342
Brain Research, 539 (1991) 342-346
Elsevier BRES 24498
a 2 Receptors
the entinociceptive action of 8-OHhot-plate test in mice
AT in the
Mark J. Millan and Francis C. Colpaert Neurobiology Division, FONDAX, Groupe de Recherche Servier, Paris (France)
(Accepted 16 October 1990) Key words: Serotonin; 8-OH-DPAT; ldazoxan; Antinociception; Pain; Hot plate; Mouse
The prototypical 5-HTIA agonist, 8-OH-DPAT, dose-dependently (0.16-10.0 mg/kg, s.c.) elicited a pronounced antinociception in the hot-plate test in mice. This action was not affected by the 5-HTIA antagonists, BMY 7378, (-)-pindolol and (-)-alprenoiol nor by selective antagonists at 5-HT~c, 5-HT2 and 5-HT3 receptors. It was also resistant to antagonists at D], D 2, ct1 and opioid receptors. In contrast, it was blocked by the a 2 antagonists, idazoxan, rauwolscine and yohimbine. L 659,066, a selective a 2 antagonist which does not enter the CNS, was ineffective. The action of 8-OH-DPAT was mimicked by the centrally acting ctz agonists, UK 14,304 and guanabenz whereas ST 91, which does not penetrate the blood-brain barrier, was inactive. The action of UK 14,304 and guanabenz was also blocked by idazoxan, rauwolscine and yohimbine but not by L 659,066. These data indicate that the antinociceptive properties of 8-OH-DPAT in the hot-plate test in mice are mediated by CNS-localized a 2 receptors, rather than 5-HT~A receptors.
The discovery of a multiplicity of serotonin (5-HT) receptors in the CNS of animals and man has encouraged the development of many ligands interacting selectively with particular 5-HT receptor subtypes 22"28. The aminotetralin, 8-OH-DPAT, has emerged of great importance in characterizing 5-HTIA receptors 21"zz. 8 - O H - D P A T shows very high affinity for 5-HT]A sites whereas it interacts with other 5-HT receptor subtypes (5-HTIB, 5-HTIc , 5-HT1D, 5-HT 2 and 5-HT3) only with much lower affinity 21'22'28. 8 - O H - D P A T acts as a potent agonist in several in vivo models of 5-HTIA activity, for example, enhancement of food intake, induction of hypothermia and facilitation of corticosterone secretion (for review, see ref. 12). An additional function in which 5-HT1A receptors have been implicated is the control of nociception: consistent with such a role, 5-HT1A receptors are located in the dorsal horn of the spinal cord wherein processing of primary afferent nociceptive information occurs 1°. However, their role in the control of nociception is still unclear since: first, electrophysiological data do not suggest a major antinociceptive role of spinal 5-HTIA receptors 14 and second, 8 - O H - D P A T and other 5-HTIA agonists have yielded a confusing pattern of test- and stimulus qualitydependent hyperaigesia, analgesia and no e f f e c t 1"2"13' 15.24-27. Further, in those cases where antinociception has been seen with 8-OH-DPAT, it has n 9t been shown by the use of appropriate antagonists that an activation of
5"HT1A receptors was responsible. This is important since 8 - O H - D P A T can exert actions at non-serotonergic receptors, notably a 2 receptors 4,5'9A1A7,23. Alpha 2 receptors are of particular relevance since, first, a 2 agonists elicit powerful antinociception 16"x9'29 and, second, there is evidence for an interaction between adrenergic and serotoninergic mechanisms for the induction of antinociception 6,27. In the present study, we examined the nature of the receptor type underlying the putative antinociceptive properties of 8 - O H - D P A T in the hot-plate test in mice. It is demonstrated that a 2 rather than 5-HTxA receptors mediate the action of 8 - O H - D P A T in this test. Male N M R I mice (Iffa Credo, IUskirchen, France) of 20-22 g were employed. They were housed in groups of 15 in sawdust-lined cages with ad lib access to chow and water. Laboratory temperature was 21 +_ 1 °C and humidity 60 + 5%. All experiments were performed between 9.30 and 12.30. The hot-plate (Ugo-Basile, Varese, Italy) consisted of a circular, copper slab of diameter 19 cm and surrounded by a perspex cylinder of height 18 cm. It was maintained at a temperature of 55.00 +_ 0.05 °C. Mice were gently placed on the centre of the hot-plate and the latency to lick either front paw recorded. The cut-off was 60 s. The influence of drugs administered alone was evaluated 30 min post-injection. For interaction studies, rats were pretreated 10 min previously with the antagonist; that is, 40 min prior to testing.
