Activation of the opioid and nonopioid hypoalgesic systems at the level of the brainstem and spinal cord: does a coulometric relation predict the emergence or form of environmentally induced hypoalgesia?

Prior research suggests that a coulometric relation (Intensity x Duration) determines whether an opioid or nonopioid hypoalgesic system is activated by afferent nociceptive information. Using a paradigm that generates a brainstem-mediated hypoalgesia on the tail-flick test, we found that a coulometric relation does not predict either the emergence or the form of shock-induced hypoalgesia in decerebrate rats. In fact, no evidence was obtained that the brainstem's opioid hypoalgesic system can be activated by ascending neurons. More severe shocks elicited hypoalgesia in spinalized rats. Although a coulometric relation did not predict the emergence of hypoalgesia in spinalized rats, shock severity did predict the form of the hypoalgesia; the least severe shocks elicited an opioid hypoalgesia, and the more severe shocks generated a nonopioid hypoalgesia. A similar pattern of data was observed in intact rats exposed to the least severe shock parameters.