Previous studies have shown that chronic i.v. treatment with morphine or heroin decreased mu opioid receptor activation of G-proteins in specific brain regions. The present study examined the effect of intrathecal (i.t.) morphine administration on receptor/G-protein coupling in the spinal cord. In spinal cord membranes, [35S]GTP gamma S binding was stimulated by agonists of several G-protein-coupled receptors, including mu opioid (DAMGO), delta opioid (DPDPE), GABA(B) (baclofen), cannabinoid CB(1) (WIN 55,212-2), muscarinic cholinergic (carbachol) and adenosine A(1) (PIA). [35S]GTP gamma S autoradiography revealed that most of this agonist activation of G-proteins was localized to laminae I and II of dorsal horn. To determine the effects of chronic morphine on these receptor activities, rats were treated for 7 days with 0.11 mg/kg/day i.t. morphine, and receptor activation of G-proteins was determined by [35S]GTP gamma S autoradiography of brain and spinal cord. In spinal cord sections, chronic morphine treatment decreased DAMGO-stimulated [35S]GTP gamma S binding in laminae I and II at all levels of spinal cord examined. There were no effects of morphine treatment on [35S]GTP gamma S stimulation in spinal cord by other receptor systems examined (Adenosine A(1) and GABA(B)), and no significant effects of chronic i.t. morphine treatment were observed in brain sections. These data show that homologous desensitization of mu receptor/G-protein coupling occurs specifically in spinal cord following chronic morphine administration.