- Email: [email protected]
Antinociceptive effect of (S)-N-desmethyl trimebutine against a mechanical stimulus in a rat model of peripheral neuropathy
Life Scii
ELSEVIER
PII !%024-3205(99)00609-8
Vol.66, No. 5, pp. 433-439.2000 COp)+htO1999Wmia.PhC8lllC. RinkdinlhsUSA. Allri@rosawd 0024-3205~ tiuntmamr
ANTINOCICEPTIVE EFFECT OF (S)-N-DESMETHYL TRIMEBUTINE AGAINST A MECHANICAL STIMULUS IN A RAT MODEL OF PERIPHERAL NEUROPATHY Valerie Kayser’, Dennis Christensen’
, Gistle Guilbaud’,
Francois Roman*
‘Unite de Recherches de Physiopharmacologie du Systeme Nerveux, I.N.S.E.R.M. U-161,2 rue d’AlCsia, 75014 Paris, France *Institut de Recherche JouveinaWarke Davis, 1 l-1 3 rue de la Loge, 94265 Fresnes Cedex, France (Ikceived
in final form &September9, 1999)
Trimebutine (2-dimethylamino-2-phenylbutyl 3,4,5-trimethoxybenzoate, hydrogen maleate) relieves abdominal pain in humans. In the present study, the antinociceptive action of systemic (S)-N-desmethyl trimebutine, a stereoisomer of N-monodesmethyl trimebutine, the main metabolite of trimebutine in humans, was studied in a rat model of neuropathic pain produced by chronic constriction injury to the sciatic nerve. Mechanical (vocalization threshold to hindpaw pressure) stimulus was used. Experiments were performed two weeks after surgery when the pain-related behaviour has fully developed. (S)-Ndesmethyl trimebutine (1, 3, 10 mg/kg s.c.) produced dose-dependent antinociceptive effects on the nerve-injured and the contralateral hindpaw. The effect of the lowest dose (1 mg/kg s.c.) of (S)-N-desmethyl trimebutine on the nerve-injured paw was equal to that seen after a ten time stronger dose on the contralateral paw. The effect of (S)-N-desmethyl trimebutine (1 mg/kg) was not naloxone reversible. The results suggest that systemic (S)-N-desmethyl trimebutine may be useful in the treatment of some aspects of neuropathic pain. Key Words: trimebutine, naloxone, mechanical stimulation, neurq?athic Pain
Trimebutine (2-dimethylamino-2-phenylbutyl 3,4,5trimethoxybenzoate hydrogen maleate) is used in the treatment of various digestive tract disorders including dyspepsia, irritable bowel syndrome and post-operative ileus. When orally administered, trimebutine is mainly metabolized to N-monodesmethyl trimebutine in the human liver. N-monodesmethyl trimebutine is the main metabolite of trimebutine with plasma levels higher than that of the parent compound. Recently, trimebutine and N-monodesmethyl trimebutine have been shown to influence the activity of visceral afferent nerves in the rat (1). It has been known for years that trimebutine exhibits weak opioid properties (2). Lately it was demonstrated in addition that trimebutine, N-monodesmethyl trimebutine and their corresponding stereoisomers display du Corresponding author: Dr. Valerie Kayser, Unite de Recherches de Physiopharmacologie systeme nerveux, INSERM U161.2, rue d’Altsia, 75014 Paris, France; Tel: 33-l-40-78-93-50, Fax: 33-l-45-88-13-04; e-mail: [email protected]
434
(S)-Ndesmethyl Trimebutine and Neuropathic Pain
Vol. 66, No. 5, 2000
sodium channels blocking properties and inhibit veratridine-induced glutamate release in vitro (3), with the relative potency being (S)-N-monodesmethyl trimebutine>N-monodesmethyl trimebutine>trimebutine. These properties of trimebutine, N-desmethyl trimebutine and their respective stereoisomers suggest that the compounds may demonstrate a therapeutic potential in humans broader than that expected for a simple spasmolytic compound. Nerve damage that affects peripheral nerves leads to abnormal pain states referred to as neuropathic pain. This pain syndrome is usually poorly controlled by currently available medications. A rat model of peripheral mononeuropathy is produced by persistent moderate constriction of the common sciatic nerve (4, 5). In this model, a mechanical test (vocalization threshold to hindpaw pressure), which reflects a centrally integrated response is especially sensitive to analgesic compounds (6-10). In the present study, we wanted to investigate the potential antinociceptive desmethyl trimebutine ((S)-N-desmethyl-TMB) in neuropathic rats.