Correspondence: M.J. Millan, Neurobiology Division, FONDAX-Groupe De Recherche Servier, 7 rue Amp&e, 92800 Puteaux, Paris, France.
0(XJ6-8993/91/$03.50 © 1991 Elsevier Science Publishers B.V. (Biomedical Division)
343 Drugs were dissolved in distilled water, if necessary plus a few drops of lactic acid, and injected subcutaneously in a volume of 10.0 ml/kg. Drug doses are in terms of the base. Drug salts and sources were as follows. Haloperidol, R ( + ) - S C H 23390 HCl and (+)-8-OHD P A T HBr (Research Biochemicals, Natick, MA, U.S.A.), (-)-alprenolol (Sigma, Chesnes, France), (-)pindolol (Sandoz, Basle, Switzerland), ketanserin and ritanserin (Janssen Pharmaceutica, Beerse, Belgium), raciopride (Astra, Sjokerthal, Sweden), L 659,066 HCI (MSD, Montpelliers, France), ST 91 (Boehringer Ingelheim, Reims, France), prazosin HCI, corynanthine HCI and rauwolscine HCl (Extrasynthese, Genay, France). BMY 7378 HCI, ICS 205,930 HCI, G R 38032F and idazoxan HCI were synthesized by Servier chemists. Chemical formulae are as follows. R ( + ) - S C H 23390, 7-chloro-8-hydroxy-3-ketyl- 1-phenyl-2,3,4,5, tetrahydro1H-3-benzazepine HCI; (+)-8-OH-DPAT, 8-hydroxy-2(di)-n-propylamino) tetralin HBr; ST 91, 2-(2,6)-(diethylphenylimino)-2-imidazoline; ICS 205,930, 3"-(tropanyl) 1H-indol-3-carboxylic acid ester; BMY 7378, (8-[2-[4(2-methoxyphenyl) - 1 - piperazinyl]ethyl] - 8-azaspirol-[4]decane-7,9-dione) 2HC1; G R 38032E 1,2,3,9-tetrahydro9-methyl-3-carboxamine; L 659,066, (2R-trans)-N-(2-(1, 3,4,6,7,12b - hexahydro - 2 "-oxospiro(2H-benzofur(2,3-a)quinolizine-2,4-imidazolidin)-3"-yl)ethyl)methanesulfon_ amide). Data were analysed by one-way A N O V A followed by Newman-Keuls test: the level of significance was set at 0.05. In the presence of 8-OH-DPAT, F9,75 = 0.93, P > 0.05. Activity
Drug
Dose
Alone
+ 8-OH-DPAT
5-HT~, 5-HT~A 5-HT1A 5-HTlc/2 5-HT 3 D1 D2/D ~ D2 a~
vehicle BMY 7378 alprenolol pindolol ritanserin GR38032F SCH 23390 haloperidol raclopride prazosin
2.5 2.5 10.0 0.63 2.5 0.63 0.63 2.5 0.63
7.0 +_ 1.4 13.9 + 1.8 10.4 + 0.9 10.5 + 2.7 8.1 + 0.5 10.1 _+2.0 11.3 + 1.3 12.1 + 1.1 8.5 _+ 1.2 7.0 + 0.4
34.6 + 3.5 34.8 + 6.2 38.3 + 5.9 47.6 + 8.1 47.8 + 6.2 38.4 + 8.0 29.8 + 4.6 42.6 + 8.2 35.1 + 9.0 36.9 + 5.4
345 structure and, second, the action of idazoxan was mimicked by the non-imidazoline a 2 antagonists, rauwolscine and yohimbine. In fact, idazoxan, rauwolscine and yohimbine all have some affinity for 5-HTIA receptors s' 17,22. However, a direct action of these at 5-HT1A sites is unlikely in view of the inactivity of 5-HT1A antagonists themselves. In contrast to idazoxan, rauwolscine and yohimbine, L 659,066 did not modify the action of 8-OH-DPAT. Clearly, this pattern of data parallels that acquired with U K 14,304 and guanabenz. Thus, it can be concluded that CNS-localized t:t2 receptors similarly mediate the antinociceptive action of 8-OH-DPAT. It has been suggested that the non-selective 5-HT1/2 agonist, 5 - M e O D M T induces antinociception by initiating a release of noradrenaline which subsequently interacts with a 2 receptors 27. Although it cannot be excluded 2°, such a mechanism seems unlikely in the case of 8-OH-DPAT. First, B M Y 7378, (-)-alprenolol and (-)pindolol fail to inhibit the antinociceptive action of 8 - O H - D P A T : this suggests that 8 - O H - D P A T is not acting via 5-HTIA sites to induce a release of noradrenaline. Second, a 5-HTIA receptor-mediated action 8 - O H - D P A T inhibits rather than enhances the activity of noradrenergic neurones in the locus coeruleus, the origin of major ascending and descending projections involved in the mediation of antinociception 3. Rather, it is likely that 8 - O H - D P A T directly occupies ct2 sites. Indeed 8-OHD P A T does possess significant affinity at ct2 receptors,
