Neurons of the acoustic thalamus that project to the amygdala contain glutamate

Long-Term Value Memory in the Primate Posterior Thalamus for Fast Automatic Action

The thalamus is known to process information from various brain regions and relay it to other brain regions, serving an essential role in sensory perception and motor execution. The thalamus also receives inputs from basal ganglia nuclei (BG) involved in value-based decision making, suggesting a role in the value process. We found that neurons in a particular area of the rhesus macaque posterior thalamus encoded the historical value memory of visual objects. Many of these value-coding neurons were located in the suprageniculate nucleus (SGN). This thalamic area directly received anatomical input from the superior colliculus (SC), and the neurons showed visual responses with contralateral preferences. Notably, the value discrimination activity of these thalamic neurons increased during learning, with the learned values stably retained even more than 200 days after learning. Our data indicate that single neurons in the posterior thalamus not only processed simple visual information but also represented historical values. Furthermore, our data suggest an SC-posterior thalamus-BG-SC subcortical loop circuit that encodes the historical value, enabling a quick automatic gaze by bypassing the visual cortex.

Plasticity in Limbic Regions at Early Time Points in Experimental Models of Tinnitus

Tinnitus is one of the most prevalent auditory disorders worldwide, manifesting in both chronic and acute forms. The pathology of tinnitus has been mechanistically linked to induction of harmful neural plasticity stemming from traumatic noise exposure, exposure to ototoxic medications, input deprivation from age-related hearing loss, and in response to injuries or disorders damaging the conductive apparatus of the ears, the cochlear hair cells, the ganglionic cells of the VIIIth cranial nerve, or neurons of the classical auditory pathway which link the cochlear nuclei through the inferior colliculi and medial geniculate nuclei to auditory cortices. Research attempting to more specifically characterize the neural plasticity occurring in tinnitus have used a wide range of techniques, experimental paradigms, and sampled at different windows of time to reach different conclusions about why and which specific brain regions are crucial in the induction or ongoing maintenance of tinnitus-related plasticity. Despite differences in experimental methodologies, evidence reveals similar findings that strongly suggest that immediate and prolonged activation of non-classical auditory structures (i.e., amygdala, hippocampus, and cingulate cortex) may contribute to the initiation and development of tinnitus in addition to the ongoing maintenance of this devastating condition. The overarching focus of this review, therefore, is to highlight findings from the field supporting the hypothesis that abnormal early activation of non-classical sensory limbic regions are involved in tinnitus induction, with activation of these regions continuing to occur at different temporal stages. Since initial/early stages of tinnitus are difficult to control and to quantify in human clinical populations, a number of different animal paradigms have been developed and assessed in experimental investigations. Reviews of traumatic noise exposure and ototoxic doses of sodium salicylate, the most prevalently used animal models to induce experimental tinnitus, indicate early limbic system plasticity (within hours, minutes, or days after initial insult), supports subsequent plasticity in other auditory regions, and contributes to the pathophysiology of tinnitus. Understanding this early plasticity presents additional opportunities for intervention to reduce or eliminate tinnitus from the human condition.

Reciprocal connectivity between secondary auditory cortical field and amygdala in mice

Recent studies have examined the feedback pathway from the amygdala to the auditory cortex in conjunction with the feedforward pathway from the auditory cortex to the amygdala. However, these connections have not been fully characterized. Here, to visualize the comprehensive connectivity between the auditory cortex and amygdala, we injected cholera toxin subunit b (CTB), a bidirectional tracer, into multiple subfields in the mouse auditory cortex after identifying the location of these subfields using flavoprotein fluorescence imaging. After injecting CTB into the secondary auditory field (A2), we found densely innervated CTB-positive axon terminals that were mainly located in the lateral amygdala (La), and slight innervations in other divisions such as the basal amygdala. Moreover, we found a large number of retrogradely-stained CTB-positive neurons in La after injecting CTB into A2. When injecting CTB into the primary auditory cortex (A1), a small number of CTB-positive neurons and axons were visualized in the amygdala. Finally, we found a near complete absence of connections between the other auditory cortical fields and the amygdala. These data suggest that reciprocal connections between A2 and La are main conduits for communication between the auditory cortex and amygdala in mice.

Fear potentiated startle increases phospholipase D (PLD) expression/activity and PLD-linked metabotropic glutamate receptor mediated post-tetanic potentiation in rat amygdala

Long-term memory (LTM) of fear stores activity dependent modifications that include changes in amygdala signaling. Previously, we identified an enhanced probability of release of glutamate mediated signaling to be important in rat fear potentiated startle (FPS), a well-established translational behavioral measure of fear. Here, we investigated short- and long-term synaptic plasticity in FPS involving metabotropic glutamate receptors (mGluRs) and associated downstream proteomic changes in the thalamic-lateral amygdala pathway (Th-LA). Aldolase A, an inhibitor of phospholipase D (PLD), expression was reduced, concurrent with significantly elevated PLD protein expression. Blocking the PLD-mGluR signaling significantly reduced PLD activity. While transmitter release probability increased in FPS, PLD-mGluR agonist and antagonist actions were occluded. In the unpaired group (UNP), blocking the PLD-mGluR increased while activating the receptor decreased transmitter release probability, consistent with decreased synaptic potentials during tetanic stimulation. FPS Post-tetanic potentiation (PTP) immediately following long-term potentiation (LTP) induction was significantly increased. Blocking PLD-mGluR signaling prevented PTP and reduced cumulative PTP probability but not LTP maintenance in both groups. These effects are similar to those mediated through mGluR7, which is co-immunoprecipitated with PLD in FPS. Lastly, blocking mGluR-PLD in the rat amygdala was sufficient to prevent behavioral expression of fear memory. Thus, our study in the Th-LA pathway provides the first evidence for PLD as an important target of mGluR signaling in amygdala fear-associated memory. Importantly, the PLD-mGluR provides a novel therapeutic target for treating maladaptive fear memories in posttraumatic stress and anxiety disorders.