effect of (S)-N-
Methods
The Committee for Research and Ethical Issues of the International Association for the Study of Pain (IASP) Ethical Guidelines (11) were adhered to in these studies. In particular, the duration of the experiments was as short as possible and the number of animals was kept to a minimum. Animals. Male Sprague-Dawley rats (Charles River, France, strain designation Crl :CD(SD)BR), n= 54, weighing 175-200 g on arrival were used. The rats were housed at the experimental facilities for a week prior to the experiments. They were maintained on a 12 h light/dark cycle and had free access to standard laboratory food and tap water. The ambient temperature was 20-22°C. Surgery. The unilateral peripheral mononeuropathy was produced on the right hind limb according to the method described by Bennett and Xie (4) and Attal et al. (5). Rats were anaesthetized with sodium pentobarbitone (Nembutal, 50 mg/kg Lp.). The common sciatic nerve was exposed by blunt dissection at the level of the mid-tigh; four ligatures (S-O chromic catgut, about l-mm spacing) were placed around the nerve. Antinaciceptive testing. Experiments were carried out in a quiet room. Animals were not acclimatized to the test situations beforehand. The experimenter was unaware of the drug and doses used. Each animal received drugs only once and was used in only one experiment. The antinociceptive action was determined by measuring the vocalization threshold elicited by pressure on both the nerve-injured and the contralateral hindpaw, using the Ugo Basile (Comerio, Italy) analgesymeter. This instrument generates a linearly increasing mechanical force applied by a dome-shaped plastic tip (diameter=1 mm) on the dorsal surface of the paw. The tip was positioned between the third and fourth metatarsus (into the sciatic nerve territory) and force was applied until the rat squeaked. For each rat, a control threshold (mean of two consecutive stable thresholds expressed in g) was determined before injecting the drug. The vocalization thresholds were then measured every 10 min, until they had returned to the level of the control values. Drugs. (S)-N-desmethyl-TMB maleate (Sipsy, Avrille, (Narcan@, Du Pont Pharma, Paris, France) were diluted co-injected with (S)-N-desmethyl-TMB. The 0.1 mg/kg it has prevented the effect of 1 mg/kg iv. of morphine
France) and naloxone hydrochloride in saline (0.9% NaCl). Naloxone was dose of naloxone was chosen, because in neuropathic rats in previous studies
Vol.66, No. 5,2ooO
(S)-NdesmethylTrimebutine and Neuropathic Pain
435
(6, 8-g). In each group, the control rats received the same volume of saline.
S~utistics: Data are expressed as means f S.E.M. The areas under the curves (AUC) were calculated using the trapezoidal rule. Statistical significance of the data was analysed by oneway analysis of variance (ANOVA). The observed significances were then confirmed with Tukey’s test. Simple regressions (linear model) were performed to establish dose-dependent effects. Statistical analyses were carried out using a statistical computer program (Statgraphics Plus, Manugistics, Rockville, MD). P&O5 was used as the criterion for statistical significance. Results A 600 z -
0
Nerve-injured
paw
1
I,
I
,
,
,
,
,
60
40 Contralateral
,
,
,
60
, 100
l
8 , 0
20
( 120
paw * saline A @I-N-drsmethyl-TMB 0 (S)-N-dormethyl-TMB 0 H-N-drsmrthyl-TMB
oJ,
,
*
9
1 mg/kg 3 mg/kg t 0 mg/kg
++
,
-
,
8
60
40
Time
,
60
-
,
100
r
,
120
(mid
Fig. 1.
