Estrogen-induced alteration of μ-opioid receptor immunoreactivity in the medial preoptic nucleus and medial amygdala

Abstract

The μ-opioid receptor (μ-OR), like most G-protein-coupled receptors, is rapidly internalized after agonist binding. Although opioid peptides induce internalization in vivo, there are no studies that demonstrate μ-OR internalization in response to natural stimuli. In this study, we used laser- scanning microscopy to demonstrate that estrogen treatment induces the translocation of μ-OR immunoreactivity (μ-ORi) from the membrane to an internal location in steroid-sensitive cell groups of the limbic system and hypothalamus. Estrogen-induced internalization was prevented by the opioid antagonist naltrexone, suggesting that translocation was largely dependent on release of endogenous agonists. Estrogen treatment also altered the pattern of μ-ORi at the bright-field light microscopic level. In the absence of stimulation, the majority of immunoreactivity is diffuse, with few definable μ-OR+ cell bodies or processes. After stimulation, the density of distinct processes filled with μ-ORi was significantly increased. We interpreted the increase in the number of μ-OR+ processes as indicating increased levels of internalization. Using this increase in the density of μ-OR+ fibers, we showed that treatment of ovariectomized rats with estradiol benzoate induced a rapid and reversible increase in the number of fibers. Significant internalization was noted within 30 min and lasted for >24 hr after estrogen treatment in the medial preoptic nucleus, the principal part of the bed nucleus, and the posterodorsal medial amygdala. Naltrexone prevented the increase of μ-OR+ processes. These data imply that estrogen treatment stimulates the release of endogenous opioids that activate μ-OR in the limbic system and hypothalamus providing a 'neurochemical signature' of steroid activation of these circuits.