Distribution of 5-HT2A receptor immunoreactivity in the rat and monkey amygdaloid complex

The amygdaloid complex comprises a variety of nuclei and cortical regions, each of which has different cytoarchitectonic, chemoarchitectonic, and connectional characteristics (Pitkänen, J. Comp. Neurol., 398, 1998). 5-HT2A receptor is a G-protein coupled and phospholipase C-stimolating receptor which produces a depolarization due to a decrease in potassium conductances (Barnes, Neuropharmacol., 38, 1999). In the present study single- and double-labelling immunohistochemical techniques were employed to determine the distribution and the neuronal localization of 5-HT2A receptor in the rat and monkey amygdala. 5-HT2A receptor-immunoreactivity was found in many amygdaloid nuclei of both species. Within the rat amygdaloid complex, the neuronal density was high in the lateral, basal, accessory basal, and medial nuclei. In the monkey amygdaloid complex, the highest neuronal density was observed in the magnocellular subdivision of the basal nucleus, in the magnocellular subdivision of the accessory basal nucleus, in the periamygdaloid cortex, and in the amygdalohippocampal area. Morphological analysis and dual-labelling immunofluorescence experiments indicated that 5-HT2A receptor is localized in excitatory and inhibitory neurons. Our results, together with previous studies, suggest also that 5-HT2A receptor could modulate synchronized rhythmic oscillations in some amygdaloid nuclei.