Effect of S.C. (S)-N-desmethyl trimebutine ((S)-N-desmethyl-TMB) on the vocalization threshold to pressure on the nerve-injured (A) and the contralateral (B) hindpaw in neuropathic rats. Each point represents mean f SEM of 9 animals. * P< 0.05, ** P< 0.01 vs. pre-injection value.
436
(S)-Ndesmethyl Trimebutine and Neuropathic Pain
Vol. 66, No. $2000
In agreement with previous studies, (see 9 and references therein) the vocalization threshold of the nerve-injured hindpaw was decreased 2 weeks after the surgery (209 f 4 g vs the preconstriction value 277 f 8 g, P < 0.001, n= 54), by 25%. The threshold of the contralateral hindpaw was not modified (265 f 5 g vs the pre-operative value 270 k 6 g, n= 54).
The effect of (S)-IV-desmethyl-TMB. In the nerve-injured paw, saline had no effect on the vocalization threshold (Fig 1A). (S)-N-desmethyl-TMB (1 mg/kg) produced an antinociceptive effect, that peaked (330 f 30 g, 180% of the pre-injection value) at 30 min and lasted for 60 min (P < 0.001). The effect of (S)-N-desmethyl-TMB (3 mg/kg) peaked (540 + 42 g, 250%) at 20 min and lasted for 60 min (P < 0.001). The effect of (S)-N-desmethyl-TMB (10 mg/kg) peaked (570 f 60 g, 253%) at 30 min and lasted for 100 min (P < 0.001, Fig. 1A).
In the contralateral paw, saline or (S)-N-desmethyl-TMB at 1 mg/kg had no effect (Fig 1B). (S)-N-desmethyl-TMB (3 mg/kg) produced an effect that peaked (402 + 40 g, 148%) at 20 min and lasted for 30 min, whereas the effect of (S)-N-desmethyl-TMB at 10 mgkg peaked (450 k 40 g, 166%) at 30 min and lasted for 50 min (P < 0.001 for both doses) (Fig. 1B). Significant dose-effect relationships were observed for both the nerve-injured (r= 0.77, P < 0.001) and the contralateral (r= 0.79, P < 0.001) paw. As shown in Fig. 2, the overall effect of (S)-Ndesmethyl-TMB was enhanced on the nerve-injured paw compared to the contralateral paw for all the three doses. 30
1 1
0 Nerve-injured
paw
-2 .E x 2o WI g r 0
IO-
:
o;
lb
; Dose
of (SFN-desmethyl-TMB
(mg/kg)
Fig. 2. AUCs (100 g x min) of the respective time-curves in Fig. 1. *P
Vol. 66, No. 5, 2000
WNdesmeIhyl
Trimebutine.and Neuropathic Pain
The antinociceptive effect of (S)-N-desmethyl-TMB sedation or motor disturbances.
was not associated
437
with any detectable
The effect of the combination of (S)-N-desmethyl-TMB and naloxone. We observed no change in the effect of (S)-N-desmethyl-TMB after injecting naloxone (0.1 mgkg i.v.) in combination with (S)-N-desmethyl-TMB (1 mgkg, Fig. 3).
600 3
5 E
0
5
Nerve-injured A
paw
(S)-N-desmethyl-TM6 + saline
1 mg/kg
* (Sl-N-desmethyl-TMB
1 mg/kg
Time (min) Fig. 3.
Effect of (S)-N-desmethyl-TMB alone and of the combination of (S)-N-desmethyl-TMB and naloxone (Nx) on the vocalization threshold to pressure of the nerve-injured hindpaw in neuropathic rats. For further details, see Fig. 1.
