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Effect of the opiate antagonist naloxone on body temperature in rats
Life Sciences Vol . 17, pp . 927-932 Printed in the U.S .A .
Perqamon Press
EFFECT OF THE OPIATE ANTAGONIST NALO%ONE ON BODY TEMPERATURE IN RATS Avram Goldstein and Patricia J . Lowery Addiction Research Foundation, Palo Alto, California 94304 . (Received in final form Auqust 15, 1975)
The specific opiate antagonist naloxone was used to assess the hypothesis that an endogenous opioid playa a significant role in temperature regulation is the rat . Avery alight hypothermic effect was observed at a naloaone dose sufficient to block the opiate receptors. Eves under conditions of cold stress, the magnitude of the effect was so small ae to lend little support to the hypothesis . The identification of specific opiate receptors (1-4) and the demonstration of their regional distribution in brain (5 ,6) suggested the possibility of a naturally occurring ligand interacting with them (7) . A peptide with morphine-like pharmacologic actions has now been identified, both in brain tissue (8,9) and in pituitary gland (10,11) ; we call such a substance endorphin .l Its physiologic role remains obscure . Naloaone is considered to be a virtually "pure" narcotic antagonist, i.e ., it seems to occupy opiate receptors passively, thus blocking the effects of opiate agonista . Presumably, naloaone would also block the effects of endorphin, yet curiously, this drug appears to have no pharmacologic effects of its own, other than to block and reverse the actions of exogenous opiates (12-14) . This paradoz suggests that special conditions are required to activate the endorphin system ; under such conditions aaloxone effects might be demonstrable . Opiate agonieta, at low doses, cause hyperthermia in rate and certain other species (15-18) . It may be, therefore, that endorphin plays a role in normal thermoregulation, sustaining the body temperature in cool environments . Ia the eaperimenta reported here we examined the effects of naloxone at 23° and also at 2 °. Although the drug did cause significant hypothermia, the magnitude of the effect was so small as to seem almost negligible .
l This name was devised by Eric J. Simon from endo signifying endogenous (as contrasted with eaogenous) and orphine (a common suffis in the names of opioids like morphine and etorphine) . The final e ie dropped in conformity with the convention for nomenclature of natural peptides and biogenic amines . The name conveys the meaning "an endogenous substance with opioid activity", thereby describing the unique relationship to opiate receptors and the distinctive pharmacologic action .
927
928
Naloxone on Hody Temperature
Vol . 17, No . 6
Methode Male Wiatar rata (approa. 350 g) were housed at 23 ° (all temperatures given here are °C.) with free access to food and water for at least 24 hr before the start of an experiment . A 12-hr light cycle was in effect . Rectal temperatures were obtained by inserting a pre-calibrated thermistor probe (Yellw Springs Instrument Co .) 6 cm for 30 sec. The first temperature (Tl) was recorded at 5 p.m. Animals were marked with a number code, weighed, placed in individual cages, and either left at 23 ° or moved into a cold room The next morning at 9 a.m . temperatures were recorded again (T 2) and at 2° . each animal was injected subcutaaeously with either 0.9X saline solution or naloxone hydrochloride (10 mg/kg) , Assignments to the treatments were by means of a table of random numbers, maintaining equal sized groups (usually 4 or 8 rate per group) . Exactly 30 min later, temperatures were recorded again (T3), and the animals were discarded . Pilot trials at 30, 60, and 120 min had shown that the peak effect of naloxone occurred at 30 min. Each animal was handled only three times, ae described. Over a period of 5 months, seven experiments were carried out at 23° (N-44) and 13 at 2° (N~70) . In an additional set of three experiments at 2°, naloxone was compared with saline and with an equimolar dose of dextrallorphaa, the inert enantiomer of the antagonist levallorphaa . The purity of the naloxone hydrochloride was verified by NMR, by melting point, and by high performance liquid chromatography (better than 99 .SX pure) . These determinations were carried out by courtesy of Dr . J. I . DeGraw, Stanford Research Institute . By gas-liquid chromatography at least 99 .6X of the material was in a single peak with the characteristic retention time of nalosone (PharmChem Laboratories, Palo Alto) . Results The main data are presented in Fig . 