Serotonin Receptors As Cardiovascular Targets Carlos M

Serotonin receptors as cardiovascular targets Carlos M. Villalh, Peter de Vries and Pramod R. Saxena

Serotonin exerts complex effects in the cardiovascular Classification and nomenclature of 5HT system, including hypotension or hypertension, vaso- receptors The first vital step towards characterizing 5-HT receptors dilatation or vasoconstriction, and/or bradycardia or was undoubtedly that undertaken in 1957 by Gaddum tachycardia; the eventual response depends primarily on and Picarelli’, who reported the existence of musculotropic the nature of the 5-HT receptors involved. In the light of ‘D’ and neurotropic ‘M’ receptors for 5-HT in the guinea- pig ileum. However, subsequent evidence showed that current 5-HT receptor classification, the authors re- some musculotropic responses to 5-HT could not be placed analyse the cardiovascular responses mediated by 5-HT within this schemex. Then, in 1979, Peroutka and Snyder receptors and discuss the established and potential challenged this classification, showing the existence of therapeutic applications of 5-HT ligands in the treatment 5-HT, (low affinity for [3H]spiperone) and 5-HT, (high affinity for [3Hlspiperone) binding sites in cerebral of some cardiovascular pathologies. membraness. The advent of ketanserinlo confirmed that the 5-HT, site corresponded to most ‘D’ receptors. The Serotonin Club Nomenclature Committee built upon erotonin (5-hydroxytryptamine, 5-HT), a biogenic this information and, on the basis of the conjunction of monoamine widely distributed in the animal and structural (amino acid sequence), transductional (receptor plant kingdoms, exerts complex effects in the coupling) and operational (drug-related) criteria, 5-HT S periphery and in the central nervous system. receptors are now categorized into five main type&-l3 (see Although the effects attributable to 5-HT have been known Table 1): since the 19th century l2, its physiological and pathophysio- logical importance, including its role in cardiovascular regu- l 5-HT,. which corresponds to some ‘D’ receptors and lation, remain unclear3.*. However, in many respects, 5-HT 5-HT, binding sites and can be subdivided into func- would seem an ideal neurohumoral transmitter candidate tional subtypes 5-HT,,, 5-HT,, (previously 5-HT,,$ and for the cardiovascular system. Secreted from enterochromaf- 5-HT,, (previously 5-HT,,,); fin cells into the blood, 5-HT is stored by platelets. During l 5-HT,, which corresponds to most ‘D’ receptors and aggregation, usually at a site of vascular injury, platelets 5-HT, binding sites and can be subdivided into 5-HTzh, release 5-HT, thus providing a unique mechanism for local 5-HT,, and 5-HT,, subtypes: delivery. Moreover, circulating 5-HT can be taken up by l 5-HT,, which is equivalent to ‘M’ receptors; sympathetic neurons and vascular endothelial cells and l 5-HT,; could, subsequently, be coreleased5,“. l 5-HT,.

Carlos M. Villalh, Department of Pharmacology,Faculty of Medicine and Health Sciences, ErasmusUniversity Rotterdam, PO Box 1738, 3000 DR Rotterdam, The Netherlands,and Secci6n de Terapbutica Experimental, Departamento de Farmacologiay Toxicologia, CINVESTAV, I.P.N., Apdo. Postal 22026, 14000 MEXICO D.F., MBxico. Peter de Vries and Pramod R. Saxena*, Department of Pharmacology, Faculty of Medicine and Health Sciences, Erasmus University Rotterdam, PO Box 1738, 3000 DR Rotterdam, The Netherlands. *tel: +31 10 408 7537/47, fax: +31 10 436 6839, e-mail: [email protected]

Receptor Agonists Antagonists Transduction Localization Function 5-HT,, 8-OH-DPAT WAY100135 f-1 Adenylate cyclase Raphe nucleus Autoreceptor, centrally mediated hypotension 5-HT, ac 5-CT>5-HT>sumatriptan GR127935 f-1 Adenylate cyclase Cranial blood vessels Vasoconstriction 5-HT, a~ 5-CTz5-HT>sumatriptan GRI 27935 (-1 Adenylate cyclase Presynaptic neurons Autoreceptor 5-ht,, 5-HT Methiothepin (-1 Adenylate cyclase Cortex Unknown 5-ht, r 5-HT, sumatriptan Methysergide (-1 Adenylate cyclase CNS, periphery (?I Vasoconstriction (?I, inhibition of plasma extravasation (?I 5-HT,, cr-Methyld-HT, DOI Ketanserin (+I Phospholipase C Smooth muscle, Contraction, platelets aggregation 5-HT,, a-Methyld-HT, DOI SB200646 (+) Phospholipase C Rat fundus, Contraction, release of endothelium of nitric oxide some blood vessels 5-HT,, a-MethylEHT, DOI Mesulergine (+I Phospholipase C Choroid plexus CSF production (?I 5-HT, 2-Methyld-HT; Ondansetron, Na+/K+ channel Peripheral nerves (+I Neuronal activity 5-MeOT inactive tropisetron

