Chetan C. Gandhi’s research while affiliated with Rutgers, The State University of New Jersey and other places

Human performance on diverse tests of intellect are impacted by a "general" regulatory factor that accounts for up to 50% of the variance between individuals on intelligence tests. Neurobiological determinants of general cognitive abilities are essentially unknown, owing in part to the paucity of animal research wherein neurobiological analyses are possible. We report a methodology with which we have assessed individual differences in the general learning abilities of laboratory mice. Abilities of mice on tests of associative fear conditioning, operant avoidance, path integration, discrimination, and spatial navigation were assessed. Tasks were designed so that each made unique sensory, motor, motivational, and information processing demands on the animals. A sample of 56 genetically diverse outbred mice (CD-1) was used to assess individuals' acquisition on each task. Indicative of a common source of variance, positive correlations were found between individuals' performance on all tasks. When tested on multiple test batteries, the overall performance ranks of individuals were found to be highly reliable and were "normally" distributed. Factor analysis of learning performance variables determined that a single factor accounted for 38% of the total variance across animals. Animals' levels of native activity and body weights accounted for little of the variability in learning, although animals' propensity for exploration loaded strongly (and was positively correlated) with learning abilities. These results indicate that diverse learning abilities of laboratory mice are influenced by a common source of variance and, moreover, that the general learning abilities of individual mice can be specified relative to a sample of peers. Matzel,L.D.,Han YR,Grossman H,Karnik MS,Patel D,Scott N,Specht SM,Gandhi CC.

Using an identified synapse in the nervous system of the mollusc Hermissenda, the influence of somatic calcium accumulation on regulated synaptic transmission was investigated. Hair cells in Hermissenda project onto postsynaptic B photoreceptors where they mediate inhibitory postsynaptic potentials (IPSPs). Intracellular recordings in combination with bath perfusion of calcium channel modulators indicated that L-type channels were present on the hair cell soma but not on the terminal branches. In contrast, P/Q and an unidentified channel type (similar to N-type channels) contributed additively to transmitter release from the hair cell. Antibodies raised against rat brain channel proteins detected L- (alpha1

In laboratory studies, the assessment of memory is typically associated with overt behavioral responses. Thus, it has been difficult to determine whether the enhancement of hippocampal sensory-evoked potentials that often accompany memory formation are the neurophysiological manifestation of a memory "trace" or are a secondary product of the behavioral expression of the memory. We addressed this issue by examining changes in evoked hippocampal field potentials during sensory preconditioning, a form of behaviorally silent relational learning that requires an intact hippocampus for execution. Rats were exposed to presentations of a white noise (S1) that terminated with a tone (S2). These pairings of ostensibly "neutral" stimuli supported no change in the behavior elicited by the noise. However, if the tone was subsequently paired with mild footshock (US), suppression of ongoing licking behavior (indicative of fear) was elicited by the noise, indicating that the animal had associated the noise with tone (S1-S2), and had represented the noise-tone-shock (S1-S2-US) relationship. Pre-training neurotoxic lesions of the hippocampus had no effect on conditioned suppression to tone after tone-shock (S2-US) pairings, but disrupted the expression of continued suppression to noise (S1) after tone-shock pairings. In a second experiment, sensory-evoked field potentials in the dorsal hippocampus were recorded with extracellular electrodes. No changes in the hippocampal response evoked by white noise were observed after pairings of noise and tone, i.e., no evidence for a memory trace could be detected. In contrast, after tone was paired with footshock, two short-latency negative potentials within the noise-evoked field response increased in amplitude, a response often presumed to reflect a neurophysiological correlate of memory storage. In total, these results suggest that although the hippocampus critically contributes to the processing of a behaviorally silent associative memory, there may be no role for changes in the amplitude of hippocampal sensory-evoked field potentials in storing representations of the relationships between sensory experiences.

