A correlated light and electron microscopic immunocytochemical study of cholinergic terminals and neurons in the rat amygdaloid body with special emphasis on the basolateral amygdaloid nucleus

Functional neuroanatomy of basal forebrain projections to the basolateral amygdala: Transmitters, receptors, and neuronal subpopulations

The projections of the basal forebrain (BF) to the hippocampus and neocortex have been extensively studied and shown to be important for higher cognitive functions, including attention, learning, and memory. Much less is known about the BF projections to the basolateral nuclear complex of the amygdala (BNC), although the cholinergic innervation of this region by the BF is actually far more robust than that of cortical areas. This review will focus on light and electron microscopic tract-tracing and immunohistochemical (IHC) studies, many of which were published in the last decade, that have analyzed the relationship of BF inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of BF-BNC circuitry. The results indicate that BF inputs to the BNC mainly target the basolateral nucleus of the BNC (BL) and arise from cholinergic, GABAergic, and perhaps glutamatergic BF neurons. Cholinergic inputs mainly target dendrites and spines of pyramidal neurons (PNs) that express muscarinic receptors (MRs). MRs are also expressed by cholinergic axons, as well as cortical and thalamic axons that synapse with PN dendrites and spines. BF GABAergic axons to the BL also express MRs and mainly target BL interneurons that contain parvalbumin. It is suggested that BF-BL circuitry could be very important for generating rhythmic oscillations known to be critical for emotional learning. BF cholinergic inputs to the BNC might also contribute to memory formation by activating M1 receptors located on PN dendritic shafts and spines that also express NMDA receptors.

Functional neuroanatomy of monoaminergic systems in the basolateral nuclear complex of the amygdala: Neuronal targets, receptors, and circuits

This review discusses neuroanatomical aspects of the three main monoaminergic systems innervating the basolateral nuclear complex (BNC) of the amygdala (serotonergic, noradrenergic, and dopaminergic systems). It mainly focuses on immunohistochemical (IHC) and in situ hybridization (ISH) studies that have analyzed the relationship of specific monoaminergic inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of the monoaminergic modulation of BNC circuitry. First, light and electron microscopic IHC investigations identifying the main BNC neuronal subpopulations and characterizing their local circuitry, including connections with discrete PN compartments and other INs, are reviewed. Then, the relationships of each of the three monoaminergic systems to distinct PN and IN cell types, are examined in detail. For each system, the neuronal targets and their receptor expression are discussed. In addition, pertinent electrophysiological investigations are discussed. The last section of the review compares and contrasts various aspects of each of the three monoaminergic systems. It is concluded that the large number of different receptors, each with a distinct mode of action, expressed by distinct cell types with different connections and functions, should offer innumerable ways to subtlety regulate the activity of the BNC by therapeutic drugs in psychiatric diseases in which there are alterations of BNC monoaminergic modulatory systems, such as in anxiety disorders, depression, and drug addiction. It is suggested that an important area for future studies is to investigate how the three systems interact in concert at the neuronal and neuronal network levels.

Neuronal localization of m1 muscarinic receptor immunoreactivity in the monkey basolateral amygdala

The basolateral nuclear complex (BNC) of the amygdala plays an important role in the generation of emotional/motivational behavior and the consolidation of emotional memories. Activation of M1 cholinergic receptors (M1Rs) in the BNC is critical for memory consolidation. Previous receptor binding studies in the monkey amygdala demonstrated that the BNC has a high density of M1Rs, but did not have sufficient resolution to identify which neurons in the BNC expressed them. This was accomplished in the present immunohistochemical investigation using an antibody for the m1 receptor (m1R). Analysis of m1Rs in the monkey BNC using immunoperoxidase techniques revealed that their expression was very dense in the BNC, and suggested that virtually all of the pyramidal projection neurons (PNs) in all of the BNC nuclei were m1R-immunoreactive (m1R+). This was confirmed with dual-labeling immunofluorescence using staining for calcium/calmodulin-dependent protein kinase II (CaMK) as a marker for BNC PNs. However, additional dual-labeling studies indicated that one-third of inhibitory interneurons (INs) expressing glutamic acid decarboxylase (GAD) were also m1R+. Moreover, the finding that 60% of parvalbumin (PV) immunoreactive neurons were m1R+ indicated that this IN subpopulation was the main GAD+ subpopulation exhibiting m1R expression. The cholinergic innervation of the amygdala is greatly reduced in Alzheimer's disease and there is currently considerable interest in developing selective M1R positive allosteric modulators (PAMs) to treat the symptoms. The results of the present study indicate that M1Rs in both PNs and INs in the primate BNC would be targeted by M1R PAMs.

Neuroplasticity in Cholinergic Projections from the Basal Forebrain to the Basolateral Nucleus of the Amygdala in the Kainic Acid Model of Temporal Lobe Epilepsy

The amygdala is a cerebral region whose function is compromised in temporal lobe epilepsy (TLE). Patients with TLE present cognitive and emotional dysfunctions, of which impairments in recognizing facial expressions have been clearly attributed to amygdala damage. However, damage to the amygdala has been scarcely addressed, with the majority of studies focusing on the hippocampus. The aim of this study was to evaluate epilepsy-related plasticity of cholinergic projections to the basolateral nucleus (BL) of the amygdala. Adult rats received kainic acid (KA) injections and developed status epilepticus. Weeks later, they showed spontaneous recurrent seizures documented by behavioral observations. Changes in cholinergic innervation of the BL were investigated by using an antibody against the vesicular acetylcholine transporter (VAChT). In KA-treated rats, it was found that (i) the BL shrunk to 25% of its original size (p < 0.01 vs. controls, Student’s t-test), (ii) the density of vesicular acetylcholine transporter-immunoreactive (VAChT-IR) varicosities was unchanged, (iii) the volumes of VAChT-IR cell bodies projecting to the BL from the horizontal limb of the diagonal band of Broca, ventral pallidum, and subcommissural part of the substantia innominata were significantly increased (p < 0.05, Bonferroni correction). These results illustrate significant changes in the basal forebrain cholinergic cells projecting to the BL in the presence of spontaneous recurrent seizures.

Evidence for M2 muscarinic receptor modulation of axon terminals and dendrites in the rodent basolateral amygdala: An ultrastructural and electrophysiological analysis

The basolateral amygdala receives a very dense cholinergic innervation from the basal forebrain that is important for memory consolidation. Although behavioral studies have shown that both M₁ and M₂ muscarinic receptors are critical for these mnemonic functions, there have been very few neuroanatomical and electrophysiological investigations of the localization and function of different types of muscarinic receptors in the amygdala. In the present study we investigated the subcellular localization of M₂ muscarinic receptors (M₂Rs) in the anterior basolateral nucleus (BLa) of the mouse, including the localization of M₂Rs in parvalbumin (PV) immunoreactive interneurons, using double-labeling immunoelectron microscopy. Little if any M₂R-immunoreactivity (M₂R-ir) was observed in neuronal somata, but the neuropil was densely labeled. Ultrastructural analysis using a pre-embedding immunogold-silver technique (IGS) demonstrated M₂R-ir in dendritic shafts, spines, and axon terminals forming asymmetrical (excitatory) or symmetrical (mostly inhibitory) synapses. In addition, about one-quarter of PV+ axon terminals and half of PV+ dendrites, localized using immunoperoxidase, were M₂R+ when observed in single thin sections. In all M₂R+ neuropilar structures, including those that were PV+, about one-quarter to two-thirds of M₂R+ immunoparticles were plasma-membrane-associated, depending on the structure. The expression of M₂Rs in PV+ and PV-negative terminals forming symmetrical synapses indicates M₂R modulation of inhibitory transmission. Electrophysiological studies in mouse and rat brain slices, including paired recordings from interneurons and pyramidal projection neurons, demonstrated M₂R-mediated suppression of GABA release. These findings suggest cell-type-specific functions of M₂Rs and shed light on organizing principles of cholinergic modulation in the BLa.

The M1 Muscarinic Receptor Antagonist VU0255035 Delays the Development of Status Epilepticus after Organophosphate Exposure and Prevents Hyperexcitability in the Basolateral Amygdala

Exposure to organophosphorus toxins induces seizures that progress to status epilepticus (SE), which can cause brain damage or death. Seizures are generated by hyperstimulation of muscarinic receptors, subsequent to inhibition of acetylcholinesterase; this is followed by glutamatergic hyperactivity, which sustains and reinforces seizure activity. It has been unclear which muscarinic receptor subtypes are involved in seizure initiation and the development of SE in the early phases after exposure. Here, we show that pretreatment of rats with the selective M₁ receptor antagonist, VU0255035 [N-(3-oxo-3-(4-(pyridine-4-yl)piperazin-1-yl)propyl)-benzo[c][1,2,5]thiadiazole-4 sulfonamide], significantly suppressed seizure severity and prevented the development of SE for about 40 minutes after exposure to paraoxon or soman, suggesting an important role of the M₁ receptor in the early phases of seizure generation. In addition, in in vitro brain slices of the basolateral amygdala (a brain region that plays a key role in seizure initiation after nerve agent exposure), VU0255035 blocked the effects produced by bath application of paraoxon-namely, a brief barrage of spontaneous inhibitory postsynaptic currents, followed by a significant increase in the ratio of the total charge transferred by spontaneous excitatory postsynaptic currents over that of the inhibitory postsynaptic currents. Furthermore, paraoxon enhanced the hyperpolarization-activated cation current I_h in basolateral amygdala principal cells, which could be one of the mechanisms underlying the increased glutamatergic activity, an effect that was also blocked in the presence of VU0255035. Thus, selective M₁ antagonists may be an efficacious pretreatment in contexts in which there is risk for exposure to organophosphates, as these antagonists will delay the development of SE long enough for medical assistance to arrive.