Here, we demonstrate that systemic (S)-N-desmethyl-TMB produces a profound, dosedependent antinociceptive effect against a mechanical stimulus in neuropathic rats. The overall effect of the lowest dose of (S)-N-desmethyl-TMB (1 mgikg s.c.) in the nerve-injured paw was close to what we have previously found after an i.v. injection of 1 mgkg of morphine (6,9-10). The effect of (S)-N-desmethyl-TMB was enhanced in the nerve-injured paw compared to the contralateral paw. The effect of the lowest dose (1 mgkg s.c.) of (S)-N-desmethyl trimebutine on the nerve-injured paw was equal to that seen after a ten time stronger dose on the contralateral paw. The result is consistent with a recent report (12) showing that trimebutine and
(S)-Ndesmethyl Trimebutine and Neuropathic Pain
438
N-desmethyl-TMB pain.
produce antinociceptive
Vol. 66, No. 5, 2000
effects in a rat model of persistent
inflammatory
We further found that naloxone was unable to modify the (S)-N-desmethyl-TMB-induced antinociception. This suggests that, at least at low doses, the antinociceptive effects of (S)-Ndesmethyl-TMB in neuropathic rats are mainly due to other than the opioid properties of the parent compound, trimebutine demonstrated in previous studies (2). Our data agree with recent reports suggesting that the antinociceptive effects of trimebutine and its metabolites are mainly due to non-opioid mechanisms (1,3) and are naloxone-insensitive in rat models of persistent inflammatory pain (12). (S)-N-desmethyl-TMB displays sodium channel blocking properties and has been shown to inhibit veratridine-induced glutamate release in rat spinal cord slices (3). All these mechanisms may well contribute to the profound antinociceptive effect of (S)-N-desmethyl-TMB in the present model of neuropathic pain as well as in other pain forms. The development of neuropathic pain involves an increase in the spontaneous activity of injured nerves, which may underlie the increase in excitability of neurons in the spinal cord (13). It has been shown that sodium channel blockers such as lidocaine inhibit the spontaneous nerve activity in rats with a sciatic chronic constriction injury (14). In addition, there is evidence that the hyperexcitability of spinal cord neurons are mediated, in part, by glutamate acting on the NMDA receptor in the spinal dorsal horn (15,16). Indeed, concomitant systemic administration of relative low doses of morphine and an NMDA receptor antagonist results in potent antinociception in rats with a sciatic chronic constriction injury (10). We can speculate that the compound by interfering directly with these mechanisms has an action on the hyperexcitability associated with the neuropathic pain state. In conclusion, we demonstrated a potent antinociceptive effect of (S)-N-desmethyl trimebutine against a mechanical stimulus in neuropathic rats, which suggests that the compound may be useful also in the treatment of some aspects of neuropathic pain.
References 1.
2. 3.
4. 5. 6. 7. 8. 9. 10
C. LACHEZE, A.M. COELHO, J. FIORAMONTI and L. BUENO, .I. Pharm. Pharmacol. 50 921-928 (1998) F. ROMAN, X. PASCAUD, J.E. TAYLOR and J.L. JUNIEN, J. Pharm. Pharmacol. 39 404-407 (1987). F. ROMAN, S. LANET, J. HAMON, G. BRUNELLE, A. MAURIN, P. CHAMPEROUX, S. RICHARD and M. GOLA, J. Pharm. Exp. Ther. 289 1391- 1397 (1999). G.J. BENNETT and Y.K. XIE, Pain 33 87-107 (1988). N. ATI’AL, F. JAZAT, V. KAYSER and G.GUILBAUD, Pain 41235251 (1990). N. ATTAL, Y.L. CHEN, V. KAYSER and G. GUILBAUD, Pain 47 65-70 (1991). D. ARDID and G. GUILBAUD, Pain 49 279-287 (1992). V. KAYSER, J. DESMEULES and G. GUILBAUD, Pam 60 275-285 (1995). J.J. IDANPAAN-HEIKKILA, S. PERROT, G. GUILBAUD and V. KAYSER, Eur. J. Pharmacol. 325 155-164 (1997). D. CHRISTENSEN, J.J. IDANPAAN-HEIKKILA, G. GUILBAUD and V. KAYSER, Brit. J. Pharmacol. 125 1641-1650 (1998).
