Calretinin-immunoreactive cells and fibers in the human amygdaloid complex

Quantitative cellular pathology of the amygdala in temporal lobe epilepsy and correlation with magnetic resonance imaging volumetry, tissue microstructure, and sudden unexpected death in epilepsy risk factors

OBJECTIVE

Amygdala enlargement can occur in temporal lobe epilepsy, and increased amygdala volume is also reported in sudden unexpected death in epilepsy (SUDEP). Apnea can be induced by amygdala stimulation, and postconvulsive central apnea (PCCA) and generalized seizures are both known SUDEP risk factors. Neurite orientation dispersion and density imaging (NODDI) has recently provided additional information on altered amygdala microstructure in SUDEP. In a series of 24 surgical temporal lobe epilepsy cases, our aim was to quantify amygdala cellular pathology parameters that could predict enlargement, NODDI changes, and ictal respiratory dysfunction.

METHODS

Using whole slide scanning automated quantitative image analysis methods, parallel evaluation of myelin, axons, dendrites, oligodendroglia, microglia, astroglia, neurons, serotonergic networks, mTOR-pathway activation (pS6) and phosphorylated tau (pTau; AT8, AT100, PHF) in amygdala, periamygdala cortex, and white matter regions of interest were compared with preoperative magnetic resonance imaging data on amygdala size, and in 13 cases with NODDI and evidence of ictal-associated apnea.

RESULTS

We observed significantly higher glial labeling (Iba1, glial fibrillary acidic protein, Olig2) in amygdala regions compared to cortex and a strong positive correlation between Olig2 and Iba1 in the amygdala. Larger amygdala volumes correlated with lower microtubule-associated protein (MAP2), whereas higher NODDI orientation dispersion index correlated with lower Olig2 cell densities. In the three cases with recorded PCCA, higher MAP2 and pS6-235 expression was noted than in those without. pTau did not correlate with SUDEP risk factors, including seizure frequency.

SIGNIFICANCE

Histological quantitation of amygdala microstructure can shed light on enlargement and diffusion imaging alterations in epilepsy to explore possible mechanisms of amygdala dysfunction, including mTOR pathway activation, that in turn may increase the risk for SUDEP.

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.

The Co-Expression Pattern of Calcium-Binding Proteins with γ-Aminobutyric Acid and Glutamate Transporters in the Amygdala of the Guinea Pig: Evidence for Glutamatergic Subpopulations

The amygdala has large populations of neurons utilizing specific calcium-binding proteins such as parvalbumin (PV), calbindin (CB), or calretinin (CR). They are considered specialized subsets of γ-aminobutyric acid (GABA) interneurons; however, many of these cells are devoid of GABA or glutamate decarboxylase. The neurotransmitters used by GABA-immunonegative cells are still unknown, but it is suggested that a part may use glutamate. Thus, this study investigates in the amygdala of the guinea pig relationships between PV, CB, or CR-containing cells and GABA transporter (VGAT) or glutamate transporter type 2 (VGLUT2), markers of GABAergic and glutamatergic neurons, respectively. The results show that although most neurons using PV, CB, and CR co-expressed VGAT, each of these populations also had a fraction of VGLUT2 co-expressing cells. For almost all neurons using PV (~90%) co-expressed VGAT, while ~1.5% of them had VGLUT2. The proportion of neurons using CB and VGAT was smaller than that for PV (~80%), while the percentage of cells with VGLUT2 was larger (~4.5%). Finally, only half of the neurons using CR (~53%) co-expressed VGAT, while ~3.5% of them had VGLUT2. In conclusion, the populations of neurons co-expressing PV, CB, and CR are in the amygdala, primarily GABAergic. However, at least a fraction of neurons in each of them co-express VGLUT2, suggesting that these cells may use glutamate. Moreover, the number of PV-, CB-, and CR-containing neurons that may use glutamate is probably larger as they can utilize VGLUT1 or VGLUT3, which are also present in the amygdala.

Subgenual and Hippocampal Pathways in Amygdala Are Set to Balance Affect and Context Processing

The amygdala, hippocampus, and subgenual cortex area 25 (A25) are engaged in complex cognitive-emotional processes. Yet pathway interactions from hippocampus and A25 with postsynaptic sites in amygdala remain largely unknown. In rhesus monkeys of both sexes, we studied with neural tracers how pathways from A25 and hippocampus interface with excitatory and inhibitory microcircuits in amygdala at multiple scales. We found that both hippocampus and A25 innervate distinct as well as overlapping sites of the basolateral (BL) amygdalar nucleus. Unique hippocampal pathways heavily innervated the intrinsic paralaminar basolateral nucleus, which is associated with plasticity. In contrast, orbital A25 preferentially innervated another intrinsic network, the intercalated masses, an inhibitory reticulum that gates amygdalar autonomic output and inhibits fear-related behaviors. Finally, using high-resolution confocal and electron microscopy (EM), we found that among inhibitory postsynaptic targets in BL, both hippocampal and A25 pathways preferentially formed synapses with calretinin (CR) neurons, which are known for disinhibition and may enhance excitatory drive in the amygdala. Among other inhibitory postsynaptic sites, A25 pathways innervated the powerful parvalbumin (PV) neurons which may flexibly regulate the gain of neuronal assemblies in the BL that affect the internal state. In contrast, hippocampal pathways innervated calbindin (CB) inhibitory neurons, which modulate specific excitatory inputs for processing context and learning correct associations. Common and unique patterns of innervation in amygdala by hippocampus and A25 have implications for how complex cognitive and emotional processes may be selectively disrupted in psychiatric disorders.SIGNIFICANCE STATEMENT The hippocampus, subgenual A25, and amygdala are associated with learning, memory, and emotions. We found that A25 is poised to affect diverse amygdalar processes, from emotional expression to fear learning by innervating the basal complex and the intrinsic intercalated masses. Hippocampal pathways uniquely interacted with another intrinsic amygdalar nucleus which is associated with plasticity, suggesting flexible processing of signals in context for learning. In the basolateral (BL) amygdala, which has a role in fear learning, both hippocampal and A25 interacted preferentially with disinhibitory neurons, suggesting a boost in excitation. The two pathways diverged in innervating other classes of inhibitory neurons, suggesting circuit specificities that could become perturbed in psychiatric diseases.