Localization of the M2 muscarinic cholinergic receptor in dendrites, cholinergic terminals, and noncholinergic terminals in the rat basolateral amygdala: An ultrastructural analysis

Activation of M2 muscarinic receptors (M2Rs) in the rat anterior basolateral nucleus (BLa) is critical for the consolidation of memories of emotionally arousing events. The present investigation used immunocytochemistry at the electron microscopic level to determine which structures in the BLa express M2Rs. In addition, dual localization of M2R and the vesicular acetylcholine transporter protein (VAChT), a marker for cholinergic axons, was performed to determine whether M2R is an autoreceptor in cholinergic axons innervating the BLa. M2R immunoreactivity (M2R-ir) was absent from the perikarya of pyramidal neurons, with the exception of the Golgi complex, but was dense in the proximal dendrites and axon initial segments emanating from these neurons. Most perikarya of nonpyramidal neurons were also M2R-negative. About 95% of dendritic shafts and 60% of dendritic spines were M2 immunoreactive (M2R(+) ). Some M2R(+) dendrites had spines, suggesting that they belonged to pyramidal cells, whereas others had morphological features typical of nonpyramidal neurons. M2R-ir was also seen in axon terminals, most of which formed asymmetrical synapses. The main targets of M2R(+) terminals forming asymmetrical (putative excitatory) synapses were dendritic spines, most of which were M2R(+) . The main targets of M2R(+) terminals forming symmetrical (putative inhibitory or neuromodulatory) synapses were unlabeled perikarya and M2R(+) dendritic shafts. M2R-ir was also seen in VAChT(+) cholinergic terminals, indicating a possible autoreceptor role. These findings suggest that M2R-mediated mechanisms in the BLa are very complex, involving postsynaptic effects in dendrites as well as regulating release of glutamate, γ-aminobutyric acid, and acetylcholine from presynaptic axon terminals. J. Comp. Neurol. 524:2400-2417, 2016. © 2016 Wiley Periodicals, Inc.

Mu opioid receptor localization in the basolateral amygdala: An ultrastructural analysis

Receptor binding studies have shown that the density of mu opioid receptors (MORs) in the basolateral amygdala is among the highest in the brain. Activation of these receptors in the basolateral amygdala is critical for stress-induced analgesia, memory consolidation of aversive events, and stress adaptation. Despite the importance of MORs in these stress-related functions, little is known about the neural circuits that are modulated by amygdalar MORs. In the present investigation light and electron microscopy combined with immunohistochemistry was used to study the expression of MORs in the anterior basolateral nucleus (BLa). At the light microscopic level, light to moderate MOR-immunoreactivity (MOR-ir) was observed in a small number of cell bodies of nonpyramidal interneurons and in a small number of processes and puncta in the neuropil. At the electron microscopic level most MOR-ir was observed in dendritic shafts, dendritic spines, and axon terminals. MOR-ir was also observed in the Golgi apparatus of the cell bodies of pyramidal neurons (PNs) and interneurons. Some of the MOR-positive (MOR+) dendrites were spiny, suggesting that they belonged to PNs, while others received multiple asymmetrical synapses typical of interneurons. The great majority of MOR+ axon terminals (80%) that formed synapses made asymmetrical (excitatory) synapses; their main targets were spines, including some that were MOR+. The main targets of symmetrical (inhibitory and/or neuromodulatory) synapses were dendritic shafts, many of which were MOR+, but some of these terminals formed synapses with somata or spines. All of our observations were consistent with the few electrophysiological studies which have been performed on MOR activation in the basolateral amygdala. Collectively, these findings suggest that MORs may be important for filtering out weak excitatory inputs to PNs, allowing only strong inputs or synchronous inputs to influence pyramidal neuronal firing.

Impact of basal forebrain cholinergic inputs on basolateral amygdala neurons

In addition to innervating the cerebral cortex, basal forebrain cholinergic (BFc) neurons send a dense projection to the basolateral nucleus of the amygdala (BLA). In this study, we investigated the effect of near physiological acetylcholine release on BLA neurons using optogenetic tools and in vitro patch-clamp recordings. Adult transgenic mice expressing cre-recombinase under the choline acetyltransferase promoter were used to selectively transduce BFc neurons with channelrhodopsin-2 and a reporter through the injection of an adeno-associated virus. Light-induced stimulation of BFc axons produced different effects depending on the BLA cell type. In late-firing interneurons, BFc inputs elicited fast nicotinic EPSPs. In contrast, no response could be detected in fast-spiking interneurons. In principal BLA neurons, two different effects were elicited depending on their activity level. When principal BLA neurons were quiescent or made to fire at low rates by depolarizing current injection, light-induced activation of BFc axons elicited muscarinic IPSPs. In contrast, with stronger depolarizing currents, eliciting firing above ∼ 6-8 Hz, these muscarinic IPSPs lost their efficacy because stimulation of BFc inputs prolonged current-evoked afterdepolarizations. All the effects observed in principal neurons were dependent on muscarinic receptors type 1, engaging different intracellular mechanisms in a state-dependent manner. Overall, our results suggest that acetylcholine enhances the signal-to-noise ratio in principal BLA neurons. Moreover, the cholinergic engagement of afterdepolarizations may contribute to the formation of stimulus associations during fear-conditioning tasks where the timing of conditioned and unconditioned stimuli is not optimal for the induction of synaptic plasticity.

Effects of context-drug learning on synaptic connectivity in the basolateral nucleus of the amygdala in rats

Context-drug learning produces structural and functional synaptic changes in the circuitry of the basolateral nucleus of the amygdala (BLA). However, how the synaptic changes translated to the neuronal targets was not established. Thus, in the present study, immunohistochemistry with a cell-specific marker and the stereological quantification of synapses was used to determine if context-drug learning increases the number of excitatory and inhibitory/modulatory synapses contacting the gamma-aminobutyric acid (GABA) interneurons and/or the pyramidal neurons in the BLA circuitry. Amphetamine-conditioned place preference increased the number of asymmetric (excitatory) synapses contacting the spines and dendrites of pyramidal neurons and the number of multisynaptic boutons contacting pyramidal neurons and GABA interneurons. Context-drug learning increased asymmetric (excitatory) synapses onto dendrites of GABA interneurons and increased symmetric (inhibitory or modulatory) synapses onto dendrites but not perikarya of these same interneurons. The formation of context-drug associations alters the synaptic connectivity in the BLA circuitry, findings that have important implications for drug-seeking behavior.

Muscarinic cholinergic receptor M1 in the rat basolateral amygdala: ultrastructural localization and synaptic relationships to cholinergic axons

Muscarinic neurotransmission in the anterior basolateral amygdalar nucleus (BLa) mediated by the M1 receptor (M1R) is critical for memory consolidation. Although knowledge of the subcellular localization of M1R in the BLa would contribute to an understanding of cholinergic mechanisms involved in mnemonic function, there have been no ultrastructural studies of this receptor in the BLa. In the present investigation, immunocytochemistry at the electron microscopic level was used to determine which structures in the BLa express M1R. The innervation of these structures by cholinergic axons expressing the vesicular acetylcholine transporter (VAChT) was also studied. All perikarya of pyramidal neurons were labeled, and about 90% of dendritic shafts and 60% of dendritic spines were M1R+. Some dendrites had spines suggesting that they belonged to pyramidal cells, whereas others had morphological features typical of interneurons. M1R immunoreactivity (M1R-ir) was also seen in axon terminals, most of which formed asymmetrical synapses. The main targets of M1R+ terminals forming asymmetrical synapses were dendritic spines, most of which were M1R+. The main targets of M1R+ terminals forming symmetrical synapses were M1R+ perikarya and dendritic shafts. About three-quarters of VAChT+ cholinergic terminals formed synapses; the main postsynaptic targets were M1R+ dendritic shafts and spines. In some cases M1R-ir was seen near the postsynaptic membrane of these processes, but in other cases it was found outside of the active zone of VAChT+ synapses. These findings suggest that M1R mechanisms in the BLa are complex, involving postsynaptic effects as well as regulating release of neurotransmitters from presynaptic terminals.