1 Berge, O.-G., Post, C. and Archer, T., The behavioural pharmacology of serotonin in pain processes. In P. Bevan, A.R. Cools and T. Archer (Eds.), The Behavioural Pharmacology of 5-HT, Lawrence Earlbaum Associates, New Jersey, 1989, pp. 301-320. 2 Besson, J.-M., Serotonin and Pain, Excerpta Medica, Amsterdam 1990, 329 pp. 3 Bobker, D.H. and Williams, J.T., Serotonin agonists inhibit synaptic potentials in the rat locus ceruleus in vitro via 5-hydroxytryptamine 1A and 5-hydroxytryptamine 1B receptors, J. Pharmacol. Exp. Ther., 250 (1989) 37-43. 4 Brown, C.M., MacKinnon, A.C., McGrath, J.C., Spedding, M. and Kilpatrick, A.T., Heterogeneity of a2-receptors in rat cortex but not human platelets can be defined by 8-OH-DPAT, RU 24969 and methysergide, Br. J. Pharmacol., 99 (1990) 481-486. 5 Chaouloff, E and Jeanrenaud, B., 5-HT1A and alpha-2 adrenergic receptors mediate the hyperglycemic and hypoinsulinemic effects of 8-hydroxy-2-(di-n-propylamino) tetralin in the conscious rat, J. Pharmacol. Exp. Ther., 243 (1987) 1159-1166. 6 Clatworthy, A., Williams, J.H. and Barasi, S., Intrathecal 5-hydroxytryptamine and electrical stimulation of the nucleus raphe magnus in rats both reduce the antinociceptive potency of intrathecally administered noradrenaline, Brain Research, 455 (1988) 300-306. 7 Colpaert, EC., Effects of putative a-adrenoceptor antagonists and of other compounds on the loss of the righting reflex and on exophthalmia induced by xylazine in the rat, Drug Dev. Res., 7 (1987) 125-140. 8 Convents, A., De Keyser, J., De Backer, J. and Wauquelin, G., [3H]rauwolscine labels a 2 receptors and 5-HTIA receptors in human cerebral cortex, Eur. J. Pharmacol., 159 (1989) 307-310.
some 40- to 100-fold less than at 5-HTIA receptors 17'28. This difference in potency is consistent with the fact that the antinociceptive actions of 8 - O H - D P A T are seen in a higher dose-range range than required for 5-HTIAmediated actions12: for example, 8 - O H - D P A T elicits hypothermia and elevates circulating levels of corticosterone with an ADs0 of 0.05 and 0.06 mg/kg, respectively (Millan and Rivet, submitted). In conclusion, the present data demonstrate that the antinociception induced by 8 - O H - D P A T in the hot-plate test in mice is mediated by CNS-localized a 2 receptors. Thus, they support previous suggestions that 8-OHD P A T can exert actions in vivo via a 2 receptors. In the majority of studies, 8 - O H - D P A T has, as herein, behaved as an agonistS'll'17'23: however, there are findings 9 which suggest it might be a mixed 'agonist-antagonist'. Collectively, these data suggest that studies with appropriate antagonists should be undertaken prior to the attribution to 5-HT1A receptors of 8 - O H - D P A T - m e d i a t e d actions. Further, the data are consistent with recent arguments that 5-HTIA receptors may not play a major role in the mediation of acute antinociception and that the generally assumed-antinociceptive role of 5-HT and 5-HT receptors may require reexamination (see ref. 2 for current review). C. Grevoz is thanked for excellent technical work and V. Green for secretarial assistance.
