Neuronal localization of 5-HT type 2A receptor immunoreactivity in the rat basolateral amygdala

Although it is well established that there are alterations in type 2A 5-HT receptors (5-HT2ARs) in the basolateral nuclear complex of the amygdala (BLC) in several neuropsychiatric disorders, very little is known about the neuronal localization of these receptors in this brain region. Single-labeling and dual-labeling immunohistochemical techniques were utilized in the rat to address this question. Three different 5-HT2AR antibodies were used, each producing distinct but overlapping patterns of immunostaining. Two of three 5-HT2AR antibodies mainly stained pyramidal projection neurons in the BLC. The third antibody only stained pyramidal cells in the dorsolateral subdivision of the lateral amygdalar nucleus. With one of the antibodies, the most intensely stained neurons were a population of large nonpyramidal neurons whose morphology and distribution closely resembled those shown in previous studies to project to the mediodorsal thalamic nucleus (MD). This was confirmed in the present study using a technique that combined 5-HT2AR immunohistochemistry with fluorogold retrograde tract-tracing. Two of three 5-HT2AR antibodies stained large numbers of parvalbumin-containing interneurons in the BLC. One of these two antibodies also stained a subpopulation of somatostatin-containing neurons. None of the 5-HT2AR antibodies stained significant numbers of the other two main interneuronal subpopulations, the large cholecystokinin-positive neurons or the small interneurons that exhibit extensive colocalization of calretinin and cholecystokinin. Since each of the three antibodies was raised against a distinct immunizing antigen, they may recognize different conformations of 5-HT2AR in different neuronal domains. The expression of 5-HT2ARs in pyramidal cells and parvalbumin-positive interneurons in the BLC is consistent with the results of previous electrophysiological studies, and suggests that 5-HT may produce excitation of several neuronal populations in the BLC via 5-HT2ARs.