Central amygdala corticotropin-releasing factor neurons promote hyponeophagia but do not control alcohol drinking in mice

Corticotropin-releasing factor (CRF) signaling in the central nucleus of the amygdala (CeA) plays a critical role in rodent models of excessive alcohol drinking. However, the source of CRF acting in the CeA during alcohol withdrawal remains to be identified. In the present study, we hypothesized that CeA CRF interneurons may represent a behaviorally relevant source of CRF to the CeA increasing motivation for alcohol via negative reinforcement. We first observed that Crh mRNA expression in the anterior part of the mouse CeA correlates positively with alcohol intake in C57BL/6J males with a history of chronic binge drinking followed by abstinence and increases upon exposure to chronic intermittent ethanol (CIE) vapor inhalation. We then found that chemogenetic activation of CeA CRF neurons in Crh-IRES-Cre mouse brain slices increases gamma-aminobutyric acid (GABA) release in the medial CeA, in part via CRF1 receptor activation. While chemogenetic stimulation exacerbated novelty-induced feeding suppression (NSF) in alcohol-naïve mice, thereby mimicking the effect of withdrawal from CIE, it had no effect on voluntary alcohol consumption, following either acute or chronic manipulation. Furthermore, chemogenetic inhibition of CeA CRF neurons did not affect alcohol consumption or NSF in chronic alcohol drinkers exposed to air or CIE. Altogether, these findings indicate that CeA CRF neurons produce local release of GABA and CRF and promote hyponeophagia in naïve mice, but do not drive alcohol intake escalation or negative affect in CIE-withdrawn mice. The latter result contrasts with previous findings in rats and demonstrates species specificity of CRF circuit engagement in alcohol dependence.

Revealing the Precise Role of Calretinin Neurons in Epilepsy: We Are on the Way

Epilepsy is a common neurological disorder characterized by hyperexcitability in the brain. Its pathogenesis is classically associated with an imbalance of excitatory and inhibitory neurons. Calretinin (CR) is one of the three major types of calcium-binding proteins present in inhibitory GABAergic neurons. The functions of CR and its role in neural excitability are still unknown. Recent data suggest that CR neurons have diverse neurotransmitters, morphologies, distributions, and functions in different brain regions across various species. Notably, CR neurons in the hippocampus, amygdala, neocortex, and thalamus are extremely susceptible to excitotoxicity in the epileptic brain, but the causal relationship is unknown. In this review, we focus on the heterogeneous functions of CR neurons in different brain regions and their relationship with neural excitability and epilepsy. Importantly, we provide perspectives on future investigations of the role of CR neurons in epilepsy.

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.

Neurons Expressing Parvalbumin and Calretinin in the Human Amygdaloid Complex: A Quantitative and Qualitative Analysis in Every Nucleus and Nuclear Subdivision

The primate amygdaloid complex (AC) contains projection neurons as well as subsets of interneurons (IN), many of which express calcium-binding proteins, that through their local circuits control the activity of the projection neurons. The inhibitory parvalbumin (PV) and calretinin (CR)-positive (+) AC IN have a crucial role in the appearance of synchronized oscillations in local ensembles of projection neurons that mediate the consolidation and recall of fear memories. The GABAergic transmission of these subsets of IN is modulated by dopamine. To expand the knowledge regarding the cellular composition and distribution of IN in the human AC, we focused on two non-overlapping populations: the PV+ and CR+. We have analyzed the distribution of these IN throughout the AC from subjects without any neurological or psychiatric disorders and estimated their absolute number and density using stereological methods. We have also provided percentages of the IN with respect to the total AC neurons. The CR + IN were distributed throughout the AC, whereas the PV+ were only present in the basolateral nuclear group. The quantity of CR + IN was four times higher than that of PV+ and the percentages varied from less than 1% for PV + IN to 6-20% for CR+. The differences in quantity and distribution of CR+ and PV + IN could be related to their differential inhibitory properties and to the intrinsic and extrinsic connections of every amygdaloid region.

Comparative neuroanatomical study of the amygdala and fear conditioning in Nigerian breeds of Artiodactyla: Sheep (Uda) and goats (Red Sokoto)

The aim of this study was to evaluate fear condition responses in sheep and goat and to relate this to the neuroarchitecture of their amygdala. Forty adult sheep (Uda breed) and 40 adult goats (Red Sokoto breed) were fear-conditioned by associating the sound of a car horn (neutral stimuli) with water spray (aversive stimuli) and the fear response was determined by direct observation of the behavior of the sheep and goats and measuring their flight distances and escape time. Eight groups were studied, each comprising of 10 animals (five sheep and five goats). Goats and sheep were tested alternately in the morning of every day of the week for three consecutive weeks, in which 4 days was used for habituation and 3 days for testing. Histologically, neurons in the central and basolateral complex of the amygdala were studied and analyzed using Nissl and golgi staines. Behaviorally, goats elicited an active avoidance response expressed as flight with concomitant intense flight distances (p < .001) compared to sheep. Although, sheep had larger brain parameters, it showed attenuated basolateral amygdala cytoarchitecture consistent with reduced fear perception and response. Goats had significantly more densely distributed pyramidal and spiny stellate neurons in the basolateral amygdala while sheep showed more non-pyramidal and aspiny neurons. These results provide interesting practical perspectives on how adaptions in the amygdala coincides with alterations in fear conditioning in domestic animals and may be the basis for the higher incidence of the sheep in automobile accidents than goats in developing countries especially Africa.