α7-Containing nicotinic acetylcholine receptors on interneurons of the basolateral amygdala and their role in the regulation of the network excitability

The basolateral amygdala (BLA) plays a key role in fear-related learning and memory, in the modulation of cognitive functions, and in the overall regulation of emotional behavior. Pathophysiological alterations involving hyperexcitability in this brain region underlie anxiety and other emotional disorders as well as some forms of epilepsy. GABAergic interneurons exert a tight inhibitory control over the BLA network; understanding the mechanisms that regulate their activity is necessary for understanding physiological and disordered BLA functions. The BLA receives dense cholinergic input from the basal forebrain, affecting both normal functions and dysfunctions of the amygdala, but the mechanisms involved in the cholinergic regulation of inhibitory activity in the BLA are unclear. Using whole cell recordings in rat amygdala slices, here we demonstrate that the α(7)-containing nicotinic acetylcholine receptors (α(7)-nAChRs) are present on somatic or somatodendritic regions of BLA interneurons. These receptors are active in the basal state enhancing GABAergic inhibition, and their further, exogenous activation produces a transient but dramatic increase of spontaneous inhibitory postsynaptic currents in principal BLA neurons. In the absence of AMPA/kainate receptor antagonists, activation of α(7)-nAChRs in the BLA network increases both GABAergic and glutamatergic spontaneous currents in BLA principal cells, but the inhibitory currents are enhanced significantly more than the excitatory currents, reducing overall excitability. The anxiolytic effects of nicotine as well as the role of the α(7)-nAChRs in seizure activity involving the amygdala and in mental illnesses, such as schizophrenia and Alzheimer's disease, may be better understood in light of the present findings.

Noradrenergic innervation of pyramidal cells in the rat basolateral amygdala

The basolateral nuclear complex of the amygdala (BLC) receives dense noradrenergic/norepinephrine (NE) inputs from the locus coeruleus that play a key role in modulating emotional memory consolidation. Knowledge of the extent of synapse formation by NE inputs to the BLC, as well as the cell types innervated, would contribute to an understanding of how NE modulates the activity of the BLC. To gain a better understanding of NE circuits in the BLC, dual-label immunohistochemistry was used at the light and electron microscopic levels in the present study to analyze NE axons and their innervation of pyramidal cells in the anterior subdivision of the basolateral amygdalar nucleus (BLa). NE axons and BLa pyramidal cells were labeled using antibodies to the norepinephrine transporter (NET) and Ca(2+)/calmodulin-dependent protein kinase (CaMK), respectively. Dual localization studies using antibodies to NET and dopamine-beta-hydroxylase (DBH) revealed that virtually all NE axons and varicosities expressed both proteins. The BLa exhibited a medium density of NET+ fibers. Ultrastructural analysis of serial section reconstructions of NET+ axons revealed that only about half of NET+ terminals formed synapses. The main postsynaptic targets were small-caliber CAMK+ dendritic shafts and spines of pyramidal cells. A smaller number of NET+ terminals formed synapses with unlabeled cell bodies and dendrites. These findings indicate that the distal dendritic domain of BLa pyramidal cells is the major target of NE terminals in the BLa, and the relatively low synaptic incidence suggests that diffusion from non-synaptic terminals may be important for noradrenergic modulation of the BLa.

Retrieval of contextual memories increases activity-regulated cytoskeleton-associated protein in the amygdala and hippocampus

Activity-regulated cytoskeleton-associated protein (Arc) integrates information from multiple intracellular signaling cascades and, in turn, regulates cytoskeletal proteins involved in structural synaptic modifications. The purposes of the present study were: (1) to determine if the retrieval of contextual memories would induce Arc in hippocampal and amygdalar neurons; (2) use unbiased stereology at the ultrastructural level to quantify synapses contacting Arc-labeled (Arc+) and unlabeled (Arc-) postsynaptic structures in brain regions in which the amount of Arc integrated density (ID) correlated strongly with the degree of amphetamine conditioned place preference (AMPH CPP). The retrieval of contextual memories increased the Arc ID in the dentate gyrus, cornu ammonis (CA)1, and CA3 fields of the hippocampus and the basolateral, lateral, and central nuclei of the amygdala but not the primary auditory cortex, a control region. Stereological quantification of Arc+ and Arc- synapses in the basolateral nucleus of the amygdala (BLA) was undertaken because the strongest relationship between the amount of Arc ID and AMPH CPP was observed in the BLA. The retrieval of contextual memories increased the number and density of asymmetric (presumed excitatory) synapses contacting Arc+ spines and dendrites of BLA neurons, symmetric (presumed inhibitory or modulatory) synapses contacting Arc+ dendrites of BLA neurons, and multisynaptic boutons contacting Arc+ postsynaptic structures. Thus, the retrieval of contextual memories increases Arc in the amygdala and hippocampus, an effect that could be important for approach behavior to a drug-associated context.

Neuronal localization of M2 muscarinic receptor immunoreactivity in the rat amygdala

Muscarinic cholinergic neurotransmission in the amygdala is critical for memory consolidation in emotional/motivational learning tasks, but little is known about the neuronal distribution of different receptor subtypes. Immunohistochemistry was used in the present investigation to localize the m2 receptor (M2R). Differential patterns of M2R-immunoreactivity (M2R-ir) were observed in the somata and neuropil of the various amygdalar nuclei. Neuropilar M2R-ir was strongest in rostral portions of the basolateral nuclear complex (BLC). M2R-positive (M2R+) somata were seen in low numbers in all nuclei of the amygdala. Most M2R+ neurons associated with the BLC were in the lateral nucleus and external capsule. These cells were nonpyramidal neurons that contained glutamatic acid decarboxylase (GAD), somatostatin (SOM), and neuropeptide Y (NPY), but not parvalbumin (PV), calretinin (CR), or cholecystokinin (CCK). Little or no M2R-ir was observed in GAD+, PV+, CR+, or CCK+ axons in the BLC, but it was seen in some SOM+ axons and many NPY+ axons. M2R-ir was found in a small number of spiny and aspiny neurons of the central nucleus that were mainly located along the lateral and ventral borders of its lateral subdivision. Many of these cells contained SOM and NPY. M2R+ neurons were also seen in the medial nucleus, including a distinct subpopulation of neurons that surrounded its anteroventral subdivision. The latter neurons were negative for all neuronal markers analyzed. The intercalated nuclei (INs) were associated with two types of large M2R+ neurons, spiny and aspiny. The small principal neurons of the INs were M2R-negative. The somata and dendrites of the large spiny neurons, which were actually found in a zone located just outside of the rostral INs, expressed SOM and NPY, but not GAD. These findings indicate that acetylcholine can modulate a variety of discrete neuronal subpopulations in various amygdalar nuclei via M2Rs, especially neurons that express SOM and NPY.

Postsynaptic targets of GABAergic basal forebrain projections to the basolateral amygdala

Recent studies indicate that the basolateral amygdala, like the neocortex and hippocampus, receives GABAergic inputs from the basal forebrain in addition to the well-established cholinergic inputs. Since the neuronal targets of these inputs have yet to be determined, it is difficult to predict the functional significance of this innervation. The present study addressed this question in the rat by employing anterograde tract tracing combined with immunohistochemistry at the light and electron microscopic levels of analysis. Amygdalopetal axons from the basal forebrain mainly targeted the basolateral nucleus (BL) of the amygdala. The morphology of these axons was heterogeneous and included GABAergic axons that contained vesicular GABA transporter protein (VGAT). These axons, designated type 1, exhibited distinctive large axonal varicosities that were typically clustered along the length of the axon. Type 1 axons formed multiple contacts with the cell bodies and dendrites of parvalbumin-containing (PV+) interneurons, but relatively few contacts with calretinin-containing and somatostatin-containing interneurons. At the ultrastructural level of analysis, the large terminals of type 1 axons exhibited numerous mitochondria and were densely packed with synaptic vesicles. Individual terminals formed broad symmetrical synapses with BL PV+ interneurons, and often formed additional symmetrical synapses with BL pyramidal cells. Some solitary type 1 terminals formed symmetrical synapses solely with BL pyramidal cells. These results suggest that GABAergic neurons of the basal forebrain provide indirect disinhibition, as well as direct inhibition, of BL pyramidal neurons. The possible involvement of these circuits in rhythmic oscillations related to emotional learning, attention, and arousal is discussed.

Different fear states engage distinct networks within the intercalated cell clusters of the amygdala

Although extinction-based therapies are among the most effective treatments for anxiety disorders, the neural bases of fear extinction remain still essentially unclear. Recent evidence suggests that the intercalated cell masses of the amygdala (ITCs) are critical structures for fear extinction. However, the neuronal organization of ITCs and how distinct clusters contribute to different fear states are still entirely unknown. Here, by combining whole-cell patch-clamp recordings and biocytin labeling with full anatomical reconstruction of the filled neurons and ultrastructural analysis of their synaptic contacts, we have elucidated the cellular organization and efferent connections of one of the main ITC clusters in mice. Our data showed an unexpected heterogeneity in the axonal pattern of medial paracapsular ITC (Imp) neurons and the presence of three distinct neuronal subtypes. Functionally, we observed that the Imp was preferentially activated during fear expression, whereas extinction training and extinction retrieval activated the main ITC nucleus (IN), as measured by quantifying Zif268 expression. This can be explained by the IPSPs evoked in the IN after Imp stimulation, most likely through the GABAergic monosynaptic innervation of IN neurons by one subtype of Imp cells, namely the medial capsular-projecting (MCp)-Imp neurons. MCp-Imp neurons also target large ITC cells that surround ITC clusters and express the metabotropic glutamate receptor 1α. These findings reveal a distinctive participation of ITC clusters to different fear states and the underlying anatomical circuitries, hence shedding new light on ITC networks and providing a novel framework to elucidate their role in fear expression and extinction.