1. The temperature change following injection (Td - T3 - T2) ie plotted as a function of the preceding overnight temperature change (TD ~ T2 - Tl) . The results for animals housed at 23 ° are given at the right, those at 2° at the left . The initial mean temperatures (T l) were virtually identical in the four groups -- 37 .5° to 37 .7° (s .e .m. + 0 .1 °) . The Td data were analyzed statistically using a covariance model to account for differences between eaperimente and between treatments, as well as the linear regression elope parameter. Differences between experiments were not significant . The regression of Td on TD was significant (P < 0,05) for saline and naloxone at both ambient temperatures, but the elope estimates were not significantly different from one another . The common elope estimate was approximately - 0.4 . Thus, the temperature increase after injection (and handling) was greater, the more the It ie interesttemperature had fallen (or the less it had risen) overnight. ing that at 23 ° the mean overnight change was a slight fall in body temperature (- 0.2 °), whereas under conditions of cold stress at 2 ° it was a slight rise (+ 0.1 °) . At both ambient temperatures naloxone partially but significantly (P < 0 .01) blocked the increase induced by the injection and handling, and this effect of naloxone was greater at the lower ambient temperature . The magnitude of the effect may be estimated from the y-aaie intercepts in Fig. 1. On the average, the differential effect of naloxone as compared with' saline solution amounted to - 0.2° at 23°, and - 0 .4° at 2° . Table 1 contains both a replication of the experiments at 2° and also a comparison with dextrallorphan . The naloxone and saline effects are consis-
Vol . 17, No . 6
Naloxone on Body Temperature
Td
Td
0 0 00
929
~ +LO : T ° o"o
~ °~
a
0 0 0 om o
+ ~0
0
w. .o,,
o"
TD 0
FIG. 1 Effects of saline solution and naloxoae on body temperature in rate . On ~axie, TD is temperature change overnight before injection (- TZ - T1) . All Tl were measured at 23°. T2 were measured after 16 hr (overnight) at either 23 ° (right panel) or 2 ° (left panel) . On y-axis, Td is temperature change in 30 min after e.c . injection of 0 .9X saline or naloxone hydrochloride (10 mg/kg) (- T3 - T2 ) . Each animal is represented by a single data point . Open circles ~ saline ; solid circles ~ naloxone . The beat-fit lines with common elope (- 0 .4 ) are plotted ; there were no significant differences between elope estimates. Broken lines - saline ; solid lines ~ naloxone .
TABLE 1 Effects of Saline Solution, Nalozone, and Dextrallorphaa on Body Temperature of Rate Maintained for 16 Hours at 2 ° . Mean Td
S.E .M.
Treatment
N
Saline
12
+ 0 .35
0 .17
Naloxone
12
- 0.08
0.09
Dextrallorphaa
12
+ 0.12
0 .09
Blocks of four animals were assigned randomly to each of the groups in three separate experiments . Data are mean temperature differences (Td) in 30 min after a single s .c . injection of 10 mg/kg naloxone hydrochloride, a molar equivalent dose of dextrallorphan hydrochloride, or 0.9Z NaCl .
930
Naloxone on Body Temperature
Vol. 17, No . 6
tent with those obtained earlier. Deatrallorphan produced a smaller temperature increase than did saline and a lesser effect than did naloxone, but the differences were not statistically significant (P > 0.05) .
Diecuseion When individually housed rate were removed from their cages and injected with saline solution, a small increase in body temperature occurred in the next 30 minutes. Cold stress for 16 hours resulted in a net increase of body temperature ae compared with maintenance at room temperature, i.e ., there seemed to be a slight overshoot in temperature homeostasis . Saline injection caused a slight temperature increase in these animals too. At both 23° and at 2° nalozone opposed the temperature rise produced by saline injection, and its hypothermic effect was greater at the lower temperature . However, the magnitude of the naloxone effect was exceedingly small, only a few tenths of a degree . Dextrallorphan also tended to oppose the saline hypothhermia, but its effect was even smaller than that of naloxone . Theré seems little doubt that the hypothermic effect of nalozone, small though it is, is real . The demonstrated purity of the compound makes it seem unlikely that the action is due to a contaminant . However, the fact that dextrallorphan, which certainly does not block opiate receptors, produced a very small effect of the same kind, makes it difficult to conclude unequivocally that naloaone hypothermia is due to a specific blockade of an endorphin ftmction . Moreover, the dose of nalo~oone used here (10 mg/kg s .c .) is sufficient to block the effects of quite large doses of opiates in the rat (23), while concentrations in the nanomolar range (approximately 10 u8/L " ) are sufficient to nearly saturate opiate receptors from rat brain (19) . The results obtained here give little support to the hypothesis that as endorphin plays an important role in the maintenance of body temperature . On the other hand, a role of endorphin in modulating pain has been suggested by the finding that naloxone blocks analgesia produced by electrical stimulation in the rat midbrain (20), and that it lowers the threshold to noxious stimuli in mice (21) . In humane, naloxone is reported to antagonize acupuncture analgesia (Mayer, D.J ., Price, D .D . and Raffi, A., personal communication), but not hypnotic analgesia (22) . Further investigation using naloxane as a tool to explore other functions of endorphin would seem to be worthwhile . Acknowledgements Naloxone hydrochloride was a gift of Endo Laboratories, Inc. Dextrallorphan (crude base) was a gift of Hoffmann-LaRoche Inc. We thank Dr . J .I . DeGraw for some of the naloxone analyses, and William Horns for carrying out the statistical procedures . This investigation was supported by grant DA-00972 from the National Institute on Drug Abuse. References 1. 2. 3. 4. 5.
