A comparison of the sites at which pentazocine and morphine act to produce analgesia

Abstract

Sites in the brain stem at which microinjected morphine can produce analgesia have been investigated for sensitivity to microinjections of pentazocine, which has been proposed to act at receptors different to those mediating the effects of morphine. Microinjection of 10 mgg of pentazocine into the periaqueductal grey matter (PAG), the nucleus reticularis gigantocellularis (NRGC) and nucleus reticularis paragigantocellularis (NRPG) produced analgesia as determined by the tail flick response to noxious heat. Microinjection of pentazocine into nucleus raphe magnus (NRM) did not produce any discernible change in the nociceptive threshold measured with the tail flick test. Using the pain pressure test, analgesia was observed following microinjections of pentazocine into NRGC and NRPG, but not following microinjections into PAG or NRM. Morphine (3 and 5 gmg) microinjected into PAG, NRM, NRGC or NRPG produced analgesia as determined by both heat and pressure tests. The analgesia produced by injection of pentazocine into the NRGC or NRPG was comparable to the analgesia produced by microinjection of 3 g mg of morphine into these areas. The analgesia produced by injection of pentazocine into PAG was significantly less than that produced by 3 gmg of morphine injected into PAG. Pretreatment with naloxone did not affect the analgesia produced by microinjection of pentazocine into NRGC or NRPG, but did antagonize the analgesia produced by injection of pentazocine into PAG. Naloxone blocked the analgesic effects of microinjected morphine. Analgesia produced by systemically given pentazocine was significantly reduced following microinjection of naloxone into PAG or NRM but not into NRGC or NRPG. The present data provide further evidence that the effects of pentazocine, a gk agonist drug may be mediated by mechanisms different to those mediating the action of morphine, a gm agonist.