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Keywords:

  • endogenous enkephalins;
  • mesolimbic dopamine;
  • peptidase inhibitor;
  • locomotor activity;
  • dopamine metabolism

Abstract

The present study focused on the effects of acetorphan, a parenterally active enkephalinase inhibitor, on dopaminergic transmission in rat olfactory tubercle, nucleus accumbens and striatum. Acetorphan was administered i.v. (10 mg/kg) 15 min before measurement of the in vivo specific binding of [3H]N-propyl-norapomorphine ([3H]NPA) or measurement of the levels of dopamine (DA) and its metabolites 3-methoxytyramine-homovanillic acid (3MT-HVA) in the three areas. Acetorphan decreased the in vivo specific binding of [3H]NPA in the olfactory tubercle, this effect being antagonized by naloxone 1.5 mg/kg s.c. DA release in this brain structure was also significantly increased by acetorphan 10 mg/kg, as indicated by the 3MT:DA and HVA:DA ratios. Neither the specific binding of [3H]NPA nor DA metabolism and release were modified by the inhibitor in the striatum and the nucleus accumbens. The stimulant effect of acetorphan was significantly decreased in rats in which a bilateral lesion of dopaminergic endings in the olfactory tubercle had been produced by 6-hydroxydopamine (6-OHDA). These results suggest that dopaminergic transmissions in the olfactory tubercle are particularly sensitive to the modulation exerted by endogenous enkephalins, this modulation being at least partly involved in the increased locomotion induced by the enkephalinase inhibitor.