Regulation of μ‐Opioid Receptor mRNA in Rat Globus Pallidus: Effects of Enkephalin Increases Induced by Short‐ and Long‐Term Haloperidol Administration

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Abstract
The mRNA encoding μ‐opioid receptors is expressed in neurons of the globus pallidus, a region of the basal ganglia that receives a dense enkephalinergic innervation from the striatum. The regulation of the mRNAs encoding the opioid peptide enkephalin in the striatum and the μ‐opioid receptor in the globus pallidus was examined with in situ hybridization histochemistry following short‐ or long‐term haloperidol treatments, which alter striatal enkephalin mRNA levels. Animals were administered haloperidol daily for 3 or 7 days (1 mg/kg, s.c.) or continuously for 8 months (1 mg/kg, depot followed by oral). Enkephalin and μ‐opioid receptor mRNA levels were unchanged after 3 days of haloperidol treatment. In contrast, the enkephalin mRNA level was increased in the striatum, and μ‐opioid receptor mRNA levels were markedly decreased in the globus pallidus after 7 days of haloperidol administration. Similar effects were observed in rats treated with haloperidol for 8 months. The results provide the first evidence of regulation of μ‐opioid receptor mRNA in vivo.

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