9 Crist, J. and Suprenant, A., Evidence that 8-hydroxy-2-(ndipropylamino)tetralin (8-OH-DPAT) is a selective a~ antagonist on guinea-pig submucosa neurones, Br. J. Pharmacol., 92 (1987) 341-347. 10 Daval, G., Verge, D., Basbaum, A., Bourgoin, S. and Hamon, M., Autoradiographic evidence of serotonin binding sites on primary afferent fibres in the dorsal horn of the rat spinal cord, Neurosci. Lett., 83 (1987) 71-76. 11 Doods, H.N., Boddeke, H.W.G.M., Kalkman, H.O., Hoyer, D., Mathy, M.-J. and van Zwieten, P.A., Central 5-HT1A receptors and the mechanisms of the central hypotensive effect of (+)8-OH-DPAT, DP-5-CT, R 28935 and urapidil, J. Cardiovasc. Pharmacol., 11 (1988) 432-437. 12 Dourish, T.E, Hutson, P.H. and Ahlenius, S. (Eds.), Brain 5-HTIA Receptors, Chichester Press, Horwood, U.K., 1987, 427 PP. 13 Eide, P.K. and Hole, K., Subsensitivity of serotonin and substance P receptors involved in nociception after repeated administration of a serotonin receptor agonist, J. Neural Transm., 77 (1989) 1-10. 14 El-Yassir, N., Fleetwood-Waiker, S.M. and Mitchell, R., Heterogenous effects of serotonin in the dorsal horn of the rat: the involvement of 5-HT receptor subtypes, Brain Research, 456 (1988) 147-158. 15 Fasmer, O.B., Berge, O.G., Post, C. and Hole, K., Effects of the putative 5-HTIA agonist, 8-OH-DPAT, on nociceptive sensitivity in mice, Pharmacol. Biochem. Behav., 25 (1986) 883-888. 16 Fleetwood-Waiker, S.M., Mitchell, R., Hope, P.J., Molony, V. and Iggo, A., An ct2 receptor mediates the selective inhibition by noradrenaline of nociceptive responses of identified dorsal horn
346 neurones, Brain Research, 334 (1985) 243-254. 17 Fozard, J.R., Mir, A.K. and Middlemiss, D.N., Cardiovascular response to 8-hydroxy-2-(di-2-propylamino)tetralin (8OH-DPAT) in the rat: site of action and pharmacological analysis, J. Cardiovasc. Pharmacol., 9 (1987) 328-347. 18 Goldman, M.E., Pettibone, D.J., Reagan, J.E., Clineschmidt, B.V., Baldwin, J.J. and Huff, J.R., Blockade of peripheral a2-adrenoceptors by L 659,066 enhances glucose tolerance and insulin release in mice, Drug Dev. Res., 17 (1989) 141-151. 19 Hayes, A.G., Skingle, M. and Tyers, M.B., Alpha-adrenoceptot-mediated antinociception and sedation in the rat and dog, Neuropharmacology, 25 (1986) 391-396. 20 Heal, D.J., Prow, M.R., Martin, K.E and Buckett, W.R., The interaction of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) with a2-adrenoceptor mediated behavioural responses, Br. J. Pharmacol., 96 (1989) 306E 21 Hjorth, S., Carlsson, A., Lindberg, P., Sanchez, D., Wikstrom, D., Arvidsson, L.-E., Hacksell, U. and Nilsson, J.L.G., 8-Hydroxy(di-n-propylamino)tetralin, 8-OH-DPAT: a potent and selective simplified ergot congener with central 5-HT stimulating activity, J. Neural Transm., 55 (1982) 169-188. 22 Hoyer, D., Functional correlates of 5-HT 1 recognition sites, J. Rec. Res., 8 (1988) 59-81. 23 Marsden, C.A. and Martin, K.E, Involvement of 5-HTig-and
24
25
26
27
28
29
a:-receptors in the decreased 5-hydroxytryptamine release and metabolism in rat suprachiasmatic nucleus after intravenous 8-hydroxy-2-(di-n-propylamino)tetralin, Br. J. Pharmacol., 89 (1986) 277-286. Millan, M.J., Bervoets, K. and Colpaert, EC., Apparent hyperalgesic action of the 5-HTIA agonist, 8-OH-DPAT, in the rat reflects induction of spontaneous tail-flicks, Neurosci. Lett.. 107 (1989) 227-232. Millan, M.J. and Colpaert, EC., Attenuation of opioid-induced antinociception by 5-HTIA partial agonists in the rat, Neuropharmacology, 29 (1990) 315-318. Murphy, R.M. and Zemlan, E E , Selective serotonin 1A/1B agonists differentially affect spinal nociceptive reflexes, Neuropharmacology, 29 (1990) 463-468. Post, C. and Archer, T., Interactions between 5-HT and noradrenaline in analgesia. In J.-M. Besson (Ed.), Serotonin and Pain, Excerpta Medica, Amsterdam, 1990, pp. 153-174. Wijngaarden, I., Van Tulp, M.Th. and Soudijn, W., The concept of selectivity in 5-HT receptor research, Eur. J. Pharmacol., 188 (1990) 301-312. Yasuoka, S. and Yaksh, T.L., Effects on nociceptive threshold and blood pressure of intrathecally administered morphine and a-adrenoceptor agonists, Neuropharmacology, 22 (1983) 309315.
