Site-selective acute desensitization following local administration of opioid in the hippocampus

Endomorphin-1: induction of motor behavior and lack of receptor desensitization

The endomorphins are recently discovered endogenous agonists for the mu-opioid receptor (Zadina et al., 1997). Endomorphins produce analgesia; however, their role in other brain functions has not been elucidated. We have investigated the behavioral effects of endomorphin-1 in the globus pallidus, a brain region that is rich in mu-opioid receptors and involved in motor control. Bilateral administration of endomorphin-1 in the globus pallidus of rats induced orofacial dyskinesia. This effect was dose-dependent and at the highest dose tested (18 pmol per side) was sustained during the 60 min of observation, indicating that endomorphin-1 does not induce rapid desensitization of this motor response. In agreement with a lack of desensitization of mu-opioid receptors, 3 hr of continuous exposure of the cloned mu receptor to endomorphin-1 did not diminish the subsequent ability of the agonist to inhibit adenylate cyclase activity in cells expressing the cloned mu-opioid receptor. Confirming the involvement of mu-opioid receptors, the behavioral effect of endomorphin-1 in the globus pallidus was blocked by the opioid antagonist naloxone and the mu-selective peptide antagonist Cys(2)-Tyr(3)-Orn(5)-Pen(7) amide (CTOP). Furthermore, the selective mu receptor agonist [d-Ala(2)-N-Me-Phe(4)-Glycol(5)]-enkephalin (DAMGO) also stimulated orofacial dyskinesia when infused into the globus pallidus, albeit transiently. Our findings suggest that endogenous mu agonists may play a role in hyperkinetic movement disorders by inducing sustained activation of pallidal opioid receptors.

Endogenous opiates: 1995

This article is the eighteenth installment of our annual review of research concerning the opiate system. It includes articles published during 1995 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects. The specific topics covered this year include stress: tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.