Cholecystokinin immunoreactive neurons in the basolateral amygdala of the rhesus monkey (Macaca mulatta)

Morphological heterogeneity of neurons in the human central amygdaloid nucleus

The central amygdaloid nucleus (CeA) has an ancient phylogenetic development and functions relevant for animal survival. Local cells receive intrinsic amygdaloidal information that codes emotional stimuli of fear, integrate them, and send cortical and subcortical output projections that prompt rapid visceral and social behavior responses. We aimed to describe the morphology of the neurons that compose the human CeA (N = 8 adult men). Cells within CeA coronal borders were identified using the thionine staining and were further analyzed using the "single-section" Golgi method followed by open-source software procedures for two-dimensional and three-dimensional image reconstructions. Our results evidenced varied neuronal cell body features, number and thickness of primary shafts, dendritic branching patterns, and density and shape of dendritic spines. Based on these criteria, we propose the existence of 12 morphologically different spiny neurons in the human CeA and discuss the variability in the dendritic architecture within cellular types, including likely interneurons. Some dendritic shafts were long and straight, displayed few collaterals, and had planar radiation within the coronal neuropil volume. Most of the sampled neurons showed a few to moderate density of small stubby/wide spines. Long spines (thin and mushroom) were observed occasionally. These novel data address the synaptic processing and plasticity in the human CeA. Our morphological description can be combined with further transcriptomic, immunohistochemical, and electrophysiological/connectional approaches. It serves also to investigate how neurons are altered in neurological and psychiatric disorders with hindered emotional perception, in anxiety, following atrophy in schizophrenia, and along different stages of Alzheimer's disease.

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.

Interneuron Types and Their Circuits in the Basolateral Amygdala

The basolateral amygdala (BLA) is a cortical structure based on its cell types, connectivity features, and developmental characteristics. This part of the amygdala is considered to be the main entry site of processed and multisensory information delivered via cortical and thalamic afferents. Although GABAergic inhibitory cells in the BLA comprise only 20% of the entire neuronal population, they provide essential control over proper network operation. Previous studies have uncovered that GABAergic cells in the basolateral amygdala are as diverse as those present in other cortical regions, including the hippocampus and neocortex. To understand the role of inhibitory cells in various amygdala functions, we need to reveal the connectivity and input-output features of the different types of GABAergic cells. Here, I review the recent achievements in uncovering the diversity of GABAergic cells in the basolateral amygdala with a specific focus on the microcircuit organization of these inhibitory cells.

Immunohistochemical Identification of Interneuronal Subpopulations in the Basolateral Amygdala of the Rhesus Monkey (Macaca mulatta)

Inhibitory circuits in the basolateral nuclear complex of the amygdala (BNC) critical for controlling the acquisition, expression, and extinction of emotional responses are mediated by GABAergic interneurons (INs). Studies in rodents have demonstrated that separate IN subpopulations, identified by their expression of calcium-binding proteins and neuropeptides, play discrete roles in the intrinsic circuitry of the BNC. Far less is known about IN subpopulations in primates. In order to fill in this gap in our understanding of primate INs, the present investigation used dual-labeling immunohistochemistry for IN markers to identify subpopulations expressing cholecystokinin (CCK), calbindin (CB), calretinin (CR), and somatostatin (SOM) in somata and axon terminals in the monkey BNC. In general, colocalization patterns seen in somata and axon terminals were similar. It was found that there was virtually no colocalization of CB and CR, the two calcium-binding proteins investigated. Three subtypes of CCK-immunoreactive (CCK+) INs were identified on the basis of their expression of CR or CB: (1) CCK+/CR+; (2) CCK+/CB+); and (3) CCK+/CR-/CB-. Almost no colocalization of CCK with SOM was observed, but there was extensive colocalization of SOM and CB. CCK+, CR+, and CCK+/CR+ double-labeled axon terminals were seen surrounding pyramidal cell somata in basket-like plexuses, as well as in the neuropil. CB+, SOM+, and CB+/SOM+ terminals did not form baskets, suggesting that these IN subpopulations are mainly dendrite-targeting neurons. In general, the IN subpopulations in the monkey are not dissimilar to those seen in rodents but, unlike rodents, CB+ INs in the monkey are not basket cells.

Expression of the type 1 cannabinoid receptor (CB1R) in CCK-immunoreactive axon terminals in the basolateral amygdala of the rhesus monkey (Macaca mulatta)

Studies in rodents have shown that interactions between cholecystokinin (CCK) and the endogenous cannabinoid system in the basolateral nuclear complex of the amygdala (BNC) modulate anxiety-like behavior and fear learning/expression. One of the main cell types implicated is a CCK-immunoreactive (CCK+) basket cell that innervates the somata of pyramidal projection neurons (PNs) and expresses the type 1 cannabinoid receptor (CB1R) in its axon terminals. Although numerous studies have elucidated the anatomy and physiology of these CCK+/CB1R + interneurons in rodents, it has not been determined if they exist in primates. The present investigation used immunohistochemical techniques in the monkey to answer this question. It was found that the monkey BNC, as in rodents, has a very high density of CB1R + axons, including CB1R + axon terminals that form basket-like plexuses contacting somata of PNs. These axons, as well as axons in the neuropil, exhibit extensive colocalization of CCK and CB1R. These findings suggest that the same synaptic mechanisms involved in CCK-CB1R interactions in rodents may also apply to primates, and that therapies that target the cannabinoid system in the BNC may be useful for treating fear and anxiety in human patients.

Nonpyramidal neurons in the primate basolateral amygdala: A Golgi study in the baboon (Papio cynocephalus) and long-tailed macaque (Macaca fascicularis)

Nonpyramidal GABAergic interneurons in the basolateral nuclear complex (BNC) of the amygdala are critical for the regulation of emotion. Remarkably, there have been no Golgi studies of these neurons in nonhuman primates. Therefore, in the present study we investigated the morphology of nonpyramidal neurons (NPNs) in the BNC of the baboon and monkey using the Golgi technique. NPNs were scattered throughout all nuclei of the BNC and had aspiny or spine-sparse dendrites. NPNs were morphologically heterogeneous and could be divided into small, medium, large, and giant types based on the size of their somata. NPNs could be further divided on the basis of their somatodendritic morphology into four types: multipolar, bitufted, bipolar, and irregular. NPN axons, when stained, formed dense local arborizations that overlapped their dendritic fields to varying extents. These axons always exhibited varying numbers of varicosities representing axon terminals. Three specialized NPN subtypes were recognized because of their unique anatomical features: axo-axonic cells, neurogliaform cells, and giant cells. The axons of axo-axonic cells formed "axonal cartridges," with clustered varicosities that contacted the axon initial segments of pyramidal neurons (PNs). Neurogliaform cells had small somata and numerous short dendrites that formed a dense dendritic arborization; they also exhibited a very dense axonal arborization that overlapped the dendritic field. Giant cells had very large irregular somata and long, thick dendrites; their distal dendrites often branched extensively and had long appendages. In general, the NPNs of the baboon and monkey BNC, including the specialized subtypes, were similar to those of rodents.