It is well established that the hippocampal formation is critically involved in the acquisition of trace memories, a paradigm in which the conditioned (CS) and unconditioned stimuli (US) are separated by a temporal gap (Solomon et al., 1986). The structure is reportedly not critical for the acquisition of delay memories, where the CS and the US overlap in time (Berger & Orr, 1983; Schmaltz & Theios, 1972). Based on these results, it is often stated that the hippocampus is involved in "filling the gap" or otherwise associating the two stimuli in time. However, in addition to the presence of a temporal gap, there are other differences between trace and delay conditioning. The most apparent difference is that animals require many more trials to learn the trace task, and thus it is inherently more difficult than the delay task. Here, we tested whether the hippocampus was critically involved in delay conditioning, if it was rendered more difficult such that the rate of acquisition was shifted to be analogous to trace conditioning. Groups of rats received excitotoxic lesions to the hippocampus, sham lesions or were left intact. Using the same interstimulus intervals (ISI), control animals required more trials to acquire the trace than the delay task. As predicted, animals with hippocampal lesions were impaired during trace conditioning but not delay conditioning. However, when the delay task was rendered more difficult by extending the ISI (a long delay task), animals with hippocampal lesions were impaired. In addition, once the lesioned animal learned the association between the CS and the US during delay conditioning, it could learn and perform the trace CR. Thus, the role of the hippocampus in classical conditioning is not limited to learning about discontiguous events in time and space; rather the structure can become engaged simply as a function of task difficulty.

Type B photoreceptors in Hermissenda exhibit increased excitability (e.g., elevated membrane resistance and lowered spike thresholds) consequent to the temporal coincidence of a light-induced intracellular Ca(2+) increase and the release of GABA from presynaptic vestibular hair cells. Convergence of these pre- and postsynaptically stimulated biochemical cascades culminates in the activation of protein kinase C (PKC). Paradoxically, exposure of the B cell to light alone generates an inositol triphosphate-regulated rise in diacylglycerol and intracellular Ca(2+), co-factors sufficient to stimulate conventional PKC isoforms, raising questions as to the unique role of synaptic stimulation in the activation of PKC. GABA receptors on the B cell are coupled to G proteins that stimulate phospholipase A(2) (PLA(2)), which is thought to regulate the liberation of arachidonic acid (AA), an "atypical" activator of PKC. Here, we directly assess whether GABA binding or PLA(2) stimulation liberates AA in these cells and whether free AA potentiates the stimulation of PKC. Free fatty-acid was estimated in isolated photoreceptors with the fluorescent indicator acrylodan-derivatized intestinal fatty acid-binding protein (ADIFAB). In response to 5 microM GABA, a fast and persistent increase in ADIFAB emission was observed, and this increase was blocked by the PLA(2) inhibitor arachidonyltrifluoromethyl ketone (50 microM). Furthermore, direct stimulation of PLA(2) by melittin (10 microM) increased ADIFAB emission in a manner that was kinetically analogous to GABA. In response to simultaneous exposure to the stable AA analogue oleic acid (OA, 20 microM) and light (to elevate intracellular Ca(2+)), B photoreceptors exhibited a sustained (>45 min) increase in excitability (membrane resistance and evoked spike rate). The excitability increase was blocked by the PKC inhibitor chelerythrine (20 microM) and was not induced by exposure of the cells to light alone. The increase in excitability in the B cell that followed exposure to light and OA persisted for > or =90 min when the pairing was conducted in the presence of the protein synthesis inhibitor anisomycin (1 microm), suggesting that the synergistic influence of these signaling agents on neuronal excitability did not require new protein synthesis. These results indicate that GABA binding to G-protein-coupled receptors on Hermissenda B cells stimulates a PLA(2) signaling cascade that liberates AA, and that this free AA interacts with postsynaptic Ca(2+) to synergistically stimulate PKC and enhance neuronal excitability. In this manner, the interaction of postsynaptic metabotropic receptors and intracellular Ca(2+) may serve as the catalyst for some forms of associative neuronal/synaptic plasticity.