5-HT, 5-MeOT, renzapride GRI 13808 (+I Adenylate cyclase Gastrointestinal tract, (+I Neuronal activity, pig and human atrium positive inotropy and inotropy

5-ht54/5B 5-HT, ergotamine LSD ? CNS Unknown 5-ht, 5-MeOTz5-HT>5-CT; Methiothepin, (+I Adenylate cyclase CNS Unknown sumatriptan inactive clozapine

5-HT, 5-CT>>S-HTr5-MeOT> Mesulergine, (+I Adenylate cyclase CNS, smooth muscle, Circadian rhythm, 8-OH-DPAT; sumatriptan methiothepin, cat atrium relaxation, positive inactive clozapine inotropy and chronotropy aModifIed from Saxena and Vrllalons and Hover et a/.‘*. KNS, central nervous system; CSF, cerebrosprnal flurd; DOI. 1-(2,5-dimethoxy-4-rodophenyl)-2-am1nopropane; 8-OH-DPAT, 8-hydroxy-2-idr-n-propylamrno)tetralin; LSD, lysergrc acid diethylamrde, 5-MeOT, 5methoxytryptamine; 5-CT, 5-carboxamidotryptamrne; (-), decrease, (+I. Increase. CThe human 5-HT,,,,, receptors are so far pharmacologrcally indrstrngurshable. However, the rodent homologue (r5-HT,,) has a drfferent pharmacological profrle (agonist: CP93129; antagonist: SD221 009)

In addition, this current classification includes certain Cardiovascular responses mediated by 5-HT receptors recombinant (5-ht,,, 5-ht,,, 5-ht,,, 5-ht,, and 5-h@ and The effects of 5-HT in the cardiovascular system are com- ‘orphan’ receptors awaiting definitive characterization. plex because they consist of bradycardia or tachycardia, It is noteworthy that the arrival of sumatriptanr* led to the hypotension or hypertension, and vasodilatation or vaso- subdivision of 5-carboxamidotryptamine-sensitive 5-HT,- constriction. Considering the above classification criteria, like receptors into 5-HT,, (sumatriptan-sensitive) and the cardiovascular effects of j-HT are mediated by five main 5-HT,, (sumatriptan-insensitive) receptord. Utilizing their sets of receptors: j-HT, (5-HT,, and 5-HT,,&, 5-HT,, differential coupling to adenylate cyclase and employing 5-HT,, j-HT, and 5-HT, (Refs 3,4,15-20). new cloning techniques, it was shown that j-HT,, (vaso- Intravenous injection of j-HT usually lowers heart rate via constrictor) and j-HT,, (vasodilator) receptors are different the von Bezold-Jarisch reflex, mediated by 5-HT, receptors and appear to correspond to j-HTrB1rD (Refs 15,16) and on sensory vagal nerves in the heart, When this reflex is sup- j-HT, (Ref. 17) receptors, respectively. pressed (e.g. by vagotomy or spinal section), 5-HT increases

