Decreased mu-opioid receptor binding in the globus pallidus of rats treated with chronic haloperidol

Rationale: Chronic neuroleptic treatment produces a movement disorder in ra ts characterized by vacuous chewing movements (VCMs). Neuroleptics also pro duce a variety of changes in opioid neurotransmission in several regions of the basal ganglia. Rats with the VCM syndrome show elevated mRNA for enkep halin in striatopallidal neurons, suggesting a possible role for enkephalin in the pathophysiology of VCMs. Objective: This study investigated the rol e of mu-opioid receptor density in the basal ganglia on the expression of V CMs. Methods: Rats were treated with haloperidol for 24 weeks and withdrawn for 9 weeks. Mu (mu) receptors were labeled with [H-3]-DAMGO. Results: Hal operidol treatment produced a significant reduction in mu-receptor binding in the globus pallidus (P<0.05). There was, however, no relationship betwee n mu-opioid receptor density and VCMs in this or any other region of the ba sal ganglia. Conclusion: These results replicate prior findings of a neurol eptic-induced reduction in [H-3]-DAMGO binding in the globus pallidus. The lack of association between VCMs and [H-3]-DAMGO binding in the globus pall idus or any other region suggests that prior reports of enkephalinergic mRN A changes in the striatum are not accompanied by compensatory changes in po stsynaptic neurons.