes how error detection and reward loss, indicating declining performance, are integrated with motivational mechanisms on the basis of neuronal circuits between prefrontal/anterior cingulate and mesolimbic regions. The cortical cholinergic input system is activated by projections of mesolimbic structures to the basal forebrain cholinergic system. In prefrontal regions, increases in cholinergic activity are hypothesized to contribute to the activation of the anterior attention system and associated executive functions, particularly the top-down optimization of input processing in sensory regions. Moreover, and influenced in part by prefrontal projections to the basal forebrain, increases in cholinergic activity in sensory and other posterior cortical regions contribute directly to the modification of receptive field properties or the suppression of contextual information and, therefore, to the mediation of top-down effects. The definition of attentional effort as a cognitive incentive, and the description of a neuronal circuitry model that integrates brain systems involved in performance monitoring, the processing of incentives, activation of attention systems and modulation of input functions, suggest that 'attentional effort' represents a viable construct for cognitive neuroscience research.
The psychological construct 'sustained attention' describes a fundamental component of attention characterized by the subject's readiness to detect rarely and unpredictably occurring signals over prolonged periods of time. Human imaging studies have demonstrated that activation of frontal and parietal cortical areas, mostly in the right hemisphere, are associated with sustained attention performance. Animal neuroscientific research has focused on cortical afferent systems, particularly on the cholinergic inputs originating in the basal forebrain, as crucial components of the neuronal network mediating sustained attentional performance. Sustained attention performance-associated activation of the basal forebrain corticopetal cholinergic system is conceptualized as a component of the 'top-down' processes initiated by activation of the 'anterior attention system' and designed to mediate knowledge-driven detection and selection of target stimuli. Activated cortical cholinergic inputs facilitate these processes, particularly under taxing attentional conditions, by enhancing cortical sensory and sensory-associational information processing, including the filtering of noise and distractors. Collectively, the findings from human and animal studies provide the basis for a relatively precise description of the neuronal circuits mediating sustained attention, and the dissociation between these circuits and those mediating the 'arousal' components of attention.
We have previously described a model of limbic status epilepticus in which chronic prolonged seizure states of immobile, exploratory, minor convulsive or clonic convulsive behavior are induced by intracerebral electrical stimulation; these states appear to belong to the same behavioral progression as kindled seizures. We postulated that the underlying seizure substrates, as mapped by the 14C-2-deoxyglucose method, should reflect a corresponding anatomic progression of discharge spread. Status epilepticus was induced in rat by pulsed-train current delivered for up to 90 min to one of several subcortical areas. Autoradiographs revealed that most of the observed patterns of seizure-induced metabolic activation comprised a hierarchical sequence, such that progressively more extensive patterns subsumed anatomic territories activated in less extensive patterns, thus allowing inferences as to the progression of discharge spread. In this sequence, the basolateral amygdala ipsilateral to the induction electrode was among the first structures to be activated. In successively larger activation patterns a small unilateral network related to basolateral amygdala was involved; this evolved through a transitional state to a unilateral extensive limbic pattern; which in turn was succeeded by bilateral extensive limbic activation. This hierarchical sequence culminated in a neocortical activation pattern, in which most of the forebrain was involved in intense seizure-induced activation. Seizure behaviors increased in severity in correspondence with the underlying seizure-activated anatomic substrate. In contrast, patterns of seizure activation were observed which did not fit within the early stages of the above sequence, although analysis indicates that the later stages of spread may be shared. The study of these patterns and those reported in the literature indicates that although limbic seizure networks may be anatomically distinct at their origination, further expansion is characterized by overlap; upon assumption of extensive patterns of activation the number of nuclei participating is so vast that the identity of the limbic originator is lost and common convulsive manifestations occur.
Patients with anterior limbic damage may present a distinct syndrome, spontaneous confabulation: they fail in common memory tests, act on the basis of previous habits rather than currently relevant memories, produce confabulations composed of elements of past true events, are disorientated, and are absolutely convinced about the veracity of their perceived reality. Spontaneous confabulation is independent of other false memories, such as, provoked confabulations or illusory recognition. Studies showed that spontaneous confabulators fail to suppress (inactivate) evoked memories that do not pertain to ongoing reality. Rehabilitation differs from other memory failures. Prognosis depends on the lesion site, but recovery is always associated with recovery of this suppression capacity. Lesions typically involve the posterior medial orbitofrontal cortex or its connections in the basal forebrain. Imaging and evoked potential studies in healthy subjects support the idea that the anterior limbic system provides a reality monitoring mechanism which selects memories of current relevance by suppressing (inactivating) currently irrelevant memories. This mechanism appears to adjust the cortical representation of activated memories before their content is recognised and consolidated. Comparison with animal studies suggests that human reality monitoring is a property of the brain's reward system.