Growth and refinement of excitatory synapses in the human auditory cortex

We had earlier demonstrated a neurofilament-rich plexus of axons in the presumptive human auditory cortex during fetal development which became adult-like during infancy. To elucidate the origin of these axons, we studied the expression of the vesicular glutamate transporters (VGLUT) 1 and 2 in the human auditory cortex at different stages of development. While VGLUT-1 expression predominates in intrinsic and cortico-cortical synapses, VGLUT-2 expression predominates in thalamocortical synapses. Levels of VGLUT-2 mRNA were higher in the auditory cortex before birth compared to postnatal development. In contrast, levels of VGLUT-1 mRNA were low before birth and increased during postnatal development to peak during childhood and then began to decrease in adolescence. Both VGLUT-1 and VGLUT-2 proteins were present in the human auditory cortex as early as 15GW. Further, immunohistochemistry revealed that the supra- and infragranular layers were more immunoreactive for VGLUT-1 compared to that in Layer IV at 34GW and this pattern was maintained until adulthood. As for VGLUT-1 mRNA, VGLUT-1 synapses increased in density between prenatal development and childhood in the human auditory cortex after which they appeared to undergo attrition or pruning. The adult pattern of VGLUT-2 immunoreactivity (a dense band of VGLUT-2-positive terminals in Layer IV) also began to appear in the presumptive Heschl's gyrus at 34GW. The density of VGLUT-2-positive puncta in Layer IV increased between prenatal development and adolescence, followed by a decrease in adulthood, suggesting that thalamic axons which innervate the human auditory cortex undergo pruning comparatively late in development.

Basolateral amygdala rapid glutamate release encodes an outcome-specific representation vital for reward-predictive cues to selectively invigorate reward-seeking actions

Environmental stimuli have the ability to generate specific representations of the rewards they predict and in so doing alter the selection and performance of reward-seeking actions. The basolateral amygdala participates in this process, but precisely how is unknown. To rectify this, we monitored, in near-real time, basolateral amygdala glutamate concentration changes during a test of the ability of reward-predictive cues to influence reward-seeking actions (Pavlovian-instrumental transfer). Glutamate concentration was found to be transiently elevated around instrumental reward seeking. During the Pavlovian-instrumental transfer test these glutamate transients were time-locked to and correlated with only those actions invigorated by outcome-specific motivational information provided by the reward-predictive stimulus (i.e., actions earning the same specific outcome as predicted by the presented CS). In addition, basolateral amygdala AMPA, but not NMDA glutamate receptor inactivation abolished the selective excitatory influence of reward-predictive cues over reward seeking. These data support [corrected] the hypothesis that transient glutamate release in the BLA can encode the outcome-specific motivational information provided by reward-predictive stimuli.

New perspectives in glutamate and anxiety

Anxiety and stress-related disorders, namely posttraumatic stress disorder (PTSD), generalized anxiety disorder (GAD), obsessive-compulsive disorder (ODC), social and specific phobias, and panic disorder, are a major public health issue. A growing body of evidence suggests that glutamatergic neurotransmission may be involved in the biological mechanisms underlying stress response and anxiety-related disorders. The glutamatergic system mediates the acquisition and extinction of fear-conditioning. Thus, new drugs targeting glutamatergic neurotransmission may be promising candidates for new pharmacological treatments. In particular, N-methyl-d-aspartate receptors (NMDAR) antagonists (AP5, AP7, CGP37849, CGP39551, LY235959, NPC17742, and MK-801), NMDAR partial agonists (DCS, ACPC), α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) antagonists (topiramate), and several allosteric modulators targeting metabotropic glutamate receptors (mGluRs) mGluR1, mGluR2/3, and mGluR5, have shown anxiolytic-like effects in several animal and human studies. Several studies have suggested that polyamines (agmatine, putrescine, spermidine, and spermine) may be involved in the neurobiological mechanisms underlying stress-response and anxiety-related disorders. This could mainly be attributed to their ability to modulate ionotropic glutamate receptors, especially NR2B subunits. The aim of this review is to establish that glutamate neurotransmission and polyaminergic system play a fundamental role in the onset of anxiety-related disorders. This may open the way for new drugs that may help to treat these conditions.

A role for nitric oxide-driven retrograde signaling in the consolidation of a fear memory

In both invertebrate and vertebrate models of synaptic plasticity, signaling via the putative “retrograde messenger” nitric oxide (NO) has been hypothesized to serve as a critical link between functional and structural alterations at pre- and postsynaptic sites. However, while in vitro models of synaptic plasticity have consistently implicated NO signaling in linking postsynaptic induction mechanisms with accompanying presynaptic changes, a convincing role of such “retrograde signaling” in mammalian memory formation has remained elusive. Using auditory Pavlovian fear conditioning, we show that synaptic plasticity and NO signaling in the lateral nucleus of the amygdala (LA) regulate the expression of the ERK-driven immediate early gene early growth response gene I (EGR-1) in regions of the auditory thalamus that are presynaptic to the LA. Further, antisense knockdown of EGR-1 in the auditory thalamus impairs both fear memory consolidation and the training-induced elevation of two presynaptically localized proteins in the LA. These findings indicate that synaptic plasticity and NO signaling in the LA during auditory fear conditioning promote alterations in ERK-driven gene expression in auditory thalamic neurons that are required for both fear memory consolidation as well as presynaptic correlates of fear memory formation in the LA, and provide general support for a role of NO as a “retrograde signal” in mammalian memory formation.

Regional and laminar distribution of the vesicular glutamate transporter, VGluT2, in the macaque monkey auditory cortex

The auditory cortex of primates contains 13 areas distributed among 3 hierarchically connected regions: core, belt, and parabelt. Thalamocortical inputs arise in parallel from four divisions of the medial geniculate complex (MGC), which have regionally distinct projection patterns. These inputs terminate in layers IIIb and/or IV, and are assumed to be glutamatergic, although this has not been verified. In the present study, immunoreactivity (-ir) for the vesicular glutamate transporter, VGluT2, was used to estimate the regional and laminar distribution of the glutamatergic thalamocortical projection in the macaque auditory cortex. Coronal sections containing auditory cortex were processed for VGluT2 and other markers concentrated in the thalamorecipient layers: cytochrome oxidase, acetylcholinesterase, and parvalbumin. Marker expression was studied with wide field and confocal microscopy. The main findings were: (1) VGluT2-ir was highest in the core, intermediate in the belt, and sparse in the parabelt; (2) VGluT2-ir was concentrated in the neuropil of layers IIIb/IV in the core and layer IIIb in the belt; (3) VGluT2-ir matched regional and laminar expression of the other chemoarchitectonic markers. The results indicate that the glutamatergic thalamic projection to auditory cortex, as indexed by VGluT2-ir, varies along the core-belt-parabelt axis in a manner that matches the gradients of other markers. These chemoarchitectonic features are likely to subserve regional differences in neuronal activity between regions of auditory cortex.