Vol. 64, No. $2000
11. 12. 13. 14. 15. 16.
(S)-Ndesmethyl Trimebutine and Neumpathic Pain
439
COMMITTEE FOR RESEARCH AND ETHICAL ISSUES OF THE IASP, Pain 16 109-l 10 (1983). L. DIOP, C. LAMBERT, G. BRUNELLE and F. ROMAN, Neurogastroenterology and motility 10462 (1998). C.J. WOOLF, Nature 306 686-688 (1983). M. L. SOTGIU, G. BIELLA, A. CASTAGNA, M. LACERENZA and P. MARCHETIINI, Neuroreport 5 873-876 (1994). C.J. WOOLF and S.W.N. THOMPSON, Pain 44 293-299 (1991). A.H. DICKENSON and A.F. SULLIVAN, Neuropharmacol. 26 1235 1238 ( 1987).
ELSEVIER
PII !%024-3205(99)00609-8
Vol.66, No. 5, pp. 433-439.2000 COp)+htO1999Wmia.PhC8lllC. RinkdinlhsUSA. Allri@rosawd 0024-3205~ tiuntmamr
ANTINOCICEPTIVE EFFECT OF (S)-N-DESMETHYL TRIMEBUTINE AGAINST A MECHANICAL STIMULUS IN A RAT MODEL OF PERIPHERAL NEUROPATHY Valerie Kayser’, Dennis Christensen’
, Gistle Guilbaud’,
Francois Roman*
‘Unite de Recherches de Physiopharmacologie du Systeme Nerveux, I.N.S.E.R.M. U-161,2 rue d’AlCsia, 75014 Paris, France *Institut de Recherche JouveinaWarke Davis, 1 l-1 3 rue de la Loge, 94265 Fresnes Cedex, France (Ikceived
in final form &September9, 1999)
Trimebutine (2-dimethylamino-2-phenylbutyl 3,4,5-trimethoxybenzoate, hydrogen maleate) relieves abdominal pain in humans. In the present study, the antinociceptive action of systemic (S)-N-desmethyl trimebutine, a stereoisomer of N-monodesmethyl trimebutine, the main metabolite of trimebutine in humans, was studied in a rat model of neuropathic pain produced by chronic constriction injury to the sciatic nerve. Mechanical (vocalization threshold to hindpaw pressure) stimulus was used. Experiments were performed two weeks after surgery when the pain-related behaviour has fully developed. (S)-Ndesmethyl trimebutine (1, 3, 10 mg/kg s.c.) produced dose-dependent antinociceptive effects on the nerve-injured and the contralateral hindpaw. The effect of the lowest dose (1 mg/kg s.c.) of (S)-N-desmethyl trimebutine on the nerve-injured paw was equal to that seen after a ten time stronger dose on the contralateral paw. The effect of (S)-N-desmethyl trimebutine (1 mg/kg) was not naloxone reversible. The results suggest that systemic (S)-N-desmethyl trimebutine may be useful in the treatment of some aspects of neuropathic pain. Key Words: trimebutine, naloxone, mechanical stimulation, neurq?athic Pain
Trimebutine (2-dimethylamino-2-phenylbutyl 3,4,5trimethoxybenzoate hydrogen maleate) is used in the treatment of various digestive tract disorders including dyspepsia, irritable bowel syndrome and post-operative ileus. When orally administered, trimebutine is mainly metabolized to N-monodesmethyl trimebutine in the human liver. N-monodesmethyl trimebutine is the main metabolite of trimebutine with plasma levels higher than that of the parent compound. Recently, trimebutine and N-monodesmethyl trimebutine have been shown to influence the activity of visceral afferent nerves in the rat (1). It has been known for years that trimebutine exhibits weak opioid properties (2). Lately it was demonstrated in addition that trimebutine, N-monodesmethyl trimebutine and their corresponding stereoisomers display du Corresponding author: Dr. Valerie Kayser, Unite de Recherches de Physiopharmacologie systeme nerveux, INSERM U161.2, rue d’Altsia, 75014 Paris, France; Tel: 33-l-40-78-93-50, Fax: 33-l-45-88-13-04; e-mail: [email protected]
434
(S)-Ndesmethyl Trimebutine and Neuropathic Pain
Vol. 66, No. 5, 2000