Cholinergic innervation of pyramidal cells and parvalbumin-immunoreactive interneurons in the rat basolateral amygdala

The basolateral nucleus of the amygdala receives an extremely dense cholinergic innervation from the basal forebrain that is critical for memory consolidation. Although previous electron microscopic studies determined some of the postsynaptic targets of cholinergic afferents, the majority of postsynaptic structures were dendritic shafts whose neurons of origin were not identified. To make this determination, the present study analyzed the cholinergic innervation of the anterior subdivision of the basolateral amygdalar nucleus (BLa) of the rat using electron microscopic dual-labeling immunocytochemistry. The vesicular acetylcholine transporter (VAChT) was used as a marker for cholinergic terminals; calcium/calmodulin-dependent protein kinase II (CaMK) was used as a marker for pyramidal cells, the principal neurons of the BLa; and parvalbumin (PV) was used as a marker for the predominant interneuronal subpopulation in this nucleus. VAChT(+) terminals were visualized by using diaminobenzidine as a chromogen, whereas CAMK(+) or PV(+) neurons were visualized with Vector very intense purple (VIP) as a chromogen. Quantitative analyses revealed that the great majority of dendritic shafts receiving cholinergic inputs were CAMK(+) , indicating that they were of pyramidal cell origin. In fact, 89% of the postsynaptic targets of cholinergic terminals in the BLa were pyramidal cells, including perikarya (3%), dendritic shafts (47%), and dendritic spines (39%). PV(+) structures, including perikarya and dendrites, constituted 7% of the postsynaptic targets of cholinergic axon terminals. The cholinergic innervation of both pyramidal cells and PV(+) interneurons may constitute an anatomical substrate for the generation of oscillatory activity involved in memory consolidation by the BLa.

Neuronal localization of m1 muscarinic receptor immunoreactivity in the rat basolateral amygdala

Muscarinic cholinergic neurotransmission in the basolateral nuclear complex (BLC) of the amygdala is critical for memory consolidation in emotional/motivational learning tasks. Although knowledge of the localization of muscarinic receptor subtypes in the BLC would contribute to an understanding of the actions of acetylcholine in mnemonic function, previous receptor binding and in situ hybridization studies lacked the resolution necessary to identify which neurons in the BLC express different receptor subtypes. In the present study immunohistochemistry was used to study the neuronal localization of the m1 receptor. The intensity of m1 immunoreactivity varied in different nuclei of the amygdala, and was most robust in the BLC, and in the adjacent posterolateral cortical nucleus. The density and morphology of labeled neurons in the BLC suggested that the m1+ neuronal population included pyramidal cells, the principal neurons in this amygdalar region. In addition, there was dense punctate m1 immunoreactivity in the neuropil of the BLC. Dual labeling immunofluorescence studies of the BLC using antibodies to cell type specific markers were performed to more definitively determine the phenotype of m1-positive (m1+) neurons. An antibody to calcium/calmodulin protein kinase II (CaMK) was used to label pyramidal cells, whereas an antibody to glutamic acid decarboxylase was used to label interneurons. Virtually all of the intensely labeled m1+ neurons of the BLC were CaMK+ pyramidal cells. These data suggest that the ability of M1 receptor antagonists to impair memory consolidation in the BLC is mainly due to blockade of cholinergic influences on the activity of pyramidal neurons.

Immunohistochemical characterization of parvalbumin-containing interneurons in the monkey basolateral amygdala

Interneurons expressing the calcium-binding protein parvalbumin (PV) are a critical component of the inhibitory circuitry of the basolateral nuclear complex (BLC) of the mammalian amygdala. These neurons form interneuronal networks interconnected by chemical and electrical synapses, and provide a strong perisomatic inhibition of local pyramidal projection neurons. Immunohistochemical studies in rodents have shown that most parvalbumin-positive (PV+) cells are GABAergic interneurons that co-express the calcium-binding protein calbindin (CB), but exhibit no overlap with interneuronal subpopulations containing the calcium-binding protein calretinin (CR) or neuropeptides. Despite the importance of identifying interneuronal subpopulations for clarifying the major players in the inhibitory circuitry of the BLC, very little is known about these subpopulations in primates. Therefore, in the present investigation dual-labeling immunofluorescence histochemical techniques were used to characterize PV+ interneurons in the basal and lateral nuclei of the monkey amygdala. These studies revealed that 90-94% of PV+ neurons were GABA+, depending on the nucleus, and that these neurons constituted 29-38% of the total GABAergic population. CB+ and CR+ interneurons constituted 31-46% and 23-27%, respectively, of GABAergic neurons. Approximately one quarter of PV+ neurons contained CB, and these cells constituted one third of the CB+ interneuronal population. There was no colocalization of PV with the neuropeptides somatostatin or cholecystokinin, and virtually no colocalization with CR. These data indicate that the neurochemical characteristics of the PV+ interneuronal subpopulation in the monkey BLC are fairly similar to those seen in the rat, but there is far less colocalization of PV and CB in the monkey. These findings suggest that PV+ neurons are a discrete interneuronal subpopulation in the monkey BLC and undoubtedly play a unique functional role in the inhibitory circuitry of this brain region.

Intra-amygdala injections of CREB antisense impair inhibitory avoidance memory: role of norepinephrine and acetylcholine

Infusions of CREB antisense into the amygdala prior to training impair memory for aversive tasks, suggesting that the antisense may interfere with CRE-mediated gene transcription and protein synthesis important for the formation of new memories within the amygdala. However, the amygdala also appears to modulate memory formation in distributed brain sites, through mechanisms that include the release of norepinephrine and acetylcholine within the amygdala. Thus, CREB antisense injections may affect memory by interfering with mechanisms of modulation, rather than storage, of memory. In the present experiment, rats received bilateral intra-amygdala infusions of CREB antisense (2 nmol/1 microL) 6 h prior to inhibitory avoidance training. In vivo microdialysis samples were collected from the right amygdala before, during, and following training. CREB antisense produced amnesia tested at 48 h after training. In addition, CREB antisense infusions dampened the training-related release of norepinephrine, and to a lesser extent of acetylcholine, in the amygdala. Furthermore, intra-amygdala infusions of the beta-adrenergic receptor agonist clenbuterol administered immediately after training attenuated memory impairments induced by intra-amygdala injections of CREB antisense. These findings suggest that intra-amygdala treatment with CREB antisense may affect processes involved in modulation of memory in part through interference with norepinephrine and acetylcholine neurotransmission in the amygdala.

Amygdala tractography predicts functional connectivity and learning during feedback-guided decision-making

Flexibly adapting behavior in dynamic environments relies on fronto-limbic networks that include the amygdala, orbitofrontal cortex, and striatum. Animal work demonstrates that interactions among these regions are critical for flexible feedback-guided learning, but it remains unknown to what extent such anatomical-functional interactions operate in humans. Here, we use connectivity analyses in both structural and functional MRI to further our understanding of how brain circuits work in conjunction to promote goal-directed behavior. In particular, fiber tracking based on diffusion-weighted imaging provides information about anatomical connectivity between brain structures, and functional MRI provides estimates of functional connectivity between structures. We found that, during a feedback-guided reversal learning task, the strength of estimated white matter tracts from the amygdala to the hippocampus, orbitofrontal cortex, and ventral striatum predicted both how subjects adapted their behavior following positive and negative feedback, and the functional connectivity (estimated from functional MRI time series) between the amygdala and these regions. In addition, we identified a dissociation between an amygdala-hippocampus circuit that predicted response switching, and an amygdala-orbitofrontal cortex circuit that predicted learning following rule reversals. These findings provide novel insights into how the anatomy and functioning of amygdala-related brain circuits mediate different aspects of feedback-guided learning behavior.

Differential modes of termination of amygdalothalamic and amygdalocortical projections in the monkey

The amygdala complex participates in multiple systems having to do with affective processes. It has been implicated in human disorders of social and emotional behavior, such as autism. Of the interconnected functional networks, considerable research in rodents and primates has focused on connections between the amygdala and orbitofrontal cortex (OFC). The amygdala projects to OFC by both a direct amygdalocortical (AC) pathway and an indirect pathway through mediodorsal thalamus. In the rat, retrograde tracer experiments indicate that the AC and amygdalothalamic (AT) pathways originate from separate populations, and may therefore convey distinctive information, although the characteristics of these pathways remain unclear. To investigate this issue in monkeys we made anterograde tracer injections in the basolateral amygdala complex (BLC; n = 3). Three distinctive features were found preferentially associated with the AT or AC pathways. First, AT terminations are large (average diameter = 3.5 microm; range = 1.2-7.0 microm) and cluster around proximal dendrites, in contrast with small-bouton AC terminations. Second, AT terminations form small arbors (diameter approximately 0.1 mm), while AC are widely divergent (often >1.0 mm long). The AT terminations features are reminiscent of large bouton, "driver" corticothalamic terminations. Finally, AC but not AT terminations are positive for zinc (Zn), a neuromodulator associated with synaptic plasticity. From these results we suggest that AC and AT terminations originate from distinct populations in monkey as well as in rodent. Further work is necessary to determine the degree and manner of their segregation and how these subsystems interact within a broader connectivity network.

