Inescapable Shock-Induced Potentiation of Morphine Analgesia in Rats: Sites of Action

Inescapable shock (IS) enhances analgesia to systemic morphine (MOR) 24 hr later. IS activates serotonin neurons in the dorsal raphe nucleus (DRN), rendering them hyperexcitable. These studies tested whether IS potentiates the analgesic effect of MOR microinjected in the DRN, as predicted by this hypothesis. To test site specificity, the effect of previous IS was examined on MOR microinjected lateral to the DRN and into 2 other sites that support MOR analgesia, the nucleus raphe magnus (NRM) and spinal cord. Twenty-four hours after IS, potentiated analgesia was observed after 0.5 μg MOR microinjected into, but not lateral to, the DRN. Potentiated analgesia was also observed after NRM (1.0 μg) and spinal cord (3.0 μg) MOR microinjections. These data suggest that IS-induced excitability changes within the DRN synergize with opiates microinjected in other analgesia areas and that this potentiates the responses to opiates 24 hr after IS.