Effect of opiate receptor agonists on striatally-mediated head turning: an in vivo model of opiate delta receptor activation?

Electrical stimulation of the striatum in conscious rats elicits a contralateral head-turn. Modification of this response by opiate drugs injected sub-cutaneously or into the ipsilateral globus pallidus has been investigated. Morphine, normorphine and ethylketocyclazocine had no effect on the head-turn latency but met-enkephalin and D-Ala2, D-Leu5-enkephalin, injected into the pallidum, and smalL doses of etorphine injected subcutaneously, produced a dose-related slowing of the response. Large doses of the opiate receptor antagonist naloxone antagonised the effect of etorphine. These results suggest the presence of opiate delta receptors in the rat globus pallidus and this model may therefore prove useful for studying the effects of other agents acting at the delta receptor.