Distribution of IP3-mediated calcium responses and their role in nuclear signalling in rat basolateral amygdala neurons

Metabotropic receptor activation is important for learning, memory and synaptic plasticity in the amygdala and other brain regions. Synaptic stimulation of metabotropic receptors in basolateral amygdala (BLA) projection neurons evokes a focal rise in free Ca(2+) in the dendrites that propagate as waves into the soma and nucleus. These Ca(2+) waves initiate in the proximal dendrites and show limited propagation centrifugally away from the soma. In other cell types, Ca(2+) waves have been shown to be mediated by either metabotropic glutamate receptor (mGluR) or muscarinic receptor (mAChR) activation. Here we show that mGluRs and mAChRs act cooperatively to release Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive intracellular Ca(2+) stores. Whereas action potentials (APs) alone were relatively ineffective in raising nuclear Ca(2+), their pairing with metabotropic receptor activation evoked an IP(3)-receptor-mediated Ca(2+)-induced Ca(2+) release, raising nuclear Ca(2+) into the micromolar range. Metabotropic-receptor-mediated Ca(2+)-store release was highly compartmentalized. When coupled with metabotropic receptor stimulation, large robust Ca(2+) rises and AP-induced amplification were observed in the soma, nucleus and sparsely spiny dendritic segments with metabotropic stimulation. In contrast, no significant amplification of the Ca(2+) transient was detected in spine-dense high-order dendritic segments. Ca(2+) rises evoked by photolytic uncaging of IP(3) showed the same distribution, suggesting that IP(3)-sensitive Ca(2+) stores are preferentially located in the soma and proximal dendrites. This distribution of metabotropic-mediated store release suggests that the neuromodulatory role of metabotropic receptor stimulation in BLA-dependent learning may result from enhanced nuclear signalling.

Nitric oxide synthase-containing neurons in the amygdaloid nuclear complex of the rat

The nitric oxide-producing neurons in the rat amygdala (Am) were studied, using reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry. Almost all nuclei of the Am contained NADPHd-positive neurons and fibers, but the somatodendritic morphology and the intensity of staining of different subpopulations varied. The strongly stained neurons displayed labeling of the perikaryon and the dendritic tree with Golgi impregnation-like quality, whilst the dendrites of the lightly stained neurons were less successfully followed. Many strongly positive neurons were located in the external capsule and within the intraamygdaloid fiber bundles. A large number of small, strongly stained cells was present in the amygdalostriatal transition area. In the Am proper, a condensation of deeply stained cells occurred in the lateral amygdaloid nucleus. In the basolateral nucleus, the strongly NADPHd-positive neurons were few, and were located mainly along the lateral border of the nucleus. These cells clearly differed from the large, pyramidal, and efferent cells. The basomedial nucleus contained numerous positive cells but most of them were only lightly labeled. A moderate number of strongly stained neurons appeared in the medial division of the central nucleus, and a larger accumulation of strongly positive cells was present in the lateral and the capsular divisions. The medial amygdaloid nucleus contained numerous moderately stained neurons and displayed the strongest diffuse neuropil staining in Am. In the nucleus of the lateral olfactory tract, the first layer contained only NADPHd-stained axons, in the second layer, there were numerous moderately stained cells, and in the third layer, a few but deeply stained neurons. From the cortical nuclei, the most appreciable number of stained neurons was seen in the anterior cortical nucleus. The anterior amygdaloid area contained numerous NADPHd-positive neurons; in its dorsal part the majority of cells were only moderately stained, whereas in the ventral part the neurons were very strongly stained. The intercalated amygdaloid nucleus lacked NADPHd-positive neurons but an appreciable plexus of fine, tortuous axons was present. In the intra-amygdaloid part of the bed nucleus of the stria terminalis (st) some lightly stained cells were seen but along the entire course of st strongly stained neurons were observed. Some Am nuclei, and especially the central lateral nucleus and the intercalated nucleus, display considerable species differences when compared with the primate Am. The age-related changes of the nitrergic Am neurons, as well as their involvement in neurodegenerative diseases is discussed.

Pyramidal cells of the rat basolateral amygdala: synaptology and innervation by parvalbumin-immunoreactive interneurons

The generation of emotional responses by the basolateral amygdala is determined largely by the balance of excitatory and inhibitory inputs to its principal neurons, the pyramidal cells. The activity of these neurons is tightly controlled by gamma-aminobutyric acid (GABA)-ergic interneurons, especially a parvalbumin-positive (PV(+)) subpopulation that constitutes almost half of all interneurons in the basolateral amygdala. In the present semiquantitative investigation, we studied the incidence of synaptic inputs of PV(+) axon terminals onto pyramidal neurons in the rat basolateral nucleus (BLa). Pyramidal cells were identified by using calcium/calmodulin-dependent protein kinase II (CaMK) immunoreactivity as a marker. To appreciate the relative abundance of PV(+) inputs compared with excitatory inputs and other non-PV(+) inhibitory inputs, we also analyzed the proportions of asymmetrical (presumed excitatory) synapses and symmetrical (presumed inhibitory) synapses formed by unlabeled axon terminals targeting pyramidal neurons. The results indicate that the perisomatic region of pyramidal cells is innervated almost entirely by symmetrical synapses, whereas the density of asymmetrical synapses increases as one proceeds from thicker proximal dendritic shafts to thinner distal dendritic shafts. The great majority of synapses with dendritic spines are asymmetrical. PV(+) axon terminals form mainly symmetrical synapses. These PV(+) synapses constitute slightly more than half of the symmetrical synapses formed with each postsynaptic compartment of BLa pyramidal cells. These data indicate that the synaptology of basolateral amygdalar pyramidal cells is remarkably similar to that of cortical pyramidal cells and that PV(+) interneurons provide a robust inhibition of both the perisomatic and the distal dendritic domains of these principal neurons.

Neural substrates of cocaine-cue associations that trigger relapse

Learned associations that occur during the process of repeated drug use in addiction can later manifest as trigger factors in relapse to renewed drug-seeking and drug-taking behavior. The process of conditioned-cued relapse of drug-seeking behavior has been successfully modeled in animals using the reinstatement procedure, in which chronic drug self-administration can be extinguished or withheld, and then reinstated using conditioned stimuli previously paired with the drug. Our laboratory has extensively studied the neural circuitry underlying conditioned-cued drug-seeking during the expression of reinstatement. In order to study the learning process of drug-cue pairings, we further developed a procedure whereby discrete cocaine-cue pairings can be conducted in a single pavlovian training session in animals previously trained to self-administer cocaine. Presentation of these cues during later reinstatement trials produces robust responding over extinction levels at levels similar to those seen when animals experience the cues on a daily basis. In a series of experiments, we have shown that reversible pharmacological inactivation of the basolateral complex of the amygdala just prior to acquisition of cocaine-cue associations blocks the ability of cocaine-paired stimuli to elicit conditioned-cued reinstatement. This learning process is mediated in part by muscarinic acetylcholine and dopaminergic inputs to the basolateral complex of the amygdala, as intra-amygdala infusion of selective receptor antagonists at the time of acquisition significantly affects reinstatement. We have also recently found that disruption of neural activity within the basolateral complex of the amygdala at the time of consolidation (just after cocaine-cue pairings) will disrupt reinstatement. Taken together, these results reveal the importance of the amygdala in the acquisition, consolidation, and expression of drug-stimulus learning that drives relapse to drug-seeking behavior.

Brain cholinergic vulnerability: Relevance to behavior and disease

The major populations of cholinergic neurons in the brain include two "projection" systems, located in the pontine reticular formation and in the basal forebrain. These two complexes comprise, in part, the anatomical substrates for the "ascending reticular activating system" (ARAS). The pontine cholinergic system relays its rostral influences mainly through thalamic intralaminar nuclei, but it also connects to the basal forebrain and provides a minor innervation of cortex. The basal forebrain cholinergic complex (BFCC) projects directly to cortex and hippocampus, and has a minor connection with the thalamus. Recent data reveal that a parallel system of basal forebrain GABAergic projection neurons innervates cortex/hippocampus in a way that seems to complement the BFCC. Generally, the picture developed from more than 50 years of research is consistent with a "global" influence of these two ascending cholinergic projections on cortical and hippocampal regions. Seemingly, the BFCC acts in tandem or in parallel with the pontine cholinergic projection to activate the electro-encephalogram, increase cerebral blood flow, regulate sleep-wake cycling, and modulate cognitive function. There are quite a number and variety of human brain conditions, notably including Alzheimer's disease, in which degeneration of basal forebrain cholinergic neurons has been documented. Whether the corticopetal GABA system is affected by disease has not been established. Studies of degeneration of the pontine projection are limited, but the available data suggest that it is relatively preserved in Alzheimer's disease. Hypotheses of BFCC degeneration include growth factor deprivation, intracellular calcium dysfunction, amyloid excess, inflammation, and mitochondrial abnormalities/oxidative stress. But, despite considerable research conducted over several decades, the exact mechanisms underlying brain cholinergic vulnerability in human disease remain unclear.

