Overview

Abstract

The unconditioned behavioural effects of two non-peptide δ-opioid receptor agonists, BW 373U86 and SNC 80, were studied in the intact rat. BW 373U86 (0.1–2.5 mg/kg s.c.) and SNC 80 (2.5–10 mg/kg s.c.) dose-dependently elicited locomotion, rearing, stereotyped sniffing, licking and gnawing. These effects were abolished by pretreatment with the δ-opioid receptor antagonist naltrindole (5.0 mg/kg s.c.). In view of the phenomenological similarities between this syndrome and that elicited by dopamine-receptor agonists, the role played by dopamine receptors was investigated. The specific dopamine D1 receptor antagonist SCH 23390 and the specific dopamine D2/D3 receptor antagonist raclopride reduced or even abolished the behavioural stimulation induced by lower doses of BW 373U86 and SNC 80. When higher doses of BW 373U86 were used (2.5 mg/kg), however, raclopride, even at high cataleptic doses (6.0 mg/kg), only partly prevented the behavioural stimulation induced by the δ-opioid receptor agonist. The behavioural stimulation remaining after high doses of raclopride was abolished by the administration of SCH 23390. These results show that δ-opioid receptor stimulation elicits dopamine-dependent behavioural activation in the rat that depends on dopamine receptors, particularly of the D1 subtype.