A high resolution PHAL analysis of axonal connections suggests the existence of a visceromotor pattern generator network in the periventricular region of the rat hypothalamus (HVPG), and a preliminary account of its structure is provided here. Six nodes identified thus far include the dorsomedial nucleus and five small nuclei in the preoptic region (anteroventral and anterodorsal preoptic, parastrial, median preoptic, and anteroventral periventricular). Aside from its location between the neuroendocrine motor zone and the medial hypothalamic nuclei (behavior control column), three other primary features characterize the HVPG network. First, each HVPG nucleus generates a pattern of terminal fields that differentially targets a unique set of hypothalamic neuroendocrine motoneuron pools, and of preautonomic parts of the paraventricular nucleus. Second, the six HVPG nuclei are massively interconnected themselves. And third, the majority of projections from the HVPG nuclei remain within the medial half of the hypothalamus; additional outputs reach the septum, other parts of the diencephalon, and the brainstem central gray. Possible control of activity in the HVPG by neural inputs from the cerebral hemispheres, sensory systems, behavioral state-related cell groups, and the hypothalamic behavior or motivation control column is discussed, along with certain key functional data related to HVPG nuclei. Finally, the HVPG is incorporated into a working model of hypothalamic organization.
There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.
The difficult clinical situation still associated with most types of primary human brain tumors has fostered significant interest in defining novel therapeutic modalities for this heterogeneous group of neoplasms. Beginning in the 1980s chemotherapy has been incorporated into the treatment protocol of a number of intractable brain tumors. However, it has predominantly failed to improve patient outcome. The unsatisfactory results with chemotherapeutic intervention have chiefly been attributed to tumor cell resistance. In recent years, there has been a literal explosion in our understanding about the mechanisms by which cancer cells become chemoresistant. During the course of their evolution (intrinsic resistance) or in response to chemotherapy (acquired resistance) these cells may follow a number of pathways of genetic alterations to possess a common (multidrug) or drug-specific (individual drug) resistant phenotype. Genomic aberrations, deregulation of membrane transporting proteins and cellular enzymes, and an altered susceptibility to commit to apoptosis are among the steps on the way that contribute to the genesis of chemotherapeutic treatment failure. Although, through the years we have come to yield information and inferences as to the roles that different molecular events may have in the resistance phenotype of cancer cells, the actual involvement of single genetic alterations in conferring drug resistance in primary brain tumors remains debatable. This uncertainty and, besides, the lack of proper drug resistance diagnostics, in a vicious circle, hinder the development of effective resistance-modulation strategies. Clinical non-responsiveness to chemotherapy remains a formidable obstacle to the successful treatment of brain tumors and one of the most serious problems to be solved in the therapy of these lesions. Future advances in the chemotherapeutic management of these neoplasms will come with an improved understanding of the significance and interrelationship of the multiple biological systems operative in promoting resistance to this treatment modality. The focus of this review is to summarize current knowledge concerning major drug resistance-related markers, to describe their functional interaction en route to chemoresistance, and to discuss their implication in rendering human brain tumor cells resistant to chemotherapy.
In recent years much has become known about the substrates in the brain involved in the regulation of masculine sexual behavior and the involvement of specific neurochemicals in these brain areas. In the present paper the experimental data concerning the involvement of a number of brain areas in sexual behavior are reviewed, in relation to an incentive motivational theory of sexual behavior. The review is restricted to the involvement of opioids and dopamine, of which the role in sexual motivation and behavior is best documented. Opioids in the medial preoptic area (mPOA) impair sexual performance, although the endogenous opioids systems may be quiescent in normal, sexually active rats. Dopamine in the mPOA has a facilitative role in the masculine sexual performance. The corticomedial amygdala is involved in processing of sensory information, especially olfactory stimuli, which are subsequently directed towards the mPOA. Local beta-endorphin infusion interferes with this processing. Endogenous opioids in the ventral tegmental area activate the mesoaccumbens dopamine system and stimulate the sexual motivation. Increased dopamine transmission in the nucleus accumbens correlates with increased sexual motivation and vice versa. The basolateral amygdala plays an essential role in the association of environmental stimuli with reward and therefore in the expression of conditioned sexual motivation. Finally, the reviewed data are integrated and a comprehensive view on the relations between various neural substrates is composed.
An experimental method and its associated mathematical model are described to quantitate in vivo incorporation rates into and turnovers of fatty acids (FAs) within stable brain metabolic compartments, particularly phospholipids. A radiolabeled FA is injected i.v. in a rat, and arterial plasma unacylated FA radioactivities and unlabeled concentrations are sampled until the animal is killed after 15 min, when the brain is analyzed biochemically or with quantitative autoradiography. Unbound unacylated label in blood easily crosses the blood-brain barrier; rapidly equilibrates in the unacylated FA, acyl-CoA and phosphatidate-diacylglycerol brain pools; then is incorporated into phospholipids and other stable metabolic compartments. Uptake and incorporation of labeled FAs are independent of cerebral blood flow at constant brain blood volume. Different labeled FAs enter specific sn positions of different brain phospholipids, suggesting that a combination of probes can be used to investigate metabolism of these phospholipids. Thus, [9,10-3-H]palmitate preferentially labels the sn1 position of phosphatidylcholine; [1-14C]arachidonate the sn2 positions of phosphatidylinositol and phosphatidylcholine; and [1-14C]docosahexaenoate the sn2 positions of phosphatidylethanolamine and phosphatidylcholine. The FA model provides an operational equation for rates of incorporation of FAs into brain phospholipids, taking into account intracerebral recycling and de novo synthesis of the FA, as well as entry into brain of FA from acylated blood sources. The equation is essentially independent of specific details of the proposed model, and can be used to calculate turnovers and half-lives of FAs within different phospholipid classes. For the model to be most applicable, experiments should satisfy conditions for pulse-labeling of the phospholipids, with brain sampling times short enough to minimize exchange of label between stable metabolic compartments. A 15-20 min sampling time satisfies these criteria. The FA method has been used to elucidate the dynamics of brain phospholipids metabolism in relation to brain development, brain tumor, chronically reduced auditory input, transient ischemic insult, axotomy with and without nerve regeneration, and cholinergic stimulation in animals with or without a chronic unilateral lesion of the nucleus basalis magnocellularis.
