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Anticonvulsant effects of clonidine mediated through central α2-adrenoceptors
European Journal of Pharmacology, 77 (1982) 163-166 Elsevier/North-Holland Biomedical Press
163
Short communication
ANTICONVULSANT EFFECTS OF CLONIDINE MEDIATED THROUGH CENTRAL a 2-ADRENOCEPTORS JENNY PAPANICOLAOU, ROGER J. SUMMERS, F R A N K J.E. VAJDA and W I L L I A M S J. LOUIS *
University of Melbourne, Clinical Pharmacology and Therapeutics Unit, Department of Medicine, Austin Hospital, Heidelberg, Victoria, 3084, Australia Received 10 November 1981, accepted 17 November 1981
J. PAPANICOLAOU, R.J. SUMMERS, F.J.E. VAJDA and W.L. LOUIS, A nticonvulsant effects of clonidine mediated through central a 2-adrenoceptors, European J. Pharmacol. 77 (1982) 163-166. The effect of centrally (0.3-30 ng/kg) and peripherally (0.01 g g / k g - 1 . 0 mg/kg) administered clonidine on PTZ-induced seizures was studied in rats. At low doses, dose-dependent decreases in the duration of seizures was observed but at higher doses the duration returned to control levels. Anticonvulsant activity was antagonized by yohimbine (100 p,g/kg i.p.) indicating a2-adrenoceptor involvement, whereas the second phase of the response was antagonized by prazosin (10 n g / k g i.c.v.) indicating that it involved a~ -adrenoceptors. Convulsions
Pentylenetetrazol
Clonidine
a-Adrenoceptors
1. Introduction
There is considerable evidence to support the idea that central catecholamines play a role in the control of convulsions produced by electroshock (ECS) or drugs. In both mice and rats lowering o f central monoamine levels by reserpine or amethylparatyrosine produces a lowering of the threshold for ECS or drugs (Chen et al., 1954; Yeoh and Wolf, 1968; Kilian and Frey, 1973; Jobe et al., 1974). Central noradrenaline appears to be particularly important since selective lesioning of the dorsal ascending noradrenergic forebrain bundle by 6-hydroxydopamine injections enhances pentylenetetrazole (PTZ) seizures in rats (Mason and Corcoran, 1979). Both a- and fl-adrenoceptors may be involved since a-adrenoceptor antagonists enhance and fl-adrenoceptor antagonists prevent convulsions (Yeoh and Wolf, 1968; Kilian and Frey, 1973; Papanicolaou et al., 1981). Recently we reported that the inhibition of PTZ convulsions produced by low doses of fladrenoceptor blocking drugs was stereospecific * To whom all correspondence should be addressed.
suggesting a role for fl-adrenoceptors in this model (Papanicolaou et al., 1981). The study reported here examines the role of a-adrenoceptors in this model and provides evidence that the az-agonist clonidine has a powerful protective effect PTZinduced convulsions in the rat.
2. Materials and methods
The effect of clonidine was studied in fed Sprague-Dawley rats (male or female, 150-200 g). PTZ was injected (50 mg/kg i.p.) and the duration of the seizure in sec was measured with a stopwatch from the onset of the clonic phase to the beginning of the post-ictal phase of the convulsions. Drugs or an equivalent volume of drug vehicle (0.9% saline) were given i.p. 15 min before PTZ challenge. Intraventricular injection of drugs was made through indwelling intraventricular cannulae (Yeda R. and D. Ltd., type 92-103A) implanted at least 3 days before the experiment. For implantation, rats were anaesthetised with Alfatheson (Glaxo) 3 ml/kg i.p., placed in a headholder and the can-
0014-2999/82/0000-0000/$02.75 © 1982 Elsevier/North-Holland Biomedical Press
164 nulae placed 1.5 m m lateral and caudal to the coronal and sagittal sutures. Cannulae were supported and the wound closed with dental cement. Drugs (or vehicle) were injected in a volume of 25 / d / k g in 0.9% NaC1 using a 5/tl microlitre syringe (Scientific Glass Engineering Pty. Ltd., Melbourne) with a needle cut to protrude 2-2.5 m m beyond the cannulae. The position of the cannulae was checked after each experiment by dye injection. Significance of differences between means was assessed using Student's t-test. The following drugs were used: clonidine hydrochloride (Boehringer Ingelheim); PTZ (Sigma); prazosin hydrochloride (Pfizer, U.K., Ltd.); A1fathesin (Glaso); diphenylhydantoin (ParkeDavis); diazepam (Roche); sterile pyrogen free 0.9% saline (David Bull Laboratories).
