Organization of connections of the basal and accessory basal nuclei in the monkey amygdala

Mesoscopic-scale functional networks in the primate amygdala

The primate amygdala performs multiple functions that may be related to the anatomical heterogeneity of its nuclei. Individual neurons with stimulus- and task-specific responses are not clustered in any of the nuclei, suggesting that single-units may be too-fine grained to shed light on the mesoscale organization of the amygdala. We have extracted from local field potentials recorded simultaneously from multiple locations within the primate (Macaca mulatta) amygdala spatially defined and statistically separable responses to visual, tactile, and auditory stimuli. A generalized eigendecomposition-based method of source separation isolated coactivity patterns, or components, that in neurophysiological terms correspond to putative subnetworks. Some component spatial patterns mapped onto the anatomical organization of the amygdala, while other components reflected integration across nuclei. These components differentiated between visual, tactile, and auditory stimuli suggesting the presence of functionally distinct parallel subnetworks.

Multisensory Neurons in the Primate Amygdala

Animals identify, interpret, and respond to complex, natural signals that are often multisensory. The ability to integrate signals across sensory modalities depends on the convergence of sensory inputs at the level of single neurons. Neurons in the amygdala are expected to be multisensory because they respond to complex, natural stimuli, and the amygdala receives inputs from multiple sensory areas. We recorded activity from the amygdala of 2 male monkeys (Macaca mulatta) in response to visual, tactile, and auditory stimuli. Although the stimuli were devoid of inherent emotional or social significance and were not paired with rewards or punishments, the majority of neurons that responded to these stimuli were multisensory. Selectivity for sensory modality was stronger and emerged earlier than selectivity for individual items within a sensory modality. Modality and item selectivity were expressed via three main spike-train metrics: (1) response magnitude, (2) response polarity, and (3) response duration. None of these metrics were unique to a particular sensory modality; rather, each neuron responded with distinct combinations of spike-train metrics to discriminate sensory modalities and items within a modality. The relative proportion of multisensory neurons was similar across the nuclei of the amygdala. The convergence of inputs of multiple sensory modalities at the level of single neurons in the amygdala rests at the foundation for multisensory integration. The integration of visual, auditory, and tactile inputs in the amygdala may serve social communication by binding together social signals carried by facial expressions, vocalizations, and social grooming.SIGNIFICANCE STATEMENT Our brain continuously decodes information detected by multiple sensory systems. The emotional and social significance of the incoming signals is likely extracted by the amygdala, which receives input from all sensory domains. Here we show that a large portion of neurons in the amygdala respond to stimuli from two or more sensory modalities. The convergence of visual, tactile, and auditory signals at the level of individual neurons in the amygdala establishes a foundation for multisensory integration within this structure. The ability to integrate signals across sensory modalities is critical for social communication and other high-level cognitive functions.

Multisynaptic inputs from the medial temporal lobe to V4 in macaques

Retrograde transsynaptic transport of rabies virus was employed to undertake the top-down projections from the medial temporal lobe (MTL) to visual area V4 of the occipitotemporal visual pathway in Japanese monkeys (Macaca fuscata). On day 3 after rabies injections into V4, neuronal labeling was observed prominently in the temporal lobe areas that have direct connections with V4, including area TF of the parahippocampal cortex. Furthermore, conspicuous neuron labeling appeared disynaptically in area TH of the parahippocampal cortex, and areas 35 and 36 of the perirhinal cortex. The labeled neurons were located predominantly in deep layers. On day 4 after the rabies injections, labeled neurons were found in the hippocampal formation, along with massive labeling in the parahippocampal and perirhinal cortices. In the hippocampal formation, the densest neuron labeling was seen in layer 5 of the entorhinal cortex, and a small but certain number of neurons were labeled in other regions, such as the subicular complex and CA1 and CA3 of the hippocampus proper. The present results indicate that V4 receives major input from the hippocampus proper via the entorhinal cortex, as well as “short-cut” pathways that bypass the entorhinal cortex. These multisynaptic pathways may define an anatomical basis for hippocampal-cortical interactions involving lower visual areas. The multisynaptic input from the MTL to V4 is likely to provide mnemonic information about object recognition that is accomplished through the occipitotemporal pathway.