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.

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

Several distinct subpopulations of interneurons (INs) in the amygdalar basolateral nuclear complex (BNC) of the rat can be recognized on the basis of their expression of calcium-binding proteins and neuropeptides, including parvalbumin (PV), somatostatin (SOM), calretinin (CR), and cholecystokinin (CCK). In the rat BNC CCK is expressed in two separate IN subpopulations, termed large (CCKL ) and small (CCKS ). These subpopulations exhibit distinct connections indicative of discrete functional roles in the circuitry of the BNC. Although there have been several studies of PV+, SOM+, and CR+ INs in the primate BNC, there is almost no information regarding CCK+ INs in these species. Therefore, in the present study the distribution and morphology of CCK+ INs and their axon terminals in the BNC of the monkey was investigated. CCK immunoreactivity in the BNC was observed in somata and proximal dendrites of nonpyramidal neurons, as well as in axon terminals. A moderate density of CCK+ INs was found in all nuclei of the BNC. CCK+ INs in the BNC were morphologically heterogeneous, with both small and large varieties observed. All CCK+ somata gave rise to 2-4 dendrites that branched sparingly and were aspiny. CCK+ axon terminals in the BNC were found both in the neuropil and forming pericellular baskets contacting somata of pyramidal cells. In addition, many CCK+ neurons were contacted by multiple CCK+ terminals, indicative of the existence of a CCK interneuronal network. These data indicate that the morphology of CCK+ INs in the monkey is very similar to that of the rat.

Neuronal types of the human cortical amygdaloid nucleus

The human cortical amygdaloid nucleus (CoA) receives exteroceptive sensory stimuli, modulates the functions coded by the intrinsic amygdaloid circuit, and constitutes the beginning of the limbic lobe continuum with direct and indirect connections toward subcortical, allocortical, and higher order neocortical areas. To provide basic data on the human CoA, we characterized and classified the neurons using the thionin and the "single-section" Golgi method adapted for postmortem brain tissue and light microscopy. We found 10 different types of neurons named according to the morphological features of the cell body, dendritic branches, and spine distribution. Most cells are multipolar spiny neurons with two or more primary dendrites, including pyramidal-like ones. Three-dimensional reconstructions evidenced the types and diversity of the dendritic spines in each neuron. The unlike density of spines along dendritic branches, from proximal to distal ones, indicate that the synaptic processing and plasticity can be different in each CoA neuron. Our study provides novel data on the neuronal composition of the human CoA indicating that the variety of cells in this region can have phylogenetic, ontogenetic, morphological, and likely functional implications for the integrated human brain function. This can reflect both a more complex subcortical synaptic processing of sensory and emotional information and an adaptation for species-specific social behavior display.

Regional chemoarchitecture of the brain of lungfishes based on calbindin D-28K and calretinin immunohistochemistry

Lungfishes are the closest living relatives of land vertebrates, and their neuroanatomical organization is particularly relevant for deducing the neural traits that have been conserved, modified, or lost with the transition from fishes to land vertebrates. The immunohistochemical localization of calbindin (CB) and calretinin (CR) provides a powerful method for discerning segregated neuronal populations, fiber tracts, and neuropils and is here applied to the brains of Neoceratodus and Protopterus, representing the two extant orders of lungfishes. The results showed abundant cells containing these proteins in pallial and subpallial telencephalic regions, with particular distinct distribution in the basal ganglia, amygdaloid complex, and septum. Similarly, the distribution of CB and CR containing cells supports the division of the hypothalamus of lungfishes into neuromeric regions, as in tetrapods. The dense concentrations of CB and CR positive cells and fibers highlight the extent of the thalamus. As in other vertebrates, the optic tectum is characterized by numerous CB positive cells and fibers and smaller numbers of CR cells. The so-called cerebellar nucleus contains abundant CB and CR cells with long ascending axons, which raises the possibility that it could be homologized to the secondary gustatory nucleus of other vertebrates. The corpus of the cerebellum is devoid of CB and CR and cells positive for both proteins are found in the cerebellar auricles and the octavolateralis nuclei. Comparison with other vertebrates reveals that lungfishes share most of their features of calcium binding protein distribution with amphibians, particularly with salamanders.

Receptor-driven, multimodal mapping of the human amygdala

The human amygdala consists of subdivisions contributing to various functions. However, principles of structural organization at the cellular and molecular level are not well understood. Thus, we re-analyzed the cytoarchitecture of the amygdala and generated cytoarchitectonic probabilistic maps of ten subdivisions in stereotaxic space based on novel workflows and mapping tools. This parcellation was then used as a basis for analyzing the receptor expression for 15 receptor types. Receptor fingerprints, i.e., the characteristic balance between densities of all receptor types, were generated in each subdivision to comprehensively visualize differences and similarities in receptor architecture between the subdivisions. Fingerprints of the central and medial nuclei and the anterior amygdaloid area were highly similar. Fingerprints of the lateral, basolateral and basomedial nuclei were also similar to each other, while those of the remaining nuclei were distinct in shape. Similarities were further investigated by a hierarchical cluster analysis: a two-cluster solution subdivided the phylogenetically older part (central, medial nuclei, anterior amygdaloid area) from the remaining parts of the amygdala. A more fine-grained three-cluster solution replicated our previous parcellation including a laterobasal, superficial and centromedial group. Furthermore, it helped to better characterize the paralaminar nucleus with a molecular organization in-between the laterobasal and the superficial group. The multimodal cyto- and receptor-architectonic analysis of the human amygdala provides new insights into its microstructural organization, intersubject variability, localization in stereotaxic space and principles of receptor-based neurochemical differences.