Though once of central importance to psychologists and neurophysiologists alike, the elucidation of neural substrates for individual differences in learning no longer attracts a broad research effort and occupies a place of largely historical interest to the contemporary disciplines. The decline in interest in this subject ensued in part from the perception, arrived at decades ago, that individual differences in learning were not quantified as easily as had once been presumed. Furthermore, the dominant hypotheses in the field defied testing within the constraints imposed by the complex and largely inaccessible vertebrate nervous system. Using a 'model systems' approach where the individual cells and synaptic interactions that comprise a neural network can be identified, we have returned to this question and have established a framework by which we can begin to discern the basis for much of the variability between individuals in their capacity to learn. In the marine mollusc Hermissenda, we have found that a common influence on transmitter exocytosis is expressed homogeneously throughout the nervous system regardless of transmitter system or receptor class. Though uniformly expressed within an individual, this influence on synaptic efficacy is differentially expressed between animals. Importantly, the basal efficiency of exocytosis expressed in an individual nervous system is strongly correlated with the degree to which activity-dependent forms of neuronal/synaptic facilitation can be induced in that nervous system, and predicts the capacity for the intact animal to learn a Pavlovian association. Furthermore, we have established that a decline in basal synaptic efficacy in aged animals, arising from chronic presynaptic Ca(2+) 'leak', may contribute to age-related learning impairments. Because certain fundamental components of the exocytotic cascade are conserved widely across cell types, transmitter systems and species, the principles that we describe may have broad implications for understanding normal variability in learning, but also, in the development of specific strategies to compensate for mild learning deficits and age-related cognitive decline.

Spatial learning in the Morris water maze task is believed to be dependent on an intact hippocampal system. However, evidence from human studies and animal experiments suggests a potential cerebellar involvement in spatial processing, place learning, and other types of 'higher-order' cognition. In order to investigate this possibility, intraventricular injections (ICV) of the anti-neuronal immunotoxin OX7-saporin were used to selectively destroy cerebellar Purkinje cells, without affecting other brain areas believed to be critically involved in spatial learning and memory. Bilateral ICV injections of 2 microg OX7-saporin (4 microg total) in adult male rats produced substantial loss of Purkinje cells (56%) throughout the cerebellum without affecting hippocampal morphology or biochemical indices of cholinergic, serotonergic, or catecholaminergic function in the hippocampus, frontal cortex, or striatum. ICV OX7-saporin significantly impaired acquisition and performance of the standard Morris water maze task (though the impairment was less severe than reported in earlier studies that used alternate lesion methods or mutant mice species), but did not alter performance on the cued version of the task, or locomotor activity. In addition, lesioned animals spent significantly less time in the target quadrant on probe trial days 4 and 7 and the average distance to target scores (ADT) were significantly greater than controls on those days. Swim speed was not affected. Based on the specificity of the behavioral and neurobiological alterations, these data support the hypothesis that the cerebellum is involved in spatial processing and place learning.

The hypothesis that an individual's capacity for learning might be predicted or influenced by basal levels of synaptic efficacy has eluded empirical tests, owing in part to the inability to compare between animals single identified synaptic responses in the mammalian brain. To overcome this limitation, we have focused our analysis on the invertebrate Hermissenda, whose nervous system is composed of identifiable cells and synaptic interactions. Hermissenda were exposed to paired presentations of light and rotation such that the light came to elicit a learned defensive motor response. An animal's rate of learning was strongly correlated with the amplitude of the synaptic potential evoked in that animal's visual (light sensitive) receptors in response to stimulation of presynaptic vestibular (rotation sensitive) hair cells. In naive animals, strong correlations between the amplitude of both inhibitory and excitatory synaptic potentials were observed between synapses distributed throughout an animal's nervous system, and this conservation of synaptic efficacy was largely attributable to a common influence on transmitter release. These observations suggest that basal synaptic efficacy may be uniformly regulated throughout a nervous system, and provide direct evidence that the basal efficacy of synaptic transmission predicts, and possibly contributes to, individual differences between animals in their capacity to learn.