Auditory, somatosensory, and multisensory insular cortex in the rat

Compared with other areas of the forebrain, the function of insular cortex is poorly understood. This study examined the unisensory and multisensory function of the rat insula using high-resolution, whole-hemisphere, epipial evoked potential mapping. We found the posterior insula to contain distinct auditory and somatotopically organized somatosensory fields with an interposed and overlapping region capable of integrating these sensory modalities. Unisensory and multisensory responses were uninfluenced by complete lesioning of primary and secondary auditory and somatosensory cortices, suggesting a high degree of parallel afferent input from the thalamus. In light of the established connections of the posterior insula with the amygdala, we propose that integration of auditory and somatosensory modalities reported here may play a role in auditory fear conditioning.

Prenatal stress affects the developmental trajectory of the rat amygdala

The amygdala plays a critical role in generating the emotion of fear, and alterations in amygdala fear processing are thought to underlie the acquisition and maintenance of anxiety disorders. The prenatally stressed (PS) rat displays hormonal, behavioral and brain anatomical similarities to anxious humans and is useful to study the neurobiological underpinnings of pathological anxiety. We studied PS and control male rats at postnatal days 7 (P7), P25, P45 and P60. Using unbiased stereological analyses we examined the volumes, anterior-posterior lengths and total numbers of neurons and glia of the basolateral (BL), central (Ce) and lateral (La) amygdalar nuclei. We found prenatal stress-associated differences in the developmental trajectories of each nucleus. These were apparent in some measures as early as P7, most extensive at P25 and resolved by P45, at least as seen by Nissl staining. These changes were not a result of differential brain growth. This early divergence in developmental trajectories seen here may be the harbinger of PS rat amygdalas that ultimately function very differently in adulthood.

Localization of glucocorticoid receptors at postsynaptic membranes in the lateral amygdala

Glucocorticoids, released in high concentrations from the adrenal cortex during stressful experiences, bind to glucocorticoid receptors in nuclear and peri-nuclear sites in neuronal somata. Their classically known mode of action is to induce gene promoter receptors to alter gene transcription. Nuclear glucocorticoid receptors are particularly dense in brain regions crucial for memory, including memory of stressful experiences, such as the hippocampus and amygdala. While it has been proposed that glucocorticoids may also act via membrane bound receptors, the existence of the latter remains controversial. Using electron microscopy, we found glucocorticoid receptors localized to non-genomic sites in rat lateral amygdala, glia processes, presynaptic terminals, neuronal dendrites, and dendritic spines including spine organelles and postsynaptic membrane densities. The lateral nucleus of the amygdala is a region specifically implicated in the formation of memories for stressful experiences. These newly observed glucocorticoid receptor immunoreactive sites were in addition to glucocorticoid receptor immunoreactive signals observed using electron and confocal microscopy in lateral amygdala principal neuron and GABA neuron soma and nuclei, cellular domains traditionally associated with glucocorticoid immunoreactivity. In lateral amygdala, glucocorticoid receptors are thus also localized to non-nuclear-membrane translocation sites, particularly dendritic spines, where they show an affinity for postsynaptic membrane densities, and may have a specialized role in modulating synaptic transmission plasticity related to fear and emotional memory.

Learning the morphine conditioned cue preference: Cue configuration determines effects of lesions

The morphine conditioned cue preference was investigated using two different apparatus configurations. In one configuration, with a clear Plexiglas partition separating the drug-paired and unpaired compartments, rats could see the cues in both compartments while in either one. In the other configuration, with an opaque wood partition separating the two compartments, rats could see the cues in only one compartment at a time. The experiment had three phases: a session of pre-exposure to the entire apparatus; four 2-day training trials during each of which rats received pairings of 5 mg/Kg morphine sulphate with one compartment and saline with the other (compartments and order counterbalanced), and a test session in which the undrugged rats moved freely between the compartments while the time spent in each was measured. Four groups of rats were trained using the opaque partition in all three phases. Normal rats and rats with amygdala or nucleus accumbens lesions exhibited preferences for their morphine-paired compartments; rats with fimbria-fornix lesions had no preferences. Four additional groups were trained using the clear partition during pre-exposure, the opaque partition during training and the clear partition during testing. Normal rats and rats with fimbria-fornix lesions exhibited preferences, rats with amygdala or nucleus accumbens lesions had no preferences. This interaction between lesioned structures and the apparatus configuration is accounted for by the idea that different types of learning produced the preference for morphine-paired cues in the two apparatus configurations. Each type was learned in a different memory system and so was impaired by different lesions. These findings contribute to understanding the nature of the learning processes that produce the morphine CCP.

Distribution of calcitonin gene-related peptide-like immunoreactivity in the brain of the lizard Podarcis hispanica

The present work studies the distribution of calcitonin gene-related peptide-immunoreactive (CGRP-li) neurons and fibers in the brain of a reptile, the lizard Podarcis hispanica. CGRP-li perikarya were not present in the telencephalon. In the thalamus, CGRP-li perikarya were restricted to the posteromedial and posterolateral nuclei. In the hypothalamus, CGRP-li cells were found mainly in the supramammillary and mammillary nuclei. In the midbrain and brainstem, CGRP-li cells appeared in the ventral tegmental area, the parabrachial nucleus, and the motor nuclei of the III-VII, IX, X, and XII cranial nerves. Motoneurons of the ventral horn of the spinal cord were also immunoreactive for CGRP. CGRP-li fibers were seen in the telencephalic hemispheres, where a dense plexus of reactive fibers appeared in the septum and in the lateral striatoamygdaloid transition area. From the latter, CGRP-li fibers entered the posterior dorsal ventricular ridge, the cell layer and deep stratum of the ventral lateral cortex, and various amygdaloid nuclei. Parts of the striatum (nucleus accumbens) and pallidum also displayed CGRP-li innervation. In the diencephalon, CGRP-li innervation was observed in parts of the dorsal thalamus and in the periventricular and medial hypothalamus. The pretectum and deep layers of the optic tectum also showed CGRP-li fibers, and numerous CGRP-li fibers were observed in the midbrain central gray, tegmentum, and pons. Some of the sensory fibers of the trigeminal, vagal, and spinal nerves were also CGRP-li. These results show that the distribution of CGRP-li structures in the reptilian brain is similar to that described for other vertebrates and suggest that the thalamotelencephalic CGRPergic projections appear to be conserved among amniote vertebrates.

