A CLASSIFICATION OF OPIATE RECEPTORS THAT MEDIATE

Transcription

1 Br. J. Phrm. (198), 69, A CLASSIFICATION OF OPIATE RECEPTORS THAT MEDIATE ANTINOCICEPTION IN ANIMALS M.B. TYERS1 Phrmology Dept., Glxo Group Reserh Ltd., Wre, Hertfordshire 1 To investigte the opite reeptors tht medite ntinoieption, the tivity profiles of opioid nlgesi drugs hve been determined ginst different noieptive stimuli in the mouse nd rt. 2 In tests tht employ het s the noieptive stimulus, p-opite reeptor gonists, suh s morphine, pethidine nd dextropropoxyphene, hd steep nd prllel dose-response urves nd were pble of hieving mximum effets. In ddition, the ntinoieptive poteny rtios of these drugs in het tests were similr to those for nlgesi in mn. 3 The K-gonists, suh s ethylketzoine, nlorphine, Mr234 nd pentzoine, were essentilly intive ginst het noieption exept t doses tht used sedtion nd motor inpittion. 4 In the writhing nd pw pressure tests both j- nd --gonists produed steep nd prllel dose-response urves. 5 It is onluded tht both t- nd K-opite reeptors medite ntinoieption in nimls nd tht the intertions of nlgesi drugs with these reeptors my be lssified in terms of their ntinoieptive tivities ginst qulittively different noieptive stimuli. Introdution There is onsiderble evidene tht opite reeptors exist in differing forms. Mrtin, Edes, Thompson, Huppler & Gilbert (1976) nd Gilbert & Mrtin (1976) showed tht opioid drugs ould be lssified in terms of their effets on severl prmeters in the hroni spinl dog s being either morphine-like (j-reeptor), nlorphine-like (K-reeptor) or N-llylnormetzoine-like (-reeptor). Huthinson, Kosterlitz, Leslie, Terenius & Wterfield (1975), Lord, Wterfield, Hughes & Kosterlitz (1976) nd Kosterlitz & Leslie (1978) hve identified further reeptor in the mouse vs deferens for the binding sites of the nturlly ourring opioid peptides. This ltter reeptor ws designted the 6-reeptor. The funtions of these different reeptors in reltion to the mny phrmologil tions of opioid drugs remins somewht obsure. On mesuring the ntinoieptive tivities of opioid nlgesi drugs in nimls it hs been observed tht ertin drugs tht hve potent nlgesi tivity in mn suh s nlorphine (Lsgn & Beeher, 1954) nd pentzoine (Arher, Albertson, Hrris, Pierson, Bird, Kets, Telford & Ppdopoulos, rdint het til flik tests in rodents. In ontrst, the ntinoieptive tivities of these drugs n redily be deteted in writhing tests in the mouse (Tber, Greenhouse & Irwin, 1964) nd ginst tooth pulp stimultion in the dog (Skingle & Tyers, 1979). ' Present ddress: Phrmology Dept., Glxo Group Reserh Ltd., Greenford, Middx. UB6 OHE. 1962) re essentilly intive in hot plte nd /8/753-1$1. The im of the present study ws to investigte the differenes between ntinoieptive tests nd to identify the opite reeptors tht medite the ntinoieptive tions of opioid nlgesi drugs. Methods The methods used to evlute ntinoieptive tivity were seleted to inlude tests whih employed different types of noxious stimuli, i.e., het, hemil, nd pressure. Tests in mie (mle, AHM/I/ICI-derived, 18 to 22 g) nd rts (mle, AH rndom bred Hooded, 35 to 8 g) were ll rried out under essentilly the sme experimentl onditions. Individul tests were rried out using dose-groups of 6 nimls. Dt for the lultion of ED5 vlues were umulted from 2 or 3 individul tests rried out on different dys (< 7 dys prt), suh tht the finl dose-groups omprised 12 or 18 nimls respetively. To eliminte ge intertion the nimls were rndomized into different ges suh tht eh ge ontined nimls reeiving different tretments. Beuse tests normlly lsted for full working dy this ltter proedure lso rndomized tretments with respet to time of dy nd thus eliminted ny bis ssoited with temporl yles of pin sensitivity. Animls nd drug solutions were olour oded suh tht the opertors were unwre of the tretments the nimls were reeiving. In eh test, one or more drugs were ompred with mor- Mmilln Journls Ltd 198

