The effects of locally applied capsaicin on conduction in

Transcription

1 Br. J. Phrmc. (1986), 89, The effects of loclly pplied cpsicin on conduction in cutneous nerves in four mmmlin species Richrd Brnowski, Bruce Lynn & Adrin Pini' Deprtment of Physiology, University College London, Gower Street, London WC1E 6BT 1 By exmintion of compound ction potentils in the sphenous nerve of the nesthetized rt it hs been shown tht cpsicin cuses rpid, dose-dependent, filure of conduction in mny C-fibres when pplied directly to the nerve. A lrge reduction in C-fibre conduction occurs with concentrtions s low s 11JiM. After 15-3 min exposure to cpsicin, only prtil recovery occurs in 1 h. 2 Similr block of C-fibre conduction occurs in the ferret. However, only smller, reversible, reductions in C-fibre conduction were seen in the guine-pig nd rbbit, even t the highest concentrtion of cpsicin used (33 mm). 3 A smll reduction in the A6 component of the compound ction potentil occurred in ll four species. In the rt nd ferret the effects were much less thn those on C-fibres. 4 At high doses, smll reversible effects were lso seen on the fstest conducting Aop component of the compound ction potentil in the rt, rbbit nd guine-pig; no effects were seen on the Ap fibres in the ferret. 5 Decreses in mplitude of the compound ction potentil were ccompnied by some slowing of conduction in most cses. The slowing ws less thn 5% except for the rt AXIP nd C-fibres nd the ferret C-fibres where 9-15% chnges occurred t the highest doses of cpsicin. 6 Opening the connective tissue sheth of the nerve did not significntly increse the effectiveness of cpsicin. Introduction Cpsicin tretment, either by locl ppliction to nerve trunks or by subcutneous injection, leds to () the depletion of substnce P nd other neuropeptides from somtic nd viscerl fferent neurones nd (b) reduction in behviourl responses to noxious stimuli nd in neurogenic inflmmtory rections (e.g. see Fitzgerld, 1983). This nd other evidence hs led to the suggestion tht substnce P plys n importnt role in nociception nd neurogenic inflmmtion (Lembeck & Gmse, 1982). However, there re number of situtions where the extent or time course of peptide depletion does not mtch the functionl chnges. For exmple, when cpsicin is pplied loclly to cutneous nerve in the rt the effect of ntidromic stimultion on vsculr permebility is bolished within few hours, well before ny mjor chnge is seen in peptide levels (Gmse et l., 1982). Becuse of these mismtches, the functionl role of substnce P nd other neuropeptides in nociception 'Present ddress: Dept of Phrmcology, University of Cmbridge, Hills Rod, Cmbridge CB2 2QD. nd neurogenic inflmmtion hs been questioned (Wll & Fitzgerld, 1982). In the cse of locl ppliction it hs been proposed tht the erly functionl chnges might rise if, in ddition to its long-term ction on peptide levels, cpsicin produced n immedite block of nerve conduction in fferent C-fibres (Gmse et l., 1982). Direct electrophysiologicl exmintion of conduction in rt nerves exposed to cpsicin hs, however, yielded conflicting results. Wll & Fitzgerld (1981) reported no effects on conduction whilst Petsche et l. (1983) found substntil, but rther vrible, conduction block in C-fibres. Our preliminry results in the rt indicted tht mrked effects on both C-fibre nd A-fibre conduction did occur (Pini, 1983; Lynn et l., 1984). As well s being relevnt to the question of how cpsicin produces its effects on nociception nd neurogenic inflmmtion, these results re of interest in their own right. Cpsicin ppers to produce prticulrly powerful nd long-lsting block of conduction in fferent C-fibres nd is therefore potentilly The Mcmilln Press Ltd 1986

2 268 R. BARANOWSKI et l. of interest s selective locl nesthetic gent. For this reson we hve looked im detil t the dosedependence of cpsicin's ction on conduction in A nd C-fibres in nerves in the rt. Preliminry experiments in the rbbit filed to show ny nerve blocking ction (Lynn & Pini, 1985b). In order to estblish tht cpsicin did ct in rnge of mmmlin species, we hve looked t the rbbit in more detil nd t two other species, the ferret nd the guine-pig. Methods Experiments hve been performed on hindlimb nerves of deeply nesthetized nimls. Rts, ferrets nd rbbits were nesthetized with urethne ( gkg-') given intrperitonelly (rts nd ferrets) or intrvenously vi the mrginl er vein (rbbits). Guine-pigs were nesthetized by the neuroleptnesthesi procedure described by Evns & Hrrison (198); initil i.p. doses of droperidol (5 mg kg- '), phenoperidene (1 mg kg-') nd sodium pentobrbitone (3mgkg-') were followed by pproximtely hourly injections of phenoperidene (1 mg kg- ') nd pentobrbitone (3mg kg-1). Blood pressure ws monitored vi the crotid rtery; systolic pressure ws usully bove 1 mmhg nd recordings were stopped if it fell below 6 mmhg. Rectl temperture ws mesured with thermistor probe nd ws mintined t 37.5 ± 1 C. Recordings were mde from the sphenous nerve in the upper leg in ll four species nd, in ddition, from the surl nerve in the poplitel foss in some rbbits. The rrngement of the electrodes nd the tretment site were similr in ll preprtions nd re shown in Figure 1. The nerve ws clered proximlly from the surrounding tissues over length of bout 7 mm nd cut t the centrl end. Recordings were mde using two pltinum wire electrodes, one t the cut end of the nerve nd one 2-4 mm distl. In some rt experiments the nerve ws deshethed t the recording site to improve the signl-to-noise rtio. In some ferret experiments the recordings were mde from the nerve distlly to the stimultion points nd with it left in continuity. Neither of these chnges ltered the generl pttern of the results obtined. A pir ofpltinum wires were used for stimultion nd were plced 9-54 mm from the recording point. A 3-5 mm length of nerve between the distl stimultion point nd the recording site ws clered for ppliction ofcpsicin. In some experiments second pir of stimulting electrodes ws positioned nerer to the recording site nd proximl to the tretment point, enbling conduction in untreted nerve to be monitored. A brrier (Figure l) ws positioned immeditely proximl to the tretment site to stop spred of cpsicin to the proximl stimulting electrodes or to the recording point. As fr s possible the blood supply to the nerve ws preserved. The temperture close to the nerve ws checked in series of similr experiments nd ws lwys within 1 C of the rectl temperture. Cpsicin ws dissolved in olive oil, solvent tht ws found in control experiments to be without effect on conduction. The solution ws pplied with smll pieces of cotton wool which were pcked round the nerve. The nerve sheth ws normlly left intct t the tretment site. However, in smll number of rbbit nd guine-pig experiments the sheth ws slit longitudinlly in the tretment re with smll piece of rzor blde. Cpsicin ws pplied for 15 or 3 min. Very little extr effect occurred fter 15 min except in the rbbit nd the time course of recovery ws similr for both tretment times. Dt from both tretment times hs been pooled where pproprite. After removl of the cpsicin-soked cotton wool the treted re ws wshed twice with olive oil nd twice with sline. Only one tretment ws pplied to ech nerve except where the first tretment ws low dose tht hd no effect, in which cse second tretment ws pplied fter n intervl of t lest 3 min. Conduction ws ssessed by recording compound ction potentils using n mplifier with low frequency time constnt of 1 s nd high frequency time constnts of 3 ls for A-fibre nd 1 ms for C-fibre potentils. The C nd Ab components of the compound ction potentil (c..p.) were usully diphsic in form, despite recording from cut end (e.g. see Figure l b). The most relible mesure of the size of the C potentil ws its pek-to-pek mplitude. Attempts to use re mesurements were not pursued becuse of the problems rising from the complex shpe of mny of the potentils. Amplitudes were mesured with just mximl stimulus strengths for ech mjor component of the c..p. nd ltencies were mesured to the beginning nd/or the pek ofech component, usully t twice the threshold stimulus strength. Chnges in pek-to-pek mplitude will only ccurtely reflect chnges in numbers offibres conducting if there re no mrked shifts in the spred of unit conduction delys. As check, smll smple of single fibres hs been exmined during ppliction of mximl dose of cpsicin in the rt nd rbbit. In the rt mny C-fibres were blocked by cpsicin, mostly those of the polymodl nociceptor type, s reported by Petsche et l. (1983). In the rbbit both c..ps nd single units were exmined in two preprtions. In one preprtion there ws no fll in the c..p. mplitude nd 3 single C-units exmined were lso not blocked. In the second preprtion lrge, reversible reduction in the c..p. occurred nd 5 single units were lso blocked with subsequent recovery. So in this smll smple there ws n excellent correltion between c..p. mesurements nd the behviour of single C-units.