Synaptic plasticity in the lateral amygdala: a cellular hypothesis of fear conditioning

Fear conditioning is a form of associative learning in which subjects come to express defense responses to a neutral conditioned stimulus (CS) that is paired with an aversive unconditioned stimulus (US). Considerable evidence suggests that critical neural changes mediating the CS-US association occur in the lateral nucleus of the amygdala (LA). Further, recent studies show that associative long-term potentiation (LTP) occurs in pathways that transmit the CS to LA, and that drugs that interfere with this LTP also disrupt behavioral fear conditioning when infused into the LA, suggesting that associative LTP in LA might be a mechanism for storing memories of the CS-US association. Here, we develop a detailed cellular hypothesis to explain how neural responses to the CS and US in LA could induce LTP-like changes that store memories during fear conditioning. Specifically, we propose that the CS evokes EPSPs at sensory input synapses onto LA pyramidal neurons, and that the US strongly depolarizes these same LA neurons. This depolarization, in turn, causes calcium influx through NMDA receptors (NMDARs) and also causes the LA neuron to fire action potentials. The action potentials then back-propagate into the dendrites, where they collide with CS-evoked EPSPs, resulting in calcium entry through voltage-gated calcium channels (VGCCs). Although calcium entry through NMDARs is sufficient to induce synaptic changes that support short-term fear memory, calcium entry through both NMDARs and VGCCs is required to initiate the molecular processes that consolidate synaptic changes into a long-term memory.

N-Methyl-D-aspartate receptor antagonism blocks contextual fear conditioning and differentially regulates early growth response-1 messenger RNA expression in the amygdala: implications for a functional amygdaloid circuit of fear

N-Methyl-D-aspartate receptors in the amygdala are known to be crucial for the learning of conditioned fear, although the molecular cascades that N-methyl-D-aspartate receptors regulate are not well understood. Recent experiments from our laboratory have shown that messenger RNA expression of the immediate-early messenger gene, early growth response gene 1, increases in the lateral nucleus of the amygdala following contextual fear conditioning. However, the regulation of the increase in early growth response gene 1 expression is not known. To determine if N-methyl-D-aspartate receptors regulate both fear conditioning and the increase in early growth response gene 1 expression in the lateral nucleus of the amygdala, rats were infused i.c.v. with 2.5microg of the N-methyl-D-aspartate antagonist, DL-2-amino-5-phosphonovalerate. Most rats were killed 30min following one-trial contextual fear conditioning and their brains were processed for in situ hybridization detection of early growth response gene 1 messenger RNA expression. The remainder of the rats was tested for retention of fear conditioning 24h later. In DL-2-amino-5-phosphonovalerate-treated rats, post-shock freezing remained intact, whereas fear-conditioned freezing during the retention test was abolished. Image analysis of early growth response gene 1 messenger RNA revealed that DL-2-amino-5-phosphonovalerate blocked the fear-conditioning-associated increase in early growth response gene 1 expression in the lateral nucleus of the amygdala. In addition, DL-2-amino-5-phosphonovalerate significantly increased early growth response gene 1 expression in the central nucleus of the amygdala. The results reveal differential regulation of early growth response gene 1 messenger RNA in the amygdala by N-methyl-D-aspartate receptors and argue for a functional role of early growth response gene 1 in the formation of long-term memory for contextual fear. Furthermore, the results indicate a functional neuroanatomical circuit within the amygdala that includes dampening of excitatory and activation of inhibitory processes in distinct amygdala nuclei, resulting in the reduction of fear conditioning.

Afferents from the auditory thalamus synapse on inhibitory interneurons in the lateral nucleus of the amygdala

Physiological studies suggest that afferents to the lateral nucleus of the amygdala (LA) from the auditory thalamus initiate feedforward inhibition [Li et al. (1996b)]. This model of neural processing requires that thalamic afferents synapse directly onto inhibitory interneurons. To determine whether such synaptic contacts occur, we combined anterograde tract tracing with interneuron immunocytochemistry. The anterograde tracer biotinylated dextran amine (BDA) was injected into the auditory thalamus. Inhibitory interneurons in the LA were identified using antibodies directed against gamma aminobutyric acid (GABA) or one of the calcium binding proteins (CBPs), parvalbumin (PARV), calbindin (CALB), or calretinin (CALR), since CBPs identify distinct populations of GABAergic cells within the amygdala. The distribution of GABAergic and CBP interneurons in each subregion of the LA was examined by light microscopy and the relationships between thalamo-amygdala terminals and interneurons were examined by confocal and electron microscopy. Immunoreactive cells were distributed in all three subdivisions of LA, except for CALR-ir neurons, which were sparse in the dorsal subregion and were found mainly in the ventromedial and ventrolateral subregions. Confocal microscopy revealed some thalamo-amygdala terminals in close proximity to LA interneurons, while electron microscopy showed that thalamo-amygdala terminals made direct synaptic contacts onto distal dendritic processes of inhibitory neurons. These data provide morphological evidence that thalamic afferents synapse directly onto inhibitory interneurons in LA, and are consistent with the possibility that inputs from the auditory thalamus initiate feedforward inhibition in LA. This architecture could play an important role in the suppression of background neural noise, thereby enhancing the response of LA cells to incoming auditory stimuli.

Ultrastructural organization of transmitters in the cat lateralis medialis-suprageniculate nucleus of the thalamus: an immunohistochemical study

The lateralis medialis-suprageniculate nuclear (LM-Sg) complex of the cat's posterior thalamus receives a rather wide variety of inputs from diverse cortical and subcortical areas. Previous ultrastructural studies of this nucleus demonstrated the presence of four types of vesicle-containing profiles and characterized some of these as gamma-aminobutyric acid (GABA)-containing terminals (Norita and Katoh [1987] J. Comp. Neurol. 263:54-67; Norita and Katoh [1988] Prog. Brain Res. 75:109-118). The present study has extended these observations by examining the immunoreactivity (ir) of LM-Sg, with antibodies raised against aspartate (Asp), glutamate (Glu), GABA, the acetylcholine (ACh) marker, choline acetyltransferase (ChAT), and substance P (SP), by using light and electron microscopy. Neuronal somata immunopositive for the excitatory amino acids (EAAs) Asp and Glu, were of medium size. EAA-ir terminals also were of medium size and contained round synaptic vesicles; they made asymmetrical synaptic contacts with dendritic profiles. Neuronal somata immunopositive for GABA were small. GABA-positive terminals also were small and contained pleomorphic synaptic vesicles; they formed symmetrical synaptic contacts with dendritic profiles. No neurons immunolabeled for ChAT were found. Terminals immunopositive for ChAT were small and contained round synaptic vesicles; these made symmetrical synaptic contacts, asymmetrical synaptic contacts, or both, of the en passant type with dendritic profiles. SP-immunolabeled neuronal somata were not found. Immunolabeled terminals were small, contained round synaptic vesicles, and made asymmetrical synaptic contacts with dendritic profiles. ChAT-ir and SP-ir axon terminals were not expressed evenly within LM-Sg. This difference in distribution suggests that within the LM-Sg, there may be a difference in specific sensory processing functions which correlate with transmitter type.