Descriptions of conditioned response generation in Hermissenda stipulate that the synaptic interaction between type B and A photoreceptors should be enhanced after associative pairings of light and rotation. Although evidence from several laboratories has confirmed this assumption, the mechanism underlying this synaptic facilitation has not been elucidated. Here we report that in vitro conditioning (i.e., light paired with stimulation of vestibular hair cells) modifies the kinetics of presynaptic action potentials in the B photoreceptor in a manner sufficient to account for this synaptic facilitation. After paired training, we observed an increase in the duration of evoked action potentials and a decrease in the amplitude of the spike afterhyperpolarization in the B-cell. As previously reported, paired training also enhanced the excitability (i.e., input resistance and evoked spike rate) of the B photoreceptor. In a second experiment, simultaneous recordings were made in type B and A photoreceptors, and paired training was found to produce an increase in the amplitude of the IPSP in the A photoreceptor in response to an evoked spike in the B-cell. Importantly, there was no change in the initial slope of the postsynaptic IPSP in the A photoreceptor, suggesting that spike duration-independent mechanisms of neurotransmitter exocytosis or postsynaptic receptor sensitivity did not contribute to the observed synaptic facilitation. Perfusion of 4-aminopyridine (4-AP) mimicked a known effect of behavioral conditioning in that it specifically reduced the amplitude of the transient voltage-dependent K(+) current (I(A)) in the B-cell, but in addition, produced action potential broadening and synaptic facilitation that was analogous to that observed after in vitro conditioning. Finally, the effect of 4-AP on B-cell action potentials and on the postsynaptic IPSP in the A-cell was occluded by previous paired (but not unpaired) training, suggesting that the prolongation of the B-cell action potential by a reduction of I(A) was sufficient to account for the observed synaptic facilitation. The occlusion of the effects of 4-AP by paired training was not attributable to a saturation of the capacity of the B-cell for transmitter exocytosis, because it was observed that tetraethylammonium (TEA)-induced inhibition of the delayed voltage-dependent K(+) current induced both spike broadening and synaptic facilitation regardless of training history. Collectively, these results demonstrate that training-induced facilitation at B-cell synapses is attributable to the effects of a reduction of a presynaptic K(+) conductance on action potential kinetics and suggest another critical similarity between the cellular basis for learning in Hermissenda and other invertebrate systems.

Injection of GABA(A)/benzodiazepine receptor ligands into the medial septum (MS) alters the activity of cholinergic neurons that innervate the hippocampus and can produce bidirectional modulation of spatial memory. Recent evidence suggests that two subtypes of the GABA(A) receptor are differentially localized to either GABAergic (alpha(1)/beta(2)/gamma(2)) or cholinergic (alpha(3)/beta(3)/gamma(2)) neurons within the MS. The present studies characterized the dose-related behavioral and neurochemical effects of intraseptal infusions of two benzodiazepine (BDZ) agonists that appear to exhibit different profiles of pharmacological specificity for these receptor subtypes. Male Sprague-Dawley rats were cannulated and then artificial CSF, chlordiazepoxide (CDP: 8 or 12 microg), or zolpidem (4, 8, or 12 microg) was injected into the MS. Spatial working memory was assessed in a delay radial-arm maze task and the activity of cholinergic neurons in the MS was evaluated by high-affinity choline uptake (HA-ChU) in the hippocampus. Intraseptal injection of either CDP or zolpidem produced dose-related impairments in spatial working memory and decreases in hippocampal HAChU. Both BDZ agonists were found to produce retrograde memory deficits and a decrease in HAChU following the highest dose tested (12 microg). However, intraseptal injection of 8 microg of zolpidem produced a behavioral deficit comparable to the high dose of CDP, but did not alter HAChU within the HPC. Although the cholinergic component of the septohippocampal pathway has been shown to be important in modulating hippocampal physiology and spatial memory processes, data from the present experiments suggest that the GABAergic component may also play an important role in the behavioral functions of the septohippocampal pathway.