3 NERVE CONDUCTION BLOCK BY CAPSAICIN 269 Appliction Proximl Distl stim site Stim Record b Figure 1 () Digrmmtic representtion of the preprtion. The brrier consisted of pcked pieces of cotton wool soked in sline in most experiments. In some rt experiments with 33 mm cpsicin, piece of thin cette sheet embedded in lginte dentl impression mteril ws used. P = polythene sheet to reduce spred to djcent tissues. In some experiments only the distl stimulting electrodes were used. Distnce from distl electrodes to recording electrodes rnged from 9-54 mm. (b) The different components of the compound ction potentil in the rt sphenous nerve before nd during the ppliction of.33 mm cpsicin in olive oil. Left pnel, control trces immeditely before ppliction; right pnel, 12 min fter pplying cpsicin. In ech pnel, upper trces show the Ax component, time clibrtion.4 ins, mplitude clibrtion 2 mv; middle trces show the A6 component, clibrtions 1. ms,.4 mv; lower trces show C component, clibrtions 1Oms, 1 pv. Further spects of this problem of relting conduction block to c..p. mplitude will be considered in the Discussion. The significnce of differences between mens ws determined by use of t tests unless otherwise stted. Drugs used were: cpsicin (8-methyl-N-vnillyl-6- nonenmide), Merck; droperidol (Droleptn), Jnssen; phenoperidene hydrochloride (Operidine), Jnssen; sodium pentobrbitone (Sgtl), My & Bker. Results The effect of cpsicin on conduction in cutneous nerves in four mmmlin species hs been exmined by recording compound ction potentils (c..ps). The generl shpe of the c..p. ws similr in ll the species. The A-fibres gve mjor fst-conducting pek tht will be referred to s the otp component. The slower A- fibres gve series of peks, with usully one cler-cut component t 8-13 m s conduction velocity. This ws the slowest component nd will be clled here the 6 c..p. However, it should be noted tht in ll nerves there were other smller peks tht lso conducted with velocities in the 6 rnge. The C-fibre component of the c..p. ws lwys clerly seprted from the A- fibre components. Its fstest fibres conducted t m s- nd the shpe ws di- or multi-phsic. Compound ction potentil mplitudes were smller for distl thn for proximl stimultion, presumbly becuse the ction potentils in individul fibres become desynchronized nd so summte less well. Pek-to-pek mplitudes of both A-fibre nd C- fibre components were pproximtely inversely proportionl to the conduction distnce over the rnge of conduction distnces used in this study. For exmple, in 8 rt experiments with two sets of electrodes the verge control c..p. mplitudes were 6.7 mv (A) nd 264 IV (C) for proximl stimultion with n verge conduction distnce of 1.8 mm nd 2.25 mv (A) nd 11 pv (C) for distl stimultion t 26.3 mm. Thus for n pproximte 2.5 fold increse in

4 27 R. BARANOWSKI et l. conduction distnce, nd so in verge conduction time, the c..p. mplitudes fell to 37-38%. In the four species exmined the pttern of c..p. chnges following cpsicin ppliction differed, so the results from ech will be considered in turn. Rt As shown in Figure lb, cpsicin pplied directly to the sphenous nerve produced profound reduction in the size of the C-fibre component of the c..p. with only smll shift in ltency, indicting substntil degree of conduction block in the treted segment of the nerve. Conduction in the untreted, proximl, prt of the nerve ws unffected. The onset of the conduction block ws rpid, within 1 min t high concentrtions. As shown in Figure 2c, the mximum effect ws reched in 5-15 min. On removl of the cpsicin the c..p. slowly incresed in size but 1 h lter ws still substntilly below control levels. This filure of recovery ws not due to ny generl deteriortion since the fst Amp component recovered fully nd lso in 13 experiments where the A nd C components were recorded with stimultion proximl to the tretment site, these remined stble throughout the period studied. The effect of different concentrtions of cpsicin is shown in Figure 2c. In the rt, gm produced no significnt effect whilst 11OgM produced lrge reduction in C-fibre c..p. Interestingly, even t the highest concentrtions used (1%; 33 mm) the C-c..p. ws not usully completely bolished. Recovery, even from low doses, ws slow nd substntil effect ws still present 45 min fter removing cpsicin (see Figure 2c). As in Figure Ib, the C-c..p. often comprised two components in which cse the lrger erly component ws usully reduced more thn the lte component. Chnges in C-c..p. mplitude were ccompnied by increses in conduction dely nd these re plotted in Figure 2f. With 33 mm cpsicin there ws n verge increse of 19% (± 4%, s.e.men, n = 4) whilst with 11IM the increse ws only 7% (± 3%, n= 5). The Aop component of the c..p. ws not significntly ffected by low doses of cpsicin (e.g. see Figure Ib) but ws reduced by 33 mm, the lrgest concentrtion used. As cn be seen from Figure 2, this component fell by 3% during 3min ppliction nd then lrgely recovered to control levels in 4-6min. As with the C-c..p., these mplitude chnges were ccompnied by ltency increses verging 12% (± 3%, n = 7) (see Figure 2d). A B b A 8 C C c = 4 ---I EL 2 d + o 13 I.... i... e + f, t o 12 S i. IK/t A1--} -l oo -J -3 Q 3Q/C 3 6 tv- Cps Cps on off -3 3/ 3 Cps Cps on off Time (min) -3 3/ Cps Cps on off 3 6 Figure 2 Amplitudes (-c) nd ltencies (d-f) of Ax (,d), Ab (b,e) nd C (c,f) components of the compound ction potentil (c..p.) in the rt sphenous nerve t vrious times before, during nd fter the ppliction ofcpsicin (Cps) t 3 different concentrtions: (----) 33mM, n = 7; (--) mm, n= 8-9; (...O..) 33 LM, n= 2-3. Error brs show s.e. When only 15 min pplictions were used, this is shown by brek in the grph nd postppliction points hve been plotted from the sme reference time s with 3 min pplictions.