Posterior Orbitofrontal and Anterior Cingulate Pathways to the Amygdala Target Inhibitory and Excitatory Systems with Opposite Functions

The bidirectional dialogue of the primate posterior orbitofrontal cortex (pOFC) with the amygdala is essential in cognitive-emotional functions. The pOFC also sends a uniquely one-way excitatory pathway to the amygdalar inhibitory intercalated masses (IM), which inhibit the medial part of the central amygdalar nucleus (CeM). Inhibition of IM has the opposite effect, allowing amygdalar activation of autonomic structures and emotional arousal. Using multiple labeling approaches to identify pathways and their postsynaptic sites in the amygdala in rhesus monkeys, we found that the anterior cingulate cortex innervated mostly the basolateral and CeM amygdalar nuclei, poised to activate CeM for autonomic arousal. By contrast, a pathway from pOFC to IM exceeded all other pathways to the amygdala by density and size and proportion of large and efficient terminals. Moreover, whereas pOFC terminals in IM innervated each of the three distinct classes of inhibitory neurons, most targeted neurons expressing dopamine- and cAMP-regulated phosphoprotein (DARPP-32+), known to be modulated by dopamine. The predominant pOFC innervation of DARPP-32+ neurons suggests activation of IM and inhibition of CeM, resulting in modulated autonomic function. By contrast, inhibition of DARPP-32 neurons in IM by high dopamine levels disinhibits CeM and triggers autonomic arousal. The findings provide a mechanism to help explain how a strong pOFC pathway, which is poised to moderate activity of CeM, through IM, can be undermined by the high level of dopamine during stress, resulting in collapse of potent inhibitory mechanisms in the amygdala and heightened autonomic drive, as seen in chronic anxiety disorders.SIGNIFICANCE STATEMENT The dialogue between prefrontal cortex and amygdala allows thoughts and emotions to influence actions. The posterior orbitofrontal cortex sends a powerful pathway that targets a special class of amygdalar intercalated mass (IM) inhibitory neurons, whose wiring may help modulate autonomic function. By contrast, the anterior cingulate cortex innervates other amygdalar parts, activating circuits to help avoid danger. Most IM neurons in primates label for the protein DARPP-32, known to be activated or inhibited based on the level of dopamine. Stress markedly increases dopamine release and inhibits IM neurons, compromises prefrontal control of the amygdala, and sets off a general alarm system as seen in affective disorders, such as chronic anxiety and post-traumatic stress disorder.

The human medial amygdala: structure, diversity, and complexity of dendritic spines

The medial nucleus of the amygdala (Me) is a component of the neural circuit for the interpretation of multimodal sensory stimuli and the elaboration of emotions and social behaviors in primates. We studied the presence, distribution, diverse shape, and connectivity of dendritic spines in the human Me of adult postmortem men. Data were obtained from the five types of multipolar neurons found in the Me using an adapted Golgi method and light microscopy, the carbocyanine DiI fluorescent dye and confocal microscopy, and transmission electron microscopy. Three-dimensional reconstruction of spines showed a continuum of shapes and sizes, with the spines either lying isolated or forming clusters. These dendritic spines were classified as stubby/wide, thin, mushroom-like, ramified or with an atypical morphology including intermediate shapes, double spines, and thorny excrescences. Pleomorphic spines were found from proximal to distal dendritic branches suggesting potential differences for synaptic processing, strength, and plasticity in the Me neurons. Furthermore, the human Me has large and thin spines with a gemmule appearance, spinules, and filopodium. The ultrastructural data showed dendritic spines forming monosynaptic or multisynaptic contacts at the spine head and neck, and with asymmetric or symmetric characteristics. Additional findings included en passant, reciprocal, and serial synapses in the Me. Complex long-necked thin spines were observed in this subcortical area. These new data reveal the diversity of the dendritic spines in the human Me likely involved with the integration and processing of local synaptic inputs and with functional implications in physiological and various neuropathological conditions.

Cellular components of the human medial amygdaloid nucleus

The medial nucleus (Me) is a superficial component of the amygdaloid complex. Here we assessed the density and morphology of the neurons and glial cells, the glial fibrillary acidic protein (GFAP) immunoreactivity, and the ultrastructure of the synaptic sites in the human Me. The optical fractionator method was applied. The Me presented an estimated mean neuronal density of 1.53 × 10⁵ neurons/mm³ (greater in the left hemisphere), more glia (72% of all cells) than neurons, and a nonneuronal:neuronal ratio of 2.7. Golgi-impregnated neurons had round or ovoid, fusiform, angular, and polygonal cell bodies (10-30 μm in diameter). The length of the dendrites varied, and pleomorphic spines were found in sparsely spiny or densely spiny cells (1.5-5.2 spines/dendritic μm). The axons in the Me neuropil were fine or coarsely beaded, and fibers showed simple or notably complex collateral terminations. The protoplasmic astrocytes were either isolated or formed small clusters and showed GFAP-immunoreactive cell bodies and multiple branches. Furthermore, we identified both asymmetrical (with various small, clear, round, electron-lucent vesicles and, occasionally, large, dense-core vesicles) and symmetrical (with small, flattened vesicles) axodendritic contacts, also including multisynaptic spines. The astrocytes surround and may compose tripartite or tetrapartite synapses, the latter including the extracellular matrix between the pre- and the postsynaptic elements. Interestingly, the terminal axons exhibited a glomerular-like structure with various asymmetrical contacts. These new morphological data on the cellular population and synaptic complexity of the human Me can contribute to our knowledge of its role in health and pathological conditions.

