A Dopaminergic Axon Lattice in the Striatum and Its Relationship with Cortical and Thalamic Terminals
ABSTRACT Interactions between glutamatergic corticostriatal afferents and dopaminergic nigrostriatal afferents are central to basal ganglia function. The thalamostriatal projection provides a glutamatergic innervation of similar magnitude to the corticostriatal projection. We tested the hypotheses that (1) thalamostriatal synapses have similar spatial relationships with dopaminergic axons as corticostriatal synapses do and (2) the spatial relationships between excitatory synapses and dopaminergic axons are selective associations. We examined at the electron microscopic level rat striatum immunolabeled to reveal vesicular glutamate transporters (VGluTs) 1 and 2, markers of corticostriatal and thalamostriatal terminals, respectively, together with tyrosine hydroxylase (TH) to reveal dopaminergic axons. Over 80% of VGluT-positive synapses were within 1 microm of a TH-positive axon and >40% were within 1 microm of a TH-positive synapse. Of structures postsynaptic to VGluT1- or VGluT2-positive terminals, 21 and 27%, respectively, were apposed by a TH-positive axon and about half of these made synaptic contact. When structures postsynaptic to VGluT-positive terminals and VGluT-positive terminals themselves were normalized for length of plasma membrane, the probability of them being apposed by, or in synaptic contact with, a TH-positive axon was similar to that of randomly selected structures. Extrapolation of the experimental data to more closely reflect the distribution in 3D reveals that all structures in the striatum are within approximately 1 microm of a TH-positive synapse. We conclude that (1) thalamostriatal synapses are in a position to be influenced by released dopamine to a similar degree as corticostriatal synapses are and (2) these associations arise from a nonselective dopaminergic axon lattice.
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- "However, in contrast to the traditional view of inter-neuronal chemical excitatory synaptic transmission in which structural and functional specializations are observed at the postsynaptic domains, striatal dopaminergic transmission does not always require such level of postsynaptic structural specialization (Rice and Cragg, 2008; Fuxe et al., 2012). Instead, release occurs in a diffuse manner, DA receptors are extrasynaptic and ultrastructural studies on the extension and density of DA neuron axonal arborization in the striatum point to broad, intricate projections that cover vast areas (Pickel et al., 1981; Smith et al., 1994; Moss and Bolam, 2008; Matsuda et al., 2009). This diffusely spread mode of transmission (in contrast to localized, highly spatially restricted communication), is termed “volume transmission”, and is a feature of a number of transmitters such as acetylcholine, norepinephrine, DA and serotonin (Taber and Hurley, 2014). "
ABSTRACT: The mesolimbic and nigrostriatal dopamine (DA) systems play a key role in the physiology of reward seeking, motivation and motor control. Importantly, they are also involved in the pathophysiology of Parkinson's and Huntington's disease, schizophrenia and addiction. Control of DA release in the striatum is tightly linked to firing of DA neurons in the ventral tegmental area (VTA) and the substantia nigra (SN). However, local influences in the striatum affect release by exerting their action directly on axon terminals. For example, endogenous glutamatergic and cholinergic activity is sufficient to trigger striatal DA release independently of cell body firing. Recent developments involving genetic manipulation, pharmacological selectivity or selective stimulation have allowed for better characterization of these phenomena. Such termino-terminal forms of control of DA release transform considerably our understanding of the mesolimbic and nigrostriatal systems, and have strong implications as potential mechanisms to modify impaired control of DA release in the diseased brain. Here, we review these and related mechanisms and their implications in the physiology of ascending DA systems.Frontiers in Behavioral Neuroscience 05/2014; 8:188. DOI:10.3389/fnbeh.2014.00188 · 4.16 Impact Factor