sodium channels blocking properties and inhibit veratridine-induced glutamate release in vitro (3), with the relative potency being (S)-N-monodesmethyl trimebutine>N-monodesmethyl trimebutine>trimebutine. These properties of trimebutine, N-desmethyl trimebutine and their respective stereoisomers suggest that the compounds may demonstrate a therapeutic potential in humans broader than that expected for a simple spasmolytic compound. Nerve damage that affects peripheral nerves leads to abnormal pain states referred to as neuropathic pain. This pain syndrome is usually poorly controlled by currently available medications. A rat model of peripheral mononeuropathy is produced by persistent moderate constriction of the common sciatic nerve (4, 5). In this model, a mechanical test (vocalization threshold to hindpaw pressure), which reflects a centrally integrated response is especially sensitive to analgesic compounds (6-10). In the present study, we wanted to investigate the potential antinociceptive desmethyl trimebutine ((S)-N-desmethyl-TMB) in neuropathic rats.
effect of (S)-N-
Methods
The Committee for Research and Ethical Issues of the International Association for the Study of Pain (IASP) Ethical Guidelines (11) were adhered to in these studies. In particular, the duration of the experiments was as short as possible and the number of animals was kept to a minimum. Animals. Male Sprague-Dawley rats (Charles River, France, strain designation Crl :CD(SD)BR), n= 54, weighing 175-200 g on arrival were used. The rats were housed at the experimental facilities for a week prior to the experiments. They were maintained on a 12 h light/dark cycle and had free access to standard laboratory food and tap water. The ambient temperature was 20-22°C. Surgery. The unilateral peripheral mononeuropathy was produced on the right hind limb according to the method described by Bennett and Xie (4) and Attal et al. (5). Rats were anaesthetized with sodium pentobarbitone (Nembutal, 50 mg/kg Lp.). The common sciatic nerve was exposed by blunt dissection at the level of the mid-tigh; four ligatures (S-O chromic catgut, about l-mm spacing) were placed around the nerve. Antinaciceptive testing. Experiments were carried out in a quiet room. Animals were not acclimatized to the test situations beforehand. The experimenter was unaware of the drug and doses used. Each animal received drugs only once and was used in only one experiment. The antinociceptive action was determined by measuring the vocalization threshold elicited by pressure on both the nerve-injured and the contralateral hindpaw, using the Ugo Basile (Comerio, Italy) analgesymeter. This instrument generates a linearly increasing mechanical force applied by a dome-shaped plastic tip (diameter=1 mm) on the dorsal surface of the paw. The tip was positioned between the third and fourth metatarsus (into the sciatic nerve territory) and force was applied until the rat squeaked. For each rat, a control threshold (mean of two consecutive stable thresholds expressed in g) was determined before injecting the drug. The vocalization thresholds were then measured every 10 min, until they had returned to the level of the control values. Drugs. (S)-N-desmethyl-TMB maleate (Sipsy, Avrille, (Narcan@, Du Pont Pharma, Paris, France) were diluted co-injected with (S)-N-desmethyl-TMB. The 0.1 mg/kg it has prevented the effect of 1 mg/kg iv. of morphine
France) and naloxone hydrochloride in saline (0.9% NaCl). Naloxone was dose of naloxone was chosen, because in neuropathic rats in previous studies
Vol.66, No. 5,2ooO
(S)-NdesmethylTrimebutine and Neuropathic Pain
435
(6, 8-g). In each group, the control rats received the same volume of saline.