seizures (fig. 1). At 1.0-50.0 /xg/kg i.p. seizures were markedly reduced and in some animals completely abolished. At 50-1000 ~ g / k g there was a dose-dependent reduction in anticonvulsant effect such that at the highest dose level the duration of the seizures was not significantly different from control. To determine whether these effects of clonidine were centrally mediated, the a adrenoceptor agonist was administered i.c.v. 3 rain before PTZ challenge. Clonidine given i.c.v, was 140 times more effective than when given i.p., producing dose-dependent anticonvulsant effects at dose levels of 0.3-3.0 n g / k g (fig. 1). Convulsions were abolished in all animals treated with 3.0 n g / k g i.c.v. Higher dose levels had less anticonvulsant effect and animals given 30 n g / k g clonidine i.c.v, showed a duration of seizures approaching control values. Additional evidence for a central action was obtained using another a 2-adrenoceptor agonist, oxymetazoline. Unlike clonidine, the more polar compound oxymetazoline does not penetrate the blood-brain barrier. When given i.p. at dose levels at which clonidine produced a maximal reduction in the duration of convulsions, oxyme-
3. Results Clonidine given i.p. at 0.01-1.0 # g / k g 15 min before PTZ challenge (50 m g / k g i.p.) produced a dose-dependent reduction in the duration of the
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clonidine dose (pg/k(J) (Io(j scale) Fig. I. The effect of clonidine administered intraventricularly(i.c.v., broken line) or intraperitoneally (i.p., solid line) on the duration of convulsions following pentylenetetrazol (PTZ, 50 mg/kg i.p.) in rats. In control animals (drug vehicle 0.9% saline injection) the duration of seizures was 61.0+2.0 sec (n=3; i.c.v.) and 57.8--+3.0 (n= 14; i.p.). I-bars represent the S.E.M. and the numbers in parentheses the number of animals. Significance of differenceswas assessed by Student's unpaired t-test (= P
165
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g Fig. 2. Effect of clonidine (Clon) on the duration of pentylenetetrazol (PTZ)-induced convulsions in rats. In the left panel is shown the effect of intraperitoneal (i.p.) and intracerebroventricular (i.c.v.) injection of sterile pyrogen free saline (vehicle). In the middle panel the effect of clon i.c.v., i.p. at low and high dose levels, prazosin (Praz) i.c.v, and Praz i.c.v. + high dose clonidine i.p. In the right panel the effect of oxymetazoline (Oxy) and the anticonvulsants diphenylhydantoin (DPH) and diazepam (Diaz) are shown for comparison with clonidine. The numbers refer to the number of animals used (* P<0.001).
tazoline had no significant effect (fig. 2). These anticonvulsant effects are probably mediated through central a2-adrenoceptors since the a 2 -adrenoceptor antagonist yohimbine (100/~ g / k g i.p., 5 min before clonidine and 20 min before PTZ) produced a parallel 6.6 fold shift to the right of the dose-response curve. To investigate the possibility that reversal of the anticonvulsant effect of clonidine with high doses was mediated through an action on central a~adrenoceptors, the selective a~ -adrenoceptor antagonist, prazosin (10 n g / k g i.c.v.) was given 3 min before clonidine (1 m g / k g i.p.) and 18 min before PTZ challenge. This reduced the duration of seizures from 68.5 ± 4.5 (n = 6) to 19.7 ± 1.3 sec (n = 3; P < 0.001) (fig. 2). Prazosin alone (10 n g / k g i.c.v. 18 min before PTZ) had no significant effect on the duration of seizures (61.5 ± 1.5 sec, n = 4, NS) compared to control. The protective effect of clonidine was then compared to the established anticonvulsants diphenylhydantoin (DPH) and diazepam (Diaz) (fig. 2). The dose levels (IP) required to decrease
the duration of seizures by 50% were for D P H 5 mg/kg, Diaz 0.1 m g / k g and clonidine 0.18 /~g/kg. Thus in this model clonidine was 550 times more effective than Diaz and 27500 times more effective than DPH.
