ACTIVATION OF THE OPIOID AND NONOPIOID HYPOALGESIC SYSTEMS AT THE LEVEL OF THE BRAIN-STEM 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 Durat ion).determines whether an opioid or nonopioid hypoalgesic system is a ctivated by afferent nociceptive information. Using a paradigm that ge nerates a brainstem-mediated hypoalgesia on the tail-flick test, we fo und 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 syst em can be activated by ascending neurons. More severe shocks elicited hypoalgesia in spinalized rats. Although a coulometric relation did no t predict the emergence of hypoalgesia in spinalized rats, shock sever ity did predict the form of the hypoalgesia; the least severe shocks e licited an opioid hypoalgesia, and the more severe shocks generated a nonopioid hypoalgesia. A similar pattern of data was observed in intac t rats exposed to the least severe shock parameters.