Etorphine increases the number of mu-opioid receptor-positive cells in thecerebral cortex

Imunocytochemical techniques were used to determine whether agonist-induced activation of mu -opioid receptors alters the number and distribution of m u -opioid receptor-positive cells in the rat cerebral cortex. In untreated rats, mu -opioid receptor immunoreactivity was localized to neuronal perika rya and dendrites and to neuropilar punctate structures. mu -Opioid recepto r positive neurons were mostly in layers II and III and exhibited a bipolar or bitufted morphology. In rats treated with the mu -opioid receptor agoni st etorphine (0.1 mg/kg intraperitoneally) and perfused after different sur vival periods, there was an enhancement of immunostaining for mu -opioid re ceptors observed at 15 min, reaching a maximum at 60 min, and which returne d to normal at 480 min. Etorphine-induced effects included an increase in t he intensity of cellular and neuropil staining; statistical analysis showed that the number of mu -opioid receptor-positive cells in etorphine-treated groups was significantly higher than in controls or saline-treated rats. I n animals that received both etorphine and the mu -opioid receptor antagoni st naloxone, the pattern of mu -opioid receptors immunoreactivity was simil ar to that of untreated animals. This study shows that the number of mu -opioid receptor-positive cells is s ignificantly increased following etorphine treatment and suggests that agon ist treatment may be exploited to increased immunostaining of mu -opioid re ceptors and also of other G-protein coupled receptors. (C) 2000 IBRO. Publi shed by Elsevier Science Ltd. All rights reserved.