Inputs from the amygdaloid and extraamygdaloid areas terminate in various divisions of the central nucleus. To elucidate the interconnections between the different regions of the central nucleus and its connectivity with the other amygdaloid areas, we injected the anterograde tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L) into the capsular, lateral, intermediate, and medial divisions of the central nucleus in rat. There were a number of labeled terminals near the injection site within each division. The intrinsic connections between the various divisions of the central nucleus were organized topographically and originated primarily in the lateral division, which projected to the capsular and medial divisions. Most of the connections were unidirectional, except in the capsular division, which received a light reciprocal projection from its efferent target, the medial division. The intermediate division did not project to any of the other divisions of the central nucleus. Extrinsic projections from the central nucleus to the other amygdaloid nuclei were meager. Light projections were observed in the parvicellular division of the basal nucleus, the anterior cortical nucleus, the amygdalohippocampal area, and the anterior amygdaloid area. No projections to the contralateral amygdala were found. These data show that the central nucleus has a dense network of topographically organized intradivisional and interdivisional connections that may integrate the intraamygdaloid and extraamygdaloid information entering the different regions of the central nucleus. The sparse reciprocal connections to the other amygdaloid nuclei suggest that the central nucleus does not regulate the other amygdaloid regions but, rather, executes the responses evoked by the other amygdaloid nuclei that innervate the central nucleus.

The amygdaloid complex plays an important role in the detection of emotional stimuli, the generation of emotional responses, the formation of emotional memories, and perhaps other complex associational processes. These functions depend upon the flow of information through intricate and poorly understood circuitries within the amygdala. As part of an ongoing project aimed at further elucidating these circuits, we examined the intra-amygdaloid connections of the accessory basal nucleus in the rat. In addition, we examined connections of the anterior cortical nucleus and amygdalahippocampal area to determine whether portions of these nuclei should be included in the accessory basal nucleus (as some earlier studies suggest). Phaseolus vulgaris leucogglutinin was injected into different rostrocaudal levels of the accessory basal nucleus (n = 12) or into the anterior cortical nucleus (n = 3) or amygdalahippocampal area (n = 2). The major intra-amygdaloid projections from the accessory basal nucleus were directed to the medial and capsular divisions of the central nucleus, the medial division of the amygdalohippocampal area, the medial division of the lateral nucleus, the central division of the medial nucleus, and the posterior cortical nucleus. The projections originating in the anterior cortical nucleus and the lateral division of the amygdalohippocampal area differed from those originating in the accessory basal nucleus, which suggests that these areas are not part of the accessory basal nucleus. The present findings and our previous data suggest that each of the deep amygdaloid nuclei have different intra-amygdaloid connections. The pattern of these various connections suggests that information entering the amygdala from different sources can be integrated only in certain amygdaloid regions.

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Inhibitory neurons in the entorhinal cortex control information flow between the cortical areas and the hippocampus. We characterized the inhibitory circuits in the rat entorhinal cortex by analyzing the distribution of calretinin-immunoreactivity and its colocalization with glutamate decarboxylase (GAD) and gamma-aminobutyric acid (GABA). The location of calretinin-immunoreactive (IR) neurons and terminals varies between the different layers and subfields of the entorhinal cortex. The immunopositive neurons can be divided into two major morphological classes: bipolar and multipolar, which have two or more long, aspiny or sparsely spiny dendrites that extend through several layers. In addition, there are unclassified immunopositive neurons that have large lightly stained somata. They are located primarily in layer V. Colocalization analyses with GAD and GABA revealed that approximately 40% (657 out of 1,777) of all calretinin-IR cells within the entorhinal cortex contain GAD or GABA. In layers I-III, over 90% of the calretinin-IR neurons contain GAD or GABA. In layers V-VI, however, most of the calretinin-IR neurons do not colocalize with either GAD or GABA. The distribution patterns of calretinin-immunoreactivity in the entorhinal cortex is consistent with the partitioning of the rat entorhinal cortex into six subfields. Furthermore, calretinin is expressed in a morphologically heterogeneous population of cells in the rat entorhinal cortex which includes both GABAergic and non-GABAergic neurons.