2 54 M.B. TYERS phine nd plebo ontrol in whih.9% w/v NCI solution (sline) ws given in the sme dose-volume. In ll tests, drugs were given subutneously 3 min before testing for ntinoieptive tivity. Drugs were dissolved in sline nd injeted in dose volume of.2 ml/2 g body wt. The results presented refer to the free id/bse equivlent. Antinoieptive tivities (ED5), 95% onfidene limits, regression slopes, linerity nd poteny rtios, where pplible, were lulted by methods of Finney (1964). Aetylholine-indued writhing in the mouse Tests were rried out to determine the inhibitory effets of the test drugs ginst writhing indued by etylholine in the mouse. Thirty minutes fter subutneous injetion of drug or plebo, mie were injeted with etylholine, 3 mg/kg intrperitonelly. The number of writhes ourring in the first 5 min therefter ws reorded. A writhe ws defined s ontrtion of the bdominl musles ompnied by n extension of the hind limbs. The ED5 vlue ws defined s the dose of test drug pble of reduing by 5% the number of writhes ourring in plebo-treted mie. Hot plte test in the mouse In the hot plte test, retion times of mie pled on opper plte heted to men (±rnge) temperture of C were determined. A 'front pw lik' ws tken s the noieptive response t whih time the nimls were rpidly removed from the hot plte; 6 s ws tken s the mximum retion time. The ED5 vlue ws rbitrrily defined s the dose of test drug pble of elevting the retion time to twie tht determined for plebo-treted nimls. Pw pressure test in the rt The effets of test drugs on noieptive pressure thresholds of both yest-inflmed nd non-inflmed hind pws were determined in rts. Initil noieptive thresholds were determined for both hind pws in ll of the rts using n 'Anlgesymeter' (Ugo Bsile, Miln). Eh rt reeived n injetion of.1 ml of 2% w/v suspension of Brewer's yest (CWE Ltd., Cwston, Norfolk) given subutneously into the subplntr surfe of the right hind pw nd then returned to their test ges. Test drugs were injeted subutneously 3.5 h fter the yest injetion. Noieptive thresholds were re-determined 3 min lter, i.e. 4 h fter the yest injetions. The noieptive response ws usully shrill volistion whih ws lerly distinguishble from tht whih often ourred with norml hndling. A few nimls did not volize but reted to the stimulus with strong ttempts to withdrw their pws. Either retion ws tken s the noieptive response. For lultion of ED5 vlues, results relte to the men pre-yest noieptive responses, lled the 'men h responses', determined for both pws in ll of the rts in the test. For yest-inflmed pws the ED5 vlue ws defined s the dose of test drug tht rised the noieptive threshold to vlue hlf-wy between the men hyperesthesi determined for the yest-inflmed pws of plebo-treted rts nd the 'men h response'. For non-inflmed pws the ED5 ws rbitrrily defined s the dose of test drug tht rised the noieptive threshold by 1 g bove the 'men h response'. Til immersion tests in the rt In the til immersion tests the noxious stimulus ws hot wter mintined t either 5 or 55 C. Noieptive retion times, whih were the times tken for the rts to 'flik' their tils or to withdrw them from the hot wter, were determined. The ED5 vlue ws rbitrrily defined s the dose of test drug tht rised the retion time to vlue twie tht determined for plebo ontrol nimls. Drugs The following drugs were used: morphine hydrohloride, odeine phosphte, pethidine hydrohloride (MFrln Smith); dextropropoxyphene (Dist); AH7921 [3,4-dihloro-N[(l-(dimethylmino) ylohexyl)methyl]benzmide hydrohloride] (Glxo Group Reserh Ltd.); (±)-ethylketzoine methnesulphonte, (±)-ketzoine methnesulphonte; pentzoine s the free bse (Sterling-Winthrop); buprenorphine hydrohloride (Rekitt nd lmn); Mr234 s the free bse [(-)-4lR, 5R, 9R)-5,9-dimethyl-2-(L-tetrhydrofurfuryl)-2'-hydroxy-6, 7-benzomorphn], Mr 1353 [(± )-x-5, 9-dimethyl-2-(3-methylfurfuryl}2'-hydroxy-6, 7-benzomorphn methne-sulphonte] (Dr H. Merz, Boehringer-Ingelheim); nlorphine hydrobromide (Burroughs Wellome); etylholine hloride (Sigm). Results Hot plte nd til immersion tests The ntinoieptive tivities (ED5) of the nlgesi drugs tested in the hot plte (55 C) test in the mouse nd til immersion (5 nd 55 C) tests in the rt re given in Tble 1. Dose-response urves re shown in Figures I to 3. In these tests morphine, odeine, D-propoxyphene, pethidine nd AH7921 produed

