Open elevated plus maze-induced antinociception in rats: a non-opioid type of pain inhibition?

CORNELIO, A. M. AND NUNES-DE-SOUZA, R. L. Open elevated plus maze-induced antinociception in rats: A non-opioid type of pain inhibition? PHYSIOL BEHAV 00 (0) 000-000, 2008. This study investigated whether antinociception induced by exposure to an open elevated plus-maze (oEPM: four open arms) (i) shows cross-tolerance to morphine (Exp. 1 and 2), (ii) is attenuated by repeated exposure to the oEPM (Exp. 3), (iii) is blocked by systemic treatment with naltrexone (Exp. 4), and (iv) is affected by adrenalectomy (Exp. 5) in rats. Animals were daily treated with morphine (M, 5 mg/kg, i.p.) or distilled water (DW) for 5 consecutive days (antinociceptive tolerance assessed by tail-flick test). Then, rats received formalin 2.5% injection (50 microl) into the right hind paw followed by M or DW injection and 25 min later, time spent licking the injected paw was recorded for 10 min (Exp. 1). Similar procedure was followed in Experiment 2, except that licking response was recorded during exposure to an oEPM or enclosed EPM (eEPM: four enclosed arms) in undrugged rats. Experiment 3 evaluated nociception in rats submitted to 1, 2, 3, 4 or 6 exposures to either eEPM or oEPM (formalin was injected only during the last exposure). Experiment 4 investigated the effects of naltrexone (2.5 mg/kg; s.c.) on nociception during eEPM or oEPM exposure. Nociception was also assessed during eEPM or oEPM exposure in sham and adrenalectomized rats (Exp. 5). The results shown that oEPM-induced antinociception (i) did not display cross-tolerance to morphine, (ii) was unchanged by at least 6 maze re-exposures, (iii) failed to be reversed by naltrexone, and (iv) was not prevented by adrenalectomy.