Hypoalgesia elicited by a conditioned stimulus is blocked by a mu, but nota delta or a kappa, opioid antagonist injected into the rostral ventromedial medulla

The present study investigated the role of mu, delta, and kappa receptors w ithin the RVM in mediating expression of conditional hypoalgesia (CHA). Fiv e groups of rats with RVM cannulae were given daily sessions of paired or u npaired presentations of an auditory CS (white noise) and foot shock across three consecutive days. On the test day, rats in the Paired condition were injected with the mu antagonist CTAP, the delta antagonist naltrindole, th e kappa antagonist nor-BNI, or saline. Rats in the Unpaired condition were injected with saline. TFLs were measured before and after injections, as we ll as during and after presentations of the CS. The results showed that non e of the drugs affected baseline TFLs. During CS presentation, rats in the Paired condition injected with saline showed longer TFLs than those in the Unpaired condition given saline, confirming the presence of CHA. Expression of this response was blocked by CTAP, but was unaffected by naltrindole or nor-BNI. These results suggest that mu, but not delta or kappa, opioid rec eptors in the RVM mediate expression of CHA. (C) 1999 International Associa tion for the Study of Pain. Published by Elsevier Science B.V.