3 OPIATE RECEPTORS THAT MEDIATE ANTINOCICEPTION 55 Tble 1 rt steep dose-response urves nd rehed mximum effets without using ny observble motor inpittion (Figure 1). On lowering the temperture in the til immersion test from 55 C to 5 C there ws no signifint hnge in the ntinoieptive poteny of these drugs. Buprenorphine,.5 to 5. mg/kg ws effetive in the hot plte test but produed shllower dose-response urve thn morphine nd did not reh mximum effet (Figure 2). Higher doses of buprenorphine, 5 to 2 mg/kg, in the hot plte test used less effet, produing bell-shped dose-response urve. In the til immersion (55 C) test (Figure 2) buprenorphine,.3 to 3. mg/kg, produed rther vrible effets with shllow dose-regression nd low mximum (Figure 2b). In the lower temperture til immersion (5 C) test buprenorphine,.6 to.5 mg/kg, produed more onsistent effet; it ws bout 4 times more potent thn in the higher temperture test, but the dose-response urve ws muh shllower thn tht for morphine rehing only 4% of mximum. Furthermore, s in the hot plte test, the highest dose of buprenorphine,.5 mg/kg, produed less effet thn the next lower dose of.25 mg/kg. The benzomorphns, ethylketzoine nd. Mrl353, produed steep dose-response urves in both the hot plte nd til immersion tests (Figure 2 nd 3) but in ontrst to the effets of morphine the effetive doses of these drugs lso produed mrked motor inpittion, i.e., the nimls were lerly unble to respond to the noieptive stimulus. Pentzoine inresed hot plte retion lteny nd used sedtion in mie but hd no effet in the til immersion tests in the rt (Figure 2). Similrly, ketzoine nd Mr234 produed very shllow dose-response urves in the hot plte test in the mouse s well s motor inpittion (Figure 3 nd 2) but were intive t 8 nd 2 mg/kg respetively in both til immersion tests in the rt. Nlorphine,.1 to 8 mg/kg ws intive in the hot plte nd til immersion tests nd hd no observble effets on behviour. Aetylholine-indued writhing nd pw pressure tests The ntinoieptive tivities (ED5) of the nlgesi drugs tested in the etylholine-indued writhing test in the mouse nd inflmed nd non-inflmed pw pressure tests in the rt re given in Tble 2. Doseresponse urves re shown in Figures 4 to 6. With the exeption of ketzoine nd nlorphine, ll drugs tested produed prllel dose-response urves nd rehed mximum effets.when tested in the etylholine-indued writhing nd pw pressure tests. The responses to ketzoine,.1 to- 1 mg/kg, in the writhing test (Figure 6) were similr to those for the other drugs but in the pw pressure tests doseresponse urves were shllow nd did not reh mximum effet (Figure 6b, ). Nlorphine,.3 to 1. mg/kg, in the writhing test hieved only 75% inhibition (Figure 5) nd in the yest inflmed nd noninflmed pw pressure tests the dose-response urves rehed eiling t 35 nd 55% of mximum respetively. Higher doses (> 1. mg/kg) of nlorphine in the Antinoieptive potenies of opioid nlgesi drugs ginst het-indued noieption in the mouse nd Drug Morphine deine D-Propoxyphene Pethidine AH7921 Nlorphine Ketzoine Pentzoine Buprenorphine Ethylketzoine Mr234 Mr1353 AntinoieptivepoteniesED5 mg/kg subutneously (95% onfidene limits) Hot plte (55 C) test (mouse) 1.7 ( ) 15.3 ( ) 11.3 ( ) 7.8 ( ) 1.8 ( ) Intive.1-8 t ( ) * 1.5 (.1-39.).41 (.2-.9) t 3. 2.( ) Til immersion tests (rt) 5 C 55 C.7 (.3-1.5) 1.1 ( ) 4.1 ( ) 5.4 ( ).6 (.3-1.) Intive.1-8 Intive 1.-8 Intive 1.-4 *.9 (.1.4) 1.3 ( ) Intive ( ).6 (.3-1.2) 16.1 (5.5-42) 6.2 ( ) 6. ( ).8 (.3-1.6) Intive.1-8 Intive 1.-8 Intive 1.-4 *.4 (.1-3.5) 1.4 ( ) Intive ( ) Drugs were given subutneously, 3 min before evlution for ntinoieptive tivity. Dose-response urves re given in Figures 1-3. * Vlues for buprenorphine were lulted from the rising liner portions of the doseresponse urves. t Shllow dose-response urves, only the highest doses were signifint, P <.5 (see Figures 2 nd 3).