Coupled networks of parvalbumin-immunoreactive interneurons in the rat basolateral amygdala

Recent studies indicate that the basolateral amygdala exhibits fast rhythmic oscillations during emotional arousal, but the neuronal mechanisms underlying this activity are not known. Similar oscillations in the cerebral cortex are generated by a network of parvalbumin (PV)-immunoreactive interneurons interconnected by chemical synapses and dendritic gap junctions. The present immunoelectron microscopic study revealed that the basolateral amygdalar nucleus (BLa) contains a network of parvalbumin-immunoreactive (PV+) interneurons interconnected by chemical synapses, dendritic gap junctions, and axonal gap junctions. Twenty percent of synapses onto PV+ neurons were formed by PV+ axon terminals. All of these PV+ synapses were symmetrical. PV+ perikarya exhibited the greatest incidence of PV+ synapses (30%), with lower percentages associated with PV+ dendrites (15%) and spines (25%). These synapses comprised half of all symmetrical synapses formed with PV+ cells. A total of 18 dendrodendritic gap junctions between PV+ neurons were observed, mostly involving secondary and more distal dendrites (0.5-1.0 microm thick). Dendritic gap junctions were often in close proximity to PV+ chemical synapses. Six gap junctions were observed between PV+ axon terminals. In most cases, one or both of these terminals formed synapses with the perikarya of principal neurons. This is the first study to describe dendritic gap junctions interconnecting PV+ interneurons in the basolateral amygdala. It also provides the first documentation of gap junctions between interneuronal axon terminals in the mammalian forebrain. These data provide the anatomical basis for a PV+ network that may play a role in the generation of rhythmic oscillations in the BLa during emotional arousal.

Muscarinic receptor antagonism in the basolateral amygdala blocks acquisition of cocaine-stimulus association in a model of relapse to cocaine-seeking behavior in rats

Recent evidence has demonstrated a critical role for the basolateral amygdala complex in the reinstatement of extinguished drug-seeking behavior produced by drug-paired cues. In the current study, we utilized a model of the acquisition and expression of cocaine-stimulus associative pairing in order to study the role of cholinergic input to the basolateral amygdala in mediating conditioned-cued reinstatement. Male, Sprague-Dawley rats were first trained daily to self-administer i.v. cocaine on a fixed ratio 1 schedule of reinforcement. The muscarinic acetylcholine receptor antagonist, scopolamine, was directly infused into the basolateral amygdala prior to: a) classically conditioned pairing of a tone+light stimulus with cocaine infusions (acquisition), or b) testing of conditioned-cued reinstatement following a period of withdrawal from cocaine and extinction of cocaine-paired lever responding. Infusion of scopolamine just prior to the classical conditioning trial produced a dose-dependent disruption of cocaine-seeking behavior maintained by cocaine-paired cues during the reinstatement test. In contrast, infusion of scopolamine prior to the reinstatement test had no effect on conditioned-cued reinstatement of cocaine-seeking behavior. These results indicate a crucial role for cholinergic innervation of muscarinic acetylcholine receptors in the basolateral amygdala during the formation, but not the expression, of stimulus-reward associations that mediate cue-induced cocaine-seeking behavior.

Cholinergic modulation of memory in the basolateral amygdala involves activation of both m1 and m2 receptors

Muscarinic cholinergic activation is a critical component of basolateral amygdala (BLA)-mediated modulation of memory consolidation. The receptor(s) mediating this activation during consolidation have not been elucidated. This study investigated the roles of muscarinic subtype 1 (m1) and subtype 2 (m2) receptors in memory enhancement, by post-training intra-BLA infusions of the non-selective muscarinic agonist oxotremorine. Rats received intra-BLA infusions of either oxotremorine alone (10 microg in 0.2 microl per side), oxotremorine together with the selective m1 antagonist telenzipine (1.7, 5.0, 17 or 50 nmol/side), oxotremorine with the selective m2 antagonist methoctramine (1.7, 5.0, 17 or 50 nmol/side), oxotremorine with a combination of the above doses of telenzipine and methoctramine, or only vehicle, immediately after inhibitory avoidance training. Performance on a 48-hour retention test was significantly enhanced in oxotremorine-treated rats relative to vehicle-infused controls. Intra-BLA co-infusion of oxotremorine with either telenzipine (5, 17 or 50 nmol/side) or methoctramine (17 or 50 nmol/side) blocked the oxotremorine-induced enhancement. Combinations of these antagonists did not act additively to block memory enhancement by oxotremorine. These findings indicate that modulation of memory consolidation induced by cholinergic influences within the BLA requires activation of both m1 and m2 receptor synapses. Plausible mechanisms for m1- and m2-mediated influences on BLA circuitry are discussed.

Drug addiction, relapse, and the amygdala

Evidence has extensively implicated the amygdala in the associative learning process for appetitive reinforcers. Recent interest has focused on the role of the amygdala in the learned associations that occur during the process of drug addiction and relapse. Using an animal model of relapse after chronic cocaine self-administration, we found that rats reinstate extinguished lever responding for conditioned stimuli (tone + light) previously paired with cocaine or heroin ("conditioned-cued reinstatement"). The basolateral amygdala (BLA) complex plays a critical role in this behavior, because permanent lesions or reversible pharmacologic inactivation of the BLA attenuates conditioned-cued reinstatement without affecting cocaine self-administration or cocaine-primed reinstatement. Conditioned-cued reinstatement appears to be mediated in part by dopamine inputs to the BLA, as intra-BLA infusion of a dopamine D1 receptor antagonist blocks reinstatement, whereas intra-BLA infusion of amphetamine potentiates reinstatement. Furthermore, the BLA is also necessary for acquisition of associative learning with cocaine-paired stimuli. Disruption of neural activity within the BLA by sodium channel blockade or muscarinic receptor blockade just before acquisition of stimulus-cocaine associations blocks the ability of conditioned stimuli to elicit conditioned-cued reinstatement after extinction. Together, these results reveal the importance of the amygdala as part of a corticolimbic circuit mediating both the acquisition and the expression of conditioning that plays a critical role in relapse to drug-seeking behavior.

Synaptic connections of distinct interneuronal subpopulations in the rat basolateral amygdalar nucleus

Although it is well established that the activity of pyramidal projection neurons in the basolateral amygdala (ABL) is controlled by gamma-aminobutyric acid (GABA)ergic inhibitory interneurons, very little is known about the connections of specific interneuronal subpopulations in this region. In the present study, immunohistochemical techniques were used at the light and electron microscopic levels to identify specific populations of interneurons and to analyze their connections with each other and with unlabeled presumptive pyramidal neurons. Double-labeling immunofluorescence experiments revealed that antibodies to vasoactive intestinal peptide (VIP) and calbindin-D28K (CB) labeled two separate interneuronal subpopulations in the ABL. Light microscopic double-labeling immunoperoxidase experiments demonstrated that many VIP-positive (VIP+) axon terminals formed intimate synaptic-like contacts with the CB-positive (CB+) neurons and that both CB+ and VIP+ terminals often contributed to the formation of pericellular baskets that surrounded unlabeled perikarya of pyramidal neurons. By using a dual immunoperoxidase/immunogold-silver procedure at the ultrastructural level, it was found that 30% of VIP+ terminals in the anterior subdivision of the basolateral nucleus innervated interneurons that were either CB+ (25%) or VIP+ (5%). A smaller percentage (15%) of CB+ terminals formed synapses with labeled interneurons. Both VIP+ and CB+ terminals also innervated unlabeled perikarya, dendrites, and spines, most of which probably belonged to pyramidal neurons. The interconnections between interneurons may be important for disinhibitory mechanisms and the mediation of rhythmic oscillations in the ABL.

Cholinergic activation of the basolateral amygdala regulates unlearned freezing behavior in rats

Recent evidence suggests that the basolateral amygdala (BLA) is involved in the expression of freezing behavior in rats. This study investigated the effects of unilateral phthalic acid (PA) lesions of the nucleus basalis magnocellularis (NBM) on fear-motivated behavior in response to a natural predator-stimulus. Such lesions preferentially disrupt the cholinergic projection to the BLA. Rats were placed in a chamber containing either real or fake cat hair, and the amount of time spent freezing and the number of contacts made with the stimulus were measured. Compared with Sham control rats, the PA NBM-lesioned rats displayed significantly less freezing in the presence of the cat hair. Both the Sham and lesioned rats made fewer contacts with the real than the fake cat hair. Pre-testing intra-BLA infusion of the direct muscarinic cholinergic agonist oxotremorine ipsilateral to the PA NBM-lesion attenuated the freezing deficit. The indirect non-specific cholinergic agonist physostigmine increased the time spent freezing in Sham rats, but did not attenuate the freezing deficit in the NBM-lesioned rats. Sham and NBM-lesioned rats given oxotremorine infusions made fewer contacts with either the real or the fake cat hair. The PA NBM-lesion did not affect open field activity. These findings indicate that muscarinic cholinergic activation in the BLA from the NBM influences fear-motivated freezing behavior.

Immunohistochemical characterization of somatostatin containing interneurons in the rat basolateral amygdala

There are discrete subpopulations of GABAergic interneurons in the basolateral amygdala (ABL) that contain particular neuropeptides or calcium-binding proteins (calbindin-D28k, parvalbumin (PV), or calretinin). The present study employed a dual-labeling immunofluorescence technique combined with confocal laser scanning microscopy to investigate the neurochemical characteristics of the interneuronal subpopulation containing somatostatin (SOM). The great majority of SOM+ neurons in the ABL exhibited GABA immunoreactivity (66-82% depending on the nucleus). These SOM+ neurons constituted 11-18% of the GABA+ population. There was also extensive colocalization of SOM with calbindin (CB) in all nuclei of the ABL, but no colocalization of SOM with parvalbumin, calretinin, or vasoactive intestinal polypeptide. In the basolateral nucleus more than 90% of SOM+ neurons also exhibited CB immunoreactivity, whereas in the lateral nucleus about two-thirds of SOM+ neurons contained significant levels of CB. These SOM/CB neurons constituted about one quarter of the CB+ population in the basolateral nucleus and about one third of the CB+ population in the lateral nucleus. These results, in conjunction with the findings of previous studies, indicate that there are at least three major subpopulations of GABAergic interneurons in the ABL: (i) SOM+ neurons (most of which also contain CB and/or neuropeptide Y); (ii) PV+ neurons (most of which also contain CB); and (iii) CR+ neurons (most of which also contain vasoactive intestinal polypeptide).