Brain Research, 539 (1991) 342-346
Elsevier BRES 24498
a 2 Receptors
the entinociceptive action of 8-OHhot-plate test in mice
AT in the
Mark J. Millan and Francis C. Colpaert Neurobiology Division, FONDAX, Groupe de Recherche Servier, Paris (France)
(Accepted 16 October 1990) Key words: Serotonin; 8-OH-DPAT; ldazoxan; Antinociception; Pain; Hot plate; Mouse
The prototypical 5-HTIA agonist, 8-OH-DPAT, dose-dependently (0.16-10.0 mg/kg, s.c.) elicited a pronounced antinociception in the hot-plate test in mice. This action was not affected by the 5-HTIA antagonists, BMY 7378, (-)-pindolol and (-)-alprenoiol nor by selective antagonists at 5-HT~c, 5-HT2 and 5-HT3 receptors. It was also resistant to antagonists at D], D 2, ct1 and opioid receptors. In contrast, it was blocked by the a 2 antagonists, idazoxan, rauwolscine and yohimbine. L 659,066, a selective a 2 antagonist which does not enter the CNS, was ineffective. The action of 8-OH-DPAT was mimicked by the centrally acting ctz agonists, UK 14,304 and guanabenz whereas ST 91, which does not penetrate the blood-brain barrier, was inactive. The action of UK 14,304 and guanabenz was also blocked by idazoxan, rauwolscine and yohimbine but not by L 659,066. These data indicate that the antinociceptive properties of 8-OH-DPAT in the hot-plate test in mice are mediated by CNS-localized a 2 receptors, rather than 5-HT~A receptors.
The discovery of a multiplicity of serotonin (5-HT) receptors in the CNS of animals and man has encouraged the development of many ligands interacting selectively with particular 5-HT receptor subtypes 22"28. The aminotetralin, 8-OH-DPAT, has emerged of great importance in characterizing 5-HTIA receptors 21"zz. 8 - O H - D P A T shows very high affinity for 5-HT]A sites whereas it interacts with other 5-HT receptor subtypes (5-HTIB, 5-HTIc , 5-HT1D, 5-HT 2 and 5-HT3) only with much lower affinity 21'22'28. 8 - O H - D P A T acts as a potent agonist in several in vivo models of 5-HTIA activity, for example, enhancement of food intake, induction of hypothermia and facilitation of corticosterone secretion (for review, see ref. 12). An additional function in which 5-HT1A receptors have been implicated is the control of nociception: consistent with such a role, 5-HT1A receptors are located in the dorsal horn of the spinal cord wherein processing of primary afferent nociceptive information occurs 1°. However, their role in the control of nociception is still unclear since: first, electrophysiological data do not suggest a major antinociceptive role of spinal 5-HTIA receptors 14 and second, 8 - O H - D P A T and other 5-HTIA agonists have yielded a confusing pattern of test- and stimulus qualitydependent hyperaigesia, analgesia and no e f f e c t 1"2"13' 15.24-27. Further, in those cases where antinociception has been seen with 8-OH-DPAT, it has n 9t been shown by the use of appropriate antagonists that an activation of
5"HT1A receptors was responsible. This is important since 8 - O H - D P A T can exert actions at non-serotonergic receptors, notably a 2 receptors 4,5'9A1A7,23. Alpha 2 receptors are of particular relevance since, first, a 2 agonists elicit powerful antinociception 16"x9'29 and, second, there is evidence for an interaction between adrenergic and serotoninergic mechanisms for the induction of antinociception 6,27. In the present study, we examined the nature of the receptor type underlying the putative antinociceptive properties of 8 - O H - D P A T in the hot-plate test in mice. It is demonstrated that a 2 rather than 5-HTxA receptors mediate the action of 8 - O H - D P A T in this test. Male N M R I mice (Iffa Credo, IUskirchen, France) of 20-22 g were employed. They were housed in groups of 15 in sawdust-lined cages with ad lib access to chow and water. Laboratory temperature was 21 +_ 1 °C and humidity 60 + 5%. All experiments were performed between 9.30 and 12.30. The hot-plate (Ugo-Basile, Varese, Italy) consisted of a circular, copper slab of diameter 19 cm and surrounded by a perspex cylinder of height 18 cm. It was maintained at a temperature of 55.00 +_ 0.05 °C. Mice were gently placed on the centre of the hot-plate and the latency to lick either front paw recorded. The cut-off was 60 s. The influence of drugs administered alone was evaluated 30 min post-injection. For interaction studies, rats were pretreated 10 min previously with the antagonist; that is, 40 min prior to testing.