Differential expression of EGR-1 mRNA in the amygdala following diazepam in contextual fear conditioning

The amygdaloid complex is thought to be a major site of action of anxiolytic benzodiazepine agonists. To investigate whether activity in the amygdaloid complex is altered with anxiolytic effects of diazepam, mRNA expression of the immediate-early gene EGR-1 was examined in the amygdala following blockade of fear conditioning by diazepam. It was previously shown that mRNA expression of EGR-1 (also called, NGFI-A, Zif 268, Krox 24) increases in the lateral nucleus of the amygdala (LA) shortly following contextual fear conditioning. It was therefore hypothesized that diazepam would block both contextual fear and the concomitant increase in EGR-1 mRNA expression in the LA induced by fear conditioning. Rats administered systemic diazepam before fear conditioning displayed both anxiolytic effects during the post-shock period and amnesic effects during a retention test 24 h later. Diazepam blocked the fear-conditioning-induced increase in EGR-1 expression in the LA. In addition, diazepam significantly increased EGR-1 mRNA expression in the central nucleus of the amygdala (CeA) in a dose-dependent manner. The results reveal differential regulation of EGR-1 by diazepam in the central and lateral nuclei of the amygdala suggesting that these two amygdala nuclei act in a reciprocal manner during the anxiolytic and amnesic action of the benzodiazepine agonist.

Differential synaptic distribution of the AMPA-GluR2 subunit on GABAergic and non-GABAergic neurons in the basolateral amygdala

The cellular and ultrastructural distribution patterns of the AMPA glutamate receptor subunit, GluR2, were determined in the rat basolateral amygdala. GluR2 immunoreactivity was widely and uniformly distributed in the basolateral nucleus, with both pyramidal and non-pyramidal neurons labelled. In fact, double label immunohistochemical analyses demonstrated that over 90% of the GABAergic interneurons were labelled for GluR2. Electron microscopic analyses further confirmed the presence of GluR2 in the soma and dendrites of GABAergic interneurons as well as in the soma, spines and dendritic shafts of pyramidal cells. As in our parallel study in the rat hippocampus, immunogold analyses revealed that GluR2 immunoreactivity was frequently preferentially located at asymmetric synapses on both pyramidal cell spines and shafts, as well as the dendritic processes and soma of GABAergic interneurons. However, the number of immunogold particles per labelled synapse on GABAergic neurons was significantly lower than at similar labelled asymmetric synapses on spines of presumed pyramidal cells. Given that the presence of GluR2 within the AMPA receptor complex decreases calcium flux, these data indicate that GABAergic local circuit neurons might possess AMPA receptors with higher calcium permeability on average than pyramidal cells, as has been suggested for hippocampus. Such cell class-specific differences in the subunit representation and resultant channel properties of AMPA receptors have implications for response properties as well as selective vulnerability of neurons within the basolateral nucleus of the amygdala.

Neural architecture of the rat medial geniculate body

The rat medial geniculate body was subdivided using Nissl preparations to establish nuclear boundaries, with Golgi-Cox impregnations to identify projection and local circuit neurons, and in fiber stained material to delineate the fiber tracts and their distribution. Three divisions were recognized (ventral, dorsal and medial): the first two had subdivisions. The ventral division had lateral and medial parts. The main cell type had bushy tufted dendrites which, with the afferent axons, formed fibrodendritic laminae oriented from dorso-lateral to ventro-medial; such laminae were not as regular medially, in the ovoid nucleus. The dorsal division contained several nuclei (dorsal superficial, dorsal, deep dorsal, suprageniculate, and ventrolateral) and neurons with radiating or bushy dendrites; the nuclear subdivisions differed in the concentration of one cell type or another, and in packing density. A laminar organization was present only in the dorsal superficial nucleus. Medial division neurons were heterogeneous in size and shape, ranging from tiny cells to magnocellular neurons; the various cell types intermingled. so that no further subdivision could be made. This parcellation scheme was consistent with, and supported by, the findings from plastic embedded or fiber stained material. There were very few small neurons with locally ramifying axons and which could perform an intrinsic role like that of Golgi type II cells. Their rarity was consistent with the small number of such profiles in plastic embedded or Nissl material and the few GABAergic medial geniculate body neurons seen in prior immunocytochemical work. While similar neuronal types and nuclear subdivisions are recognized in the rat and cat, there may be major interspecific differences with regard to interneuronal organization in the auditory thalamus whose functional correlates are unknown.

Origins of medial geniculate body projections to physiologically defined zones of rat primary auditory cortex

Medial geniculate body neurons projecting to physiologically identified subregions of rat primary auditory cortex (area 41, Te1) were labeled with horseradish peroxidase in adult rats. The goals were to determine the type(s) of projection neuron and the spatial arrangement of these cells with respect to thalamic subdivisions. Maps of best frequency were made with single neuron or unit cluster extracellular recording at depths of 500-800 microm, which correspond to layers III-IV in Nissl preparations. Tracer injections were made in different cortical isofrequency regions (2, 11, 22, or 38 kHz, respectively). Labeled neurons were plotted on representative sections upon which the architectonic subdivisions were drawn independently. Most of the cells of origin lay in the ventral division in every experiment. Injections at low frequencies labeled bands of neurons laterally in the ventral division; progressively more rostral deposits at higher frequencies labeled bands or clusters more medially in the ventral division, and through most of its caudo-rostral extent. Medial division labeling was variable. Labeled cells were always in the lateral half of the nucleus and were often scattered. There were few labeled cells in the dorsal division. Seven types of thalamocortical neuron were identified: ventral division cells had a tufted branching pattern, while medial division neurons have heterogeneous shapes and sizes and were larger. Dorsal division neurons had a radiate branching pattern. The size range of labeled neurons spanned that of Nissl stained neuronal somata. Area 41 may receive two types of thalamic projection: ventral division input is strongly convergent, highly topographic, spatially focal, and restricted to one type of neuron only, while the medial division projection is more divergent, coarsely topographical, involves multiple cortical areas, and has several varieties of projection neuron. Despite species differences in local circuitry, many facets of thalamocortical organization are conserved in phylogeny.