5 NERVE CONDUCTION BLOCK BY CAPSAICIN 271 The slowest component of the A-fibre c..p., the 6 component, ws studied in most rts, but not in ny of those treted with 33 mm cpsicin. This component ws too smll to mesure in some nerves nd ws often the first component to suffer generl deteriortion during long recordings. Dt on the 6 fibres is thus rther less complete thn tht on the ot3 or C-fibres. However, significnt reductions were seen t concentrtions of 11 pm-1.1 mm in severl nerves (lthough not the one in Figure Ib) nd on verge there ws fll of 25% (±+ I 1%, n = 6). These reductions in c..p. mplitude re significntly smller thn the fll in C- fibre c..p., but re in turn significntly greter thn the effect on x fibres t these concentrtions. The reltively smll chnges in 6-c..p. mplitude were not ccompnied by ny significnt increses in ltency, lthough there ws slight trend in this direction (see Figure 2e). Ferret (Mustel putorius furo L) The min results from the ferret re summrized in Figure 3 together with trces showing typicl c..ps from the ferret sphenous nerve. In generl, ferret c..ps were much like those in the rt, but the A-fibres conducted bout 2% fster. The C-c..p. ws reduced mrkedly by cpsicin just s ws found in the rt, lthough n pproximtely 3 times greter concentrtion ws required to produce lrge effect (see Figure 3b). The durtion of the reduction gin lsted for t b ' _ 8 E C 11 I. Ji m J~~~~~~~~ 1 c : ''x. I / 3 6 Time (min) Cps Cps on off Figure 3 Effect of cpsicin (Cps) on c..ps recorded from the sphenous nerve of the ferret. () Typicl trces recorded 7 min before (upper pnel), 13 min fter strting (middle pnel) nd 58 min fter ending (lower pnel) n ppliction of 33 mm cpsicin in olive oil. In ech pnel trces re s follows: upper = Amxp component, time clibrtion.4 ms, mplitude clibrtion 2 mv; middle = Ad component, clibrtions 2 ms, 1 sv; lower = C component, clibrtions 1ms 1 iv. (b,c) Time course of mplitude (b) nd ltency (c) shifts of C-c..p. t 3 concentrtions: (-- A --) 33 mm, n = 4; (-A-).33 mm, n = 3; (...A *.. ) jim, n = 3. Error brs show s.e.