The endocannabinoid system consists of two cannabinoid (CB) receptors, seven ligands, and ligand-catabolizing enzymes such as fatty acid amid hydrolase (FAAH) and monoglyceride lipase (MGL). The system's phylogenetic distribution is poorly known. The ligands cannot be molecularly investigated because they are not polypeptides and their specific synthetic enzymes have not been identified, so no sequences are available. Ligand phylogenetics can be inferred, nonetheless, by their presence in a range of extant organisms. Thus a meta-analysis of ligand extraction studies was performed (chemotaxonomy), and compared to a molecular search for homologs of CB receptors, vanilloid receptors (VR1), FAAH, and MGL in the genomes of sequenced organisms (phylogenomics). Putative homologs underwent functional mapping to ascertain the presence of critical amino acid motifs known to impart protein functionality. From an evolutionary perspective it appears that (1) endocannabinoid ligands evolved before CB receptors; (2) the ligands evolved independently multiple times; (3) CB receptors evolved prior to the metazoan-bilaterian divergence (ie, between extant Hydra and leech), but were secondarily lost in the Ecdysozoa; (4) VR1 may predate CB receptors but its affinity for endocannabinoids is a recent acquisition, appearing after the lower vertebrate-mammal divergence; (5) MGL may be as old as the ligands, whereas FAAH evolved recently, after the appearance of vertebrates. FAAH's emergence correlates with VR1's newly-found affinity for anandamide; this overlap in evolutionary time is recapitulated by complementary distribution patterns of FAAH, VR1, and anandamide in the brain. Linking FAAH, VR1, and anandamide implies a coupling among the remaining "older" parts of the endocannabinoid system, MGL, CB receptors, and 2-AG.
The third paper by Camillo Golgi on his new method was on the olfactory bulb. This paper has never been translated into English, but is of special interest both for its pioneering description of olfactory bulb cells and for containing the first illustration by Golgi of cells stained with his new method. A translation into English is provided in this paper, together with commentaries on the significant points in his descriptions. These results are placed in the perspective of Cajal's subsequent first publication on the olfactory bulb and brief mention of the work of other early histologists. This perspective allows one to see more clearly Golgi's fundamental contributions to the olfactory bulb in particular and to the description of the neuronal architecture of the brain in general.
Long-term potentiation (LTP) of synaptic transmission is considered a reliable cellular model of several forms of learning and memory. Described for the first time in 1973, this synaptic phenomenon consists in the enduring facilitation of the communication between two neurons in response to the sustained activation of the synapses by which they are interconnected. In a book of 1895 entitled Project for a Scientific Psychology, Sigmund Freud theorized about the possibility of representing memory at the synaptic level as "a permanent alteration following an event", and anticipated several crucial physiological properties of LTP. In the present article we aim at presenting Freudian theory on the functional organization of the nervous system developed in the Project, with particular respect to his ideas of the cellular bases of memory.
In 1899, at the invitation of G. Stanley Hall, the great psychologist and President of Clark University, Santiago Ramón y Cajal, and four other European scientists of significant note, were invited to participate in the Decennial Celebration of Clark. Cajal, accompanied by his wife, arrived in Worcester, via New York, to much acclaim and praise in the local press. His three lectures, all delivered in French and illustrated with large color drawings made upon his arrival at Clark, were concerned with previously unpublished observations on the structure of the human cerebral cortex. The full text of these lectures and 31 illustrations (in black and white) were published, in English, in a large Decennial Volume prepared by Clark University. At the culmination of the Clark Celebration, Cajal, and the other invited attendees, received the honorary Doctor of Laws degree. Cajal, ever the scholar, visited many sites of interest in the Northeastern US prior to his return to Spain including Columbia, Harvard, and the University of New York. This paper details the events surrounding Cajal's visit to Clark University, his only visit to the United States.