A. GOLDSTEIN, L .I . LOWNEY, and B.K . PAL, Proc . Nat . Acad . Sci. U .S .A . 68, 1742-1747 (1971) . C .B . PERT and S.H . SNYDER, Science 179, 1011-1014 (1973) . E .J . SIMON, J .M . HILLER, and I. EDELMAN, Proc . Nat . Acad . Sci. U .S .A . 70, 1947-1949 (1973) . L.I . LOSiNEY, K. SCHULZ, P .J . LOWERY, and A. GOLDSTEIN, Science 183, 749753 (1974) . M.J . KUHAR, C .B . PERT, and S .H . SNYDER, Nature 245, 447-450 (1973) .
Vol . 17, No . 6 6. 7.
8. 9. 10 . 11 . 12 . 13 . 14 . 15 . 16 . 17 . 18 . 19 . 20 . 21 . 22 . 23 .
Naloxone on Body Temperature
931
J.M. HILLER, J. PEARSON, and E.J . SIMON, Res . Commun . Chem . Pathol . Pharmacol. 6, 1052-1062 (1973) . A. GOLDSTEIN, in Biological and Behavorial Approaches to Drug Dependence, eds. H. Cappel and A.E . LeBlanc . Proc . International Symposia on Alcohol and Drug Research, Toronto, October 23-25, 1973, Addiction Research Foundation, Toronto . J. HUGHES, Brain Res . 88, 295-308 (1975) . J . HUGHES, T . SMITH, B. MORGAN, and L . FOTHERGILL, Life Sci. 16, 17531758 (1975) . H . TESCHEMACHER, K.E . OPHEIM, B .M . COX, and A. GOLDSTEIN, Life Sci . 16, 1771-1775 (1975) . B .M . COX, K.E . OPHEIM, H . TESCHEMACHER, and A. GOLDSTEIN, Life Sci . 16, 1777-1782 (1975) . M.C . BRAUDE, L .S . HARRIS, E.L . MAY, J .P . SMITH, and J.E . VILLARREAL, eds . Narcotic Antagonists . Adv . Biochem. Psychopharmacol ., Vol. 8, Raven Press, New York (1973) . H .W . KOSTERLITZ and A.J . WATT, Brit . J. Pharmacol . 33, 266-276 (1968) . D.R . JASINSRI, W .R. MARTIN, and C.A . HAERTZEN, J. Pharmacol . Exp . Ther . 157, 420-426 (1967) . M. SHARKAWI, Brit . J . Pharmacol. 44, 544-548 (1972) . L .-M. GUNNE, Arch . In t . Pharmacodyn. 129, 416-428 (1960) . V.J . LOTTI, in the Pharmacology of Thermoregulation, E. Schonbaum and P . Lomaa, eds ., pp . 382-394, S . Karger, Basel (1973) . J.B . HERMANN, J. Pharmacol . Exp. Ther . 76, 309-315 (1942) . C.B . PERT and S.H . SNYDER, Proc . Nat . Acad . Sci . U .S .A . 70, 2243-2247 (1973) . H . AKIL, D .J . MAYER, and J .C . LIEBESKIND, Comptes Rend . Acad . Sci . D274, 3603-3605 (1972) . J.J . JACOB, E .C . TREMBLAY, and M. COLOMBEL, Psychopharmacologia 37, 217223 (1974) . A. GOLDSTEIN and E .R . HILGARD, Proc . Nat . Acad . Sci. U .S .A . 72, 20412043 (1975) . H. SLUMBERG and H .B . DAYTON, in Aganist and Antagonist Actions of Narcotic Analgesic Drugs, H.W . Kosterlitz, H.O .J . Collier and J.E . Villarreal, eds., pp . 110-119, University Park Press, Baltimore (1973) .
