Repetitive exposure to the hot-plate test produces stress induced analgesia and alters β-endorphin neuronal transmission within the periaqueductal gray of the rat

Repetitive exposure of rats to a hot plate induced a novel non-opioid form of stress induced analgesia. The exposure caused a persistent 1.5–2 s increase in tail flick latency which was not attenuated by systemic naltrexone, but was completely inhibited by systemic MK-801. Concomitantly, alterations occurred in the ability to pharmacologically distinguish multiple β-endorphin receptors in the periaqueductal gray. Thus, in response to different forms of stress, different pathways may be activated by β-endorphin, resulting in stress induced analgesias with varied pharmacological characteristics (e.g., opioid and non-opioid).