Microinjection of opioid antagonists into the substantia nigra reduces stress-induced eating in rats

Abstract

Stress produced by pinching the tail has been shown to cause satiated animals to eat and to display oral stereotypies. Endogenous opioids and central dopamine systems have been implicated in the mediation of these effects. In order to test the possibility that the substantia nigra (SN) might be involved, the amount of food intake and gnawing produced by mild tail pinch were assessed following bilateral microinjections of opioid antagonists into the SN. Evaluations of nociceptive thresholds were also conducted using tail flick and hot plate tests. Eating induced by tail pinch was reduced by microinjections of the non-selectve opioid antagonist naloxone (3, 10, 20 and 30 nmol) and by the μ-selective antagonist Cys2, Tyr3, Orn5, Pen7 Amide (CTOP) (1, 3, and 10 nmol). These effects on eating occured in the absence of effects of gnawing. κ- and δ-antagonists (10 nmol) had no effect on eating or gnawing. Naloxone did not alter either tail flick or hot-plate response latencies. The highest dose of CTOP increased response latency on the hot-plate test only. The results are interpreted as suggesting that the SN may be an important central site of action for opioid antagonists in reducing stress-induced eating. The possibility that the SN may be a central site mediating the effects of dopamine on this phenomenon is also discussed. © 1992.