6 272 R. BARANOWSKI et l. lest 1 h fter removl of the cpsicin, lthough fter the lowest effective dose (33 gm) complete recovery ws mde by some nerves in 4 min. Some slowing of conduction occurred t the highest concentrtion, lthough the chnges in ltency were rther vrible (see Figure 3c) nd only rech sttisticl significnce if the dt from 33 gm nd 33 mm re pooled. There were no significnt effects on either the Aolp or A6 components of the c..p. t ny of the concentrtions tried. Guine-pig Typicl c..ps from the guine-pig sphenous nerve re shown in Figure 4. The potentils re notble for the reltively lrge mplitude of the C-fibre component which ws typiclly 2-3 times lrger thn in the rt or ferret. Cpsicin hd little effect on this potentil, s cn be seen from Figure 4d. The verge reduction with the highest concentrtion (33 mm) ws only 31% (+ 7%, n = 7), which lthough highly sttisticlly significnt (P <.1) is much smller thn the reduction seen in the rt nd ferret. There were lso no significnt shifts in ltency, the verge increse with 33 mm being only 1.7% (± 1.7%, n = 7). Finlly, the effects on C-c..p. mplitude recovered fully in 3 min, gin unlike the long-lsting chnges seen in the rt nd ferret. The Ab c..p. (Figure 4c) showed exctly the sme pttern of chnges s the C-c..p., lthough the dt vry rther more becuse of the greter difficulty in mesuring the smller Ad potentil. The Axp c..p. mplitude ws lso significntly reduced by 33 mm cpsicin (Figure 4b). However, pired comprison within nerves showed tht on verge the effect on the AP c..p. ws significntly less thn tht on the Ad nd C components. 1 - b ro/ 8 6 I.= c.5 Q _ E ] 1 8 6L I tj~~~ / 15 3 * + Time (min) Cps Cps on off Effect of cpsicin on c..ps from the sphenous nerve of the guine-pig. () Typicl trces recorded before Figure 4 (upper pnel) nd during (lower pnel) the ppliction of33 mm cpsicin. Top trces Axp component, time clibrtion.4 ms, mplitude clibrtion 1 mv; middle trces, Ab component, clibrtions 1 ms,.2 mv; lower trces, C component, clibrtions 1 ms,.4 mv. Conduction distnce 15 mm. Note reltively lrge C-c..p. nd lck ofeffect of cpsicin. (b-d) Averge (± s.e.) c..p. mplitudes before, during nd fter 15 min exposure to 33 mm (, n = 7) or 3.3 mm (-, n = 5) cpsicin; (b) Ap-fibres; (c) Ab-fibres; (d) C-fibres.

7 NERVE CONDUCTION BLOCK BY CAPSAICIN 273 Rbbit The c..p. ws exmined in both the sphenous nd surl nerves in the rbbit. In both nerves its shpe ws very like tht recorded in the guine-pig sphenous nerve nd it behved similrly in response to 33 mm cpsicin. Significnt effects were only seen t 1 nd 33 mm, these concentrtions reducing ll components of the c..p. (see Figure 5). After 1-15 min exposure to 33 mm cpsicin, the C-fibre component of the c..p. ws reduced by 34% (±9%, n = 6), the Ab component by 48% (± 12%) nd the AP component by 17% (± 7%). With longer pplictions further reduction occurred nd by 2-3 min ll components were on verge significntly reduced. The slow onset of the effect nd its continuing increse over 3 min were in contrst to the sitution in the other three species. The mplitude reductions vried mrkedly between preprtions, whilst the effects on different components in given preprtion were firly similr. Anlysis of vrince reveled tht within nerves there ws significntly smller effect on the AP component thn on the A5 nd C components. The mplitude 1 r (I u.. I _ - + b C 1 t I. <: c t Cps on ol-- I- decreses were ccompnied by smll but significnt increses in ltency of bout 5% t 2-3 min. The time course of recovery ws very vrible. The 5 nerves whose dt re pooled in Figure 5 showed no significnt remining effect t 1 h post-tretment. Two other nerves studied in preliminry experiments lso showed no fll in c..p. mplitudes with 33 mm cpsicin. However, in 2 other preprtions there were lrge irreversible flls in ll c..p. components. It is possible tht these reductions result from generl deteriortion in the preprtion, but it my lso be tht minority of rbbit nerves re considerbly more sensitive to cpsicin thn is indicted by the verges plotted in Figure 5. To test if the reltive ineffectiveness of cpsicin in the rbbit nd guine-pig ws due to the connective tissue sheth round the nerve, this ws slit open t the ppliction site in I rbbit nd 3 guine-pig nerves. Cpsicin (33 mm) pplied to these nerves produced n verge fll in c..p. mplitude (ll components) to 8.2% compred with 7.3% for nerves with sheth intct. Thus it ppers tht the nerve sheth does not ct s significnt brrier to the diffusion ofcpsicin. Comprison between the four species The C-c..p. mplitudes nd ltencies during the ppliction of different concentrtions of cpsicin in ll the species exmined re plotted in Figure 6. The ferret nd rt re clerly much more sensitive to direct xoril ppliction of cpsicin thn re the rbbit or guine-pig. Also, the effects seen in these ltter 2, species reversed