ABBREVIAT1ONS EOP, endogenous o#{231}ioki peptide; ME, methionk*.enkephaln; -EP, -endorphi; -EP1, ndorptn1; 11-50,48811, tra±)-3,4-dhloro-N-methy1-N-[2-(1-pyrrolldkiy1)cyclohexy1]-benzeneacetamIdemethane sultonate; NTX, naltrexone; CTOP, D-Phe-Cys-Tyr-o-Trp- Om-TIw-Pen-Tlw-NH2; NTI, naltrindole; nor-BNI, nor-binaftorphETilne; PAG, perlaqUedUCtal gray; CNS, central nervous system; l.c., Intracerebral. 596 THE Jouan.i. OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Copyright C 1994 by The American Society for Pharmamiogj and Experimental Therapeutics VoL 269, No.2 Printed in USA Antagonism of Nitrous Oxide Antinociception in the Rat Hot Plate Test by Site-Specific Mu and Epsilon Opioid Receptor Blockade13 BRADLEY L. HODGES,4 MICHAEL J. GAGNON, THOMAS A. GILLESPIE,45 JEFFREY R. BRENEISEN,6 DAVID F. O’LEARY,7 SHUICHI HARA8 and RAYMOND M. QUOCK D,artment of Biomedical Sciences, University of Illinois College of Medicine at Rockford, Rockford, Illinois Accepted for publication January 31 , 1994 ABSTRACT The an&gesic property of the anesthetic gas N20 has long been known and used to treat pain in clinical medicine and dentistry. The present study was cOndUCted to ldenttfy by subtype and possible location the brain opiold receptors that mediate N2O antinociception in rats.A 5-mm exposure to 70% N20 consist- ently evoked an antinodceptive effect in the hot plate test. This drug effect was partly antagonized in dose-related fashion by i.c.v. pretreatment with naltrexone, D-Phe-Cys-Tyr-D-Trp-Om- Thr-Pen-Thr-NH2 and -endorphin1., which block multiple, mu and epsilon oc1 receptors, respectively. However, the N20- evoked antinociception was unaffected by i.c.v. pretreatment with either the delta oploid antagonist naftrindole or the kappa oploid antagonist nor-binaltorphimine. When D-Phe-Cys-Tyr-D- Trp-Om-Thr-Pen-Thr-NH2 was administered intracerebrally di- recdy Into the pedequeductal gray, N20 antinociception was partly antagonized in a dose-dependent manner. The antinoci- captive response to N2O was uninfluenced by -endorphin1. administered into the petiaqueductal gray. The findings of these pharmacologicai antagonism studies are consistent with the hypothesis that exposure to N2O causes a neuronal release of $-endorphin. These results indicate that supraspinal mu and epsilon oc1 receptors medte N2O antinockeption in the rat hot plate paradIgm and that one central site of such mu but not epsilon oploid receptors is the periaqueductal gray. Subanesthetic concentrations of the anesthetic gas N20 pro- duce a prominent analgesic effect in both human subjects and experimental animals (Finck, 1985; Marshall and Longnecker, 1990). Inhalation of 20% N20 in 02 reportedly produces an Received for publication June 28, 1993. 1This research was supported by Public Health Service grant DE-06894. 2 ‘p.: was approved by the Biologic Resource Committee of the University of Illinois College of Medicine at Rockford and was conducted in accordance with the Guidefor the Care and Use ofliiborotoryAnimois as adopted and promulgated by the National Institutes of Health. I Preliminary reports of these results were presented at the 1992 annual meeting of the Federation of American Societies for Experimental Biology (O’Leary, et al., 1992), the 1992 fall meeting of the American Society for Phar- macology and Experimental Therapeutics (Hodges and Quock, 1992) and the Experimental Biology 93 Meeting (Hodges et aL, 1993). 4Present address: University oflllinois at Urbana-Champaign, Urbana-Chain- pair’, IL I Supported by a High School Student Summer Apprentice Fellowship from the Rockford Regional Academic-Industrial Consortium (RAIC, Rockford, IL). S Present addresaC David Grant U.S.A.F. MediCal Center, Travis Air Force Base, CA. 7 Supported by a High School Science Teacher Summer Research Fellowship from the Pierce Chemical Company (Rockford, IL). Present address: Crystal Lake Central High School, Crystal Lake, IL S Visiting Scientist from the DepartmentofForensic Medicine, Tokyo MediCal College, Tokyo, Japan. analgesic effect in human subjects approximately equivalent to that evoked by 15 mg of morphine sulfate (Chapman et aL, 1943; Parbrook et aL, 1964). The clinical literature includes reports of the specific application of N20 analgesia in dental surgery (Dworkin et aL, 1983; Hammond and Full, 1984; Smith and Beirne, 1985), in obstetrics (Marx and Bassell, 1985), in emergency medicine (Thai et at., 1979; Donen et at., 1982) and in the treatment of selected types of pain (Thompson and Locon, 1976; Fosburg and Crone, 1983; Henderson et aL, 1990). The mechanism of this analgesic effect of N20 has long been attributed to a nonspecific depresaion of CNS function (Son- nenschein et aL, 1948). An opioid mechanism was initially suggested by Berkowitz et aL (1976), who first reported that N20 antinociception in mice was sensitive to antagonism by naloxone, an opioid receptor antagonist. This finding was quickly verified by other studies, both in experimental animals (Smith and Bees, 1981; Lawrence and Livingston, 1981) and human subjects (Chapman and Benedetti, 1979; Giliman et aL, 1980; Yang et at., 1980). The discovery of opioid receptor multiplicity and the differ- ential sensitivity of these receptor subtypes to various opioid