While one of the original underpinnings of the dopamine theory of schizophrenia was the paranoid psychosis which often develops during the binges or speed runs of chronic amphetamine addicts (and, more recently, in cocaine addicts), neurochemical studies of such drug abusers or from animals given continuous stimulants in an effort to model stimulant psychoses have not played a major role in the further evolution of this theory. One clear persisting alteration produced by continuous amphetamine is a neurotoxicity to dopaminergic innervations in caudate. Yet continuous cocaine administration apparently does not induce a similar neurotoxicity and this makes this effect a poor candidate for an underpinning of stimulant psychoses. However, it has recently been found that both continuous amphetamine and cocaine induce a strong pattern of degeneration which is highly confined to the lateral habenula and its principal output pathway, fasciculus retroflexus. This finding has led to a reconsideration of the role of these structures in psychoses. The habenula, as the chief relay nucleus of the descending dorsal diencephalic system (consisting of stria medullaris, habenula and fasciculus retroflexus), is an important link between limbic and striatal forebrain and lower diencephalic and mesencephalic centers. Studies of glucose utilization have consistently shown the habenula to be highly sensitive to dopamine agonists and antagonists. Lesions of habenula produce a wide variety of behavioral alterations. The dorsal diencephalic system has major and predominantly inhibitory connections onto dopamine-containing cells and it mediates part of the negative feedback from dopamine receptors onto dopamine cell bodies. It represents one of the major inputs in brain to the raphe nuclei and has anatomical and functional connections to modulate important functions such as sensory gating through thalamus, pain gating through central gray and raphe and motor stereotypies and reward mechanisms through substantia nigra and the ventral tegmental area. It is argued that alterations in these pathways are ideal candidates for producing the behaviors which occur during psychosis and that future considerations of the circuitry underlying psychoses need to include this highly important but relatively neglected system.
Research on the neurobiology of learning and memory has been guided by two major theories: (i) memory as a psychological process and (ii) memory as a change in synaptic neural connectivity. It is not widely recognised that not only are these theories different but, moreover, they are fundamentally incompatible. Confusion concerning basic concepts in the learning and memory field in mammals has lead to the creation of an extensive but often inconclusive experimental literature. However, one important conclusion suggested by recent work in this field is that experience-dependent changes in neural connectivity occur in many different brain systems. Particular brain structures, such as the hippocampus, do not play any uniquely important role in experience-dependent behavior. Research in learning and memory can be best pursued on the basis of biological studies of animal behavior and a cellular approach to brain function.
In 1906 the Nobel Prize in Physiology or Medicine was shared between Camillo Golgi and Ramón y Cajal in recognition of their work on the structure of the nervous system. Golgi's most impressive contribution was his method, described in 1873. This was applied in studies of the cerebellum, the olfactory bulb, hippocampus and the spinal cord. These studies together with his earlier work were included in his Opera Omnia, published in 1903. His method was highly praised by Cajal. His adherence to the reticular theory was opposed by Cajal, however, who had spelled out the neuron theory already in the late 1800s. Cajal's extraordinary contributions to the structure of the nervous system, based largely on the Golgi method and Ehrlich's methylene blue stain, were published in his Textura del Sistema Nerviosa de Hombre y de los Vertebrados, three volumes published from 1897 to 1904. Documents from the Nobel Archives reveal that Kölliker, Retzius and Fürst were the ones who proposed Golgi and Cajal for a shared prize. Golgi was nominated by Hertwig, as well. Cajal was proposed by Ziehen and Holmgren, and also by Retzius, as an alternative to a shared prize. Holmgren, who was commissioned to write the report to the Nobel Committee, found Cajal far superior to Golgi. Sundberg, asked for another evaluation, was more positive to Golgi's contributions than Holmgren. Gadelius supported Holmgren's views. The final vote gave a majority for a shared prize. The prize ceremony and the lectures were described in detail in Cajal's autobiography.
Among the 20 proposed members of the connexin family of proteins that form gap junctional intercellular communication (GJIC) channels in mammalian tissues, over half are reported to be expressed in the nervous system. There have been conflicting observations, however, concerning the particular connexins expressed by astrocytes, oligodendrocytes, Schwann cells and neurons. Identification of the several connexin proteins at gap junctions between each neuronal and glial cell type is essential for the rational design of investigations into the functions of GJIC between glial cells and into the functional contributions of electrical and "mixed" (chemical plus electrical) synapses to communication between neurons in the mammalian nervous system. In this report, we provide a summary of recent findings regarding the localization of connexins in gap junctions between glial cells and between neurons. Attention is drawn to technical considerations involved in connexin localization by light and electron microscope immunohistochemistry and to limitations of physiological methods and approaches currently used to analyze neuronal and glial coupling. Early physiological studies that provided evidence for the presence of gap junctions and electrical synapses in isolated regions of the mammalian brain and spinal cord are reexamined in light of recent evidence for widely expressed neuron-specific connexins and for the existence of several newly discovered types of gap junctions linking neurons.
This article reviews the scientific contributions of Jacques Paillard (1920-2006), who strengthened substantially the role of physiological psychology in the field of movement neuroscience. His research began in 1947 under the direction of the French neurophysiologist, Alfred Fessard (1900-1982), with whom he then collaborated for 9 years while an undergraduate and then graduate student and junior faculty member in psychology at the University of Paris (the Sorbonne). Paillard moved to the University of Marseille in 1957 as a Professor of Psychophysiology. In parallel, he became a founding member and administrator of the Institute of Neurophysiology and Psychophysiology, which began in 1963 on the Marseille campus of the National Center of Scientific Research (CNRS). Paillard retired from his university and CNRS positions in 1991 but he continued seminal research until his demise. Paillard advanced understanding of higher brain influences on human spinal motor mechanisms and the functional role of proprioception as revealed in patients deprived of such sensibility. He remains best known, however, for his work on human motor cognition. He reasoned that brain "maps" of the external world are constructed by the body's own movements and the central effects of their resulting central and peripheral feedback. He proposed two levels of interactive brain processing for the planning and/or execution of a reaching movement: 1) a sensorimotor level, using body posture as a key reference; and 2) a "higher" cognitive level for accurate movement performance, using learned representations of the position and shape of the environmental components, including the body, itself.