Destruction of the auditory thalamus disrupts the production of fear but not the inhibition of fear conditioned to an auditory stimulus

The auditory thalamus is part of a neural circuit that mediates the expression of fear to auditory stimuli. Bilateral lesions of the auditory thalamus prevent the expression of fear to an auditory stimulus paired with shock. The present study assessed whether bilateral lesions of the auditory thalamus would also disrupt the inhibition of fear to an auditory stimulus paired with the absence of shock. Rats were given bilateral lesions of the auditory thalamus followed by Pavlovian conditioned inhibition training in which a light was paired with shock and a noise and light compound was presented in the absence of shock. Fear and the inhibition of fear were measured with the fear-potentiated startle effect. Lesions of the auditory thalamus did not disrupt the ability of the noise to inhibit the expression of fear to the light. However, these lesions did disrupt the ability of the noise to produce fear-potentiated startle after it had been subsequently paired with shock. These results suggest that although the auditory thalamus is an essential part of a neural circuit that mediates the expression of fear to auditory stimuli, it is not an essential part of the circuit that mediates the inhibition of fear to auditory stimuli.

Cortical pathways to the mammalian amygdala

The amygdaloid nuclear complex is critical for producing appropriate emotional and behavioral responses to biologically relevant sensory stimuli. It constitutes an essential link between sensory and limbic areas of the cerebral cortex and subcortical brain regions, such as the hypothalamus, brainstem, and striatum, that are responsible for eliciting emotional and motivational responses. This review summarizes the anatomy and physiology of the cortical pathways to the amygdala in the rat, cat and monkey. Although the basic anatomy of these systems in the cat and monkey was largely delineated in studies conducted during the 1970s and 1980s, detailed information regarding the cortico-amygdalar pathways in the rat was only obtained in the past several years. The purpose of this review is to describe the results of recent studies in the rat and to compare the organization of cortico-amygdalar projections in this species with that seen in the cat and monkey. In all three species visual, auditory, and somatosensory information is transmitted to the amygdala by a series of modality-specific cortico-cortical pathways ("cascades") that originate in the primary sensory cortices and flow toward higher order association areas. The cortical areas in the more distal portions of these cascades have stronger and more extensive projections to the amygdala than the more proximal areas. In all three species olfactory and gustatory/visceral information has access to the amygdala at an earlier stage of cortical processing than visual, auditory and somatosensory information. There are also important polysensory cortical inputs to the mammalian amygdala from the prefrontal and hippocampal regions. Whereas the overall organization of cortical pathways is basically similar in all mammalian species, there is anatomical evidence which suggests that there are important differences in the extent of convergence of cortical projections in the primate versus the nonprimate amygdala.

Emotional memory and psychopathology

A leading model for studying how the brain forms memories about unpleasant experiences is fear conditioning. A cumulative body of work has identified major components of the neural system mediating this form of learning. The pathways involve transmission of sensory information from processing areas in the thalamus and cortex to the amygdala. The amygdala's lateral nucleus receives and integrates the sensory inputs from the thalamic and cortical areas, and the central nucleus provides the interface with motor systems controlling specific fear responses in various modalities (behavioural, autonomic, endocrine). Internal connections within the amygdala allow the lateral and central nuclei to communicate. Recent studies have begun to identify some sites of plasticity in the circuitry and the cellular mechanisms involved in fear conditioning. Through studies of fear conditioning, our understanding of emotional memory is being taken to the level of cells and synapses in the brain. Advances in understanding emotional memory hold out the possibility that emotional disorders may be better defined and treatment improved.

AMPA receptor facilitation accelerates fear learning without altering the level of conditioned fear acquired

Rats treated with the AMPA receptor-facilitating drug 1-(quinoxolin-6-ylcarbonyl)piperidine (BDP-12) during training acquired fear conditioning to a tone faster than vehicle-treated controls. The effect on acquisition was dependent on the dose given. BDP-12-treated rats and vehicle-treated controls reached the same level of conditioned fear and extinguished at the same rate. The dissociation of learning rate from these other normally covariant measures suggests that the drug had a specific and isolated effect on acquisition. Controls for drug effects on stimulus sensitivity, locomotor activity, generalized fearfulness, and other performance factors support this interpretation. The known action of BDP-12 on receptor dynamics suggests that its effect on acquisition may be attributed to specific modulation of an AMPA and NMDA receptor-dependent plasticity mechanism. The finding that the drug accelerates acquisition but does not affect the level of conditioned fear acquired parallels the effect of the drug on long-term potentiation (LTP) (increasing the rate but not the ceiling of potentiation) and suggests that common mechanisms may underlie fear conditioning and LTP.

NMDA and AMPA receptors in the lateral nucleus of the amygdala are postsynaptic to auditory thalamic afferents

Projections from the medial geniculate body (MGB) to the lateral nucleus of the amygdala (LA) have been implicated in the conditioning of emotional reactions to acoustic stimuli. Anatomical and physiological studies indicate that this pathway uses the excitatory amino acid L-glutamate as a transmitter. Recent physiological studies have demonstrated that synaptic transmission in the thalamo-amygdala pathway requires the activation of both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors, two of the major classes of ionotrophic glutamate receptors. In order to characterize the nature of thalamoamygdala interactions, we examined the synaptic associations between thalamic afferents and amygdala neurons that contain at least one glutamate receptor subtype. Thalamic afferents to the amygdala were identified by lesion-induced anterograde degeneration and anterograde transport of biotinylated dextran-amine, while postsynaptic glutamate receptors were labeled immunocytochemically using antisera directed the R1 subunit of the NMDA receptor and the GluR1 and GluR2/3 subunits of the AMPA receptors. Both methods demonstrated that the majority (77%) of thalamic afferents contact dendritic spines, and most (60%) of these spines express at least one glutamate receptor subtype. To a lesser extent, identified afferents also contacted small and large dendritic shafts, and many of these were immunoreactive. Thalamic afferents terminated on approximately the same proportion (60%) of immunoreactive targets for each glutamate receptor studied. These data provide morphological evidence that thalamic afferents directly synapse onto amygdala neurons that express glutamate receptors and suggest ways in which thalamic afferents activate and influence amygdala circuitry.