Correspondence: M.J. Millan, Neurobiology Division, FONDAX-Groupe De Recherche Servier, 7 rue Amp&e, 92800 Puteaux, Paris, France.
0(XJ6-8993/91/$03.50 © 1991 Elsevier Science Publishers B.V. (Biomedical Division)
343 Drugs were dissolved in distilled water, if necessary plus a few drops of lactic acid, and injected subcutaneously in a volume of 10.0 ml/kg. Drug doses are in terms of the base. Drug salts and sources were as follows. Haloperidol, R ( + ) - S C H 23390 HCl and (+)-8-OHD P A T HBr (Research Biochemicals, Natick, MA, U.S.A.), (-)-alprenolol (Sigma, Chesnes, France), (-)pindolol (Sandoz, Basle, Switzerland), ketanserin and ritanserin (Janssen Pharmaceutica, Beerse, Belgium), raciopride (Astra, Sjokerthal, Sweden), L 659,066 HCI (MSD, Montpelliers, France), ST 91 (Boehringer Ingelheim, Reims, France), prazosin HCI, corynanthine HCI and rauwolscine HCl (Extrasynthese, Genay, France). BMY 7378 HCI, ICS 205,930 HCI, G R 38032F and idazoxan HCI were synthesized by Servier chemists. Chemical formulae are as follows. R ( + ) - S C H 23390, 7-chloro-8-hydroxy-3-ketyl- 1-phenyl-2,3,4,5, tetrahydro1H-3-benzazepine HCI; (+)-8-OH-DPAT, 8-hydroxy-2(di)-n-propylamino) tetralin HBr; ST 91, 2-(2,6)-(diethylphenylimino)-2-imidazoline; ICS 205,930, 3"-(tropanyl) 1H-indol-3-carboxylic acid ester; BMY 7378, (8-[2-[4(2-methoxyphenyl) - 1 - piperazinyl]ethyl] - 8-azaspirol-[4]decane-7,9-dione) 2HC1; G R 38032E 1,2,3,9-tetrahydro9-methyl-3-carboxamine; L 659,066, (2R-trans)-N-(2-(1, 3,4,6,7,12b - hexahydro - 2 "-oxospiro(2H-benzofur(2,3-a)quinolizine-2,4-imidazolidin)-3"-yl)ethyl)methanesulfon_ amide). Data were analysed by one-way A N O V A followed by Newman-Keuls test: the level of significance was set at 0.05. In the presence of 8-OH-DPAT, F9,75 = 0.93, P > 0.05. Activity
Drug
Dose
Alone
+ 8-OH-DPAT
5-HT~, 5-HT~A 5-HT1A 5-HTlc/2 5-HT 3 D1 D2/D ~ D2 a~
vehicle BMY 7378 alprenolol pindolol ritanserin GR38032F SCH 23390 haloperidol raclopride prazosin
2.5 2.5 10.0 0.63 2.5 0.63 0.63 2.5 0.63
7.0 +_ 1.4 13.9 + 1.8 10.4 + 0.9 10.5 + 2.7 8.1 + 0.5 10.1 _+2.0 11.3 + 1.3 12.1 + 1.1 8.5 _+ 1.2 7.0 + 0.4
34.6 + 3.5 34.8 + 6.2 38.3 + 5.9 47.6 + 8.1 47.8 + 6.2 38.4 + 8.0 29.8 + 4.6 42.6 + 8.2 35.1 + 9.0 36.9 + 5.4
345 structure and, second, the action of idazoxan was mimicked by the non-imidazoline a 2 antagonists, rauwolscine and yohimbine. In fact, idazoxan, rauwolscine and yohimbine all have some affinity for 5-HTIA receptors s' 17,22. However, a direct action of these at 5-HT1A sites is unlikely in view of the inactivity of 5-HT1A antagonists themselves. In contrast to idazoxan, rauwolscine and yohimbine, L 659,066 did not modify the action of 8-OH-DPAT. Clearly, this pattern of data parallels that acquired with U K 14,304 and guanabenz. Thus, it can be concluded that CNS-localized t:t2 receptors similarly mediate the antinociceptive action of 8-OH-DPAT. It has been suggested that the non-selective 5-HT1/2 agonist, 5 - M e O D M T induces antinociception by initiating a release of noradrenaline which subsequently interacts with a 2 receptors 27. Although it cannot be excluded 2°, such a mechanism seems unlikely in the case of 8-OH-DPAT. First, B M Y 7378, (-)-alprenolol and (-)pindolol fail to inhibit the antinociceptive action of 8 - O H - D P A T : this suggests that 8 - O H - D P A T is not acting via 5-HTIA sites to induce a release of noradrenaline. Second, a 5-HTIA receptor-mediated action 8 - O H - D P A T inhibits rather than enhances the activity of noradrenergic neurones in the locus coeruleus, the origin of major ascending and descending projections involved in the mediation of antinociception 3. Rather, it is likely that 8 - O H - D P A T directly occupies ct2 sites. Indeed 8-OHD P A T does possess significant affinity at ct2 receptors,
