Approach and escape responses to mesencephalic central gray stimulation in rats: effects of morphine and naloxone.
Two experiments were conducted to examine whether regional differences in mesencephalic central gray (CG) could be delineated according to the reinforcing (rewarding and/or punishing) effects of electrical stimulation of CG, and whether such reinforcing properties of CG stimulation would be mediated by endogenous opioid system. In Expt. I, rats with electrodes in CG were trained to turn CG stimulation on and off by running from one compartment to another in a shuttlebox. Rats whose electrode tips were verified at the ventral part of CG, showed both approach and escape responses to the brain stimulation, while those stimulated to more dorsal CG yielded only escape response. In Expt. II, effects of morphine (4 and 8 mg/kg) and naloxone (2 and 10 mg/kg) were investigated for 3 h after injection in the rats which showed both approach and escape responses to CG stimulation in Expt. I. Naloxone significantly increased approach latency to CG stimulation without any effect on escape latency. On the other hand, morphine markedly enhanced the number of shuttling responses and tended to decrease approach latency to stimulation in the late postinjection period, although it increased both approach and escape latencies in the early period. The results were discussed in terms of rewarding and punishing effects of CG stimulation and their underlying endogenous opioid-opiate receptor mechanism.
D000818:Animals; D001931:Brain Mapping; D004924:Escape Reaction / Q000502:physiology; D008297:Male; D009020:Morphine / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D010487:Periaqueductal Gray / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains; D011957:Receptors, Opioid / Q000502:physiology*; D012653:Self Stimulation / Q000187:drug effects / Q000502:physiology*
D011957:Receptors, Opioid; D009270:Naloxone; D009020:Morphine