4 56 M.B. TYERS UL) n CL) C - C.) 6) 4 I 6 r F 2 F 1F ' U) C.) C 4 C) I 6 r -1. I I. _ F 2 F 11F () C1) I- L)._ b 6 r 5 [ 4-3 F 2 F 11F -~ 6 C) 5 C 4 C) 2) 3 3 C X 2 E E- F IF 1- I Dose (mg/kg s..) Figure I Effets of morphine (A), AH7921 (), pethidine (v) nd D-propoxyphene (U) on noieptive retion ltenies in the hot plte (55 C) test in the mouse () nd til immersion (55 C) test in the rt (b). Broken lines re responses of plebo-treted nimls; vertil lines show s.e. men Dose (mg/kg s..) Figure 2 Effets of buprenorphine (1), Mr234 (O), nlorphine (V) nd pentzoine () on noieptive retion ltenies in the hot plte (55 C) test in the mouse () nd til immersion (55 C) test in the rt (b). Broken lines re responses of plebo-treted nimls; vertil lines show s.e. men. pw pressure tests produed less effet, nd in the non-inflmed pw dose of 3 mg/kg ws not signifintly different (P >.5) from plebo. The poteny differenes for the tions of drugs in het ompred with non-het tests rnged from one to six fold for morphine, odeine, D-propoxyphene, pethidine nd AH7921; tht is, these drugs were slightly more tive in the pressure nd writhing tests thn in the hot plte nd til immersion tests. For the remining drugs the poteny differenes rnged from 1 for Mrl353 whih produed motor inpittion in the mouse nd rt t the sme doses tht prolonged retion ltenies, to greter thn 4 (buprenorphine). Sttistil nlyses A prinipl omponents nlysis ws rried out on the ntinoieptive dt from rodent tests ssuming tht the log ED5s were from multivrint norml distribution. The im of this nlysis ws to find smll number of liner ombintions of the originl 6 vribles (tests) whih ould explin the mjority of the totl vrine. Exmintion of the oeffiients of these priniple omponents reveled tht 93% of the totl vrine fell within two omponents. The first (Y1) explins 6% of the vrine nd is mesure of the overll ntinoieptive tivity in ll of the tests. The seond omponent (Y2) ounts for 33% of the

5 OPIATE RECEPTORS THAT MEDIATE ANTINOCICEPTION U) J C._ Q CI. 5-41F 31F L C n._ 3:._. 6 F 4* L I I I b b ~r 6 4 F C) C 2 C.) E._ fi 4) Lo. 1 O Z_ Dose (mg/kg s..) Figure 3. Effets of ethylketzoine (Q), ketzoine ([El) nd Mr 1353 (A) on noieptive retion ltenies in the hot plte (55 C) test in the mouse () nd til immersion (55 C) test in the rt (b). Broken lines re responses of plebo-treted nimls; vertil lines show s.e. men. Q. - C 5 r 4 F 3 F 2 F vrine nd lerly distinguishes the het nd nonhet tests. The remining four omponents re less importnt. The priniple omponent sores for eh ompound re given in Tble 3. High vlues for Y1 indite high overll tivity while high vlues for Y2 indite reltively high non-het; het tivity. Vlues for Y2 lerly distinguish p-gonists (morphine, odeine, pethidine, AH7921, D-propoxyphene) nd K-gonists (nlorphine, buprenorphine nd the benzomorphns tested). Disussion The lssifition of opite reeptors from dt obtined in the hroni spinl dog (Mrtin et l., 1F 1 II I Dose (mg/kg s..) Figure 4 Antinoieptive tions of morphine (A), AH7921 (), pethidine (V) nd D-propoxyphene (U) in the etylholine-indued writhing test in the mouse () nd in the inflmed (b) nd non-inflmed () pw pressure tests in the rt. Broken lines re responses of plebo-treted nimls; vertil lines show s.e. men. 1976) is bsed on the phrmologil profiles of rnge of opioid nlgesi drugs on severl prmeters. Of these prmeters, two were inluded to determine the ntinoieptive tivities of the drugs. These were the 'flexor reflex' response to pressure stimulus nd