Regional distribution of calretinin and calbindin-D28k expression in the brain of the urodele amphibian Pleurodeles waltl during embryonic and larval development

The sequence of appearance of calretinin and calbindin-D28k immunoreactive (CRir and CBir, respectively) cells and fibers has been studied in the brain of the urodele amphibian Pleurodeles waltl. Embryonic, larval and juvenile stages were studied. The early expression and the dynamics of the distribution of CBir and CRir structures have been used as markers for developmental aspects of distinct neuronal populations, highlighting the accurate extent of many regions in the developing brain, not observed on the basis of cytoarchitecture alone. CR and, to a lesser extent, CB are expressed early in the central nervous system and show a progressively increasing expression from the embryonic stages throughout the larval life and, in general, the labeled structures in the developing brain retain their ability to express these proteins in the adult brain. The onset of CRir cells primarily served to follow the development of the olfactory bulbs, subpallium, thalamus, alar hypothalamus, mesencephalic tegmentum, and distinct cell populations in the rhombencephalic reticular formation. CBir cells highlighted the development of, among others, the pallidum, hypothalamus, dorsal habenula, midbrain tegmentum, cerebellum, and central gray of the rostral rhombencephalon. However, it was the relative and mostly segregated distribution of both proteins in distinct cell populations which evidenced the developing regionalization of the brain. The results have shown the usefulness in neuroanatomy of the analysis during development of the onset of CBir and CRir structures, but the comparison with previous data has shown extensive variability across vertebrate classes. Therefore, one should be cautious when comparing possible homologue structures across species only on the basis of the expression of these proteins, due to the variation of the content of calcium-binding proteins observed in well-established homologous regions in the brain of different vertebrates.

Where and what is the paralaminar nucleus? A review on a unique and frequently overlooked area of the primate amygdala

The primate amygdala is composed of multiple subnuclei that play distinct roles in amygdala function. While some nuclei have been areas of focused investigation, others remain virtually unknown. One of the more obscure regions of the amygdala is the paralaminar nucleus (PL). The PL in humans and non-human primates is relatively expanded compared to lower species. Long considered to be part of the basal nucleus, the PL has several interesting features that make it unique. These features include a dense concentration of small cells, high concentrations of receptors for corticotropin releasing hormone and benzodiazepines, and dense innervation of serotonergic fibers. More recently, high concentrations of immature-appearing cells have been noted in the primate PL, suggesting special mechanisms of neural plasticity. Following a brief overview of amygdala structure and function, this review will provide an introduction to the history, embryology, anatomical connectivity, immunohistochemical and cytoarchitectural properties of the PL. Our conclusion is that the PL is a unique subregion of the amygdala that may yield important clues about the normal growth and function of the amygdala, particularly in higher species.

Stereological analysis of the rat and monkey amygdala

The amygdala is part of a neural network that contributes to the regulation of emotional behaviors. Rodents, especially rats, are used extensively as model organisms to decipher the functions of specific amygdala nuclei, in particular in relation to fear and emotional learning. Analysis of the role of the nonhuman primate amygdala in these functions has lagged work in the rodent but provides evidence for conservation of basic functions across species. Here we provide quantitative information regarding the morphological characteristics of the main amygdala nuclei in rats and monkeys, including neuron and glial cell numbers, neuronal soma size, and individual nuclei volumes. The volumes of the lateral, basal, and accessory basal nuclei were, respectively, 32, 39, and 39 times larger in monkeys than in rats. In contrast, the central and medial nuclei were only 8 and 4 times larger in monkeys than in rats. The numbers of neurons in the lateral, basal, and accessory basal nuclei were 14, 11, and 16 times greater in monkeys than in rats, whereas the numbers of neurons in the central and medial nuclei were only 2.3 and 1.5 times greater in monkeys than in rats. Neuron density was between 2.4 and 3.7 times lower in monkeys than in rats, whereas glial density was only between 1.1 and 1.7 times lower in monkeys than in rats. We compare our data in rats and monkeys with those previously published in humans and discuss the theoretical and functional implications that derive from our quantitative structural findings.

Developmental changes in the distribution of calretinin-immunoreactive cells in human fetal nasal epithelium

Immunoreactivity of the calcium binging protein calretinin is often used as a marker of olfactory neurons. Although the immunoreactivity and density of olfactory neurons are known to change between developmental stages in the human fetus, previous descriptions have been limited to the olfactory epithelium and/or the nasal septum and have not included the entire nasal cavity. Using horizontal semi serial sections of heads of six mid-term fetuses (9-15 weeks of gestation), we examined the topographical anatomy of calretinin-positive olfactory neurons. By 9 weeks of gestation, the distribution of calretinin-positive cells reached levels inferior to the developing inferior meatus. By 12 weeks, concentrations in the inferior end had reached the level of the inferior meatus and the middle meatus carried abundant positive cells. However, by 15 weeks, calretinin positive cells were restricted to levels superior to the middle meatus and in the vomeronasal organ. Placode-derived cells are initially distributed antero-inferiorly along the nasal epithelium, but most lose their calretinin immunoreactivity. They might differentiate into the neuroendocrine cells embedded between nasal respiratory epithelial cells. The final differentiation of calretinin-positive cells was likely to require connection to the olfactory bulb and accessory bulb.