DDT Vol. 2, No 7 July 1997 295 the heart rate in different species by a variety of direct and are currently interested in serotonergic drugs+, and the indirect mechanisms*,r8.21, including: patenting activity indicates that these companies expect to convert new chemical entities into marketable products for l myocardial 5-HT, (rat), 5-HT, (human, pig) and 5-HT, therapeutic use. The established, as well as some potential, (cat) receptors; therapeutic uses of 5-HT receptor agonists and antagonists l neurotropic 5-HT, (rabbit) receptors; and are listed in Table 2. l tyramine-like (guinea-pig) or unknown (mollusc) mechanisms. Migraine Many studies have conclusively shown that sumatriptan, an On the other hand, the blood pressure response to 5-HT agonist at the 5-HT,,;,, receptors mediating constriction of is usually triphasic and consists of an initial short-lasting cranial large arteries and arteriovenous anastomosesi*,rj,34,is hypotension caused by bradycardia (von Bezold-Jarisch effective in aborting migraine attackG3j. The successof this reflex), a middle pressor phase and a late, longer-lasting, drug has prompted a large number of pharmaceutical com- hypotension. The pressor response is a result of vaso- panies to develop novel 5-HT,,,,, receptor agonists* constriction mediated mainly by 5-HT, receptors3. Finally, (Table 2). In particular, efforts are being directed towards the late depressor response may involve four different making more lipid-soluble and selective compounds to im- mechanisms: prove oral bioavailability and to avoid coronary artery vasoconstriction. Although the new 5-HT,,,,, receptor agonists l stimulation of central 5-HT,,\ receptors, producing a de- such as rizatriptan, zolmitriptan and naratriptan (Figure 1 crease in sympathetic and an increase in vagal activityQ23; and Table 3) appear to have a better oral bioavailability33, l activation of prejunctional sympathoinhibitory 5-HT,a,,, they do not seem to differ with respect to their coronary side effect potentiap6. l activation of 5-HT, receptors on vascular smooth We believe that vasoconstriction in the extracerebral muscle17.20.26; and (intra- and extracranial) blood vessels, which appear to be l stimulation of certain receptors on vascular endothelium dilated during migraine, is mainly responsible for the thera- (5-HT,, in some blood vessels), resulting in the release peutic effect of sumatriptan33. However, other mechanisms, of a relaxant factor, probably nitric oxider2.2’. such as inhibition of central trigeminal neuron@ or neuro- genie plasma extravasation38, have also been suggested. In addition, 5-HT-induced constriction in a number of Phebus and coworkers39 have argued that the inhibition of blood vessels, particularly the large arteries and arterio- plasma extravasation as well as the therapeutic activity of venous anastomoses of the cranial extracerebral region (e.g. sumatriptan is mediated via the 5-ht,, receptor. Therefore, human middle meningeal arteryzx-Jo and pig and dog exter- they surmise that LY334370 (Figure l), which lacks vaso- nal carotid arterial bed’?), is mediated by j-HT,,,,, (most constrictor activity (on rabbit saphenous vein) but has a likely 5-HT,,$ receptors. Although mRNA for the 5-ht,, selective agonist action at the 5-ht,, receptor (Table 3) and receptor has been located in the human middle meningeal blocks plasma extravasation following trigeminal nerve artery29, activation of this receptor does not appear to con- stimulation, may potentially be a better antimigraine agent tribute to 5-HT- or sumatriptan-induced contractions of this than sumatriptan3”. However, in our opinion, the following vesseljo. facts do not support the above hypothesis:

5-HT-related drugs in cardiovascular pathologies l Some compounds, such as neurokinin NK, and endo- The cardiovascular pharmacology of 5-HT suggests that thelin ET,,a receptor antagonists and CP122288(Figure 1 compounds acting on 5-HT receptors can be employed for and Table 3), all of which potently inhibit plasma therapeutic use in the treatment of migraine, hypertension extravasation4w2, failed to show clinical efficacy in and some peripheral vascular diseasesA,+z3i-33.In particular, migraine4*6. the recent recognition of the 5-HT, receptor as a functional l Sumatriptan (~4: 7.63, 7.94) has a higher affinity than receptor will undoubtedly disclose more therapeutic possi- ergotamine (~4: 6.76) for the i-ht,, receptor (N. Adham bilities. Indeed, a large number of pharmaceutical companies et al.+‘; PJ. Pauwels, pers. commun.), yet sumatriptan is

296 DDT Vol. 2, No. 7 July 1997 Table 2. Therapeutic uses of serotonergic drugs in cardiovascular diseasesa

Drug Company Status Acute migraine

5-HT lB/lD agonist Sumatriptan Glaxo Wellcome Marketed 5-HT,,,,, agonist Naratriptan Glaxo Wellcome Clinical Phase III 5-HT,B,,D agonist Zolmitriptan Zeneca Clinical Phase III 5-HT , B,1D agonist Rizatriptan Merck Clinical Phase IIB 5-HT , B,1D agonist Eletriptan Pfizer Clinical Phase IIB 5-HT 1B,1 D agonist Alniditan Janssen Clinical Phase IIA 5-HT, B,1o agonist SB209509 SmithKline Beecham Clinical Phase IIA 5-HT, F agonist LY334370 Lilly Clinical Phase IIA Migraine prophylaxis 5-HT ,e,,n agonist/BHT, antagonist Methysergideb Novartis Marketed 5-HT, antagonist Pizotifenb Novartis Marketed 5-HT,, antagonists Questionable