GABAergic innervation of alpha type II calcium/calmodulin-dependent protein kinase immunoreactive pyramidal neurons in the rat basolateral amygdala

Although calcium/calmodulin-dependent protein kinase II (CaMK) has been shown to play a critical role in long-term potentiation (LTP) and emotional learning mediated by the basolateral amygdala, little is known about its cellular localization in this region. We have utilized immunohistochemical methods to study the neuronal localization of CaMK, and its relationship to gamma-aminobutyric acid (GABA)-ergic structures, in the rat basolateral amygdala (ABL). Light microscopic observations revealed dense CaMK staining in the ABL. Although the cell bodies and proximal dendrites of virtually every pyramidal cell appeared to be CaMK(+), the cell bodies of small nonpyramidal neurons were always unstained. Dual localization of CaMK and GABA immunoreactivity with confocal immunofluorescence microscopy revealed that CaMK and GABA were found in different neuronal populations in the ABL. CaMK was contained only in pyramidal neurons; GABA was contained only in nonpyramidal cells. At the ultrastructural level, it was found that CaMK was localized to pyramidal cell bodies, thick proximal dendrites, thin distal dendrites, most dendritic spines, axon initial segments, and axon terminals forming asymmetrical synapses. These findings suggest that all portions of labeled pyramidal cells, with the exception of some dendritic spines, can exhibit CaMK immunoreactivity. By using a dual immunoperoxidase/immunogold-silver procedure at the ultrastructural level, GABA(+) axon terminals were seen to innervate all CaMK(+) postsynaptic domains, including cell bodies (22%), thick (>1 microm) dendrites (34%), thin (<1 microm) dendrites (22%), dendritic spines (17%), and axon initial segments (5%). These findings indicate that CaMK is a useful marker for pyramidal neurons in ultrastructural studies of ABL synaptology and that the activity of pyramidal neurons in the ABL is tightly controlled by a high density of GABAergic terminals that target all postsynaptic domains of pyramidal neurons.

Phthalic acid amygdalopetal lesion of the nucleus basalis magnocellularis induces reversible memory deficits in rats

The basolateral amygdala (BLA) is extensively implicated in emotional learning and memory. The current study investigated the contribution of cholinergic afferents to the BLA from the nucleus basalis magnocellularis in influencing aversive learning and memory. Sprague-Dawley rats were given permanent unilateral phthalic acid (300 ng) lesions of the nucleus basalis magnocellularis and were chronically implanted with cannulas aimed at the ipsilateral BLA. Lesioned rats showed a pronounced inhibitory avoidance task retention deficit that was attenuated by acute posttraining infusions of the muscarinic cholinergic agonist oxotremorine (4 ng) or the indirect agonist physostigmine (1 microg) into the BLA. Continuous multiple-trial inhibitory avoidance training and testing revealed that lesioned rats have a mild acquisition deficit, requiring approximately 1 additional shock to reach the criterion, and a pronounced consolidation deficit as indicated by a shorter latency to enter the shock compartment on the retention test. Because lesioned rats did not differ from sham-operated controls in performance on a spatial water maze task or in shock sensitivity, it is not likely that the memory impairments produced by the phthalic acid lesions are due to any general sensory or motor deficits. These findings suggest that the dense cholinergic projection from the nucleus basalis magnocellularis to the BLA is involved in both the acquisition and the consolidation of the aversive inhibitory avoidance task.

Localization of the CB1 type cannabinoid receptor in the rat basolateral amygdala: high concentrations in a subpopulation of cholecystokinin-containing interneurons

The neuronal localization of the CB1 cannabinoid receptor in the rat basolateral amygdala was studied using peroxidase and fluorescence immunohistochemical techniques. All nuclei of the basolateral amygdala contained a large number of lightly stained pyramidal neurons and a small number of more intensely stained non-pyramidal neurons. Most of the latter cells had medium-sized to large multipolar somata and three to four aspiny dendrites, but some exhibited smaller oval somata. The axon initial segments of some of these non-pyramidal neurons exhibited large swollen varicosities in colchicine-injected animals, suggesting that much of the CB1 receptor protein is transported down the axons of these cells. Double-labeling studies using immunofluorescence histochemistry combined with confocal laser scanning microscopy revealed that the great majority of non-pyramidal neurons with CB1 receptor immunoreactivity belonged to a cholecystokinin-containing subpopulation. Whereas none of the other subpopulations of non-pyramidal neurons (exhibiting immunoreactivity for calretinin, parvalbumin, or somatostatin) expressed high levels of CB1 receptor immunoreactivity, a small percentage of these cells exhibited low levels of immunoreactivity. The results indicate that cannabinoids may modulate the activity of pyramidal projection neurons as well as a subpopulation of cholecystokinin-containing non-pyramidal neurons in the basolateral amygdala. Previous studies indicate that most of the latter are inhibitory interneurons that utilize GABA as a neurotransmitter. The intense staining of the cholecystokinin-containing interneurons and the evidence that large amounts of CB1 receptor protein are transported down the axons of these cells suggests that, as in the hippocampus, cannabinoids may inhibit the release of GABA from the axon terminals of these neurons.

Neural substrates of conditioned-cued relapse to drug-seeking behavior

Relapse to drug use following abstinence is a significant impediment in the long-term treatment of drug abuse and dependence. Conditioned stimuli are believed to be critically involved in activating drug craving and relapse to compulsive drug-taking behavior. Studies in humans and animal models have recently begun to identify the fundamental neural circuitry that mediates relapse following withdrawal from chronic drug self-administration. The current review summarizes key findings in this area that have converged on the amygdalar complex and regions of the frontal lobe as critical structures in conditioned-cued relapse. It is proposed that the amygdala is a key regulator of discrete stimulus-reinforcer associations, while the anterior cingulate and orbitofrontal cortex are critical regulators of relapse evoked by conditioned stimuli that predict drug availability. This corticolimbic circuitry may form the neural basis of multiple long-term conditioned associations produced by a variety of drugs of abuse ranging from psychostimulants to opiates. Future studies aimed at discerning the functional roles of these pathways will provide critical direction for the development of treatments for the prevention of relapse.

Colocalization of calcium-binding proteins and GABA in neurons of the rat basolateral amygdala

The basolateral amygdala contains subpopulations of non-pyramidal neurons that express the calcium-binding proteins parvalbumin, calbindin-D28k (calbindin) or calretinin. Although little is known about the exact functions of these proteins, they have provided useful markers of specific neuronal subpopulations in studies of the neuronal circuitry of the cerebral cortex and other brain regions. The purpose of the present study was to investigate whether basolateral amygdalar non-pyramidal neurons containing parvalbumin, calbindin, or calretinin exhibit immunoreactivity for GABA, and to determine if calretinin is colocalized with parvalbumin or calbindin in the rat basolateral amygdala. Pyramidal neurons were distinguished from non-pyramidal neurons on the basis of staining intensity. Using immunofluorescence confocal laser scanning microscopy, as well as the 'mirror technique' on immunoperoxidase-stained sections, it was found that there was virtually no colocalization of calretinin with parvalbumin or calbindin, but that the great majority of basolateral amygdalar non-pyramidal neurons containing parvalbumin, calbindin, or calretinin exhibited GABA immunoreactivity. Calbindin-positive neurons constituted almost 60% of the GABA-containing population in both subdivisions of the basolateral nucleus and more than 40% of the GABA-containing population in the lateral nucleus. Parvalbumin-positive neurons constituted 19-43% of GABA-immunoreactive neurons in the basolateral amygdala, depending on the nucleus. Calretinin-positive non-pyramidal neurons constituted about 20% of the GABA-positive neuronal population in each nucleus of the basolateral amygdala. These findings indicate that non-pyramidal neurons containing parvalbumin, calbindin, or calretinin comprise the majority of GABA-containing neurons in the basolateral amygdala, and that the calretinin subpopulation is distinct from non-pyramidal subpopulations containing parvalbumin and calbindin. These separate neuronal populations may play unique roles in the inhibitory circuitry of the amygdala.