With the growing realization in the 1930s that the brain played a crucial role in regulating the secretions of the pituitary gland, neuroendocrinology as we now know it developed from two rather separate directions. One approach relied heavily on morphological techniques to define neurosecretion; a novel, but for many years flawed model that was originally developed to explain the presence of gland-like cells in the diencephalon. During its first 20 years neurosecretion, as a concept, made no significant contribution to our understanding of how the pituitary was controlled. Then, following the identification by Sanford Palay and Wolfgang Bargmann of a continuous neurosecretory pathway from the hypothalamus to the neural lobe, neurosecretion became incorporated into a more broadly based concept of pituitary function, particularly regarding the neural lobe. The second approach integrated structural and functional methods to investigate neural regulation of the pituitary. This work eventually explained how the pituitary was controlled by the brain. It led directly to our understanding of the control of vasopressin and oxytocin release by neuroendocrine terminals in the neural lobe, the neurohumoral control of the pars distalis, and eventually to a detailed description of the neural networks that control pituitary function. As increasingly sophisticated morphological, neurophysiological, and eventually molecular biological techniques were applied to the problem, the original notion of the diencephalic gland and neurosecretion became unsustainable. The gland-nerve cells of the 1930s became the neurosecretory cells of the 1940s and 1950s, and then finally neuroendocrine neurons in the 1960s. From then on neuroendocrinology developed into the more unified discipline we know today. The chronology of these two approaches will be examined here using examples from research that occurred approximately between 1920 and 1965. The goal is not to give a comprehensive history of pituitary function or neuroendocrinology. Instead, the focus will be to compare the rationales and effectiveness of two contrasting experimental approaches: predominantly structural analyses as opposed to more integrated approaches.
Retroviruses are biologically complex infectious agents which are capable of cellular infection and subsequent integration into the host genome. Retroviruses can exist in an endogenous form in which viral sequences are integrated into the human germline and are vertically transmitted in a Mendelian fashion. The transcriptional activation of these viral sequences in cells within the central nervous system can affect the transcriptional regulation of adjacent genes and result in alterations of neural functioning. This report discusses evidence for a possible role of endogenous retroviruses in the etiopathogenesis of schizophrenia and other human brain diseases. Evidence of endogenous retrovirus activity is manifested by the identification of viral sequences in the brains and cerebrospinal fluids of affected individuals. In addition, affected individuals display evidence of increased activity of virally-encoded reverse transcriptase. The identification of a retroviral component of schizophrenia would be consistent with genetic, environmental, and neurodevelopmental aspects of the disease process. The delineation of a role for retroviruses in disease pathogenesis might lead to new methods for the diagnosis and treatment of schizophrenia.
The extracellular levels of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian cerebral cortex, are regulated by specific high-affinity, Na+/Cl- dependent transporters. Four distinct genes encoding GABA transporters (GATs), named GAT-1, GAT-2, GAT-3, and BGT-1 have been identified using molecular cloning. Of these, GAT-1 and -3 are expressed in the cerebral cortex. Studies of the cortical distribution, cellular localization, ontogeny and relationships of GATs with GABA-releasing elements using a variety of light and electron microscopic immunocytochemical techniques have shown that: (i) a fraction of GATs is strategically placed to mediate GABA uptake at fast inhibitory synapses, terminating GABA's action and shaping inhibitory postsynaptic responses; (ii) another fraction may participate in functions such as the regulation of GABA's diffusion to neighboring synapses and of GABA levels in cerebrospinal fluid; (iii) GATs may play a role in the complex processes regulating cortical maturation; and (iv) GATs may contribute to the dysregulation of neuronal excitability that accompanies at least two major human diseases: epilepsy and ischemia.
Motivated behavior requires coordinated somatic, autonomic, and endocrine responses, and may be divided into initiation, procurement, and consummatory phases (Swanson, L.W. and Mogenson, G.J., Neural mechanisms for the functional coupling of autonomic, endocrine and somatomotor responses in adaptative behavior, Brain Res. Rev., 3 (1981) 1-34). Obviously, such behavior may involve the entire central nervous system, although it is important to identify circuitry or systems that mediate the behavior directed toward specific goal objects. This problem has recently been clarified by the identification of hypothalamic subsystems important for the execution of instinctive behaviors related to ingestion, reproduction, and defense. These subsystems are modulated by sensory (reflex), central control (e.g., circadian), and voluntary (cortical) inputs. The latter are dominated by inputs from the ventral temporal lobe and medial prefrontal region, which are both direct and via associated parts of the basal nuclei (ganglia). Hypothalamic output is characterized by descending projections to brainstem and spinal motor systems, and by projections back to the cerebral cortex, which are both direct and via a continuous rostromedial part of the dorsal thalamus. This thalamic region includes the anterior, medial, and midline groups, which in turn innervate a continuous ring of cortex that includes the hippocampal formation and the cingulate, prefrontal, and insular regions. Parts of this thalamic region also innervate the ventral striatum, which receives a massive input from the cortical rings as well.
