DISTRIBUTION OF 5-HT2A RECEPTOR IMMUNOREACTIVITY IN THE RAT HIPPOCAMPAL FORMATION

The hippocampal formation is composed of three layered allocortical structures, including the dentate gyrus, the hippocampus proper (which is subdivided into tree fields: CA3, CA2, and CA1), and the subiculum (Sharfmann, Ann. N.Y. Acad. Sci., 911, 2000). 5-HT2A receptor is a G-protein coupled receptor. Activation of this receptor produces neuronal depolarization, a result of the closing of potassium channels (Barnes, Neuropharmacol., 38, 1999). To clarify the distribution and the neuronal localization of 5-HT2A receptor in the rat hippocampal formation, a single- and double-labelling immunohistochemical techniques were employed using non-colchicine- and colchicine-injected rats. 5-HT2A receptor immunoreactivity was widely distributed in the hippocampal formation. In untreated rats, the immunoreactivity was mainly localized to dendrites and axons. Pretreatment with colchicine enhanced the number of 5-HT2A receptor immunoreactive cell bodies. The highest intensity of stained somata were found in polymorphic and granule cells layers of the dentate gyrus, in the pyramidal cell layer of the hippocampus proper, and in the pyramidal cell layer of the subiculum. Morphological and double-immunofluorescence analyses indicated that 5-HT2Ar was located on both excitatory and inhibitory neurons, suggesting that this receptor can regulate hippocampal activity acting on different neuronal population