Light and electron microscopic study of cholinergic and noradrenergic elements in the basolateral nucleus of the rat amygdala: evidence for interactions between the two systems

Pharmacological studies have suggested that the cholinergic (ACh) and noradrenergic (NA) systems in the amygdala (AM) play an important role in learning and memory storage and that the two systems interact to modulate memory storage. To obtain anatomical evidence for the interaction, the organization of the ACh and NA fibers in rat AM was investigated by immunocytochemistry for choline acetyltransferase (ChAT) and dopamine-beta-hydroxylase (DBH) in conjunction with light, confocal laser scanning, and electron microscopy (LM, CLSM, and TEM, respectively). LM showed that the ChAT immunoreactivity was densest in the basolateral nucleus (BL), whereas the DBH immunoreactivity was densest in the posterior BL. CLSM demonstrated that the ChAT-immunoreactive profiles in the BL were frequently located in juxtaposition to the DBH-immunoreactive axons. The TEM observations were as follows: The majority of the synapses formed by ChAT-immunoreactive terminals were symmetric, but DBH-immunoreactive axons formed both asymmetric and symmetric synapses. The ChAT-immunoreactive terminals usually established the symmetric synaptic contacts with the DBH-immunoreactive terminals and varicosities. The DBH-immunoreactive terminals formed the asymmetric synapses with the ChAT-immunoreactive dendrites of the intrinsic neurons within the AM. The results provide anatomical substrates for mnemonic functions of the ACh and NA systems and for the interactions between the two systems in the AM.

Parvalbumin-containing neurons in the rat basolateral amygdala: morphology and co-localization of Calbindin-D(28k)

Parvalbumin is a calcium-binding protein that is contained in certain neuronal populations in the brain. Although the exact function of parvalbumin is not clear, it has been found to be a useful marker for studying the connections of specific cell types in immunohistochemical studies. In the present investigation immunohistochemical techniques were used to study the morphology of parvalbumin-containing neurons in the rat basolateral amygdala. These neurons were found to be a morphologically heterogeneous subpopulation of non-pyramidal interneurons. Parvalbumin-positive axons in the basolateral amygdala were observed to form "pericellular baskets" that enveloped the perikarya of pyramidal neurons. In addition, some parvalbumin-immunoreactive axons formed "cartridges" that appeared to surround non-immunoreactive processes. The morphology of parvalbumin-positive neurons closely resembled that of neurons containing calbindin, a related calcium-binding protein. Analysis of adjacent sections stained for each protein using the mirror technique revealed that approximately 80% of parvalbumin neurons also contained calbindin, and that approximately 60% of calbindin neurons also contained parvalbumin. This study demonstrates that parvalbumin-containing neurons constitute an important subpopulation of non-pyramidal interneurons in the rat basolateral amygdala. The axonal configurations of these cells indicate that they may exert a potent inhibitory influence over pyramidal projection neurons. We suggest that parvalbumin-containing neurons can control emotional responses mediated by the basolateral amygdala by controlling the output from this important brain region.

Multiple output pathways of the basal forebrain: organization, chemical heterogeneity, and roles in vigilance

Studies over the last decade have shown that the basal forebrain (BF) consists of more than its cholinergic neurons. The BF also contains non-cholinergic neurons, including gamma-aminobutyric acid-ergic neurons which co-distribute and co-project with the cholinergic neurons. Both types of neuron project, in variable proportions, to the cerebral cortex, hippocampus, thalamus, amygdala, and olfactory bulb, whereas descending projections to the posterior hypothalamus and brainstem nuclei are predominantly non-cholinergic. Some of the cholinergic and non-cholinergic projection neurons contain neuropeptides such as galanin, nitric oxide synthase, and possibly glutamate. To understand better the function of the BF, the organization of the multiple ascending and descending projections of BF neurons is reviewed along with their neurochemical heterogeneity, and possible functions of individual pathways are discussed. It is proposed that BF neurons belong to multiple systems with distinct cognitive, motivational, emotional, motor, and regulatory functions, and that through these pathways, the BF plays a role in controlling both cognitive and non-cognitive aspects of vigilance.

Glucocorticoid enhancement of memory consolidation in the rat is blocked by muscarinic receptor antagonism in the basolateral amygdala

Glucocorticoid-induced memory enhancement is known to depend on beta-adrenoceptor activation in the basolateral amygdala (BLA). Additionally, inactivation of muscarinic cholinergic receptors in the rat amygdala blocks memory enhancement induced by concurrent beta-adrenergic activation. Together, these findings suggest that glucocorticoid-induced modulation of memory consolidation requires cholinergic as well as adrenergic activation in the BLA. Two experiments investigated this issue. The first experiment examined whether blockade of muscarinic cholinergic receptors in the BLA with atropine alters the memory-enhancing effects of the systemically administered glucocorticoid dexamethasone. Dexamethasone (0.3, 1.0 or 3.0 mg/kg, s.c.) administered to rats immediately after inhibitory avoidance training produced dose-dependent enhancement of 48-h retention. Concurrent bilateral infusions of the muscarinic cholinergic antagonist atropine (0.5 microg in 0.2 microL per side) into the BLA blocked the memory enhancement. The second experiment investigated whether the BLA is a locus of interaction between glucocorticoid and muscarinic activation. The specific glucocorticoid receptor (GR or type II) agonist RU 28362 (1.0, 3.0 or 10 ng) was infused into the BLA either alone or together with atropine immediately after training. The GR agonist produced dose-dependent memory enhancement and atropine blocked the memory enhancement. These findings indicate that muscarinic cholinergic activation within the BLA is critical for enabling glucocorticoid enhancement of memory consolidation and that enhancement of memory induced by GR activation in the BLA requires cholinergic activation within the BLA.

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.

Diffuse transmission by acetylcholine in the CNS

Recent immunoelectron microscopic studies have revealed a low frequency of synaptic membrane differentiations on ACh (ChAT-immunostained) axon terminals (boutons or varicosities) in adult rat cerebral cortex, hippocampus and neostriatum, suggesting that, besides synaptic transmission, diffuse transmission by ACh prevails in many regions of the CNS. Cytological analysis of the immediate micro-environment of these ACh terminals, as well as currently available immunocytochemical data on the cellular and subcellular distribution of ACh receptors, is congruent with this view. At least in brain regions densely innervated by ACh neurons, a further aspect of the diffuse transmission paradigm is envisaged: the existence of an ambient level of ACh in the extracellular space, to which all tissue elements would be permanently exposed. Recent experimental data on the various molecular forms of AChE and their presumptive role at the neuromuscular junction support this hypothesis. As in the peripheral nervous system, degradation of ACh by the prevalent G4 form of AChE in the CNS would primarily serve to keep the extrasynaptic, ambient level of ACh within physiological limits, rather than totally eliminate ACh from synaptic clefts. Long-lasting and widespread electrophysiological effects imputable to ACh in the CNS might be explained in this manner. The notions of diffuse transmission and of an ambient level of ACh in the CNS could also be of clinical relevance, in accounting for the production and nature of certain cholinergic deficits and the efficacy of substitution therapies.

Cholinergic synapses in human cerebral cortex: an ultrastructural study in serial sections

Cholinergic axons in the human cerebral cortex were analyzed by electron microscopy. Choline acetyltransferase (ChAT) immunoreactivity was used to identify cholinergic axons in samples of anterior temporal lobe removed at surgery. A systematic survey of labeled axon varicosities, visualized in complete serial sections, showed that 67% of all varicosities formed identifiable synaptic specializations. These synapses were usually symmetric and quite small, often present in only one to two serial sections. However, an occasional synapse was asymmetric and larger, seen in five to seven serial sections. The postsynaptic processes at cholinergic synapses were often identified as spiny dendrites or spines. The existence of cholinergic axons in the human cerebral cortex has been demonstrated in numerous studies. Our findings provide the first ultrastructural evidence that these axons make synaptic contact with cortical neurons in the human brain.

Neuropeptide Y and somatostatin-like immunoreactivity in neurons of the monkey amygdala

Neurons in the monkey amygdala exhibiting neuropeptide Y-like immunoreactivity and somatostatin-like immunoreactivity were identified using an avidin-biotin immunohistochemical technique. Differential co-existence of the two peptides was demonstrated using two-color immunoperoxidase and adjacent section methods. Numerous neuropeptide Y-positive neurons were observed in the basolateral and superficial amygdaloid nuclei. A moderate number of neuropeptide Y-positive neurons was seen in the medial subdivision of the central nucleus, but only a few neurons were observed in the lateral subdivision. Numerous somatostatin-positive neurons were stained in all major amygdaloid nuclei and always outnumbered neuropeptide Y-positive cells. All amygdaloid nuclei contained numerous peptide-positive fibers whose density varied depending on the nucleus. Approximately 90% of neuropeptide Y-positive neurons also exhibited somatostatin-like immunoreactivity. The percentage of somatostatin-positive neurons that exhibited neuropeptide-Y immunoreactivity varied in different nuclei. In the superficial amygdaloid nuclei, medial subdivision of the central nucleus and most portions of the basolateral nuclei the predominant cell type stained with both the neuropeptide Y and somatostatin antibodies was a spine-sparse non-pyramidal neuron. In the dorsal portion of the lateral nucleus, however, most peptide-positive neurons had spiny dendrites. Only the cell bodies and proximal dendrites of somatostatin-positive neurons in the lateral subdivision of the central nucleus were immunostained. This study demonstrates that specific cell populations in the primate amygdala contain neuropeptide Y, somatostatin or both peptides. Most peptide-positive neurons in the basolateral and superficial amygdaloid nuclei appear to be local circuit neurons that contribute to the dense plexus of peptide-positive axons in these regions. The finding of neurons with spiny dendrites in the dorsal part of the lateral nucleus suggests that these cells may be functionally different from peptide-positive neurons in other portions of the basolateral amygdala. The lateral subdivision of the central nucleus is distinguished from other amygdaloid nuclei by containing a large population of somatostatin-positive neurons that do not exhibit neuropeptide Y immunoreactivity.