6 58 M.B. TYERS the 'skin twith' response lteny to rdint het stimulus. Drugs lssified s p-opite reeptor gonists were bout eqully effetive ginst both pressure nd rdint het stimuli while drugs tht hd high tivity on K-opite reeptors were effetive ginst pressure noieption but less effetive or intive ginst rdint het noieption. The benzomorphn SKF147 (N-llynormetzoine), -reeptor gonist, ws only very wekly tive ginst the flexor reflex in the dog nd ws intive in the phenylquinone-writhing test in the mouse (Perl & Hrris, 1966). It is unlikely therefore, tht -reeptors re involved in ntinoieption. The results obtined in the present study show tht the intertions of opioid nlgesi drugs with their reeptors my lso be defined in terms of their ntinoieptive tivity profiles ginst different noxious stimuli in the onsious mouse nd rt. The ntinoieptive tests used my be divided into two tegories ording to their sensitivities to opioid nlgesi drugs. The hot plte nd til immersion tests, in whih het is the noieptive stimulus form one tegory nd writhing nd pressure tests form nother. Morphine-like ompounds, inluding D-propoxyphene, pethidine, odeine nd AH7921 re p-opite reeptor gonists. Tht is, they strongly nd ompletely suppress the bstinene syndrome in withdrwn morphine-dependent monkeys nd/or dogs (Deneu & Seevers, 1964; Mrtin et l., 1976). In the isolted ileum of the guine-pig nd mouse vs deferens preprtions, their poteny rtios re the sme s tht for morphine (Hughes, Kosterlitz & Leslie, 1975). Results obtined in the present study show tht the p-opite reeptor gonists re pproximtely equipotent in het nd non-het ntinoieptive tests. In these tests they produed steep dose-response urves nd were ble to reh mximum effet. In ddition, the rnked order of poteny for these drugs in ll of the niml tests studied is the sme s the order of poteny for their nlgesi potenies in mn. This indites tht for p-gonists, ntinoieptive tivities in both het nd non-het tests re preditive of nlgesi potenies in mn. The remining drugs in this study re predominntly gonists on the K-reeptor. Tht is, they do not substitute for morphine in the dependent monkey (Villrrel & Seevers, 1972; Swin & Seevers 1974; 1976); in the hroni spinl dog they hve profile of tion tht differs from tht of the p-gonists (Mrtin et l., 1976) nd in the mouse isolted vs deferens they re more potent thn in the guine-pig ileum reltive to morphine; in both preprtions more nloxone is required to ntgonize their effets thn those of the p-gonists (Huthinson et l., 1975). Results obtined in the present study indite tht in ntinoieptive tests the K-gonists re signifintly more effetive ginst writhing nd pressure noxi thn ginst het-indued noieption. These drugs re lso very potent ginst tooth pulp stimultion in the dog (Skingle & Tyers, 1979; 198). Of the drugs tested, the most seletive gonist on K-reeptors ws the benzomorphn, Mr234. Ethylketzoine, pentzoine nd Mrl353 were lso more potent ginst responses to non-het stimuli thn ginst het- Tble 2 Antinoieptive potenies of opioid nlgesis drugs ginst hemil nd pressure-indued pin in the mouse nd rt Morphine deine D-Propoxyphene Pethidine AH7921 Nlorphine Ketzoine Pentzoine Buprenorphine Ethylketzoine Mr234 Mr l 353 Antinoieptivepotenies ED5 mg/kg subutneously (95% onfidene limits) Aetylholine writhing (mouse).45(.2-.9) 5. (2.-9.4) 1.5 (.7-3.2) 2.9 ( ).59 (.5-.8) *.12(.6-.3).29 (.1-.8) 1.4 (.5-4.).4(.1-.8).12 (.4-.3).6 (.2-.16) 1.1 (.4-3.) Inflmed pw pressure (rt).43 (.3-.5) 2.1 ( ) 2.7 ( ) 1.1 (.6-1.7).57 (.5-.7) *.8 (.1-.2) *. 12 (.1-.3) 1.6 (.9-2.5).1 (.5-.2).8 (.4.13).5 (.3-.8).72 (.3-1.2) Norml pw pressure (rt).5(.3-.8) 3.6 ( ) 1.2 ( ) 2. ( ).67 (.5-.8) *.3 (.5-1.7) *.37 (.1-.9) 1.9 (1.-3.3).1(.5-.4).14(.7-.27).7 (.4-.1) 1.1 (.5-1.9) Drugs were given subutneously, 3 min before ssessment of ntinoieptive tivity. Dose-response urves re shown in Figures 4-6. * Vlues for nlorphine nd ketzoine were lulted from the rising liner portions of the dose-response urves.