Cholinergic innervation of the lateralis medialis-suprageniculate nuclear complex (LM-Sg) of the cat's thalamus: a double labeling immunohistochemical study

The purpose of this study was to investigate morphological characteristics of the synaptic relations of choline acetyltransferase (ChAT)-positive terminals that are made with a variety of post-synaptic profiles in the lateralis medialis-suprageniculate nuclear complex (LM-Sg) using ChAT, gamma-aminobutyric acid (GABA) and glutamate immunohistochemistry in combination with electron microscopical observations. The ChAT immunopositive profiles make asymmetrical synaptic contacts with glutamate immunopositive dendrites that are presumably derived from projection neurons, and/or GABA immunopositive interneurons. The present results indicate that ascending cholinergic mechanisms may be important for modifying information in both the extrinsic and intrinsic circuitries of LM-Sg.

GABAA and GABAB receptors differentially regulate synaptic transmission in the auditory thalamo-amygdala pathway: an in vivo microiontophoretic study and a model

Stimulation of the medical geniculate body elicits extracellular single unit responses in the lateral nucleus of the amygdala that are dependent upon glutamatergic neurotransmission [Li et al. (1995) Exp. Brain Res., 105-87-100]. In the present study, we examined the contribution of inhibitory amino acid transmission to these excitatory responses. Antagonists of GABAA or GABAB receptors were delivered microiontophoretically to cells activated by stimulation of the medial geniculate body. Blockade of GABAA receptors with bicuculline resulted in a pronounced increase in evoked short latency unit responses (4-8 ms). In some cases, cells that were not responsive to the stimulation became responsive in the presence of bicuculline. In contrast, delivery of GABAB antagonists, Phaclofen or 2-OH-saclofen, did not affect these short-latency responses. Using paired-pulse stimulation, both short (< 30 ms) and longer (> 50 ms) latency inhibitory processes were revealed. GABAA blockade eliminated the short latency inhibition and GABAB blockade eliminated the longer latency inhibition in most cells. These results suggest that the activation of GABAA and GABAB receptors differentially regulate glutamatergic synaptic transmission in the auditory thalamo-amygdala pathway. Moreover, our findings suggest that at least part of this regulation is via a feedforward mechanism. We tested the sufficiency of feedforward inhibition to account for the data using a simple computational model that incorporates the results presented here.

Synaptic transmission and plasticity in the amygdala. An emerging physiology of fear conditioning circuits

Numerous studies in both rats and humans indicate the importance of the amygdala in the acquisition and expression of learned fear. The identification of the amygdala as an essential neural substrate for fear conditioning has permitted neurophysiological examinations of synaptic processes in the amygdala that may mediate fear conditioning. One candidate cellular mechanism for fear conditioning is long-term potentiation (LTP), an enduring increase in synaptic transmission induced by high-frequency stimulation of excitatory afferents. At present, the mechanisms underlying the induction and expression of amygdaloid LTP are only beginning to be understood, and probably involve both the N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) subclasses of glutamate receptors. This article will examine recent studies of synaptic transmission and plasticity in the amygdala in an effort to understand the relationships of these processes to aversive learning and memory.

The temporal evolution of striatal dopamine receptor binding and mRNA expression following hypoxia-ischemia in the neonatal rat

Neonatal hypoxic-ischemic (HI) brain injury in the rat alters dopamine receptors. To determine whether such changes are permanent, dopamine receptors and corresponding mRNA were examined at various time points after neonatal HI using receptor autoradiography and in situ hybridization. Rat pups underwent ligation of the left common carotid artery followed by hypoxic exposure (8.5% O2 for 3 h). Controls underwent sham surgery alone. Animals surviving for 2-80 days following HI were studied. Striatal D1 receptors (labeled by [3H]SCH23390) were reduced as early as 2 days following HI, remained depressed for 21 days, but recovered to control levels by young adulthood (3 months of age). D2 receptors (labeled by [125I] iodosulpride) did not decline until 10 days after HI, and remained uniformly depressed throughout the caudate-putamen thereafter. Changes in D1 receptor mRNA transcripts closely paralleled alterations in receptors: early reductions in D1 mRNA signal recovered by young adulthood. D2 mRNA exhibited a unique temporal profile with an early decrease (2 days following HI), and prompt, persistent recovery. Dopamine receptors and transcripts are differentially affected by HI injury early in development. Whereas D1 receptor expression recovers from neonatal HI injury, D2 receptors remain permanently affected despite the presence of normal levels of D2 receptor transcripts. A persistent, post-transcriptional effect of HI on D2 receptor expression is suggested.

Differential localization of NMDA and AMPA receptor subunits in the lateral and basal nuclei of the amygdala: a light and electron microscopic study

Anatomical and physiological studies indicate that the amino acid L-glutamate is the excitatory transmitter in sensory afferent pathways to the amygdala and in intraamygdala circuits involving the lateral and basal nuclei. The regional, cellular, and subcellular immunocytochemical localizations of N-methyl-D-aspartate (NMDA) and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), two major classes of glutamate receptors, were examined in these areas of the amygdala. A monoclonal antibody and a polyclonal antiserum directed against the R1 subunit of the NMDA receptor were used. Each immunoreagent produced distinct distributions of perikaryal and neuropilar staining. Dendritic immunoreactivity was localized primarily to asymmetric (excitatory) synaptic junctions, mostly on spines, consistent with the conventional view of the organization and function of NMDA receptors. Whereas the anti-NMDAR1 antiserum produced sparse presynaptic axon terminal labeling and extensive glial labeling, the anti-NMDAR1 antibody labeled considerably fewer glia and many more presynaptic axon terminals. Labeled presynaptic terminals formed asymmetric and symmetric synapses, suggesting presynaptic regulation of both excitatory and inhibitory transmission. Immunoreactivity for different subunits of the AMPA receptor (GluR1, GluR2/3, and GluR4) was uniquely distributed across neuronal populations, and some receptor subunits were specific to certain cell types. Immunoreactivity for GluR1 and Glu2/3 was predominantely localized to dendritic shafts and was more extensive than that of GluR4 due to heavy labeling of proximal portions of dendrites. The distribution of GluR4 immunoreactivity was similar to NMDAR1: GluR4 was seen in presynaptic terminals, glia, and dendrites and was primarily localized to spines. The presynaptic localization of GluR4 in the absence of GluR2 suggests glutamate-mediated modulation of presynaptic Ca++ concentrations. These data add to our understanding of the morphological basis of pre- and postsynaptic transmission mechanisms and synaptic plasticity in the amygdala.