Dopaminergic innervation of interneurons in the rat basolateral amygdala

The basolateral nuclear complex of the amygdala (BLC) receives a dense dopaminergic innervation that plays a critical role in the formation of emotional memory. Dopamine has been shown to influence the activity of BLC GABAergic interneurons, which differentially control the activity of pyramidal cells. However, little is known about how dopaminergic inputs interface with different interneuronal subpopulations in this region. To address this question, dual-labeling immunohistochemical techniques were used at the light and electron microscopic levels to examine inputs from tyrosine hydroxylase-immunoreactive (TH+) dopaminergic terminals to two different interneuronal populations in the rat basolateral nucleus labeled using antibodies to parvalbumin (PV) or calretinin (CR). The basolateral nucleus exhibited a dense innervation by TH+ axons. Partial serial section reconstruction of TH+ terminals found that at least 43-50% of these terminals formed synaptic junctions in the basolateral nucleus. All of the synapses examined were symmetrical. In both TH/PV and TH/CR preparations the main targets of TH+ terminals were spines and distal dendrites of unlabeled cells. In sections dual-labeled for TH/PV 59% of the contacts of TH+ terminals with PV+ neurons were synapses, whereas in sections dual-labeled for TH/CR only 13% of the contacts of TH+ terminals with CR+ cells were synapses. In separate preparations examined in complete serial sections for TH+ basket-like innervation of PV+ perikarya, most (76.2%) of TH+ terminal contacts with PV+ perikarya were synapses. These findings suggest that PV+ interneurons, but not CR+ interneurons, are prominent synaptic targets of dopaminergic terminals in the BLC.

Molecular and functional properties of neurons in the human lateral amygdala

Neuronal properties were investigated through patch-clamp recording in situ in surgical specimens of the human lateral amygdala (LA) obtained from patients with intractable temporal lobe epilepsy. Projection neurons displayed spiny dendrites, action potentials with varying degree of frequency adaptation, and an inwardly rectifying K+ (Kir) conductance coupled to GABA(B) receptors. In interneurons, dendrites were spineless or sparsely spiny, action potentials were shorter than those in projection neurons and often occurred spontaneously, and GABA(B) receptor-mediated responses were lacking. Single-cell RT-PCR demonstrated expression of Kir channel subunits Kir3.1 and Kir3.2 and of vesicular glutamate transporters VGLUT1 and VGLUT2 in projection neurons. It is concluded that projection neurons and interneurons of the human LA can be distinguished based upon morphological, electrophysiological, and molecular biological criteria. The most striking difference relates to the expression of postsynaptic GABA(B) receptors coupled to Kir3 channels in projection neurons and the lack of functional GABA(B) receptors in interneurons.

Physiological and morphological characterization of parvalbumin-containing interneurons of the rat basolateral amygdala

The basolateral amygdala (BLA) is critical for the generation of emotional behavior and the formation of emotional memory. Understanding the neuronal mechanisms that contribute to emotional information processing in the BLA will ultimately require knowledge of the anatomy and physiology of its constituent neurons. Two major cell classes exist in the BLA, pyramidal projection neurons and nonpyramidal interneurons. Although the properties of projection neurons have been studied in detail, little is known about the properties of BLA interneurons. We have used whole-cell patch clamp recording techniques to examine the physiological properties of 48 visually identified putative interneurons from the rat anterior basolateral amygdalar nucleus. Here, we report that BLA interneurons can be differentiated into four electrophysiologically distinct subtypes based on their intrinsic membrane properties and their response to afferent synaptic input. Interneuron subtypes were named according to their characteristic firing pattern generated in response to transient depolarizing current injection and were grouped as follows: 1) burst-firing interneurons (n = 13), 2) regular-firing interneurons (n = 11), 3) fast-firing interneurons (n = 10), and 4) stutter-firing interneurons (n = 14). Post hoc histochemical visualization confirmed that all 48 recorded neurons had morphological properties consistent with their being local circuit interneurons. Moreover, by using triple immunofluorescence (for biocytin, calcium-binding proteins, and neuropeptides) in conjunction with patch clamp recording, we further demonstrated that over 60% of burst-firing and stutter-firing interneurons also expressed the calcium-binding protein parvalbumin (PV(+)). These data demonstrate that interneurons of the BLA show both physiological and neurochemical diversity. Moreover, we demonstrate that the burst- and stutter-firing patterns positively correlate with PV(+) immunoreactivity, suggesting that these neurons may represent functionally distinct subpopulations.

Cyto- and chemoarchitecture of the amygdala of a monotreme, Tachyglossus aculeatus (the short-beaked echidna)

We have examined the cyto- and chemoarchitecture of the temporal and extended amygdala in the brain of a monotreme (the short-beaked echidna Tachyglossus aculeatus) using Nissl and myelin staining, enzyme histochemistry for acetylcholine esterase and NADPH diaphorase, immunohistochemistry for calcium binding proteins (parvalbumin, calbindin and calretinin) and tyrosine hydroxylase. While the broad subdivisions of the eutherian temporal amygdala were present in the echidna brain, there were some noticeable differences. No immunoreactivity for parvalbumin or calretinin for somata was found in the temporal amygdala of the echidna. The nucleus of the lateral olfactory tract could not be definitively identified and the medial nucleus of amygdala appeared to be very small in the echidna. Calbindin immunoreactive neurons were most frequently found in the ventrolateral part of the lateral nucleus, intraamygdaloid parts of the bed nucleus of the stria terminalis and the lateral part of the central nucleus. Neurons strongly reactive for NADPH diaphorase with filling of the dendritic tree were found mainly scattered through the cortical, central and lateral subnuclei, while neurons showing only somata reactivity for NADPH diaphorase were concentrated in the basomedial and basolateral subnuclei. Most of the components of the extended amygdala of eutherians could also be identified in the echidna. Volumetric analysis indicated that the temporal amygdala in both the platypus and echidna is small compared to the same structure in both insectivores and primates, with the central and medial components of the temporal amygdala being particularly small.