4. Discussion The results suggest that in the PTZ model clonidine possesses a powerful anticonvulsant effect at low dose levels mediated by an action on central a2-adrenoceptors. At these low doses clonidine was some 27500 times more effective than the established anticonvulsant D P H and 550 times more potent than Diaz. The anticonvulsant action of clonidine and Diaz in the same model (Niemegeers and Lewi, 1979) appear to be mediated through different mechanisms since clonidine does not bind to benzodiazepine receptors (Braestrup and Squires, 1978). We have recently reported that ( - ) - p r o p r a n o l o l has a similar but weaker anticonvulsant action (Papanicolaou et al.,
166 1981). T a k e n together these results suggest that central adrenoceptors play an i m p o r t a n t role in the control of convulsions in this model. The locations of the receptors is not clear from these experiments b u t convulsions p r o d u c e d by chemical or electrical m e a n s m a y be associated with the release of a variety of transmitters. P r e j u n c t i o n a l l y located a z - a d r e n o c e p t o r s are k n o w n to m o d u l a t e the release of n o r a d r e n a l i n e a n d there is n o w c o n s i d e r a b l e evidence that n o r a d r e n a l i n e m a y play a role in experimentally i n d u c e d convulsions (Mason a n d Corcoran, 1979). I n a d d i t i o n clonidine-like d r u g s m a y m o d u l a t e the release of other transmitters such as serotonin (Starke a n d Montel, 1973) a n d this could c o n t r i b u t e to their effect. T h e cause of the reversal of the a n t i c o n v u l s a n t effect of c l o n i d i n e at high dose levels was also studied. The evidence supports the hypothesis that c l o n i d i n e produces this effect b y acting o n a central a l - a d r e n o c e p t o r since the selective a n t a g o n i s t prazosin abolished this reversal. Moreover prazosin itself possessed n o a n t i c o n v u l s a n t activity. Since the high dose levels of c l o n i d i n e were similar to those used in earlier studies (Oishi et al., 1979) it w o u l d explain why these workers failed to observe the a n t i c o n v u l s a n t effect reported here with low doses of clonidine, this a n t i c o n v u l s a n t effect in the P T Z model is n o t c o n f i n e d to clonidine. A series of a 2 - a d r e n o c e p t o r agonists are p o t e n t a n t i c o n v u l s a n t s ( P a p a n i c o l a o u et al., i n p r e p a r a t i o n ) suggesting that in this m o d e l i n h i b i t i o n m e d i a t e d t h r o u g h central a z - a d r e n o c e p t o r s is a n effective m e a n s of r e d u c i n g convulsions.
Acknowledgments This work was supported by the Life Insurance Medical Research Fund of Australia and New Zealand. R.J. Summers is a Senior Research Fellow of the National Health and Medical Research Council.
References Braestrup, C. and R.F. Squires, 1978, Pharmacological characterization of benzodiazepine receptors in the brain, European J. Pharmacol. 48, 263. Chen, G., C.R. Ensor and B.A. Bohner, 1954, A facilitative action of reserpine on the central nervous system, Proc. Soc. Exp. Biol. N.Y. 86, 507. Jobe, P.C., R.E. Stull and P.F. Geiger, 1974, The relative significance of norepinephrine, dopamine and 5-hydroxytryptamine in electroshock seizure in the rat, Neuropharmacol. 13, 961. Kilian, M. and H.H. Frey, 1973, Central monoamines and convulsive thresholds in mice and rats, Neuropharmacol. 12, 681. Mason, S.T. and M.E. Corcoran, 1979, Catecholamines and convulsions, Brain Res. 170, 497. Niemegeers, C.J.E. and P.J. Lewi, 1979, The anticonvulsant properties of anxiolytic agents, in: Industrial Pharmacology, Vol. 3, Anxiolytics, eds. S. Fielding and H. Lal (Futura Publishing Co., New York) p. 141. Oishi, R., N. Suenaga, T. Hidaka and T. Fukuda, 1979, The role of a-adrenoceptors in the regulation of pentylenetetrazol convulsions in mice, J. Pharm. Pharmacol. 31,709. Papanicolaou, J., F.J.E. Vajda, R.J. Summers and W.J. Louis, 1981, Role of j~-adrenoceptors in the anticonvulsant effect of propranolol on pentylenetetrazol-induced convulsions in rats, J. Pharm. Pharmacol. (in press). Starke, K. and H. Montel, 1973, Involvement of a receptors in clonidine inhibition of transmitter release from central monoamine neurones, Neuropharmacol. 12, 1073. Yeoh, P.N. and H.H. Wolf, 1968, Effects of some adrenergic agents on low frequency electroshock seizures, J. Pharm. Sci. 57, 340.