Amygdala projections to central amygdaloid nucleus subdivisions and transition zones in the primate

In rats and primates, the central nucleus of the amygdala (CeN) is most known for its role in responses to fear stimuli. Recent evidence also shows that the CeN is required for directing attention and behaviors when the salience of competing stimuli is in flux. To examine how information flows through this key output region of the primate amygdala, we first placed small injections of retrograde tracers into the subdivisions of the central nucleus in Old world primates, and examined inputs from specific amygdaloid nuclei. The amygdalostriatal area and interstitial nucleus of the posterior limb of the anterior commissure (IPAC) were distinguished from the CeN using histochemical markers, and projections to these regions were also described. As expected, the basal nucleus and accessory basal nucleus are the main afferent connections of the central nucleus and transition zones. The medial subdivision of the central nucleus (CeM) receives a significantly stronger input from all regions compared to the lateral core subdivision (CeLcn). The corticoamygdaloid transition zone (a zone of confluence of the medial parvicellular basal nucleus, paralaminar nucleus, and the sulcal periamygdaloid cortex) provides the main input to the CeLcn. The IPAC and amygdalostriatal area can be divided in medial and lateral subregions, and receive input from the basal and accessory basal nucleus, with differential inputs according to subdivision. The piriform cortex and lateral nucleus, two important sensory interfaces, send projections to the transition zones. In sum, the CeM receives broad inputs from the entire amygdala, whereas the CeLcn receives more restricted inputs from the relatively undifferentiated corticoamygdaloid transition region. Like the CeN, the transition zones receive most of their input from the basal nucleus and accessory basal nucleus, however, inputs from the piriform cortex and lateral nucleus, and a lack of input from the parvicellular accessory basal nucleus, are distinguishing afferent features.

Amygdala volume analysis in female twins with major depression

BACKGROUND

Previous research examining the amygdala volumes in major depressive disorder (MDD) has found conflicting evidence for association. Furthermore, few of these studies have examined differences in individuals with an onset during childhood or adolescence. This study examined amygdala volume and its potential association with early onset major depression.

METHODS

A community-based sample of 47 right-handed young adult female monozygotic and dizygotic twin pairs was examined. For 29 twin pairs, one twin per pair had a lifetime history of MDD, while 18 age-matched control twin pairs had no lifetime history of MDD or other Axis I disorder. Core, noncore, and total amygdala volumes were estimated based on a combination of manual tracing, automated segmentation, and expert rater regional boundary definitions.

RESULTS

No significant differences were found in amygdala volumes between depressed, high-risk, or control subjects. However, analyses comparing control monozygotic twins to randomly created control subject pairs suggest that there are familial, perhaps genetic, influences on core and total amygdala volumes.

CONCLUSIONS

Findings suggest that although there were no significant differences in amygdala volumes between groups, familial factors influence amygdala volumes. Discrepancies between studies measuring amygdala volume in MDD may be due to differences in amygdala boundary definitions.

Amygdaloid inputs define a caudal component of the ventral striatum in primates

The ventral striatum mediates goal-directed behavior through limbic afferents. One well-established afferent to the ventral striatum is the amygdaloid complex, which projects throughout the shell and core of the nucleus accumbens, the rostral ventromedial caudate nucleus, and rostral ventromedial putamen. However, striatal regions caudal to the anterior commissure also receive inputs from the amygdala. These caudal areas contain histochemical and cytoarchitectural features that resemble the shell and core, based on our recent studies. Specifically, there is a calcium binding protein (CaBP)-poor region in the lateral amygdalostriatal area that resembles the "shell." To examine the idea that the caudal ventral striatum is part of the "classic" ventral striatum, we placed small injections of retrograde tracers throughout the caudal ventral striatum/amygdalostriatal area and charted the distribution of specific amygdaloid inputs. Amygdaloid inputs to the CaBP-poor zone in the lateral amygdalostriatal area arise from the basal nucleus, the magnocellular subdivision of the accessory basal nucleus, the periamygdaloid cortex, and the medial subdivision of the central nucleus, resembling that of the shell of the ventral striatum found in our previous studies. There are also amygdaloid inputs to CaBP-positive areas outside the shell, which originate mainly in the basal nucleus. Taken together, the "limbic-related" striatum forms a continuum from the rostral ventral striatum through the caudal ventral striatum/lateral amygdalostriatal area based on histochemical and cellular similarities, as well as inputs from the amygdala.