The cytokines are a large and diverse family of polypeptide regulators with multiple regulatory functions that have been comprehensively evaluated in the immune system under strictly controlled experimental conditions. These peptide signals exhibit often unpredictable interactions when evaluated for their pathophysiological involvement in specific inflammatory conditions in vivo. In our joint efforts to understand the basis for early pathophysiological changes in the brains of HIV-infected subjects, we have developed animal models for lentivirus infections, and assessed the actions of various cytokines acutely on transmitter release properties in vitro, and in an in vivo transgenic mouse model. IL1beta, IL2, IL6, and IFNalpha will each enhance the release of AVP in slices of rat hypothalamus and amygdala. TGFbeta selectively blocks the ability of ACh to release AVP from hypothalamus or amygdala, but has no effects on the release stimulated by other cytokines. IFNalpha, but not TGFbeta will also activate CRH release; as with AVP, TGF selectively blocks the ACh-stimulated CRH release in both amygdala and hypothalamus. The IFNalpha-stimulated release of AVP and CRH appears to be mediated by cyclic GMP production, and this release by IFNalpha and IL-2 may be mediated in part by activation of constitutive nitric oxide synthase. These combined in vitro actions would suggest that cns cytokine actions should upregulate the hypothalamic pituitary adrenal axis. In a transgenic mouse model with increased astrocytic expression and release of the cytokine IL6, the HPA axis is upregulated, but the effect seems attributable to adrenocortical hypersensitization to ACTH. Lastly, in studies of cytokine mediated effects on astrocytic uptake of the excitatory transmitter glutamate, the reactive oxygen species hydrogen peroxide and peroxynitrite, but not nitric oxide, inhibited glutamate uptake in a concentration-dependent manner. Although superoxide and nitric oxide had no effect by themselves on the rate of glutamate uptake by astrocytes, the same cultures did respond to nitric oxide with a sustained increase in cytoplasmic free calcium. Thus while reactive oxygen species do provide a potential path to neurotoxicity but one apparently not involving nitric oxide. These various data provide important opportunities for early therapeutic interventions in neuro-inflammatory states such as Neuro-AIDS.
Using immunohistochemical and in situ hybridization methodologies the localization of neuropeptide tyrosine (NPY) and two of its receptors, the Y1- and the Y2-receptor (R), has been analysed in various tissues in normal animals and animals subjected to different experimental procedures as well as animals with a genetic and an acquired disease. (1) Dorsal root ganglion (DRG) neurons are discussed with special focus on the effect of peripheral nerve injury. In normal DRG neurons NPY cannot be detected, whereas Y1-R mRNA and Y1-R-like immunoreactivity (LI) are strongly expressed. The Y1-Rs decorate the membrane of the cell soma and are not transported peripherally into the axonal branches. Y2-R mRNA levels are low. After axotomy there is a marked increase in NPY, a decrease in Y1-Rs and an increase in Y2-Rs. The Y2-R is transported centrifugally. These findings suggest that NPY-ergic mechanisms participate in the adaptive changes of sensory neurons in response to injury. (2) Using specific antibodies the cellular and subcellular localization of the Y1-R protein have been analysed in cerebral blood vessels. The results demonstrate high concentrations of receptors in smooth muscle cells around pial arterioles with lower numbers in large vessels on the basal surface of the brain. In many regions the receptors 'disappear' after the arterioles have entered the brain tissue. At the ultrastructural level the receptors are found both on the endothelial and peripheral side of the muscle cells as well as laterally, where muscle cells oppose each other. The receptor protein is often associated with small vesicles. No NPY-positive nerve fibers were found around the Y1-R-rich arterioles, but they were only seen around the arteries with low Y1-R levels. The Y1-R-rich arterioles were, however, seen close to numerous NPY-positive fibers originating from central interneurons. These findings raise the possibility that centrally originating NPY can influence cerebral blood flow, possibly by stimulating NPY-Rs on the peripheral side of the muscle cells. However, also blood borne NPY, released under special conditions, such as stress from sympathetic nerves and the adrenal medulla and transported with blood, may stimulate receptors on the endothelial side of the smooth muscle cells. (3) In the arcuate nucleus Y1- and Y2-Rs are found, whereby the Y1-Rs are located in its ventro-medial portion and co-localized with POMC peptides, and the Y2-R in its ventromedial part, partly co-localized with NPY. NPY nerve endings makes synaptic contact with the POMC/Y1-R-positive neurons. In a mouse model for genetic anorexia very high levels of NPY were observed in arcuate neurons as compared to control mice. However, NPY mRNA levels were not different between the two groups. Taken together these findings are in good agreement with the view that NPY in the