within 1 h of removl of cpsicin, whilst the effects on the rt nd ferret lsted well in -. T excess of this time. '~ ~{~ l - Significnt effects on the A-fibre components of the 4s>+_4- c..p. only occurred with the highest concentrtion..... used (33 mm). The mplitude chnges for the Aocp nd A6 components re summrized in Figure 7 long with those of the C-c..p. t this concentrtion. Ferret Acxp ~ T _ Jcomponents were unffected whilst guine-pig, rbbit I---1- nd rt Ax, c..ps, were reduced by 17-29%. On verge, the A5 c..ps were ffected more thn the Acxp ones, but were not reduced by s much s C-c..ps in the rt nd ferret. Ltency shifts were never s mrked 2 3/ s the mplitude chnges, but in most cses significnt i mplitude decreses were ccompnied by significnt Cps off Figure 5 Amplitudes of different components of the c..p. before, during nd fter 3min ppliction of 33 mm (O) or 3.3 mm (*) cpsicin to cutneous nerves in the rbbit. () Axp component; (b) Ab component; (c) C component. Averges for 4 nerves (3.3 mm) or 5 nerves (33 mm); nerves used were surl (2) nd sphenous (2 or 3). Error brs for 33 mm, show 1 s.e.; errors for 3.3 mm were smller thn the symbol size. increses in conduction dely. Discussion Assessment ofconduction blockfrom compound ction potentils It is importnt to consider first the extent to which the c..p. mplitude gives relible mesure ofconduction

8 274 R. BARANOWSKI et l 4 * E J b Concentrtion (mm) 1 F f1- -' T > Concentrtion (mm) Figure 6 C-fibre c..p. mplitudes () nd ltencies (b) during cpsicin ppliction to cutneous nerves in 4 species. Averge vlues from 3-7 nerves mesured 1-15 min fter beginning ppliction nd expressed s % of the immedite pretretment vlue: (@) rt; (A), ferret; (U), guine-pig; (*), rbbit. block. Ifconduction in single xons is not slowed then, for given conduction distnce, the c..p. mplitude will vry directly with the number of xons conducting. However, if there is differentil slowing of conduction in single xons then the c..p. mplitude will fll without ny conduction block. For popultion of xons conducting t different but constnt velocities, differentil slowing will occur simply with incresing conduction distnce. We hve found tht the mplitude of both A nd C components of the c..p. re pproximtely inversely proportionl to the conduction distnce, nd so to the conduction ltency. Cpsicin tretment cused ltency increses of less thn 2%, so if these were uniform shifts for ll fibres, then the ssocited chnges in mplitude would be smll. If shifts in ltency only occurred in subpopultion of fibres, then the ltencies of c..p. components might not show ny chnge. However, if M _~ Figure 7 C tp C 8 UP C b B C 8 d Rt Ferret Guine- Rbbit pig Reduction in size of different components of the compound ction potentil in four species mesured 1-15 min fter pplying 33mM cpsicin. Averge results, with s.e., from 7 nerves in rt nd guine-pig, 2-4 in ferret nd 5 in rbbit. *No dt re vilble for 6 fibres in the rt t 33mM, but t.i1-1.1mm n verge reduction of 24.9% (± 11.4, n = 6) ws found (see Figure 2b). lrge group of fibres were differentilly slowed this ought to produce mrked chnge in the shpe of the c..p., but such effects were not seen. It ppers, therefore, tht smll degree of the observed mplitude reduction might be due to slowing of conduction. However, the mjor effects, such s those of C-fibres in the rt nd ferret, must be due lrgely to conduction block. This is lso in greement with the results of single unit recordings (Petsche et l., 1983; Lynn & Pini, unpublished dt) which show tht mny fferent C-fibres re blocked by cpsicin. The sensitivity of cutneous nervefibres to cpsicin in different mmmlin species Our results clerly show tht cpsicin exerts powerful blocking ction on C-fibres in cutneous nerves in the rt, thus confirming the findings of Petsche et l. (1983). The negtive results of Wll & Fitzgerld (1981) my hve been due to their use of Tween 8/lcohol vehicle which itself cused some conduction block. Cpsicin does not block ll C- fibres in the rt; even t high concentrtions some remin conducting with velocities in the norml rnge. Sympthetic postgnglionic C-fibres re unffected by cpsicin (Hndwerker et l., 1984) nd it my be lrgely these fibres tht form the cpsicin-resistnt component of the C-c..p. Cpsicin lso blocks C- fibres in the ferret nd hs been reported to block C- fibres in the monkey (Chung et l., 1985) nd the ct (Such & Jncso, 1985). In contrst, in the guine-pig nd rbbit the effects of cpsicin re smller, only occur t high concentrtions nd re not selective for