Essential hypertension 5-HT,, agonist/q antagonist Urapidilc Bvk Marketed 5-HT, antagonist/a, antagonist Ketanserinc Janssen Marketed 5-HT, agonists Worth exploring Pulmonary hypertension 5-HT, antagonist/q antagonist Ketanserinc Janssen Sometimes used Portal hypertension 5-HT, antagonist/a, antagonist Ketanserinc Janssen Questionable Eclampsia of pregnancy 5-HT, antagonist/q antagonist Ketanserinc Janssen Questionable Carcinoid syndrome 5-HT, antagonist/q antagonist Ketanserin Janssen Questionable Cardiac arrhythmia 5-HT, antagonists Unlikely

Peripheral vascular diseases 5-HT, antagonist/a, antagonist Ketanserin Questionable 5-HT, agonists Worth exploring Skin transplantation 5-HT, agonists Worth exploring Cerebral ischaemia 5-HT,, agonists Unlikely

aModified from Saxena4 and Saxena and Ferra@ b5-HT, receptor antagonlst activity IS probably not Involved In therapeutic actlon.

a less potent antimigraine agent on the basisof parenteral l The 5-HT,,,,n receptor agonists rizatriptan and alniditan doses used in migraine (sumatriptan: 6 mg, subcuta- (Figure l), which are effective in migraineso, have little neously; ergotamine: 0.25-0.5 mg, intramuscular). affinity for the 5-ht,, receptor (Table 3)5l.

l The inhibitory action of sumatriptan on plasma extra- l The contraction of the human middle meningeal artery vasation, but not of CP122288, is blocked by GR127935 by sumatriptan seemsto be mediated by the 5-HT,, (not (Ref. 48); this latter compound has a high affinity for 5-h@ receptor30, indicating that the antimigraine activity 5-HTIB11nreceptors (~4: 9.9/8.9)“‘, but not for the i-ht,, of sumatriptan-like drugs is dependent upon their receptor (pKj: 7.1; H. Connor, per-s. commun.). affinity for the 5-HT,, receptor.

DDT Vol. 2, No. 7 July 1997 297 5-HTnm receptors) and antidepressant action in the case of pizotifen, may be necessary for cH,.,,,*-&3;H’ therapeutic actionjj. Similarly, because no clear- cut positive antimigraine effect has been found H H for any of the 5-HT, receptor antagonists@s’, Sumatriptan Rizatriptan this receptor does not appear to play a major role in migraine. ,W Lastly, it may be pointed out that the constriction of porcine carotid arteriovenous anastomoses

(-)-(I?) .2 HCI u elicited by ergotamine, dihydroergotamine or 5-HT (the latter in the presence of ketanserin) is Naratriptan Alniditan not very susceptible to blockade by GR127935. kc, H3C, This suggests the involvement of a receptor other than the 5-HTz, and 5-HTIB1iDreceptor@. It will be interesting to characterize this receptor CH3NHS02 further and explore whether it can be a target