arcuate nucleus plays an important role in regulating feeding behaviour. (4) After intracerebral prion inoculation in mice an upregulation of NPY mRNA levels was observed in CA3 pyramidal neurons, and this effect was seen at a time point just before the first behavioural symptoms were manifested. At approximately the same time there was a dramatic decrease in Y2-R binding in strata oriens and radiatum of the CA1 region of the hippocampus, whereas in other regions no changes or much smaller changes were observed. Also, there was only a very slight decrease in Y2-R mRNA levels in CA3 neurons. It thus appears as if the prion disease prevents ligand binding to the Y2-R, perhaps by influencing traffic of receptor proteins, possibly at the level of cell membrane-associated caveolae, which have been implicated in the conversion of normal protein to scrapie protein. It is possible that these changes in NPY-ergic mechanisms may underlie some of the central symptoms associated with the prion disease. (ABSTRACT TRUNCATED)
The D3 dopamine receptor, a D2-like receptor, is selectively expressed in the ventral striatum, particularly in the shell of nucleus accumbens and islands of Calleja, where it is found in medium sized substance P neurons. The latter co-express the D1 receptor whose interaction with the D3 receptor was studied by treating rats with selective agonists and antagonists. In agreement with the opposite cAMP response, they mediate in cultured neuroblastoma cells, the D1 and D3 receptors exerted opposite influences on c-fos expression in islands of Calleja. However, in agreement with the synergistic influence of cAMP on D3 receptor-mediated mitogenesis on the same cultured cells, D1 and D3 receptor stimulation in vivo synergistically enhanced preprotachykinin mRNA in the shell of accumbens. This indicates that the two receptor subtypes may affect neurons in either synergy or opposition according to the cell or signal generated. Levodopa-induced behavioral sensitization in hemiparkinsonian rats is another example of D1/D3 receptor interaction. Hence repeated levodopa administration induces the ectopic appearance of the D3 receptor in substance P/dynorphin, striatonigral neurons of the dorsal striatum. This induction is secondary to D1 receptor stimulation in neurons of the denervated side and fully accounts for the sensitization, i.e. the increased behavioral responsiveness to levodopa. During brain development, a similar process could operate to control the late appearance of the D3 receptor in D1-receptor bearing neurons of the ventral striatum at a time at which they start to be innervated by dopamine neurons. Finally, taking into account a variety of genetic, developmental, neuroimaging and pharmacological data, we postulate that imbalances between the levels of D1 and D3 receptors in the same neurons could be responsible for schizophrenic disorders.
Deciphering the neurobiological consequences of cerebral cytokine expression in vivo represents an important research objective which has implications for our understanding of the pathogenesis and treatment of many significant neurological disorders. In our own pursuit of this objective, studies by us have utilized a transgenic strategy employing the GFAP promoter to direct the chronic expression of the cytokines IL-3, IL-6, IFN-alpha or TNF-alpha to astrocytes in mice. Transgenic expression of each cytokine produces a unique spectrum of neuropathological and functional alterations, thereby directly implicating these mediators in the pathogenesis of CNS disease. Moreover, as exemplified here with the GFAP-IL6 transgenic mice, these models are valuable tools in which to perform multi-level analysis to link molecular and cellular alterations to specific electrophysiological, neuroendocrine and behavioral outcomes. Integrative studies such as described here in the GFAP-cytokine transgenic mice, are providing a more thorough understanding of the actions of cytokines in the CNS and bridge the gap between structural and functional neuropathology.
Glutamate-gated cation selective channels mediate fast excitatory neurotransmission in the mammalian brain. Functionally critical channel positions contain amino acid residues not predicted from the exonic sequence for the channel subunits. The codons for these residues are created in the respective primary gene transcripts by the site selective deamination of adenosine to inosine. This type of RNA editing requires a short double-stranded RNA structure formed by the exonic sequence around the adenosine targeted for deamination with a complementary sequence in the downstream intron and hence, it precedes splicing. Candidate enzymes for nuclear transcript editing currently comprise three molecularly cloned mammalian RNA-dependent adenosine deaminases. Two of these are expressed in most body tissues, perhaps indicating that adenosine deamination in transcripts is more global than has been recognized. Indeed, numerous mRNAs in different tissues may contain inosine residues and encode proteins with amino acid substitutions and different properties relative to the exonically encoded forms. If so, RNA editing by adenosine deamination may significantly enlarge the functional repertoire of the mammalian genome.
