Inescapable shock-induced potentiation of morphine analgesia in rats: involvement of opioid, GABAergic, and serotonergic mechanisms in the dorsal raphe nucleus

Exposure of rats to inescapable shock (IS) potentiated the analgesic response to a low dose (1 mg/kg) of morphine 24 hr later. This effect was blocked by naltrexone (10 micrograms), diazepam (5 micrograms), or 8-hydroxy-2-(di-n-propylamine)-tetralin (8-OH-DPAT; 1 microgram) microinjected into the dorsal raphe nucleus (DRN) 15 min before IS. When microinjected into the DRN at the time of tail-flick testing, 8-OH-DPAT also effectively prevented this effect. Further, intra-DRN administration of a beta-carboline mimicked the effects of IS, because rats treated with methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (1 microgram) and simply restrained displayed potentiated morphine analgesia 24 hr later. These data suggest that this phenomenon shares mechanisms in common with other effects of IS at the level of the DRN.