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.