Acute systemic administration of morphine selectively increases mu opioid receptor binding in the rat brain

Abstract

Opioid receptor binding, including the mu, delta, and kappa receptor subtypes, was compared in morphine-injected and control rats. Brain tissues were homogenized and centrifuged either one or two times prior to receptor binding assay. In brain membranes from morphine-injected rats centrifuged once, there was a decrease in mu, but not delta or kappa, binding compared to controls, perhaps indicating occupation of these sites by morphine. By contrast, homogenates from morphine-injected rats centrifuged twice manifested an increase inmu, but not delta or kappa, binding sites. These results suggest that pharmacological stimulation of central opioid receptors with morphine causes a rapid, selective increase in the number of available mu binding sites.