7 OPIATE RECEPTORS THAT MEDIATE ANTINOCICEPTION 59 1 r C 3 r- C.2 C 81 6 F 41 2 OL I I I b 5Mr b 5r ~ 1 o ' I I- j *4 C) t I.._ 5 4 r. 2 I C 5 r 4 F 3" 3k 21 -li 3 2 F io 1 O..I Dose (mg/kg s..) Figure 5 Antinoieptive tions of buprenorphine (1), Mr234 (o), nlorphine (V) nd pentzoine () in the etylholine-indued writhing test in the mouse () nd in the inflmed (b) nd non-inflmed () pw pressure tests in the rt. Broken lines re responses of plebo-treted nimls; vertil lines show s.e. men. indued noieption. But for these ltter drugs the poteny differenes between het nd non-het tests were less thn for Mr2O34. However, it is importnt to stress tht the doses of these ltter benzomorphns tht inresed retion ltenies in the hot plte nd 1 I Dose (mg/kg s..) Figure 6 Antinoieptive effets of ethylketzoine (O), ketzoine (O) nd Mr1353 (A) in the etylholineindued writhing test in the mouse () nd in the inflmed (b) nd non-inflmed () pw pressure tests in the rt. Broken lines re responses of plebo-treted nimls; vertil lines show se. men. til immersion tests lso used mrked sedtion, txi nd, t the higher doses, loss of righting reflex. These nimls were unble to respond to the het stimulus nd therefore the tivities of these drugs in het tests were most probbly n indition of their B.J.P. 69/3-K

8 51 M.B. TYERS effets on motor oordintion rther thn on noieption. Therefore, for these drugs, prtiulrly ethylketzoine, ny tions on.-reeptors in het tests would be msked by their depressnt properties. Similr observtions were reported by Hrris & Rosenberg (1967) who showed tht the tivities of pentzoine nd ylzoine in the hot plte test were due to sedtion or motor in-oordintion s mesured using n inlined sreen. Nlorphine ould lso be lssified s K-gonist but ompred with Mr234 the dose-response urves were shllow nd lower mximum effets were obtined. Mrtin et l. (1976) obtined similr result for nlorphine in the hroni spinl dog on the flexor reflex response. Thus, results obtined in the present study support the onept tht nlorphine is prtil gonist on K-reeptors. One finding tht is diffiult to explin is tht higher doses of nlorphine in the pw pressure test onsistently produed lesser effets thn lower doses. This bell-shped dose-response urve is similr to tht found for buprenorphine in het tests nd for inhibition of gut propulsion (wn, Lewis & MFrlne, 1977). Ketzoine ws only wekly tive ginst hetindued noieption t doses tht lso used sedtion. In the pw pressure test ketzoine ws more potent thn ginst het noieption but produed shllow dose-response urve with low mximum effet. However, in the etylholine-writhing test in the mouse the dose-response urve ws prllel to tht of Mr234 nd rehed mximum effet. It my be tht ketzoine is lso prtil gonist on K-reep- Tble 3 Priniple omponent sores for the nlgesi drugs in rodent tests Drug Morphine deine D-Propoxyphene Pethidine AH7921 Nlorphine Ketzoine Pentzoine Buprenorphine Ethylketzoine Mr234 Mrl353 Priniple omponents Y1* Y2t * High vlues of Y1 indite high overll tivity. t High vlues of Y2 indite reltively high non-het: het tivity. Y2 lerly distinguishes p- nd K-gonists. tors but hs higher intrinsi tivity thn nlorphine. Buprenorphine, t doses devoid of sedtive tivity, produed shllow dose-response urve with low mximum effet in the hot plte nd til immersion tests. This finding is onsistent with the lssifition of buprenorphine s prtil gonist on j-reeptors (Mrtin et l., 1976). However, buprenorphine ws muh more potent (5 to 4 times) ginst non-het thn ginst het noxi suggesting tht this drug is predominntly n gonist on