A BSTR ACTTo identify the cortical sites where 5-hydroxytryptamine 2A (5-HT 2A ) serotonin receptors respond to the action of hallucinogens and atypical antipsychotic drugs, we have examined the cellular and subcellular distribution of these receptors in the cerebral cortex of macaque monkeys (with a focus on prefrontal areas) by using light and electron microscopic immunocytochemical techniques. 5-HT 2A receptor immunoreactivity was detected in all cortical layers, among which layers II and III and layers V and VI were intensely stained, and layer IV was weakly labeled. The majority of the receptor-labeled cells were pyramidal neurons and the most intense immunolabeling was consistently confined to their parallelly aligned proximal apical dendrites that formed two intensely stained bands above and below layer IV. The features highlighting the importance of 5-HT 2A receptors are contrasted by a disarray of data on its cortical and cellular distribution and by a lack of information on its subcellular localization (8-19). In the rat cortex, 5-HT 2A receptors have been reported to be expressed only by pyramidal cells according to an autoradiographic study (17) or only by interneurons according to an immunocytochemical study (19), but a recent mRNA in situ hybridization study of the human cortex (11) and an immunocytochemical study of the rat cortex have demonstrated the receptor both in pyramidal and nonpyramidal cells (20). 5-HT 2 agonist drugs directly activate pyramidal cells in the rat prefrontal cortex (21, 22), but only interneurons were found to be responsive in the rat piriform cortex (23-25). The light and electron microscopic 5-HT 2A -receptor-immunocytochemical experiments presented herein have enabled us to identify the neuronal elements containing 5-HT 2A receptor in the primate cortex at a level of resolution and specificity not possible in previous autoradiographic and in situ hybridization studies.
MATERIALS AND METHODSFour adult rhesus monkeys (Macaca mulatta), housed and treated in accordance with institutional guidelines, were deeply anesthetized with Nembutal (100 mg͞kg, given intravenously) and perfused transcardially with normal saline followed by 4% paraformaldehyde͞0.1% glutaraldehyde prepared in 0.1 M sodium phosphate buffer (PB, pH 7.4). Coronal blocks containing frontal, parietal, temporal, and occipital cortical regions were cut at 50 m on a cryostat and Vibratome. The sections were treated for 1 h with a blocking serum (5% normal goat serum͞1% albumin͞0.1% lysine͞0.1% glycine in PB; also used in all antibody dilutions), incubated for 2 days at 4°C with 5-HT 2A receptor monoclonal antibodies raised in mouse (1:2,000 dilution; clone G186-1117; PharMingen), and processed by the avidin-biotin method using horse anti-mouse biotinylated antibodies (1:250 dilution; 1 h) and the Vectastain ABC Elite reagent (1:100 dilution, 1 h; Vector Laboratories). The immunoperoxidase reaction was visualized by using (i) 0.05% diaminobenzidine (DAB; Sigma) and 0.01% hydrogen peroxide in PB (resulting in a br...