S~utistics: Data are expressed as means f S.E.M. The areas under the curves (AUC) were calculated using the trapezoidal rule. Statistical significance of the data was analysed by oneway analysis of variance (ANOVA). The observed significances were then confirmed with Tukey’s test. Simple regressions (linear model) were performed to establish dose-dependent effects. Statistical analyses were carried out using a statistical computer program (Statgraphics Plus, Manugistics, Rockville, MD). P&O5 was used as the criterion for statistical significance. Results A 600 z -
0
Nerve-injured
paw
1
I,
I
,
,
,
,
,
60
40 Contralateral
,
,
,
60
, 100
l
8 , 0
20
( 120
paw * saline A @I-N-drsmethyl-TMB 0 (S)-N-dormethyl-TMB 0 H-N-drsmrthyl-TMB
oJ,
,
*
9
1 mg/kg 3 mg/kg t 0 mg/kg
++
,
-
,
8
60
40
Time
,
60
-
,
100
r
,
120
(mid
Fig. 1.
Effect of S.C. (S)-N-desmethyl trimebutine ((S)-N-desmethyl-TMB) on the vocalization threshold to pressure on the nerve-injured (A) and the contralateral (B) hindpaw in neuropathic rats. Each point represents mean f SEM of 9 animals. * P< 0.05, ** P< 0.01 vs. pre-injection value.
436
(S)-Ndesmethyl Trimebutine and Neuropathic Pain
Vol. 66, No. $2000
In agreement with previous studies, (see 9 and references therein) the vocalization threshold of the nerve-injured hindpaw was decreased 2 weeks after the surgery (209 f 4 g vs the preconstriction value 277 f 8 g, P < 0.001, n= 54), by 25%. The threshold of the contralateral hindpaw was not modified (265 f 5 g vs the pre-operative value 270 k 6 g, n= 54).
The effect of (S)-IV-desmethyl-TMB. In the nerve-injured paw, saline had no effect on the vocalization threshold (Fig 1A). (S)-N-desmethyl-TMB (1 mg/kg) produced an antinociceptive effect, that peaked (330 f 30 g, 180% of the pre-injection value) at 30 min and lasted for 60 min (P < 0.001). The effect of (S)-N-desmethyl-TMB (3 mg/kg) peaked (540 + 42 g, 250%) at 20 min and lasted for 60 min (P < 0.001). The effect of (S)-N-desmethyl-TMB (10 mg/kg) peaked (570 f 60 g, 253%) at 30 min and lasted for 100 min (P < 0.001, Fig. 1A).
In the contralateral paw, saline or (S)-N-desmethyl-TMB at 1 mg/kg had no effect (Fig 1B). (S)-N-desmethyl-TMB (3 mg/kg) produced an effect that peaked (402 + 40 g, 148%) at 20 min and lasted for 30 min, whereas the effect of (S)-N-desmethyl-TMB at 10 mgkg peaked (450 k 40 g, 166%) at 30 min and lasted for 50 min (P < 0.001 for both doses) (Fig. 1B). Significant dose-effect relationships were observed for both the nerve-injured (r= 0.77, P < 0.001) and the contralateral (r= 0.79, P < 0.001) paw. As shown in Fig. 2, the overall effect of (S)-Ndesmethyl-TMB was enhanced on the nerve-injured paw compared to the contralateral paw for all the three doses. 30
1 1
0 Nerve-injured
paw
-2 .E x 2o WI g r 0
IO-
:
o;
lb
; Dose
of (SFN-desmethyl-TMB
(mg/kg)
Fig. 2. AUCs (100 g x min) of the respective time-curves in Fig. 1. *P
Vol. 66, No. 5, 2000
WNdesmeIhyl
Trimebutine.and Neuropathic Pain
The antinociceptive effect of (S)-N-desmethyl-TMB sedation or motor disturbances.
