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

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 in mu, 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.