K-reeptors. Further evidene tht the predominnt ntinoieptive tions of morphine nd buprenorphine re medited vi different opite reeptors is shown when these drugs re injeted into the spinl subrhnoid spe in rts (Byrnt & Tyers, 1979). By this route, low doses of morphine re ntinoieptive in hot plte nd pw pressure tests but buprenorphine is intive until the intrthel dose exeeds tht whih is tive by the subutneous route. The results obtined show tht het nd non-het ntinoieptive tests in rodents distinguish between p- nd K-opite reeptors. Nlorphine, buprenorphine nd the benzomorphns tested pper to hve ntinoieptive tions tht re medited predominntly through intertions with K-opite reeptors. The rnked order of poteny for these drugs is the sme in eh of the non-het tests nd for their nlgesi potenies, where known, in mn. There is no orreltion between the ntinoieptive poteny rtios in het nd non-het tests. The extent of intertion, if ny, of the 1t-gonists with K-reeptors nnot be determined from these dt. In terms of ntinoieption-it my be tht the gonist tions on pi- nd K-reeptors seletively inhibit responses to het nd non-het noxi respetively. If this were so, then beuse the rnked order of poteny is the sme in eh of the tests for the p-gonists, they would be expeted to be eqully effiious on both 1t- nd K-reeptors. This seems unlikely in view of the widely differing hemil strutures of the t-gonists. It is more likely tht intertion with either reeptor n inhibit noieption rising from the non-het stimuli employed here while intertion with 1s-reeptors is neessry for inhibition of noieption rising from het stimuli. This postulte would lso imply tht there re t lest two neurl pthwys whih either inhibit or onvey noieption rising from het nd non-het stimuli. Experiments with opite reeptor ntgonists nd reeptor binding studies re needed to onfirm these onepts. In ontrst to some of the results presented here, severl investigtors hve shown tht the ntinoieptive tions of nlgesi drugs with dul gonist nd ntgonist tions my be evluted in hot plte (O'Cllghn & Holtzmn, 1975) til flik (Gry,

9 OPIATE RECEPTORS THAT MEDIATE ANTINOCICEPTION 511 Osterberg & Suto, 197) nd til immersion (Sewell & Spener, 1976) tests providing tht the temperture is lowered to give less intense noieptive stimulus. However, even in these tests, the ntinoieptive potenies reported for these drugs were still onsiderbly lower thn their nlgesi potenies in mn or indeed in other non-het ntinoieptive tests. In the present study, of ll the drugs tested, only buprenorphine ws slightly more potent in the lower thn in the higher temperture til immersion test. In onlusion, evidene hs been presented to show tht t lest two opite reeptors medite ntinoieption in niml tests. In ddition, it hs been shown tht opioid nlgesi drugs my be lssified in terms of their profiles of ntinoieptive tions ginst different noxi. Tests tht employ het s the noieptive stimulus, suh s the hot plte nd til immersion tests, re seletive for ji-gonists nd re insensitive to the ntinoieptive tions of K-gonists. Tests tht employ pressure, hemil writhing or tooth pulp stimultion (Skingle & Tyers, 1979) re sensitive to both p- nd K-gonists. I should like to thnk M. Skingle, G. oper, Jnine Thompson, Christine Evns, B. Hy nd C. Robertson for tehnil ssistne nd J.K. Forster of the mputing nd Sttistis Unit for the sttistil nlyses. I m lso grteful to Prof. H.W. Kosterlitz nd Dr G. Metlf for their helpful ritiisms of this mnusript. ReferenCes ARCHER, S., ALBERTSON, N.F., HARRIS, L.S., PIERSON, A.K., BIRD, J.G., KEATS, A.S. TELFORD, J. & PAPADOPOULOS, C.N. (1962). Nroti