1 Berge, O.-G., Post, C. and Archer, T., The behavioural pharmacology of serotonin in pain processes. In P. Bevan, A.R. Cools and T. Archer (Eds.), The Behavioural Pharmacology of 5-HT, Lawrence Earlbaum Associates, New Jersey, 1989, pp. 301-320. 2 Besson, J.-M., Serotonin and Pain, Excerpta Medica, Amsterdam 1990, 329 pp. 3 Bobker, D.H. and Williams, J.T., Serotonin agonists inhibit synaptic potentials in the rat locus ceruleus in vitro via 5-hydroxytryptamine 1A and 5-hydroxytryptamine 1B receptors, J. Pharmacol. Exp. Ther., 250 (1989) 37-43. 4 Brown, C.M., MacKinnon, A.C., McGrath, J.C., Spedding, M. and Kilpatrick, A.T., Heterogeneity of a2-receptors in rat cortex but not human platelets can be defined by 8-OH-DPAT, RU 24969 and methysergide, Br. J. Pharmacol., 99 (1990) 481-486. 5 Chaouloff, E and Jeanrenaud, B., 5-HT1A and alpha-2 adrenergic receptors mediate the hyperglycemic and hypoinsulinemic effects of 8-hydroxy-2-(di-n-propylamino) tetralin in the conscious rat, J. Pharmacol. Exp. Ther., 243 (1987) 1159-1166. 6 Clatworthy, A., Williams, J.H. and Barasi, S., Intrathecal 5-hydroxytryptamine and electrical stimulation of the nucleus raphe magnus in rats both reduce the antinociceptive potency of intrathecally administered noradrenaline, Brain Research, 455 (1988) 300-306. 7 Colpaert, EC., Effects of putative a-adrenoceptor antagonists and of other compounds on the loss of the righting reflex and on exophthalmia induced by xylazine in the rat, Drug Dev. Res., 7 (1987) 125-140. 8 Convents, A., De Keyser, J., De Backer, J. and Wauquelin, G., [3H]rauwolscine labels a 2 receptors and 5-HTIA receptors in human cerebral cortex, Eur. J. Pharmacol., 159 (1989) 307-310.
some 40- to 100-fold less than at 5-HTIA receptors 17'28. This difference in potency is consistent with the fact that the antinociceptive actions of 8 - O H - D P A T are seen in a higher dose-range range than required for 5-HTIAmediated actions12: for example, 8 - O H - D P A T elicits hypothermia and elevates circulating levels of corticosterone with an ADs0 of 0.05 and 0.06 mg/kg, respectively (Millan and Rivet, submitted). In conclusion, the present data demonstrate that the antinociception induced by 8 - O H - D P A T in the hot-plate test in mice is mediated by CNS-localized a 2 receptors. Thus, they support previous suggestions that 8-OHD P A T can exert actions in vivo via a 2 receptors. In the majority of studies, 8 - O H - D P A T has, as herein, behaved as an agonistS'll'17'23: however, there are findings 9 which suggest it might be a mixed 'agonist-antagonist'. Collectively, these data suggest that studies with appropriate antagonists should be undertaken prior to the attribution to 5-HT1A receptors of 8 - O H - D P A T - m e d i a t e d actions. Further, the data are consistent with recent arguments that 5-HTIA receptors may not play a major role in the mediation of acute antinociception and that the generally assumed-antinociceptive role of 5-HT and 5-HT receptors may require reexamination (see ref. 2 for current review). C. Grevoz is thanked for excellent technical work and V. Green for secretarial assistance.