163
Short communication
ANTICONVULSANT EFFECTS OF CLONIDINE MEDIATED THROUGH CENTRAL a 2-ADRENOCEPTORS JENNY PAPANICOLAOU, ROGER J. SUMMERS, F R A N K J.E. VAJDA and W I L L I A M S J. LOUIS *
University of Melbourne, Clinical Pharmacology and Therapeutics Unit, Department of Medicine, Austin Hospital, Heidelberg, Victoria, 3084, Australia Received 10 November 1981, accepted 17 November 1981
J. PAPANICOLAOU, R.J. SUMMERS, F.J.E. VAJDA and W.L. LOUIS, A nticonvulsant effects of clonidine mediated through central a 2-adrenoceptors, European J. Pharmacol. 77 (1982) 163-166. The effect of centrally (0.3-30 ng/kg) and peripherally (0.01 g g / k g - 1 . 0 mg/kg) administered clonidine on PTZ-induced seizures was studied in rats. At low doses, dose-dependent decreases in the duration of seizures was observed but at higher doses the duration returned to control levels. Anticonvulsant activity was antagonized by yohimbine (100 p,g/kg i.p.) indicating a2-adrenoceptor involvement, whereas the second phase of the response was antagonized by prazosin (10 n g / k g i.c.v.) indicating that it involved a~ -adrenoceptors. Convulsions
Pentylenetetrazol
Clonidine
a-Adrenoceptors
1. Introduction
There is considerable evidence to support the idea that central catecholamines play a role in the control of convulsions produced by electroshock (ECS) or drugs. In both mice and rats lowering o f central monoamine levels by reserpine or amethylparatyrosine produces a lowering of the threshold for ECS or drugs (Chen et al., 1954; Yeoh and Wolf, 1968; Kilian and Frey, 1973; Jobe et al., 1974). Central noradrenaline appears to be particularly important since selective lesioning of the dorsal ascending noradrenergic forebrain bundle by 6-hydroxydopamine injections enhances pentylenetetrazole (PTZ) seizures in rats (Mason and Corcoran, 1979). Both a- and fl-adrenoceptors may be involved since a-adrenoceptor antagonists enhance and fl-adrenoceptor antagonists prevent convulsions (Yeoh and Wolf, 1968; Kilian and Frey, 1973; Papanicolaou et al., 1981). Recently we reported that the inhibition of PTZ convulsions produced by low doses of fladrenoceptor blocking drugs was stereospecific * To whom all correspondence should be addressed.
suggesting a role for fl-adrenoceptors in this model (Papanicolaou et al., 1981). The study reported here examines the role of a-adrenoceptors in this model and provides evidence that the az-agonist clonidine has a powerful protective effect PTZinduced convulsions in the rat.
2. Materials and methods
The effect of clonidine was studied in fed Sprague-Dawley rats (male or female, 150-200 g). PTZ was injected (50 mg/kg i.p.) and the duration of the seizure in sec was measured with a stopwatch from the onset of the clonic phase to the beginning of the post-ictal phase of the convulsions. Drugs or an equivalent volume of drug vehicle (0.9% saline) were given i.p. 15 min before PTZ challenge. Intraventricular injection of drugs was made through indwelling intraventricular cannulae (Yeda R. and D. Ltd., type 92-103A) implanted at least 3 days before the experiment. For implantation, rats were anaesthetised with Alfatheson (Glaxo) 3 ml/kg i.p., placed in a headholder and the can-
0014-2999/82/0000-0000/$02.75 © 1982 Elsevier/North-Holland Biomedical Press
164 nulae placed 1.5 m m lateral and caudal to the coronal and sagittal sutures. Cannulae were supported and the wound closed with dental cement. Drugs (or vehicle) were injected in a volume of 25 / d / k g in 0.9% NaC1 using a 5/tl microlitre syringe (Scientific Glass Engineering Pty. Ltd., Melbourne) with a needle cut to protrude 2-2.5 m m beyond the cannulae. The position of the cannulae was checked after each experiment by dye injection. Significance of differences between means was assessed using Student's t-test. The following drugs were used: clonidine hydrochloride (Boehringer Ingelheim); PTZ (Sigma); prazosin hydrochloride (Pfizer, U.K., Ltd.); A1fathesin (Glaso); diphenylhydantoin (ParkeDavis); diazepam (Roche); sterile pyrogen free 0.9% saline (David Bull Laboratories).
