Intercalated and paracapsular cell islands of the adult rat amygdala: a combined rapid-Golgi, ultrastructural, and immunohistochemical account.

The anterior and rostral paracapsular intercalated islands (AIC and PIC, respectively) were studied in the context of the amygdaloid modulation of fear/anxiety using horizontal sections. The structural analysis carried out using silver-impregnated specimens revealed that the AIC is composed of tightly packed, medium-sized spiny neurons with distinct dendritic and axonal patterns that send projecting axons to the central nucleus of the amygdala. The AIC occupies a strategic position between the basolateral amygdaloid complex and the caudal limb of the anterior commissure from which it receives fibers en passage and axon terminals. Electron microscopic observation of terminal (i.e., synaptic) degeneration 72 h after the surgical interruption of the anterior commissure, confirms the synaptic interaction between the latter and the AIC neurons. These observations suggest that these islands may gate the activity of neurons from the contralateral basal forebrain and synchronize the anxiogenic output of both amygdalae. Immunohistochemical analysis indicated that, within the AIC and rostral PIC, the distance between tyrosine hydroxylase-immunoreactive terminals and the punctate dopamine D(1) receptor immunoreactivity, was in the micrometer range. These results indicate a short distance and a rapid extrasynaptic form of dopamine volume transmission mediated via D(1) receptors in the AIC and PIC which may enhance fear and anxiety by suppressing feed-forward inhibition in the basolateral and central amygdaloid nuclei. The strong suggestion for a commissural axon projection to the AIC documented here, coupled with the previous evidences indicting an isocortical and amygdalar contributions to the anterior commissure, opens the possibility that the AIC may be involved in decoding nerve impulses arising from both the ipsi- and contra-lateral forebrain to, in turn, modulate the homolateral amygdala.