H for developing novel antimigraine drugs. Eletriptan CP122288 0 ,CH3 Systemic, pulmonary and portal hypertension okdL:H3 Both urapidil (5-HT,, receptor agonist) and ketanserin (5-HT,, receptor antagonist) are H 0 approved for the treatment of systemic hyper- Zolmitriptan LY334370 tension Indeed, it is claimed that these drugs decrease blood pressure by stimulating 5-HT,, Figure 1. Sumatriptan and neu.: 5-HT,,, and 5-h,, receptor agonists under investigation for the treatment of migraine headaches. receptors located centrally in the nucleus tractus solitarii (urapidil) or by blocking 5-HT,, receptors mediating peripheral vasoconstriction Whether an agonist action at the 5-ht,, receptor can also con- (ketanserin). However, as discussed in detail elsewherej, it fer antimigraine action remains to be seen, and the results of seemsthat such effects are not involved to a significant degree clinical trials with LY334370 are awaited with interest. in the clinical effects of these drugs; both urapidil and Some antimigraine drugs (methysergide, pizotifen, ergota- ketanserin have a potent a,-adrenoceptor antagonist activity, mine, dihydroergotamine) are potent antagonists at 5-HT, which can adequately explain their antihypertensive effect, receptors, but many other 5-HT,, antagonists (ketanserin, In view of the arteriolar vasodilatation mediated by the cyproheptadine, mianserin, methiothepin) are not of much 5-HT, receptorj9. it may be worthwhile exploring selective use in migraine therapyjz. More recently, it has been pro- agonists at this receptor as antihypertensive agents. In this posed that 5-HT,, 2c receptors may be involved in the initi- regard, however, one should be aware of the competition ation of migraine attacksz’,j3.j4. Thus, selective antagonism of with excellent drugs already available. these receptors, in particular the 5-HT,, receptor, which may In the complex setting of cardiac surgery and cardio- mediate nitric oxide release from vascular endotheliumi2.27, pulmonary bypass, several potent mediators are released, should be effective in migraine preventionj3,j*. However, as which, in turn, may produce systemic and/or pulmonary already discussed elsewherej*. several 5-HTzB1,c receptor hypertension. One of the mediators may be 5-HT, released antagonists, including mianserin and cyproheptadine, are from aggregating platelets, causing vasoconstriction by not very effective antimigraine agents. It seems, therefore, activating 5-HT, receptors, particularly in patients with that additional properties, such as vasoconstriction in the an impaired endothelial function (e.g. atherosclerosis@). extracerebral cephalic circulation in the case of methy- Ketanserin has proved effective in the treatment of post- sergide, ergotamine and dihydroergotamine (partly via operative pulmonary hypertension61. It is far from clear,

298 DDT Vol. 2, No. 7 July 1997 Table 3. p&values at cloned human receptors for 5-HT, receptors is somewhat questionable. However, in sumatriptan and some new 5-HT,,,,, and 5-ht,, this context, selective 5-HT7 receptor antagonists may be receptor agonists under investigation for the expected to be effective. treatment of migraine headachesa Compound 5-HT,, 5-HT,, 5-h& Cardiac disorders

Sumatriptan 8.50 8.68 7.94 5-HT, receptors mediate increases in the rate and contractil- Naratriptan 9.33 9.16 8.40 ity of the human atrium (see above). Because 5-HT can Eletriptan 8.00 8.94 8.18b induce arrhythmias in the human isolated atrium, it is pro- Zolmitriptan 9.08 9.66 7.54 posed that 5-HT, receptor antagonists could be useful in the Rizatriptan 8.14 8.63 6.86 treatment of cardiac arrhythmia@. However, the role of Alniditan 8.96 9.40 6.44 5-HT, if any, in the pathogenesis of cardiac arrhythmias has CP122288 8.16 7.53 8.48b LY334370 6.86 6.90 8.96 not been established. Thus, it seemsunlikely that such drugs will be effective in this disorder. a Affinity constant data are from Dr P.J. Pauwels (pers. commun.), except for On the other hand, the ability of 5-HT, receptor agonists eletriptan. CP122288 (both Pfizer. UK). alntditan5’ and LY334370 (Ref. 39) to increase atria1contractility suggestedthat these drugs may b Value for the rat receptor. have application in the therapy of heart failure. Any such however, whether 5-HT, receptor blockade is responsible hopes were dashed, however, when it was shown that for the effectiveness of ketanserin, since no selective 5-HT, 5-HT, receptors are not present on human ventricles65.66. receptor antagonists have been shown to be effective. Perhaps less questionable is the role of 5-HTz receptors in Peripheral vascular diseases some cases of portal hypertension, where higher levels of 5-HT has been implicated in the pathophysiology of periph- free plasma 5-HT are found in the portal venous circu- eral vascular diseases,but the evidence is not compelling67. lation3*. This assumption is based on the fact that both Although some clinical trials show a moderate efficacy of ketanserin (which combines 5-HT, with a,-adrenoceptor ketanserin”‘, this drug is not registered for this indication in blockade) and ritanserin (at doses without a,-adrenoceptor The Netherlands. However, it is interesting to note that one blockade) lower portal venous pressureb2. of the prominent cardiovascular effects of 5-HT is its ability to produce vasodilatation via stimulation of 5-HT, recep- Pre-eclampsia and eclampsia tors17,20.Therefore, selective agonists at vascular 5-HT, Ketanserin has been shown to be effective in the treatment receptors may be expected to enhance capillary blood flow of pre-eclampsia and eclampsia, pathologies that seem to and be useful in the treatment of peripheral vascular dis- involve local release of 5-HT from aggregating platelets in eases, including trophic skin ulcers. This approach might the placental circulation32.63.Although this finding does not also have potential applications, as yet unexplored, in the establish the involvement of 5-HT, receptors, this hypoth- medical treatment of skin grafts and baldness. esis could be substantiated by establishing the effectiveness of selective 5-HT, receptor antagonists. Cerebral ischaemia Buspirone and ipsapirone, established anxiolytic agents with Carcinoid syndrome an action at central 5-HT,, receptors+, decrease infarct size The carcinoid syndrome is a pathology characterized pre- in animal models of focal”8 and global”” cerebral ischaemia. dominantly by gastrointestinal alterations (e.g. diarrhoea) Notwithstanding, the involvement of 5-HT,, receptors is and facial flushing, provoked by a tumour secreting a gross questionable because these drugs are nonselective agents excess of several mediators, including 5-HT (Ref. 31). and, most significantly, Shydroxy-2-(di-n-propylamino)- Ketanserin has been claimed to be effective in diminishing tetralin (S-OH-DPAT). which is also a potent 5-HT,, receptor the frequency and severity of some manifestations of this agonist12,was ineffective in these experimental models6~70. syndrome, including diarrhoea and flushings*. Nevertheless, considering that 5-HT-induced arteriolar vasodilatation is Closing remarks mediated mainly by 5-HT, receptors5”, the clinical effective- During the last decade, research in the field of 5-HT has ness of ketanserin in relieving facial flushing by blockade of been boosted by the availability of potent and selective