An analysis at the network and membrane level has provided evidence that antagonistic interactions between adenosine A2A/dopamine D2 and adenosine A1/dopamine D1 receptors in the ventral and dorsal striatum are at least in part responsible for the motor stimulant effects of adenosine receptor antagonists like caffeine and for the motor depressant actions of adenosine receptor agonists. The results obtained in stably cotransfected cells also underline the hypothesis that the intramembrane A2A/D2 and A1/D1 receptor interactions represent functionally important mechanisms that may be the major mechanism for the demonstrated antagonistic A2A/D2 and A1/D1 receptor interactions found in vivo in behavioural studies and in studies on in vivo microdialysis of the striopallidal and strioentopeduncular GABAergic pathways. A major mechanism for the direct intramembrane A2A/D2 and A1/D1 receptor interactions may involve formation of A2A/D2 and A1/D1 heterodimers leading to allosteric changes that will alter the affinity as well as the G protein coupling and thus the efficacy to control the target proteins in the membranes. This is the first molecular network to cellular integration in the nerve cell membrane and may be well suited for a number of integrated tasks and can be performed in a short-time scale, in comparison with the very long-time scale observed when receptor heteroregulation involves phosphorylation or receptor resynthesis. Multiple receptor-receptor interactions within the membranes through formation of receptor clusters may lead to the storage of information within the membranes. Such molecular circuits can represent hidden layers within the membranes that substantially increase the computational potential of neuronal networks. These molecular circuits are biased and may therefore represent part of the molecular mechanism for the storage of memory traces (engrams) in the membranes.
During neonatal hypoxic-ischemic brain injury, activation of transcription of a series of genes is induced to stimulate erythropoiesis, anti-apoptosis, apoptosis, necrosis and angiogenesis. A key factor mediating these gene transcriptions is hypoxia-inducible factor-1alpha (HIF-1alpha). During hypoxia, HIF-1alpha protein is stabilized and heterodimerizes with HIF-1beta to form HIF-1, subsequently regulating the expression of target genes. HIF-1alpha participates in early brain development and proliferation of neuronal precursor cells. Under pathological conditions, HIF-1alpha is known to play an important role in neonatal hypoxic-ischemic brain injury: on the one hand, HIF-1alpha has neuroprotective effects whereas it can also have neurotoxic effects. HIF-1alpha regulates the transcription of erythropoietin (EPO), which induces several pathways associated with neuroprotection. HIF-1alpha also promotes the expression of vascular endothelial cell growth factor (VEGF), which is related to neovascularization in hypoxic-ischemic brain areas. In addition, HIF-1alpha has an anti-apoptotic effect by increasing the expression of anti-apoptotic factors such as EPO during mild hypoxia. The neurotoxic effects of HIF-1alpha are represented by its participation in the apoptotic process by increasing the stability of the tumor suppressor protein p53 during severe hypoxia. Moreover, HIF-1alpha plays a role in cell necrosis, by interacting with calcium and calpain. HIF-1alpha can also exacerbate brain edema via increasing the permeability of the blood-brain barrier (BBB). Given these properties, HIF-1alpha has both neuroprotective and neurotoxic effects after hypoxia-ischemia. These events are cell type specific and related to the severity of hypoxia. Unravelling of the complex functions of HIF-1alpha may be important when designing neuroprotective therapies for hypoxic-ischemic brain injury.
Phospholipase A(2) catalyzes the hydrolysis of membrane glycerophospholipids leading to the production of metabolites observable by both 1H and 31P magnetic resonance spectroscopy. The signal of choline-containing compounds (Cho) observed by 1H magnetic resonance spectroscopy is constituted of metabolites of phosphatidylcholine, especially phosphocholine (PCho) and glycerophosphocholine (GPCho). The phosphomonoester (PME) and phosphodiester (PDE) signals observed by 31P magnetic resonance spectroscopy are, respectively, precursors and catabolites of phospholipids. A large number of brain diseases have been reported to cause variations in the intensity of the Cho, PME and PDE signals. Changes in the activity of phospholipase A(2) have been measured in many brain diseases. In this review, the relationships between the results of 1H and 31P magnetic resonance spectroscopy and the phospholipase A(2) assays are analyzed. In many brain diseases, the variation in the Cho signal intensity can be correlated with a stimulation or inhibition of the phospholipase A(2) activity.
The pro-inflammatory cytokine interleukin(IL)-1beta is a main component in inflammatory pathways and is overexpressed in the brain of Alzheimer's disease (AD) patients. Several studies report associations between IL-1beta polymorphisms and AD, but findings from different studies are controversial. Our aim was to verify the correlation between the single nucleotide polymorphisms (SNPs) of the IL-1beta, at sites -511 and +3953, and AD by meta-analysis. Computerized bibliographic searches of PUBMED and AlzGene database (http://www.alzgene.org) were supplemented with manual searches of reference lists. There is evidence for association between IL-1beta +3953 SNP and AD, with an OR=1.60 (95% C.I.: 1.16-2.22; Z=2.83 p=0.005) for TT genotype. No significant difference in genotype distribution of the IL-1beta -511 SNP in AD was obtained, but high between-study heterogeneity was found. To reduce heterogeneity, subgroup analyses were performed using, as stratifying variables, characteristics of the population under study (age, gender, type of AD diagnosis, Mini Mental State Examination of the controls) and characteristics related to the study design (statistical power of individual studies). The frequency of the IL-1beta -511 TT genotype resulted significantly higher than other genotypes only when the Caucasian studies with the highest statistical power were included in the subgroup analysis (OR=1.32; 95% C.I.: 1.03-1.69; p=0.03), with no evidence of between-study heterogeneity. Our data support an association between the TT genotype of IL-1beta +3953 SNP and AD, and suggest a possible association of the -511 TT genotype. Unreplicability of the results seems to be due mainly to the lack of statistical power of the individual studies.
The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."