seizures (fig. 1). At 1.0-50.0 /xg/kg i.p. seizures were markedly reduced and in some animals completely abolished. At 50-1000 ~ g / k g there was a dose-dependent reduction in anticonvulsant effect such that at the highest dose level the duration of the seizures was not significantly different from control. To determine whether these effects of clonidine were centrally mediated, the a adrenoceptor agonist was administered i.c.v. 3 rain before PTZ challenge. Clonidine given i.c.v, was 140 times more effective than when given i.p., producing dose-dependent anticonvulsant effects at dose levels of 0.3-3.0 n g / k g (fig. 1). Convulsions were abolished in all animals treated with 3.0 n g / k g i.c.v. Higher dose levels had less anticonvulsant effect and animals given 30 n g / k g clonidine i.c.v, showed a duration of seizures approaching control values. Additional evidence for a central action was obtained using another a 2-adrenoceptor agonist, oxymetazoline. Unlike clonidine, the more polar compound oxymetazoline does not penetrate the blood-brain barrier. When given i.p. at dose levels at which clonidine produced a maximal reduction in the duration of convulsions, oxyme-
3. Results Clonidine given i.p. at 0.01-1.0 # g / k g 15 min before PTZ challenge (50 m g / k g i.p.) produced a dose-dependent reduction in the duration of the
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clonidine dose (pg/k(J) (Io(j scale) Fig. I. The effect of clonidine administered intraventricularly(i.c.v., broken line) or intraperitoneally (i.p., solid line) on the duration of convulsions following pentylenetetrazol (PTZ, 50 mg/kg i.p.) in rats. In control animals (drug vehicle 0.9% saline injection) the duration of seizures was 61.0+2.0 sec (n=3; i.c.v.) and 57.8--+3.0 (n= 14; i.p.). I-bars represent the S.E.M. and the numbers in parentheses the number of animals. Significance of differenceswas assessed by Student's unpaired t-test (= P
165
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g Fig. 2. Effect of clonidine (Clon) on the duration of pentylenetetrazol (PTZ)-induced convulsions in rats. In the left panel is shown the effect of intraperitoneal (i.p.) and intracerebroventricular (i.c.v.) injection of sterile pyrogen free saline (vehicle). In the middle panel the effect of clon i.c.v., i.p. at low and high dose levels, prazosin (Praz) i.c.v, and Praz i.c.v. + high dose clonidine i.p. In the right panel the effect of oxymetazoline (Oxy) and the anticonvulsants diphenylhydantoin (DPH) and diazepam (Diaz) are shown for comparison with clonidine. The numbers refer to the number of animals used (* P<0.001).
tazoline had no significant effect (fig. 2). These anticonvulsant effects are probably mediated through central a2-adrenoceptors since the a 2 -adrenoceptor antagonist yohimbine (100/~ g / k g i.p., 5 min before clonidine and 20 min before PTZ) produced a parallel 6.6 fold shift to the right of the dose-response curve. To investigate the possibility that reversal of the anticonvulsant effect of clonidine with high doses was mediated through an action on central a~adrenoceptors, the selective a~ -adrenoceptor antagonist, prazosin (10 n g / k g i.c.v.) was given 3 min before clonidine (1 m g / k g i.p.) and 18 min before PTZ challenge. This reduced the duration of seizures from 68.5 ± 4.5 (n = 6) to 19.7 ± 1.3 sec (n = 3; P < 0.001) (fig. 2). Prazosin alone (10 n g / k g i.c.v. 18 min before PTZ) had no significant effect on the duration of seizures (61.5 ± 1.5 sec, n = 4, NS) compared to control. The protective effect of clonidine was then compared to the established anticonvulsants diphenylhydantoin (DPH) and diazepam (Diaz) (fig. 2). The dose levels (IP) required to decrease
the duration of seizures by 50% were for D P H 5 mg/kg, Diaz 0.1 m g / k g and clonidine 0.18 /~g/kg. Thus in this model clonidine was 550 times more effective than Diaz and 27500 times more effective than DPH.