DDT Vol. 2, No. 7 July 1997 299 drugs. These drugs and the increasing understanding of 26 Trevethick. MA, Feniuk. W. and Humphrey, P.P. (1986) Life Sci. 38, transduction mechanisms, as well as the structure of the 1521-1528 receptor protein, have enabled a more meaningful charac- 27 Fozard. J.R. (1995) Arch. Znt. Pharmacodyn Ther. 329, 111-119 28 Hamel. E. et al. (1993) :Cfol Pbarmacol. 44. 242-246 terization and nomenclature of 5-HT receptors, Thus, five 29 Bouchelet, I. et al (1996) Mol. Pharmacol 50, 219-223 main classes with several subtypes (5-HT,, 5-HT,, 5-HT,, 30 Razzaque. 2. et al. (1997) Br. J. Pharmacol 120, 211P 5-HT, and 5-HT,) as well as some not yet adequately 31 Robertson, J.1. (1990) Cardiovasc Dnlgs Tber + (Suppl. 1). 5558 characterized recombinant (e.g. 5-ht,,, 5-ht,,, j-ht,,, 5-ht,,, 32 Robertson, J.I. (1991) J. Cardiouzsc Pharmacol 17 (Suppl. 5). S&i3 5-ht,) and ‘orphan’ 5-HT receptors have been recognized, 33 Saxena. P.R and Ferrari. M D (1996) EQert Opin Inwst. Drugs 5,581-593 Indeed, the molecular cloning and expression of a growing 34 Humphrey, P.P. and Feniuk. W. (1991) Trend< Pharmacol. Sci. 12, 444-446 Ferrari. M.D. and Saxend. P.R (1993) Trends Pbarmacol. Sci 14, 129-133 number of 5-HT receptors in host cells now offers the possi- 35 36 MadssenVanDenBrink, M. et al. (1997) 5r.J. Pharmacol. 120,68~ bility to screen new molecules easily. This will help identify 37 Hoskm. K.L., Kaube. H. and Goadsby. PJ. (1996) Bruzn 119, 1419-1428 selective ligands, which could, in turn, provide access 38 Moskowitz. M.A. (1992) Trends Pharmacol. Sci 13, 307-311 to better drugs for a more efficient treatment of human 39 Phebus. LA et al (1996) ,Veuroscience&feeting 196. Washington, DC, USA ailments. 40 Gupta. P. et al. (1995) 5r.J. Pharmacol 116, 2385-2390 41 Brandli. P. et al (1996) Pain 64, 315-322 42 Shepheard. S.L. et al. (1995) .Veuropharmacology34. 255261 ACKNOWLEDGEMENTS 43 Diener. 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