ntgonists s nlgesis. Siene, 137, BRYANT, R.M. & TYERS M.B. (1979). Antinoieptive tions of morphine nd buprenorphine given intrthelly in onsious rts. Br. J. Phrm., 66, P. COWAN, A., LEWIS, J.W. & MACFARLANE, I.R. (1977). Agonist nd ntgonist properties of buprenorphine, new ntinoieptive gent. Br. J. Phrm., 6, DENEAU, G.A. & SEEVERS, M.H. (1964). Drug Dependene. In Evlution of Drug Ativites: Phrmometris. Vol. I, ed. (Lurene, D.R. & Bhrh, A.L. pp London: Ademi Press. FINNEY, D.J. (1964). Sttistil Method in Biologil Assy. 2nd Ed. London: Griffin. GILBERT, P.E. & MARTIN, W.R. (1976) The effets of morphine- nd nlorphine-like drugs in the nondependent, morphine-dependent nd ylzoine-dependent hroni spinl dog. J. Phrm. exp. Ther., 198, GRAY, W.D., OSTERBERG, A.C. & SCUTO, T.J. (197). Mesurement of the nlgesi effiy nd poteny of pentzoine by the D'Armour nd Smith method. J. Phrm. exp. Ther., 172, HARRIS, L.S. & ROSENBERG, F.J. (1967). CNS effets of pentzoine nd other nlgesi ntgonists. Arhs Biol. Med. Exper., 4, HUGHES, J., KOSTERLITZ, H.W. & LESLIE, F.M. (1975). Effet of morphine on drenergi trnsmission in the mouse vs deferens. Assessment of gonist nd ntgonist potenies of nroti nlgesis. Br. J. Phrm., 53, HUTCHINSON, M., KOSTERLITZ, H.W., LESLIE, F.M., WATER- FIELD, A.A. & TERENIUS, L. (1975). Assessment in the guine-pig ileum nd mouse vs deferens of benzomorphns whih hve strong ntinoieptive tivity but do not substitute for morphine in the dependent monkey. Br. J. Phrm., 55, KOSTERLITZ, H.W. & LESLIE, F.M. (1978). mprison of the reeptor binding hrteristis of opite gonists interting with j- or K-reeptors. Br. J. Phrm., 64, LASAGNA, L. & BEEECHER, H.K. (1954). The nlgesi effetiveness of nlorphine nd nlorphine-morphine ombintions in mn. J. Phrm., 112, LORD, J.A.H., WATERFIELD, A.A., HUGHES, J. & KOSTER- LITZ, H.W. (1977). Endogenous opioid peptides: multiple gonist, nd reeptors. Nture, 267, MARTIN, W.R., EADES, C.G., THOMPSON, J.A., HUPPLER, R.E. & GILBERT, P.E. (1976). The effets of morphinend nlorphine-like drugs in the nondependent nd morphine-dependent hroni spinl dog. J. Phrm. exp. Ther., 197, O'CALLAGHAN, J.P. & HOLTZMAN, S.G. (1975). Quntifition of the nlgesi tivity of nroti ntgonists by modified hot plte proedure. J. Phrm. exp. Ther., 192, PEARL, J. & HARRIS, L.S. (1966). Inhibition of writhing by nroti ntgonists. J. Phrm. exp. Ther., 154, SEWELL, R.D.E. & SPENCER, P.S.J. (1976). Antinoieptive tivity of nroti gonist nd prtil gonist nlgesis nd other gents in the til-immersion test in mie nd rts. Neurophrm., 15, SKINGLE, M. & TYERS, M.B. (1979). Evlution of ntinoieptive tivity using eletril stimultion of the tooth pulp in the onsious dog. J. Phrm. Methods, 2, SKINGLE, M. & TYERS, M.B. (198). Further studies on opite reeptors tht medite ntinoieption: tooth pulp stimultion in the dog. Br. J. Phrm. (in press). SWAIN, H.H. & SEEVERS M.H. (1974). Evlution of new ompounds for morphine-like physil dependene in the rhesus monkey. Bull. Problems Drug Dependene, 36, Addendum, 1, SWAIN, H.H. & SEEVERS, M.H. (1976). Evlution of new ompounds for morphine-like physil dependene in the rhesus monkey. Bull. Problems Drug Dependene, 38, Addendum 2,

10 512 M.B. TYERS TABER, R.I., GREENHOUSE, D.D. & IRWIN, S. (1964). Inhibition of phenylquinone-indued writhing by nroti ntgonists. Nture, 24, VILLARREAL, J.E. & SEEVERS, M.H. (1972). Evlution of new ompounds for morphine-like physil dependene in the rhesus monkey. Bull. Problems Drug Dependene 34, Addendum 7, (Reeived August 16, 1979.)