was not associated
437
with any detectable
The effect of the combination of (S)-N-desmethyl-TMB and naloxone. We observed no change in the effect of (S)-N-desmethyl-TMB after injecting naloxone (0.1 mgkg i.v.) in combination with (S)-N-desmethyl-TMB (1 mgkg, Fig. 3).
600 3
5 E
0
5
Nerve-injured A
paw
(S)-N-desmethyl-TM6 + saline
1 mg/kg
* (Sl-N-desmethyl-TMB
1 mg/kg
Time (min) Fig. 3.
Effect of (S)-N-desmethyl-TMB alone and of the combination of (S)-N-desmethyl-TMB and naloxone (Nx) on the vocalization threshold to pressure of the nerve-injured hindpaw in neuropathic rats. For further details, see Fig. 1.
Here, we demonstrate that systemic (S)-N-desmethyl-TMB produces a profound, dosedependent antinociceptive effect against a mechanical stimulus in neuropathic rats. The overall effect of the lowest dose of (S)-N-desmethyl-TMB (1 mgikg s.c.) in the nerve-injured paw was close to what we have previously found after an i.v. injection of 1 mgkg of morphine (6,9-10). The effect of (S)-N-desmethyl-TMB was enhanced in the nerve-injured paw compared to the contralateral paw. The effect of the lowest dose (1 mgkg s.c.) of (S)-N-desmethyl trimebutine on the nerve-injured paw was equal to that seen after a ten time stronger dose on the contralateral paw. The result is consistent with a recent report (12) showing that trimebutine and
(S)-Ndesmethyl Trimebutine and Neuropathic Pain
438
N-desmethyl-TMB pain.
produce antinociceptive
Vol. 66, No. 5, 2000
effects in a rat model of persistent
inflammatory
We further found that naloxone was unable to modify the (S)-N-desmethyl-TMB-induced antinociception. This suggests that, at least at low doses, the antinociceptive effects of (S)-Ndesmethyl-TMB in neuropathic rats are mainly due to other than the opioid properties of the parent compound, trimebutine demonstrated in previous studies (2). Our data agree with recent reports suggesting that the antinociceptive effects of trimebutine and its metabolites are mainly due to non-opioid mechanisms (1,3) and are naloxone-insensitive in rat models of persistent inflammatory pain (12). (S)-N-desmethyl-TMB displays sodium channel blocking properties and has been shown to inhibit veratridine-induced glutamate release in rat spinal cord slices (3). All these mechanisms may well contribute to the profound antinociceptive effect of (S)-N-desmethyl-TMB in the present model of neuropathic pain as well as in other pain forms. The development of neuropathic pain involves an increase in the spontaneous activity of injured nerves, which may underlie the increase in excitability of neurons in the spinal cord (13). It has been shown that sodium channel blockers such as lidocaine inhibit the spontaneous nerve activity in rats with a sciatic chronic constriction injury (14). In addition, there is evidence that the hyperexcitability of spinal cord neurons are mediated, in part, by glutamate acting on the NMDA receptor in the spinal dorsal horn (15,16). Indeed, concomitant systemic administration of relative low doses of morphine and an NMDA receptor antagonist results in potent antinociception in rats with a sciatic chronic constriction injury (10). We can speculate that the compound by interfering directly with these mechanisms has an action on the hyperexcitability associated with the neuropathic pain state. In conclusion, we demonstrated a potent antinociceptive effect of (S)-N-desmethyl trimebutine against a mechanical stimulus in neuropathic rats, which suggests that the compound may be useful also in the treatment of some aspects of neuropathic pain.
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