Perqamon Press
EFFECT OF THE OPIATE ANTAGONIST NALO%ONE ON BODY TEMPERATURE IN RATS Avram Goldstein and Patricia J . Lowery Addiction Research Foundation, Palo Alto, California 94304 . (Received in final form Auqust 15, 1975)
The specific opiate antagonist naloxone was used to assess the hypothesis that an endogenous opioid playa a significant role in temperature regulation is the rat . Avery alight hypothermic effect was observed at a naloaone dose sufficient to block the opiate receptors. Eves under conditions of cold stress, the magnitude of the effect was so small ae to lend little support to the hypothesis . The identification of specific opiate receptors (1-4) and the demonstration of their regional distribution in brain (5 ,6) suggested the possibility of a naturally occurring ligand interacting with them (7) . A peptide with morphine-like pharmacologic actions has now been identified, both in brain tissue (8,9) and in pituitary gland (10,11) ; we call such a substance endorphin .l Its physiologic role remains obscure . Naloaone is considered to be a virtually "pure" narcotic antagonist, i.e ., it seems to occupy opiate receptors passively, thus blocking the effects of opiate agonista . Presumably, naloaone would also block the effects of endorphin, yet curiously, this drug appears to have no pharmacologic effects of its own, other than to block and reverse the actions of exogenous opiates (12-14) . This paradoz suggests that special conditions are required to activate the endorphin system ; under such conditions aaloxone effects might be demonstrable . Opiate agonieta, at low doses, cause hyperthermia in rate and certain other species (15-18) . It may be, therefore, that endorphin plays a role in normal thermoregulation, sustaining the body temperature in cool environments . Ia the eaperimenta reported here we examined the effects of naloxone at 23° and also at 2 °. Although the drug did cause significant hypothermia, the magnitude of the effect was so small as to seem almost negligible .
l This name was devised by Eric J. Simon from endo signifying endogenous (as contrasted with eaogenous) and orphine (a common suffis in the names of opioids like morphine and etorphine) . The final e ie dropped in conformity with the convention for nomenclature of natural peptides and biogenic amines . The name conveys the meaning "an endogenous substance with opioid activity", thereby describing the unique relationship to opiate receptors and the distinctive pharmacologic action .
927
928
Naloxone on Hody Temperature
Vol . 17, No . 6
Methode Male Wiatar rata (approa. 350 g) were housed at 23 ° (all temperatures given here are °C.) with free access to food and water for at least 24 hr before the start of an experiment . A 12-hr light cycle was in effect . Rectal temperatures were obtained by inserting a pre-calibrated thermistor probe (Yellw Springs Instrument Co .) 6 cm for 30 sec. The first temperature (Tl) was recorded at 5 p.m. Animals were marked with a number code, weighed, placed in individual cages, and either left at 23 ° or moved into a cold room The next morning at 9 a.m . temperatures were recorded again (T 2) and at 2° . each animal was injected subcutaaeously with either 0.9X saline solution or naloxone hydrochloride (10 mg/kg) , Assignments to the treatments were by means of a table of random numbers, maintaining equal sized groups (usually 4 or 8 rate per group) . Exactly 30 min later, temperatures were recorded again (T3), and the animals were discarded . Pilot trials at 30, 60, and 120 min had shown that the peak effect of naloxone occurred at 30 min. Each animal was handled only three times, ae described. Over a period of 5 months, seven experiments were carried out at 23° (N-44) and 13 at 2° (N~70) . In an additional set of three experiments at 2°, naloxone was compared with saline and with an equimolar dose of dextrallorphaa, the inert enantiomer of the antagonist levallorphaa . The purity of the naloxone hydrochloride was verified by NMR, by melting point, and by high performance liquid chromatography (better than 99 .SX pure) . These determinations were carried out by courtesy of Dr . J. I . DeGraw, Stanford Research Institute . By gas-liquid chromatography at least 99 .6X of the material was in a single peak with the characteristic retention time of nalosone (PharmChem Laboratories, Palo Alto) . Results The main data are presented in Fig . 