9 NERVE CONDUCTION BLOCK BY CAPSAICIN 275 C-fibres. One possible explntion for the difference between the rt nd ferret nd the rbbit nd guinepig might be tht the ltter two species hve higher proportion of sympthetic C-fibres in their cutneous nerves. This would be consistent with the reltively lrge C-c..ps in rbbit nd guine-pig nerves. However, there is no reson to expect such difference nd previous single unit study found tht, s in the rt, the gret mjority of C-fibres in the sphenous nerve of the rbbit hd cutneous receptive fields nd so presumbly re fferent, not efferent, fibres (Lynn, 1979). In the rt the blocking ction of cpsicin occurs t concentrtions of 11IOM, some 3 times less thn the 33 mm (1%) used in mny studies nd this demonstrtes the importnce of restricting the spred of cpsicin during experiments designed to show locl effects. It lso rises the possibility tht when rts re given lrge subcutneous or intrmusculr injections of 5 mg kg- 1 or more, tissue concentrtions might rech levels tht cuse C-fibre conduction block. This is consistent with the rpid onset of the hypo-lgesi reported in dult rts fter single injection of cpsicin (Hyes et l., 198). Conduction ws ffected in most myelinted s well s unmyelinted fibres when they were exposed to high concentrtions (33 mm) of cpsicin. Reductions in A6 fibre c..p. mplitudes hve lso been found in ct nd monkey (Such & Jncso, 1985; Chung et l., 1985), nd in Aoxp c..p. mplitudes in the ct (Such & Jncso, 1985). At lower concentrtions (<1.1 mm) the only effects were on Ab fibres in the rt. The durtion of the ction on A6 fibres ws hrd to ssess in our experiments becuse this potentil tended to deteriorte during long recording periods. Whether the ction is selective for different functionl types of A6 fibres is not known. Mechnism of conduction block In vitro experiments hve shown tht cpsicin depolrizes mmmlin C-fibres (Ault & Evns, 198; Hyes et l., 1984) nd their cell bodies (Heymn & Rng, 1985) nd tht the extent of depolriztion correltes well with the degree of conduction block (Mrsh, 1985). In rt C-neurones this depolriztion is due to non-specific increse in ionic permebility (Heymn & Rng, 1985). The conduction block could rise from the xons ccommodting to the mintined depolriztion nd the slowing of conduction my lso be due to ccommodtion in the treted re. In fct the chnge in conduction velocity loclly must be substntil since this prt of the nerve formed less thn 25% of the totl conduction distnce. Cpsicin hs little effect on xonl conduction in the rbbit or guine-pig. However, rbbits nd especilly guine-pigs re sensitive to the neurotoxic nd irritnt effects of cpsicin (Jncso, 196; Buck & Burks, 1983). Possibly xons, cell bodies nd peripherl terminls show different reltive sensitivity to cpsicin in different species. On the bsis of structurectivity reltions it hs been suggested tht cpsicin intercts with specific membrne receptor (Szolcsnyi & Jncso-Gbor, 1975). If so, receptors my be evenly distributed over ll the membrne of neurones with C-xons in rt nd ferret, but be restricted to the terminls in the rbbit nd guine-pig. In contrst, rt neurones with myelinted xons show no effects from cpsicin ppliction to the cell bodies (Heymn & Rng, 1985) lthough we hve shown cler effects on xons. If these xonl effects re lso due to interction with specific receptor then it ppers tht the distribution of receptors cn vry between types of neurone s well s between species. The long durtion of the blocking ction of cpsicin in the rt nd, t high concentrtion, in the ferret is of interest. Exposure to cpsicin is known to cuse swelling of vgl nerve C-fibres (Mrsh, 1985) nd disintegrtion of processes in cultured dorsl root gnglion cells (P.G. Hogn, unpublished results). It my be, therefore, tht the long lsting reduction in C- fibre potentils results from xonl dmge cused by cpsicin. Consistent with this is our preliminry finding tht C-fibre conduction in the treted segment remins reduced for 1-2 dys fter exposure to cpsicin (Lynn et l., 1984). Conduction block nd nociceptive rections In mny previous studies of the ction of cpsicin, ttention hs focussed on its bility to deplete sensory neurones of substnce P nd other neuropeptides. However, peptide depletion is slow, becoming significnt only h fter tretment, whilst reductions in nociceptive rections nd in neurogenic inflmmtory rections my occur within few hours (Gmse et l., 1982). It is now cler tht s proposed by Gmse et l. (1982), these rpid ctions re due to n immedite effect of cpsicin on conduction in fferent C-fibres. This blocking ction cn clerly resolve mny of the discrepncies between the time course of nociceptive chnges nd of chnges in peptide levels tht were highlighted by Wll & Fitzgerld (1982). The long durtion of cpsicin's ction on C-fibres, which lsts for more thn 4 months (Lynn & Pini, 1985), lso requires considertion when interpreting the long-term functionl consequences of cpsicin tretment in dult rodents. We wish to thnk Mr Bruce Cotsell for skilled technicl ssistnce. This study ws supported by project grnt from the Medicl Reserch Council. R.B. nd A.P. were in receipt of MRC Scholrships.