Development of neurons and fibers containing calcium binding proteins in the pallial amygdala of mouse, with special emphasis on those of the basolateral amygdalar complex

We studied the development of neurons and fibers containing calbindin, calretinin, and parvalbumin in the mouse pallial amygdala, with special emphasis on those of the basolateral amygdalar complex. Numerous calbindin-immunoreactive (CB+) cells were observed in the incipient basolateral amygdalar complex and cortical amygdalar area from E13.5. At E16.5, CB+ cells became more abundant in the lateral and basolateral nuclei than in the basomedial nucleus, showing a pattern very similar to that of gamma-aminobutyric acid (GABA)ergic neurons. Many CB+ cells observed in the pallial amygdala appeared to originate in the anterior entopeduncular area/ganglionic eminences of the subpallium. The density of CB+ cells gradually increased in the pallial amygdala until the first postnatal week and appeared to decrease later, coinciding with the postnatal appearance of parvalbumin cells and raising the possibility of a partial phenotypic shift. Calretinin (CR) immunoreactivity could be observed in a few cells and fibers in the pallial amygdala at E14.5, and by E16.5 it became a good marker of the different nuclei of the basolateral amygdalar complex. Numerous CB+ and CR+ varicosities, part of which have an intrinsic origin, were observed in the basolateral amygdalar complex from E16.5, and some surrounded unstained perikarya and/or processes before birth, indicating an early formation of inhibitory networks. Each calcium binding protein showed a distinct spatiotemporal expression pattern of development in the mouse pallial amygdala. Any alteration in the development of neurons and fibers containing calcium binding proteins of the pallial amygdala may result in important disorders of emotional and social behavior.

Subtypes of substance P receptor immunoreactive interneurons in the rat basolateral amygdala

Local injections of the neurotoxin SP-saporin into the basolateral amygdala (BLA) are reported to specifically lesion substance P receptor immunoreactive (SPR-IR) interneurons, and to reduce anxiety related behavior. Hence, this technique might provide a means to study how defined interneuron populations regulate neuronal activity in the BLA. However, what interneuron subgroups in the BLA might be targeted by SP-saporin lesions has not been established. This study has used dual-labeling immunofluorescence in the rat BLA to examine SPR-IR neurons for their colocalization with the calcium-binding proteins; calbindin-D28k (CB), parvalbumin (PV), and calretinin (CR); and the neuropeptides somatostatin (SOM) and neuropeptide Y (NPY). We found that all NPY-IR neurons and 45% of SOM-IR interneurons expressed SPR-IR, and that 50% and 51% of the SPR-IR interneuron population expressed NPY- and SOM-IR, respectively. Previous studies have reported that approximately a third of SOM-IR interneurons also express NPY, which suggests a large degree of overlap between the NPY, SOM and SPR expressing neurons in the BLA. We also found that the majority of SPR-IR cells were CB-IR (62%), but that these interneurons represented only 2.8% of the total CB-IR population. Moreover, SPR-IR interneurons did not express either PV-or CR- IR. Hence, SP-saporin lesions would ablate all interneurons in the BLA that contain NPY, but leave the majority of the CB-IR cells intact, and have no effect on the CR- and PV-IR populations. Consequently, these results support the use of SP-saporin lesions as a useful technique to study the role of NPY-IR interneurons in the BLA.

Ontogeny of the human amygdala

Data on the fetal development of the human amygdala is reviewed with special reference to major ontogenetic events. In the fifth gestational month, the inferior portion of the amygdala reveals cell-dense columns merging with the ganglionic eminence (proliferative zone) in Nissl-stained sections. These columns contain vimentin-positive fibers and can therefore be regarded as migrational routes. In the sixth and seventh months, distinct reorganization of the cytoarchitectonics takes place. The sequential occurrence of afferens can be visualized using anti-GAP-43; moreover, outgrowing axons appear to reach the periphery of the ganglionic eminence. The latter may thus represent an intermediate target for growing axons using anti-calbindin and anti-calretinin. Migrating and immature amygdaloid neurons can be shown in the fifth month. From the eighth month onwards, various nonpyramidal neurons and pyramidal neurons are immunolabeled. Transient expression of calretinin in pyramidal neurons is observed. When punctate calbindin and calretinin immunostaining in the fifth and eighth months is compared, distinct redistribution is observed. On the whole, it is apparent that the amygdala has reached a high degree of maturity in the eighth month. At this developmental stage, AKAP79, being enriched in postsynaptic densities, shows a characteristic nuclear-specific distribution pattern. The latter largely corresponds to the expression pattern of NMDAR1. Thus, AKAP79 may have a preference for anchoring enzymes to glutamate receptors. The aforementioned results provide a basis for investigations on subtle changes in pathologically altered material, such as hemorrhage, in the ganglionic eminence of preterm infants.

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.

Comparison of the distribution of calcium-binding proteins and intrinsic connectivity in the lateral nucleus of the rat, monkey, and human amygdala

A large amount of anatomic, electrophysiologic, pharmacologic, and behavioral data published over the past decade has provided novel insight into the function of the amygdala in the rat. An important question remains as to how well the data obtained in the rat amygdala can be extrapolated to primates. To address this issue from a functional neuroanatomic point of view, we compared the recently published data on the distribution of calcium-binding proteins (parvalbumin, calbindin-D(28k), calretinin) and intrinsic connectivity in the rat, monkey, and human amygdala. The aim of our ongoing analysis is twofold: (1) to determine whether the nuclei with the "same name" in the three species are chemoarchitectonically similar and (2) to determine whether the intradivisional, interdivisional, and internuclear connectivity is similarly organized in the rat and monkey. We focused on the lateral nucleus, which is the major recipient of thalamic and cortical sensory information directed to the amygdala and provides the most widespread intraamygdaloid connections. Our analysis suggests many similarities in the organization of chemoarchitectonics and intrinsic connectivity of the different subdivisions of the lateral nucleus of the rat, monkey, and human amygdala. There are also dissimilarities, however, which might relate to differences in the function of the amygdala in rodents and primates.

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.

