European Journal of Pharmacology, 58 (1979) 19--25
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© Elsevier/North-Holland Biomedical Press
ANTINOCICEPTIVE ACTIVITY OF CLONIDINE AND ITS POTENTIATION OF MORPHINE ANALGESIA T H E O D O R E C. SPAULDING, STUART FIELDING, JOSEPH J. V E N A F R O and HARBANS LAL *
Department of Pharmacology, Hoechst-Roussel Pharmaceuticals, Inc., Somerville, NJ 08876, and • Department of Pharmacology, University of Rhode Island, Kingston, R102881, U.S.A. Received 13 March 1979, accepted 25 May 1979
T.C. SPAULDING, S. FIELDING, J.J. V E N A F R O and H. LAL, Antinociceptive activity of clonidine and its potentiation of morphine analgesia, European J. Pharmacol. 58 (1979) 19--25. The activity of clonidine and its interaction with morphine was assessed in the mouse tail flick assay. In this assay, clonidine was found to be 10 times more potent than morphine. Clonidine potentiated morphine antinociceptive activity approximately five-fold and morphine potentiated clonidine activity four-fold. Clonidine's agonistic activity was not reversed by naloxone hydrochloride (10 mg/kg) while the potentiating effect of clonidine by morphine was. Tolerance to the antinociceptive effect of morphine was observed in morphine pelletimplanted mice but no cross tolerance was observed for clonidine. These data indicate that clonidine-induced analgesia is not a result of an interaction at morphine receptors; but rather, c o m m o n pathway(s) are present which appear to complement the agonistic interaction of each. Antinociception
Tail flick
Clonidine
1. Introduction Clonidine, a clinically useful antihypertensive agent, has been shown to be effective in blocking the effects of noxious stimuli in mice and rats. Clonidine reduced the number of writhes induced by acetic acid, acetylcholine (Bentley et al., 1977) and phenylquinone (Fielding et al., 1977, 1978). Clonidine was effective in increasing the latency of the tail flick response in mice and the tail withdrawal from 55°C water in rats, as well as increasing the pain threshold in the RandaU--Selitto procedure in rats (Fielding et al., 1977, 1978). Further Paalzow and Paalzow (1976) showed that clonidine blocked pain-induced vocalization after discharge in rats. Assessment of clonidine interaction with opiates in various systems indicated a multiplicity of activity. The antinociceptive effects of clonidine were not reversed by naloxone (Fielding et al., 1978}; however, clonidine
Morphine potentiation
Cross tolerance
maintained self-administration behavior when substituted for fentanyl in rats (Shearman et al., 1977). Further, clonidine was effective in suppressing certain narcotic withdrawal signs in rats and effective in precipitating other withdrawal signs (Tseng et al., 1975; Meyer and Sparber, 1976; Schulz and Herz, 1977; Fielding et al., 1977, 1978). In the present communication, the interaction of clonidine with acutely administered morphine is assessed. Cross tolerance to morphine in the tail flick assay was tested to contrast the morphine--clonidine interaction after acute or chronic administration.
2. Materials and methods 2.1. Materials
Clonidine HC1 was a gift from Boehringer Ingelheim Corp. and naloxone was donated
20 b y Endo Laboratories, Garden City, NY. Morphine sulfate and morphine base (for use in the preparation of morphine pellets) were purchased from Merck and Co., Rahway, NJ. Each morphine pellet consisted of: Morphine alkaloid 50 mg, microcrystalline cellulose (Avicel PH 101) N.F. 1 0 0 m g , colloidal silicon dioxide, N.F. (Cab~-sil M5) 0.75 mg and calcium stearate, N.F. 3 mg.
T.C. SPAULDING ET AL. at the time of peak analgesia which was 15 min after clonidine and 30 min after morphine administration. Maximal potentiation was found b y keeping the doses of morphine (0.5 mg/kg) and clonidine (0.016 mg/kg) constant. The morphine pretreatment time was at 30 min while clonidine was administered at 15, 30, 60 and 90 min prior to testing. The time for maximal potentiation corresponded to times for peak analgesia.