10 276 R. BARANOWSKI et l. References AULT, B. & EVANS, R.H. (198). Depolrizing ction of cpsicin on isolted dorsl root fibres of the rt. J. Physiol., 36, 22-23P. BUCK, S.H. & BURKS, T.F. (1983). Cpsicin: hot new phrmcologicl tool. Trends in Phrmcologicl Sciences (TIPS), 4, CHUNG, J.M., LEE, K.H., HORI, Y. & WILLIS, W.D. (1985). Effects of cpsicin pplied to peripherl nerve on the responses ofprimte spinothlmic trct cells. Brin Res., 329, EVANS, E.F. & HARRISON, R.V. (198). Neuroleptnesthesi: n idel nesthetic procedure for physiologicl reserch. J. Physiol., 298, 13P. FITZGERALD, M. (1983). Cpsicin nd sensory neurones - review. Pin, 15, GAMSE, R., PETSCHE, U., LEMBECK, F. & JANSCO, G. (1982). Cpsicin pplied to peripherl nerve inhibits xoplsmic trnsport of substnce P nd somtosttin. Brin Res., 239, HANDWERKER, H.O., HOLZER-PETSCHE, U., HEYM, C. & WELK, E. (1984). C-fibre functions fter topicl ppliction of cpsicin to peripherl nerve nd fter neontl cpsicin tretment. In Antidromic Vsodilttion nd Neurogenic Inflmmtion, ed. Chhl, L.A., Szolcsnyi, J. & Lembeck, F. pp Budpest: Akdemii Kido. HAYES, A.G., HAWCOCK, A.B. & HILL, R.G. (1984). The depolrising ction of cpsicin on rt isolted scitic nerve. Life Sci., 35, HAYES, A.G., SKINGLE, M. & TYERS, M.B. (198). The ntinociceptive effects of single doses of cpsicin in the rodent. Br. J. Phrmc., 7, 96P. HEYMAN, 1. & RANG, H.P. (1985). Depolrizing responses to cpsicin in sub-popultion of rt dorsl root gnglion cells. Neurosci. Letts, 56, JANSCO, N. (196). Role of the nerve terminls in the mechnism of inflmmtory rections. Bull. Millrd Fillimore Hosp., Bufflo, NY., 7, LEMBECK, F. & GAMSE, R. (1982). Substnce P in peripherl sensory processes. In Substnce P in the Nervous System, CIBA Foundtion symposium, pp London: Pitmn. LYNN, B. (1979). The het sensitiztion of polymodl nociceptors in the rbbit nd its independence of the locl blood flow. J. Physiol., 287, LYNN, B., CARPENTER, S.E. & PINI, A. (1984). Cpsicin nd cutneous fferents. In Antidromic Vsodilttion nd Neurogenic Inflmmtion, ed. Chhl, L.A., Szolcsnyi, J. & Lembeck, F. pp Budpest: Akdemii Kido. LYNN, B. & PINI, A. (1985). Long-term block of fferent C- fibres following cpsicin tretment in the rt. J. Physiol., 362, 19P. LYNN, B. & PINI, A. (1985b). Effects of cpsicin on nociceptive fferents from the skin. Phil. Trns. R. Soc. B., 38, 191. MARSH, S.J. (1985). Depolriztion of rt vgl C-fibres nd sensory neurones by cpsicin in vitro. J. Physiol., 36, 14P. PETSCHE, U., FLEISCHER, E., LEMBECK, F. & HAND- WERKER, H.O. (1983). The effect of cpsicin ppliction to peripherl nerve on impulse conduction in functionlly identified fferent nerve fibres. Brin Res., 265, PINI, A. (1983). Effects of cpsicin on conduction in cutneous nerve of the rt. J. Physiol., 338, 6-61P. SUCH, G. & JANCSO, G. (1985). Axonl effects of cpsicin: n electrophysiologicl study. Actphysiol. hungric, 66, SZOLCSANYI, J. & JANCSO-GABOR, A. (1975). Sensory effects of cpsicin congeners. I. Reltionship between chemicl structure nd pin-producing potency of pungent ctions. Arzneim.-Forschung, 25, WALL, P.D. & FITZGERALD, M. (1981). Effects of cpsicin pplied loclly to dult peripherl nerve. I. Physiology of peripherl nerve nd spinl cord. Pin, 11, WALL, P.D. & FITZGERALD, M. (1982). If substnce P fils to fulfill the criteri s neurotrnsmitter in somtosensory fferents, wht might be its function? In Substnce P in the Nervous System, CIBA Foundtion Symposium, pp London: Pitmn. (Received December 23, Revised My 16, Accepted June 2, 1986.)