Non-opiate beta-endorphin fragments and dopamine--II. beta-Endorphin 2-9 enhances apomorphine-induced stereotypy following subcutaneous and intra-striatal injection.

The non-opiate beta-endorphin (beta E) fragment 2-16 (des Tyr1-alpha-endorphin) enhanced apomorphine-induced stereotyped sniffing in rats, but did not interfere with the hypoactivity elicited by small doses of apomorphine. Structure-activity relationship studies revealed that the active moiety of alpha-endorphin fragments with respect to their potentiating effects on apomorphine-induced stereotyped sniffing resides in the beta E fragment 2-9. Subsequent studies showed that the potentiating influence of beta E 2-9 was dependent on the dose of the peptide and that the interaction between this peptide and apomorphine may be non-competitive in nature. The stereotyped sniffing elicited by apomorphine, injected bilaterally into the striatal area of the brain, was dose-dependently enhanced by intra-striatal pretreatment with beta E 2-9. It is concluded that the influence of alpha-type endorphins and beta E 2-9 on apomorphine-induced behavioural changes, is in some aspects opposite to that of gamma-type endorphins, but may be mediated by quite different mechanisms.