2.2. Analgesic assay 2.5. Statistical methods Antinociceptive testing was performed using non-fasted male Swiss Webster CD-1 mice weighing 18--28 g. The tail flick assay was essentially the same as that described b y D ' A m o u r and Smith (1941) and the data were quantified according to Hayashi and Takemori (1971). In general, cut-off times in the analgesic tests ranged from 2.5 to 2.9 sec. with the exception of the morphine pellet-implanted mice where the range was between 3.2--3.7 sec. Times for peak antinociceptive activity were determined and the appropriate vehicle controls were run in the single
19
© Elsevier/North-Holland Biomedical Press
ANTINOCICEPTIVE ACTIVITY OF CLONIDINE AND ITS POTENTIATION OF MORPHINE ANALGESIA T H E O D O R E C. SPAULDING, STUART FIELDING, JOSEPH J. V E N A F R O and HARBANS LAL *
Department of Pharmacology, Hoechst-Roussel Pharmaceuticals, Inc., Somerville, NJ 08876, and • Department of Pharmacology, University of Rhode Island, Kingston, R102881, U.S.A. Received 13 March 1979, accepted 25 May 1979
T.C. SPAULDING, S. FIELDING, J.J. V E N A F R O and H. LAL, Antinociceptive activity of clonidine and its potentiation of morphine analgesia, European J. Pharmacol. 58 (1979) 19--25. The activity of clonidine and its interaction with morphine was assessed in the mouse tail flick assay. In this assay, clonidine was found to be 10 times more potent than morphine. Clonidine potentiated morphine antinociceptive activity approximately five-fold and morphine potentiated clonidine activity four-fold. Clonidine's agonistic activity was not reversed by naloxone hydrochloride (10 mg/kg) while the potentiating effect of clonidine by morphine was. Tolerance to the antinociceptive effect of morphine was observed in morphine pelletimplanted mice but no cross tolerance was observed for clonidine. These data indicate that clonidine-induced analgesia is not a result of an interaction at morphine receptors; but rather, c o m m o n pathway(s) are present which appear to complement the agonistic interaction of each. Antinociception
Tail flick
Clonidine
1. Introduction Clonidine, a clinically useful antihypertensive agent, has been shown to be effective in blocking the effects of noxious stimuli in mice and rats. Clonidine reduced the number of writhes induced by acetic acid, acetylcholine (Bentley et al., 1977) and phenylquinone (Fielding et al., 1977, 1978). Clonidine was effective in increasing the latency of the tail flick response in mice and the tail withdrawal from 55°C water in rats, as well as increasing the pain threshold in the RandaU--Selitto procedure in rats (Fielding et al., 1977, 1978). Further Paalzow and Paalzow (1976) showed that clonidine blocked pain-induced vocalization after discharge in rats. Assessment of clonidine interaction with opiates in various systems indicated a multiplicity of activity. The antinociceptive effects of clonidine were not reversed by naloxone (Fielding et al., 1978}; however, clonidine
Morphine potentiation
Cross tolerance
maintained self-administration behavior when substituted for fentanyl in rats (Shearman et al., 1977). Further, clonidine was effective in suppressing certain narcotic withdrawal signs in rats and effective in precipitating other withdrawal signs (Tseng et al., 1975; Meyer and Sparber, 1976; Schulz and Herz, 1977; Fielding et al., 1977, 1978). In the present communication, the interaction of clonidine with acutely administered morphine is assessed. Cross tolerance to morphine in the tail flick assay was tested to contrast the morphine--clonidine interaction after acute or chronic administration.
2. Materials and methods 2.1. Materials
Clonidine HC1 was a gift from Boehringer Ingelheim Corp. and naloxone was donated
20 b y Endo Laboratories, Garden City, NY. Morphine sulfate and morphine base (for use in the preparation of morphine pellets) were purchased from Merck and Co., Rahway, NJ. Each morphine pellet consisted of: Morphine alkaloid 50 mg, microcrystalline cellulose (Avicel PH 101) N.F. 1 0 0 m g , colloidal silicon dioxide, N.F. (Cab~-sil M5) 0.75 mg and calcium stearate, N.F. 3 mg.
T.C. SPAULDING ET AL. at the time of peak analgesia which was 15 min after clonidine and 30 min after morphine administration. Maximal potentiation was found b y keeping the doses of morphine (0.5 mg/kg) and clonidine (0.016 mg/kg) constant. The morphine pretreatment time was at 30 min while clonidine was administered at 15, 30, 60 and 90 min prior to testing. The time for maximal potentiation corresponded to times for peak analgesia.
2.2. Analgesic assay 2.5. Statistical methods Antinociceptive testing was performed using non-fasted male Swiss Webster CD-1 mice weighing 18--28 g. The tail flick assay was essentially the same as that described b y D ' A m o u r and Smith (1941) and the data were quantified according to Hayashi and Takemori (1971). In general, cut-off times in the analgesic tests ranged from 2.5 to 2.9 sec. with the exception of the morphine pellet-implanted mice where the range was between 3.2--3.7 sec. Times for peak antinociceptive activity were determined and the appropriate vehicle controls were run in the single