Comparative ultrastructural localization of the NMDAR1 glutamate receptor in the rat basolateral amygdala and bed nucleus of the stria terminalis

The N-methyl-D-aspartate (NMDA)-type glutamate receptor in the basolateral amygdala (BLA) has been implicated in activity-dependent plasticity important for cortically evoked acquisition of fear-potentiated startle response. We examined the ultrastructural immunoperoxidase labeling of the R1 subunit of the NMDA receptor in the BLA of adult rats to determine the potential cellular and subcellular sites mediating the effects generated by NMDA activation. The localization was compared with that seen in the bed nucleus of the stria terminalis (BNST), the major efferent pathway from the central nucleus of the amygdala, which has a more pronounced involvement in autonomic function. Electron microscopy established that in the BLA, 68.4% (n = 177) of the profiles showing NMDAR1-like immunoreactivity (NMDAR1-LI) were dendrites, and 19.8% were distal tips of astrocytic processes. In contrast, profiles containing NMDAR1-LI (n = 262) in the BNST were more equally distributed between dendrites (37.4%) and axons (38.2%). The subcellular localization of NMDAR1 immunoreactivity was, however, similar in both regions. Our findings provide the first ultrastructural evidence that glutamate may prominently act through NMDAR1 receptors to elicit postsynaptic actions on intrinsic neurons in the BLA and BNST. The results also indicate that, in the BLA, the NMDAR1 receptor plays an important role in astrocytic function, whereas the receptor is more preferentially a presynaptic modulator in axons which terminate in or pass through the BNST.

LTP is accompanied by commensurate enhancement of auditory-evoked responses in a fear conditioning circuit

Transmission of auditory information from the medial geniculate body to the lateral nucleus of the amygdala is believed to be involved in the conditioning of fear responses to acoustic stimuli. This pathway exhibits LTP of electrically evoked field potentials after high frequency stimulation of the medial geniculate body. High frequency stimulation of the medial geniculate body also results in a long-lasting potentiation of a field potential in the lateral amygdala elicited by a naturally transduced acoustic stimulus. This demonstrates that natural information processing can make use of the physiological mechanisms set in motion by LTP induction.

Direct projections from the non-laminated divisions of the medial geniculate nucleus to the temporal polar cortex and amygdala in the cat

The medial geniculate nucleus (MG) is well known to send projection fibers not only to the auditory cortex, but also to the limbic structures of the forebrain including the perirhinal cortex and amygdala. In the cat, the non-laminated portions of the MG are also known to project to the amygdala, as well as to the auditory cortical areas surrounding the primary auditory area. On the other hand, projections from the non-laminated MG to the limbic cortical areas have not so far been studied systematically. Thus, in the present study, direct projections from the non-laminated portions of the medial geniculate nucleus to the temporal polar cortex and amygdala were examined in the cat by retrograde and anterograde tract-tracing techniques. The temporal polar cortex is the ventral polar region of the posterior sylvian and posterior ectosylvian gyri, which is located dorsal to the posterior rhinal sulcus and includes the ectorhinal area. After injection of cholera toxin B subunit into the temporal polar cortex, retrogradely labeled neurons were seen in the caudal two-thirds of the medial geniculate nucleus ipsilateral to the injection; they were distributed in the non-laminated portions of the MG (the dorsal and medial divisions and the ventromedial part of the ventral division), but not in the laminated portion (the principal part of the ventral division). These findings were confirmed by injecting Phaseolus vulgaris leucoagglutinin into each division of the MG. After the injection into each non-laminated division, terminal labeling was observed in the temporal polar cortex. Terminal labeling was further found in the lateral amygdaloid nucleus ipsilateral to the injection. Then, cholera toxin B subunit was injected into the lateral amygdaloid nucleus; retrogradely labeled neurons were observed ipsilaterally in the non-laminated portions of the MG, as well as in the temporal polar cortex. The results indicate that the non-laminated portions of the MG send projection fibers to the temporal polar cortex and lateral amygdaloid nucleus, and that the non-laminated portions of the MG and temporal polar cortex give rise to overlapping projections to the lateral amygdaloid nucleus. These connections appear to constitute neuronal links in "emotional" and/or "motivational" circuitry in the forebrain.

Glutamate immunoreactive terminals in the lateral amygdaloid nucleus: a possible substrate for emotional memory

The ultrastructure and synaptic associations of terminals immunoreactive for L-glutamate (Glu) were examined in the lateral nucleus of the amygdala (AL). All results reported here involved tissue fixed only with paraformaldehyde. The specificity of the antiserum with paraformaldehyde fixation conditions was assessed and confirmed by immuno-dot blot analysis: the reactivity of anti-Glu to glutamic acid was at least 1,000 times greater than the reactivity to other amino acids. At the light microscopic level, Glu-immunoreactive punctate processes and somata were present in AL. At the electron microscopic level, many Glu-immunoreactive terminals were identified. Data analysis was performed on 365 of these labeled terminals. Glu-immunoreactive terminals were 0.3-1.5 microns in diameter and contained numerous small, clear vesicles as well as mitochondria. Many (77%) of the terminals analyzed had morphologically identifiable synaptic specializations. Most (90%) of the Glu-immunoreactive terminals with synaptic specializations formed asymmetric synapses on spines or small dendrites; synaptic specializations on soma or proximal dendrites were rarely seen (< 1%). Glu-immunoreactive terminals were qualitatively compared to terminals in AL labeled with two other antisera: anti-glutaminase, a marker for the enzyme that catalyzes the conversion of glutamine to the releasable or transmitter form of Glu, and anti-gamma-aminobutyric acid (anti-GABA), a marker for the major inhibitory amino acid transmitter in the brain. Terminals immunoreactive for glutaminase, like those immunoreactive for Glu, formed mostly asymmetric synaptic specializations on spines or small dendrites. In contrast, GABA-immunoreactive terminals usually formed symmetric synapses on soma or proximal dendrites and were never observed to form asymmetric axo-spinous contacts. Although Glu is a metabolic precursor to GABA, these data indicate that the majority of Glu-immunoreactive terminals reflect the site of synthesis and release of Glu and not of GABA. In addition, these results provide morphological evidence that Glu plays a role in excitatory neurotransmission at synapses in AL and support the growing body of data implicating excitatory amino acid-mediated synaptic plasticity in-emotional learning and memory processes in AL.

Brain mechanisms of emotion and emotional learning

The amygdala appears to play an essential role in many aspects of emotional information processing and behavior. Studies over the past year have begun to clarify the anatomical organization of the amygdala and the contribution of its individual subregions to emotional functions, especially emotional learning and memory. Researchers can now point to plausible circuits involved in the transmission of sensory inputs into the amygdala, between amygdaloid subregions, and to efferent targets in cortical and subcortical regions, for specific emotional learning and memory processes.