INESCAPABLE SHOCK-INDUCED POTENTIATION OF MORPHINE ANALGESIA IN RATS - INVOLVEMENT OF OPIOID, GABAERGIC, AND SEROTONERGIC MECHANISMS IN THEDORSAL RAPHE NUCLEUS

Exposure of rats to inescapable shock (IS) potentiated the analgesic r esponse to a low dose (1 mg/kg) of morphine 24 hr later This effect wa s blocked by naltrexone (10 mu g), diazepam (5 mu g), or 8-hydroxy-2-( di-n-propylamine)-tetralin (8-OH-DPAT; 1 mu g) microinjected into the dorsal raphe nucleus (DRN) 15 min before IS. When microinjected into t he DRN at the time of tail-flick testing, 8-OH-DPAT also effectively p revented this effect. Further, intra-DRN administration of a beta-carb oline mimicked the effects of IS, because rats treated with methyl 6,7 -dimethoxy-4-ethyl-beta-carboline-3-carboxylate (1 mu g) and simply re strained displayed potentiated morphine analgesia 24 hr later. These d ata suggest that this phenomenon shares mechanisms in common with othe r effects of IS at the level of the DRN.