A comparison of the ability of opioid peptides and opiates to affect active avoidance conditioning in rats

Enkephalins reduce acquisition of an active avoidance response when administered intraperitoneally shortly before training. The present study examined whether microgram or delta opiate receptors are involved in this enkephalin effect. This was done by comparing the efficacy of micro- and delta-receptor agonists; by attempting to block the enkephalin effect with micro- and delta-receptor antagonists; and by comparing the characteristics of the effects of Met-enkephalin and Leu-enkephalin. In addition, the efficacy of kappa-agonists in reducing acquisition was assessed. It was found that micro-agonists are inactive in this assay; several delta- and kappa-agonists are active. However, not all of the data are consistent with the adequacy of this receptor classification. The micro-receptor antagonist naloxone did not readily block the effect of Met- or Leu-enkephalin but neither did the micro/delta-antagonist, diprenorphine. An additional complexity is the emergence of differences in behavioral activity of Met- snd Leu-enkephalin.