Distribution of parvalbumin, calretinin, and calbindin-D(28k) immunoreactivity in the rat amygdaloid complex and colocalization with gamma-aminobutyric acid

To understand the organization of inhibitory circuitries in the rat amygdala, the distribution of parvalbumin, calretinin, and calbindin immunoreactivity was investigated in the rat amygdaloid complex. Colocalization of various calcium-binding proteins with the inhibitory transmitter gamma-aminobutyric acid (GABA) was studied by using the mirror technique. Parvalbumin-immunoreactive (-ir) elements were located mostly in the deep amygdaloid nuclei, whereas the calretinin-ir and calbindin-ir staining were most intense in the cortical nuclei as well as in the central nucleus and the amygdalohippocampal area. Second, the distribution of immunopositive neurons largely parallelled the distribution of terminal and neuropil labeling. Third, immunostained neurons could be divided into four major morphologic types (types 1-4) based on the characteristics of the somata and the dendritic trees. The fourth lightly stained neuronal type that had a pyramidal GABA-negative soma was observed only in calretinin and calbindin preparations. Fourth, parvalbumin-ir terminals formed basket-like plexus and cartridges, which suggests that parvalbumin labels GABAergic inhibitory basket cells and axo-axonic chandelier cells, respectively. Colocalization studies indicated that 521 of 553 (94%) of parvalbumin-ir, 419 of 557 (75%) of calbindin-ir, and 158 of 657 (24%) of calretinin-ir neurons were GABA-positive in the deep amygdaloid nuclei. A high density of large GABA-negative calbindin-ir neurons was observed caudally in the medial division of the lateral nucleus and GABA-negative calretinin-ir neurons were observed in the magnocellular division of the accessory basal nucleus as well as in the intermediate and parvicellular divisions of the basal nucleus. These data suggest that in various amygdaloid areas, neuronal excitability is controlled by GABAergic neurons that contain different calcium-binding proteins. The appearance of basket-like plexus and cartridges in the parvalbumin preparations, but not in calretinin preparations, suggests that like in the hippocampus, the distribution of inhibitory terminals in the dendritic and perisomatic regions of postsynaptic neurons in the rat amygdala is organized in a topographic manner.

Differential expression of calbindin and calretinin in the human fetal amygdala

The distribution patterns of the calcium-binding proteins calbindin and calretinin, both expressed early during development within the various amygdaloid nuclei and areas, have been investigated. Anti-calbindin as well as anti-calretinin mark immature, partly migrating neurons in the 5th gestational month; the number of calretinin-immunoreactive neurons is distinctly higher. In the 8th month, calbindin and calretinin are found in a small proportion of presumed pyramidal cells and in various types of non-pyramidal neurons. Small and large bipolar and small and large multipolar neurons are shown to express calbindin and calretinin. Double-labellings show that calbindin and calretinin are largely contained in different subsets of these neuronal types, which are considered to represent interneurons. These nerve cell classes are widespread within the amygdala with mainly moderate to high packing densities. Diffuse immunoreactive structures, which are found in different intensities in the various amygdaloid nuclei, display distinct redistribution during fetal development. The results show that during early fetal development calbindin and particularly calretinin may be involved in the regulation of neuronal migration. In later development, definite subsets of interneurons, which are likely to be functionally different, are marked by anti-calbindin and -calretinin. Different diffuse immunolabelling at various developmental stages probably indicates the sequential arrival of afferent input from brain areas containing calbindin- or calretinin-immunoreactive nerve cells. With the exception that calretinin may be transiently expressed in pyramidal neurons, the distribution of calbindin- and calretinin-immunoreactive structures to a large degree corresponds to that in the adult. Thus, little reorganisation is to be expected during proceeding development.

Characterization of nodular neuronal heterotopia in children

Neuronal heterotopia are seen in various pathologies and are associated with intractable epilepsy. We examined brain tissue from four children with subcortical or periventricular nodular heterotopia of different aetiologies: one with severe epilepsy following focal brain trauma at 17 weeks gestation, one with hemimegalencephaly and intractable epilepsy, one with focal cortical dysplasia and intractable epilepsy, and one dysmorphic term infant with associated hydrocephalus and polymicrogyria. The connectivity of nodules was investigated using histological and carbocyanine dye (DiI) tracing techniques. DiI crystal placement adjacent to heterotopic nodules revealed numerous DiI-labelled fibres within a 2-3 mm radius of the crystals. Although we observed labelled fibres closely surrounding nodules, the majority did not penetrate them. Placement of DiI crystals within nodules also identified a limited number of projections out of the nodules and in one case there was evidence for connectivity between adjacent nodules. The cellular and neurochemical composition of nodules was also examined using immunohistochemistry for calretinin and neuropeptide Y (NPY), which are normally expressed in GABAergic cortical interneurons. Within heterotopic nodules from all cases, numerous calretinin-positive neurons were identified, along with a few cell bodies and many processes positive for NPY. Calretinin-positive neurons within nodules were less morphologically complex than those in the cortex, which may reflect incomplete differentiation into an inhibitory neuronal phenotype. There were also abnormal clusters of calretinin-positive cells in the overlying cortical plate, indicating that the migratory defect which produces heterotopic nodules also affects development of the cortex itself. Thus, heterotopic nodules consisting of multiple neuronal cell types are associated with malformation in the overlying cortical plate, and have limited connectivity with other brain regions. This abnormal development of connectivity may affect neuronal maturation and consequently the balance of excitation and inhibition in neuronal circuits, leading to their epileptogenic potential.

Calretinin-immunoreactive terminals make synapses on calbindin D28k-immunoreactive neurons in the lateral nucleus of the human amygdala

A double-labeling immunohistochemical procedure and correlated light and electron microscopy were used to examine if calretinin-immunoreactive terminals make synapses on calbindin D28k-positive cells. In the lateral nucleus of the human amygdala, calretinin terminals make symmetric-like synapses on the somata and proximal dendrites of calbindin D28k-labeled cells. Our data provide the first evidence that neurons which contain two different calcium-binding proteins form synaptic contacts with each other in the human amygdala.