Amygdala-kindling induces alterations in neuronal density and in density of degenerated fibers

Kindling is characterized by a progressive intensification of seizure activity culminating in generalized seizures following repeated administration of an initially subconvulsive electrical or chemical stimulus. Since it is known that epilepsy induces morphological alterations in the limbic system, we examined the neuropathological consequences of kindling with a sensitive silver-staining method for the visualization of damaged neurons and Nissl staining for the estimation of the neuronal densities in different limbic areas. Wistar rats implanted with electrodes in the left basolateral nucleus were stimulated until 15 consecutive stage V seizures (scale of Racine). Amygdala-kindled animals had reduced cell density in the amygdala and increased density of fragments of degenerated axons. Reduced neuronal density and the occurrence of degenerated axons in kindled animals were more prominent in the ipsilateral than in the contralateral hemisphere. In addition, more degenerated axons were found in cortical structures of kindled than sham-operated animals. These results indicate that kindling induced morphological alterations that were not restricted to either the ipsilateral hemisphere or the stimulated region. These morphological changes might be responsible for the emotional and behavioral disturbances that can accompany epilepsy.

The extended amygdala and the dopamine system: another piece of the dopamine puzzle

The dopamine (DA) system has long been associated with the pathophysiology of psychosis. The DA theory of schizophrenia continues to find support in neuroreceptor imaging and ligand-binding studies that show excess DA transmission in patients, as compared to controls. The pathways that regulate the primate DA system, however, have yet to be fully elucidated. The amygdala, including its extended amygdala component, is involved in evaluating the emotional value of sensory stimuli. Since emotionally relevant sensory stimuli are distorted during psychotic episodes, we hypothesize that amygdaloid influences are likely to be significant modulators of the DA system. We reviewed evidence for direct projections from the central extended amygdala to specific subpopulations of DA neurons, and we discuss how these pathways may serve as important conduits of emotionally relevant information that can have immediate and long-term effects on DA regulation.

Anatomic relationships between aromatase and androgen receptor mRNA expression in the hypothalamus and amygdala of adult male cynomolgus monkeys

This study mapped the regional locations of cells expressing cytochrome P450 aromatase (P450AROM) and androgen receptor (AR) mRNAs in the adult male macaque hypothalamus and amygdala by in situ hybridization histochemistry using monkey-specific cRNA probes. High densities of P450AROM and AR mRNA-containing neurons were observed in discrete hypothalamic areas involved in the regulation of gonadotropin secretion and reproductive behavior. P450AROM mRNA-containing neurons were most abundant in the medial preoptic nucleus, bed nucleus of the stria terminalis, and anterior hypothalamic area, whereas AR mRNA-containing neurons were most numerous in the ventromedial nucleus, arcuate nucleus, and tuberomamillary nucleus. Moderate to heavily labeled P450AROM mRNA-containing cells were present in the cortical and medial amygdaloid nuclei, which are known to have strong reciprocal inputs with the hypothalamus. Heavily labeled P450AROM mRNA-containing cells were found in the accessory basal amygdala nucleus, which projects to the cingulate cortex and hippocampus, areas that are important in the expression of emotional behaviors and memory processing. In contrast to P450AROM, the highest density of AR mRNA labeling in the temporal lobe was associated with the cortical amygdaloid nucleus and the pyramidal cells of the hippocampus. All areas that contained P450AROM mRNA-expressing cells also contained AR mRNA-expressing cells, but there were areas in which AR mRNA was expressed but not P450AROM mRNA. The apparent relative differences in the expression of P450AROM and AR mRNA-containing neurons within the monkey brain suggests that T acts through different signaling pathways in specific brain areas or within different cells from the same region.