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- "The post-synaptic cortical-induced excitatory responses are modulated by dopamine acting through a wide variety of pre-and/or post-synaptic mechanisms dependent on the type and localization of dopamine receptors and the physiological state of striatal MSNs (Gonon, 1997; Reynolds et al., 2001; Cragg and Rice, 2004; Surmeier et al., 2007; Rice and Cragg, 2008; Ma et al., 2012). Although the regulatory effects of dopamine on thalamic glutamatergic transmission have not been directly assessed, the similarity between the overall pattern of synaptic connectivity of non-CM/Pf thalamic and cortical terminals with MSNs (Moss and Bolam, 2008) suggests that these two pathways may be regulated in the same manner by nigrostriatal dopamine afferents (Figure 1). The dopaminergic modulation of corticostriatal transmission relies in part on the synaptic convergence of dopaminergic and cortical synapses on individual spines of striatal MSNs (Freund et al., 1984; Bolam and Smith, 1990; Smith et al., 1994) and/or pre-synaptic dopamine-mediated regulation of glutamate release from neighboring cortical terminals (Surmeier et al., 2007). "
ABSTRACT: Because of our limited knowledge of the functional role of the thalamostriatal system, this massive network is often ignored in models of the pathophysiology of brain disorders of basal ganglia origin, such as Parkinson's disease (PD). However, over the past decade, significant advances have led to a deeper understanding of the anatomical, electrophysiological, behavioral and pathological aspects of the thalamostriatal system. The cloning of the vesicular glutamate transporters 1 and 2 (vGluT1 and vGluT2) has provided powerful tools to differentiate thalamostriatal from corticostriatal glutamatergic terminals, allowing us to carry out comparative studies of the synaptology and plasticity of these two systems in normal and pathological conditions. Findings from these studies have led to the recognition of two thalamostriatal systems, based on their differential origin from the caudal intralaminar nuclear group, the center median/parafascicular (CM/Pf) complex, or other thalamic nuclei. The recent use of optogenetic methods supports this model of the organization of the thalamostriatal systems, showing differences in functionality and glutamate receptor localization at thalamostriatal synapses from Pf and other thalamic nuclei. At the functional level, evidence largely gathered from thalamic recordings in awake monkeys strongly suggests that the thalamostriatal system from the CM/Pf is involved in regulating alertness and switching behaviors. Importantly, there is evidence that the caudal intralaminar nuclei and their axonal projections to the striatum partly degenerate in PD and that CM/Pf deep brain stimulation (DBS) may be therapeutically useful in several movement disorders.Frontiers in Systems Neuroscience 01/2014; 8:5. DOI:10.3389/fnsys.2014.00005
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- "In the NAc, GPNs target CINs nearly exclusively, with very sparse innervation of MSNs (Brown et al., 2012). Conversely, VTA DA neurons modulate both MSNs and CINs, either through volume transmission or direct synaptic contacts (Moss and Bolam, 2008). "
ABSTRACT: The mesolimbic reward system is primarily comprised of the ventral tegmental area (VTA) and the nucleus accumbens (NAc) as well as their afferent and efferent connections. This circuitry is essential for learning about stimuli associated with motivationally-relevant outcomes. Moreover, addictive drugs affect and remodel this system, which may underlie their addictive properties. In addition to dopamine (DA) neurons, the VTA also contains approximately 30% γ-aminobutyric acid (GABA) neurons. The task of signaling both rewarding and aversive events from the VTA to the NAc has mostly been ascribed to DA neurons and the role of GABA neurons has been largely neglected until recently. GABA neurons provide local inhibition of DA neurons and also long-range inhibition of projection regions, including the NAc. Here we review studies using a combination of in vivo and ex vivo electrophysiology, pharmacogenetic and optogenetic manipulations that have characterized the functional neuroanatomy of inhibitory circuits in the mesolimbic system, and describe how GABA neurons of the VTA regulate reward and aversion-related learning. We also discuss pharmacogenetic manipulation of this system with benzodiazepines (BDZs), a class of addictive drugs, which act directly on GABAA receptors located on GABA neurons of the VTA. The results gathered with each of these approaches suggest that VTA GABA neurons bi-directionally modulate activity of local DA neurons, underlying reward or aversion at the behavioral level. Conversely, long-range GABA projections from the VTA to the NAc selectively target cholinergic interneurons (CINs) to pause their firing and temporarily reduce cholinergic tone in the NAc, which modulates associative learning. Further characterization of inhibitory circuit function within and beyond the VTA is needed in order to fully understand the function of the mesolimbic system under normal and pathological conditions.Frontiers in Behavioral Neuroscience 01/2014; 8:8. DOI:10.3389/fnbeh.2014.00008 · 4.16 Impact Factor