1. The temperature change following injection (Td - T3 - T2) ie plotted as a function of the preceding overnight temperature change (TD ~ T2 - Tl) . The results for animals housed at 23 ° are given at the right, those at 2° at the left . The initial mean temperatures (T l) were virtually identical in the four groups -- 37 .5° to 37 .7° (s .e .m. + 0 .1 °) . The Td data were analyzed statistically using a covariance model to account for differences between eaperimente and between treatments, as well as the linear regression elope parameter. Differences between experiments were not significant . The regression of Td on TD was significant (P < 0,05) for saline and naloxone at both ambient temperatures, but the elope estimates were not significantly different from one another . The common elope estimate was approximately - 0.4 . Thus, the temperature increase after injection (and handling) was greater, the more the It ie interesttemperature had fallen (or the less it had risen) overnight. ing that at 23 ° the mean overnight change was a slight fall in body temperature (- 0.2 °), whereas under conditions of cold stress at 2 ° it was a slight rise (+ 0.1 °) . At both ambient temperatures naloxone partially but significantly (P < 0 .01) blocked the increase induced by the injection and handling, and this effect of naloxone was greater at the lower ambient temperature . The magnitude of the effect may be estimated from the y-aaie intercepts in Fig. 1. On the average, the differential effect of naloxone as compared with' saline solution amounted to - 0.2° at 23°, and - 0 .4° at 2° . Table 1 contains both a replication of the experiments at 2° and also a comparison with dextrallorphan . The naloxone and saline effects are consis-
Vol . 17, No . 6
Naloxone on Body Temperature
Td
Td
0 0 00
929
~ +LO : T ° o"o
~ °~
a
0 0 0 om o
+ ~0
0
w. .o,,
o"
TD 0
FIG. 1 Effects of saline solution and naloxoae on body temperature in rate . On ~axie, TD is temperature change overnight before injection (- TZ - T1) . All Tl were measured at 23°. T2 were measured after 16 hr (overnight) at either 23 ° (right panel) or 2 ° (left panel) . On y-axis, Td is temperature change in 30 min after e.c . injection of 0 .9X saline or naloxone hydrochloride (10 mg/kg) (- T3 - T2 ) . Each animal is represented by a single data point . Open circles ~ saline ; solid circles ~ naloxone . The beat-fit lines with common elope (- 0 .4 ) are plotted ; there were no significant differences between elope estimates. Broken lines - saline ; solid lines ~ naloxone .
TABLE 1 Effects of Saline Solution, Nalozone, and Dextrallorphaa on Body Temperature of Rate Maintained for 16 Hours at 2 ° . Mean Td
S.E .M.
Treatment
N
Saline
12
+ 0 .35
0 .17
Naloxone
12
- 0.08
0.09
Dextrallorphaa
12
+ 0.12
0 .09
Blocks of four animals were assigned randomly to each of the groups in three separate experiments . Data are mean temperature differences (Td) in 30 min after a single s .c . injection of 10 mg/kg naloxone hydrochloride, a molar equivalent dose of dextrallorphan hydrochloride, or 0.9Z NaCl .
930
Naloxone on Body Temperature
Vol. 17, No . 6
tent with those obtained earlier. Deatrallorphan produced a smaller temperature increase than did saline and a lesser effect than did naloxone, but the differences were not statistically significant (P > 0.05) .
Diecuseion When individually housed rate were removed from their cages and injected with saline solution, a small increase in body temperature occurred in the next 30 minutes. Cold stress for 16 hours resulted in a net increase of body temperature ae compared with maintenance at room temperature, i.e ., there seemed to be a slight overshoot in temperature homeostasis . Saline injection caused a slight temperature increase in these animals too. At both 23° and at 2° nalozone opposed the temperature rise produced by saline injection, and its hypothermic effect was greater at the lower temperature . However, the magnitude of the naloxone effect was exceedingly small, only a few tenths of a degree . Dextrallorphan also tended to oppose the saline hypothhermia, but its effect was even smaller than that of naloxone . Theré seems little doubt that the hypothermic effect of nalozone, small though it is, is real . The demonstrated purity of the compound makes it seem unlikely that the action is due to a contaminant . However, the fact that dextrallorphan, which certainly does not block opiate receptors, produced a very small effect of the same kind, makes it difficult to conclude unequivocally that naloaone hypothermia is due to a specific blockade of an endorphin ftmction . Moreover, the dose of nalo~oone used here (10 mg/kg s .c .) is sufficient to block the effects of quite large doses of opiates in the rat (23), while concentrations in the nanomolar range (approximately 10 u8/L " ) are sufficient to nearly saturate opiate receptors from rat brain (19) . The results obtained here give little support to the hypothesis that as endorphin plays an important role in the maintenance of body temperature . On the other hand, a role of endorphin in modulating pain has been suggested by the finding that naloxone blocks analgesia produced by electrical stimulation in the rat midbrain (20), and that it lowers the threshold to noxious stimuli in mice (21) . In humane, naloxone is reported to antagonize acupuncture analgesia (Mayer, D.J ., Price, D .D . and Raffi, A., personal communication), but not hypnotic analgesia (22) . Further investigation using naloxane as a tool to explore other functions of endorphin would seem to be worthwhile . Acknowledgements Naloxone hydrochloride was a gift of Endo Laboratories, Inc. Dextrallorphan (crude base) was a gift of Hoffmann-LaRoche Inc. We thank Dr . J .I . DeGraw for some of the naloxone analyses, and William Horns for carrying out the statistical procedures . This investigation was supported by grant DA-00972 from the National Institute on Drug Abuse. References 1. 2. 3. 4. 5.