4. Discussion The results suggest that in the PTZ model clonidine possesses a powerful anticonvulsant effect at low dose levels mediated by an action on central a2-adrenoceptors. At these low doses clonidine was some 27500 times more effective than the established anticonvulsant D P H and 550 times more potent than Diaz. The anticonvulsant action of clonidine and Diaz in the same model (Niemegeers and Lewi, 1979) appear to be mediated through different mechanisms since clonidine does not bind to benzodiazepine receptors (Braestrup and Squires, 1978). We have recently reported that ( - ) - p r o p r a n o l o l has a similar but weaker anticonvulsant action (Papanicolaou et al.,
166 1981). T a k e n together these results suggest that central adrenoceptors play an i m p o r t a n t role in the control of convulsions in this model. The locations of the receptors is not clear from these experiments b u t convulsions p r o d u c e d by chemical or electrical m e a n s m a y be associated with the release of a variety of transmitters. P r e j u n c t i o n a l l y located a z - a d r e n o c e p t o r s are k n o w n to m o d u l a t e the release of n o r a d r e n a l i n e a n d there is n o w c o n s i d e r a b l e evidence that n o r a d r e n a l i n e m a y play a role in experimentally i n d u c e d convulsions (Mason a n d Corcoran, 1979). I n a d d i t i o n clonidine-like d r u g s m a y m o d u l a t e the release of other transmitters such as serotonin (Starke a n d Montel, 1973) a n d this could c o n t r i b u t e to their effect. T h e cause of the reversal of the a n t i c o n v u l s a n t effect of c l o n i d i n e at high dose levels was also studied. The evidence supports the hypothesis that c l o n i d i n e produces this effect b y acting o n a central a l - a d r e n o c e p t o r since the selective a n t a g o n i s t prazosin abolished this reversal. Moreover prazosin itself possessed n o a n t i c o n v u l s a n t activity. Since the high dose levels of c l o n i d i n e were similar to those used in earlier studies (Oishi et al., 1979) it w o u l d explain why these workers failed to observe the a n t i c o n v u l s a n t effect reported here with low doses of clonidine, this a n t i c o n v u l s a n t effect in the P T Z model is n o t c o n f i n e d to clonidine. A series of a 2 - a d r e n o c e p t o r agonists are p o t e n t a n t i c o n v u l s a n t s ( P a p a n i c o l a o u et al., i n p r e p a r a t i o n ) suggesting that in this m o d e l i n h i b i t i o n m e d i a t e d t h r o u g h central a z - a d r e n o c e p t o r s is a n effective m e a n s of r e d u c i n g convulsions.
Acknowledgments This work was supported by the Life Insurance Medical Research Fund of Australia and New Zealand. R.J. Summers is a Senior Research Fellow of the National Health and Medical Research Council.
References Braestrup, C. and R.F. Squires, 1978, Pharmacological characterization of benzodiazepine receptors in the brain, European J. Pharmacol. 48, 263. Chen, G., C.R. Ensor and B.A. Bohner, 1954, A facilitative action of reserpine on the central nervous system, Proc. Soc. Exp. Biol. N.Y. 86, 507. Jobe, P.C., R.E. Stull and P.F. Geiger, 1974, The relative significance of norepinephrine, dopamine and 5-hydroxytryptamine in electroshock seizure in the rat, Neuropharmacol. 13, 961. Kilian, M. and H.H. Frey, 1973, Central monoamines and convulsive thresholds in mice and rats, Neuropharmacol. 12, 681. Mason, S.T. and M.E. Corcoran, 1979, Catecholamines and convulsions, Brain Res. 170, 497. Niemegeers, C.J.E. and P.J. Lewi, 1979, The anticonvulsant properties of anxiolytic agents, in: Industrial Pharmacology, Vol. 3, Anxiolytics, eds. S. Fielding and H. Lal (Futura Publishing Co., New York) p. 141. Oishi, R., N. Suenaga, T. Hidaka and T. Fukuda, 1979, The role of a-adrenoceptors in the regulation of pentylenetetrazol convulsions in mice, J. Pharm. Pharmacol. 31,709. Papanicolaou, J., F.J.E. Vajda, R.J. Summers and W.J. Louis, 1981, Role of j~-adrenoceptors in the anticonvulsant effect of propranolol on pentylenetetrazol-induced convulsions in rats, J. Pharm. Pharmacol. (in press). Starke, K. and H. Montel, 1973, Involvement of a receptors in clonidine inhibition of transmitter release from central monoamine neurones, Neuropharmacol. 12, 1073. Yeoh, P.N. and H.H. Wolf, 1968, Effects of some adrenergic agents on low frequency electroshock seizures, J. Pharm. Sci. 57, 340.