9 Crist, J. and Suprenant, A., Evidence that 8-hydroxy-2-(ndipropylamino)tetralin (8-OH-DPAT) is a selective a~ antagonist on guinea-pig submucosa neurones, Br. J. Pharmacol., 92 (1987) 341-347. 10 Daval, G., Verge, D., Basbaum, A., Bourgoin, S. and Hamon, M., Autoradiographic evidence of serotonin binding sites on primary afferent fibres in the dorsal horn of the rat spinal cord, Neurosci. Lett., 83 (1987) 71-76. 11 Doods, H.N., Boddeke, H.W.G.M., Kalkman, H.O., Hoyer, D., Mathy, M.-J. and van Zwieten, P.A., Central 5-HT1A receptors and the mechanisms of the central hypotensive effect of (+)8-OH-DPAT, DP-5-CT, R 28935 and urapidil, J. Cardiovasc. Pharmacol., 11 (1988) 432-437. 12 Dourish, T.E, Hutson, P.H. and Ahlenius, S. (Eds.), Brain 5-HTIA Receptors, Chichester Press, Horwood, U.K., 1987, 427 PP. 13 Eide, P.K. and Hole, K., Subsensitivity of serotonin and substance P receptors involved in nociception after repeated administration of a serotonin receptor agonist, J. Neural Transm., 77 (1989) 1-10. 14 El-Yassir, N., Fleetwood-Waiker, S.M. and Mitchell, R., Heterogenous effects of serotonin in the dorsal horn of the rat: the involvement of 5-HT receptor subtypes, Brain Research, 456 (1988) 147-158. 15 Fasmer, O.B., Berge, O.G., Post, C. and Hole, K., Effects of the putative 5-HTIA agonist, 8-OH-DPAT, on nociceptive sensitivity in mice, Pharmacol. Biochem. Behav., 25 (1986) 883-888. 16 Fleetwood-Waiker, S.M., Mitchell, R., Hope, P.J., Molony, V. and Iggo, A., An ct2 receptor mediates the selective inhibition by noradrenaline of nociceptive responses of identified dorsal horn
346 neurones, Brain Research, 334 (1985) 243-254. 17 Fozard, J.R., Mir, A.K. and Middlemiss, D.N., Cardiovascular response to 8-hydroxy-2-(di-2-propylamino)tetralin (8OH-DPAT) in the rat: site of action and pharmacological analysis, J. Cardiovasc. Pharmacol., 9 (1987) 328-347. 18 Goldman, M.E., Pettibone, D.J., Reagan, J.E., Clineschmidt, B.V., Baldwin, J.J. and Huff, J.R., Blockade of peripheral a2-adrenoceptors by L 659,066 enhances glucose tolerance and insulin release in mice, Drug Dev. Res., 17 (1989) 141-151. 19 Hayes, A.G., Skingle, M. and Tyers, M.B., Alpha-adrenoceptot-mediated antinociception and sedation in the rat and dog, Neuropharmacology, 25 (1986) 391-396. 20 Heal, D.J., Prow, M.R., Martin, K.E and Buckett, W.R., The interaction of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) with a2-adrenoceptor mediated behavioural responses, Br. J. Pharmacol., 96 (1989) 306E 21 Hjorth, S., Carlsson, A., Lindberg, P., Sanchez, D., Wikstrom, D., Arvidsson, L.-E., Hacksell, U. and Nilsson, J.L.G., 8-Hydroxy(di-n-propylamino)tetralin, 8-OH-DPAT: a potent and selective simplified ergot congener with central 5-HT stimulating activity, J. Neural Transm., 55 (1982) 169-188. 22 Hoyer, D., Functional correlates of 5-HT 1 recognition sites, J. Rec. Res., 8 (1988) 59-81. 23 Marsden, C.A. and Martin, K.E, Involvement of 5-HTig-and
24
25
26
27
28
29
a:-receptors in the decreased 5-hydroxytryptamine release and metabolism in rat suprachiasmatic nucleus after intravenous 8-hydroxy-2-(di-n-propylamino)tetralin, Br. J. Pharmacol., 89 (1986) 277-286. Millan, M.J., Bervoets, K. and Colpaert, EC., Apparent hyperalgesic action of the 5-HTIA agonist, 8-OH-DPAT, in the rat reflects induction of spontaneous tail-flicks, Neurosci. Lett.. 107 (1989) 227-232. Millan, M.J. and Colpaert, EC., Attenuation of opioid-induced antinociception by 5-HTIA partial agonists in the rat, Neuropharmacology, 29 (1990) 315-318. Murphy, R.M. and Zemlan, E E , Selective serotonin 1A/1B agonists differentially affect spinal nociceptive reflexes, Neuropharmacology, 29 (1990) 463-468. Post, C. and Archer, T., Interactions between 5-HT and noradrenaline in analgesia. In J.-M. Besson (Ed.), Serotonin and Pain, Excerpta Medica, Amsterdam, 1990, pp. 153-174. Wijngaarden, I., Van Tulp, M.Th. and Soudijn, W., The concept of selectivity in 5-HT receptor research, Eur. J. Pharmacol., 188 (1990) 301-312. Yasuoka, S. and Yaksh, T.L., Effects on nociceptive threshold and blood pressure of intrathecally administered morphine and a-adrenoceptor agonists, Neuropharmacology, 22 (1983) 309315.