A. GOLDSTEIN, L .I . LOWNEY, and B.K . PAL, Proc . Nat . Acad . Sci. U .S .A . 68, 1742-1747 (1971) . C .B . PERT and S.H . SNYDER, Science 179, 1011-1014 (1973) . E .J . SIMON, J .M . HILLER, and I. EDELMAN, Proc . Nat . Acad . Sci. U .S .A . 70, 1947-1949 (1973) . L.I . LOSiNEY, K. SCHULZ, P .J . LOWERY, and A. GOLDSTEIN, Science 183, 749753 (1974) . M.J . KUHAR, C .B . PERT, and S .H . SNYDER, Nature 245, 447-450 (1973) .
Vol . 17, No . 6 6. 7.
8. 9. 10 . 11 . 12 . 13 . 14 . 15 . 16 . 17 . 18 . 19 . 20 . 21 . 22 . 23 .
Naloxone on Body Temperature
931
J.M. HILLER, J. PEARSON, and E.J . SIMON, Res . Commun . Chem . Pathol . Pharmacol. 6, 1052-1062 (1973) . A. GOLDSTEIN, in Biological and Behavorial Approaches to Drug Dependence, eds. H. Cappel and A.E . LeBlanc . Proc . International Symposia on Alcohol and Drug Research, Toronto, October 23-25, 1973, Addiction Research Foundation, Toronto . J. HUGHES, Brain Res . 88, 295-308 (1975) . J . HUGHES, T . SMITH, B. MORGAN, and L . FOTHERGILL, Life Sci. 16, 17531758 (1975) . H . TESCHEMACHER, K.E . OPHEIM, B .M . COX, and A. GOLDSTEIN, Life Sci . 16, 1771-1775 (1975) . B .M . COX, K.E . OPHEIM, H . TESCHEMACHER, and A. GOLDSTEIN, Life Sci . 16, 1777-1782 (1975) . M.C . BRAUDE, L .S . HARRIS, E.L . MAY, J .P . SMITH, and J.E . VILLARREAL, eds . Narcotic Antagonists . Adv . Biochem. Psychopharmacol ., Vol. 8, Raven Press, New York (1973) . H .W . KOSTERLITZ and A.J . WATT, Brit . J. Pharmacol . 33, 266-276 (1968) . D.R . JASINSRI, W .R. MARTIN, and C.A . HAERTZEN, J. Pharmacol . Exp . Ther . 157, 420-426 (1967) . M. SHARKAWI, Brit . J . Pharmacol. 44, 544-548 (1972) . L .-M. GUNNE, Arch . In t . Pharmacodyn. 129, 416-428 (1960) . V.J . LOTTI, in the Pharmacology of Thermoregulation, E. Schonbaum and P . Lomaa, eds ., pp . 382-394, S . Karger, Basel (1973) . J.B . HERMANN, J. Pharmacol . Exp. Ther . 76, 309-315 (1942) . C.B . PERT and S.H . SNYDER, Proc . Nat . Acad . Sci . U .S .A . 70, 2243-2247 (1973) . H . AKIL, D .J . MAYER, and J .C . LIEBESKIND, Comptes Rend . Acad . Sci . D274, 3603-3605 (1972) . J.J . JACOB, E .C . TREMBLAY, and M. COLOMBEL, Psychopharmacologia 37, 217223 (1974) . A. GOLDSTEIN and E .R . HILGARD, Proc . Nat . Acad . Sci. U .S .A . 72, 20412043 (1975) . H. SLUMBERG and H .B . DAYTON, in Aganist and Antagonist Actions of Narcotic Analgesic Drugs, H.W . Kosterlitz, H.O .J . Collier and J.E . Villarreal, eds., pp . 110-119, University Park Press, Baltimore (1973) .