Non-opiate β-endorphin fragments and dopamine—VI: Behavioural analysis of the interaction between γ-type endorphins and dopaminergic systems in the nucleus accumbens of rats

Injection of small doses of apomorphine, bromocriptine and the new ergoline compound, GYKI-32887 into the nucleus accumbens decreased locomotor activity when rats were tested in a small open field. This effect was observed following injection of 1 pg of these substances; GYKI-32887 being more potent than bromocriptine. The hypolocomotion induced by the ergoline compound could be prevented by local pretreatment with haloperidol (30 pg), fluphenazine (30 pg), sulpiride (10pg) or desenkephalin-γ-endorphin (DEγ E; 100 pg). Large doses of apomorphine and amphetamine, injected into the nucleus accumbens, increased locomotor activity. This behavioural response was antagonized by pretreatment with haloperidol (30 pg), but not with sulpiride (10pg) or DEγE (100pg or 10ng). It is concluded that two dopaminergic receptor systems exist in the nucleus accumbens with different sensitivity to apomorphine. One of these receptor systems, which is activated by small doses of apomorphine and ergoline compounds, can be blocked by classical and atypical neuroleptics and by the neuroleptic-like peptide DEγE. This may be of relevance to the antipsychotic action of these substances.