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Reversal of reserpine-induced bradykinesia bya-methyldopa - new light on its modus operandi
178
Brain Research, 71 (1974) 178-182 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Reversal of reserpine-induced bradykinesia by a-methyldopa - new light on its modus operandi
R. C. DUVOISIN* ANDC. D. MARSDEN University Department of Neurology, King's College Hospital and Institute of Psychiatry, London S.E. 5 (Great Britain)
(Accepted January 25th, 1974)
a-Methyldopa (AMD) is used to treat hypertension but recently it has been found to enhance the effect of levodopa in the therapy of Parkinson's diseaseS,1L It has been postulated that due to its ability to inhibit dopa decarboxylase, A M D may prevent the extracerebral metabolism of levodopa, thereby allowing a larger portion of the administered dose to reach the striatum where it may then be decarboxylated to form dopamine. Certainly its hydrazine analogue, a-methyldopa hydrazine (MK-486), acts in this manner, selectively inhibiting extracerebral decarboxylase s. However, this may not be the only possible way in which AMD may potentiate levodopa therapy. Another might be the 'correction' of the striatal dopamine deficit by a-methylated amines derived from AMD. If these could assume physiological function in the brain as Carlsson and Lindqvist 3 suggested and effectively substitute for the missing dopamine, then one would anticipate that AMD alone might exert a beneficial effect in Parkinson's disease. Since clinical experiments have been uncertain on this point, A M D having been variously found to benefit 10, exacerbate 2 or simply to fail altogether to modify the Parkinsonian state 9, pharmacological experiments on the effects of A M D on reserpine induced bradykinesia in small laboratory animals should be of interest. The evidence here, however, has also been limited and conflicting. Day and Rand 4 reported that 250 mg/kg A M D given intraperitoneally (i.p.) 17 h after reserpine yielded a gradual reversal of 'reserpine sedation' and restored locomotor activity. Subsequently, Uretsky and Seiden 18, in a more extensive study, found that 200 mg/kg A M D given 15 h after reserpine reversed the suppression of motor activity and conditioned avoidance responding in mice. They also noted that the decarboxylase inhibitor RO 4-4062, given in doses known to suppress cerebral decarboxylation, prevented the reversal. However, van Rossum and Hurkmans 14 reported that AMD combined with a monoamine oxidase inhibitor had no effect on the motor activity of mice pretreated with reserpine, 1 mg/kg daily for 3 days, and Hanson and Henning 7 found that A M D * Present address: Department of Neurology, The Mount Sinai School of Medicine, Fifth Avenue at 100th Street, New York, N.Y. 10029, U.S.A.
SHORT COMMUNICATIONS
179
failed to reverse the suppression of conditioned avoidance responses induced in the cat by reserpine given 22.5-25.75 h previously. In the course of an investigation of the reserpine syndrome in the mouse, we have recently encountered some unexpected findings which place the interaction of reserpine and A M D in a new perspective and provide an explanation of these apparently conflicting reports. Reserpine, 4 mg/kg, was given by intraperitoneal injection to male, white, 'P' strain mice and the effects studied by frequent, repeated direct observation and quantitative analysis of spontaneous motor activity using the Animex counter 11. Locomotor function was scored by counting the number of mice per batch of l0 who moved off a specifically demarcated 8 in. × l0 in. area in 15 sec, testing each animal individually. Normal animals walked briskly off the area within the time limit. Two hours after receiving reserpine the mice exhibited little spontaneous motor activity and lay quietly huddled together; none walked off the demarcated area. Several features analogous to human Parkinsonism were noted including a kyphotic posture, a resting tremor o f the paws noticeable when the animals were placed in a sitting-up position, a plastic rigidity detectable by manipulation of the limbs and a marked slowness and poverty of movement. All these features of the reserpine syndrome were reversed by A M D given 4 h after reserpine by i.p. injection in doses ranging from 100 to 1000 mg/kg. The reversal was first evident about 1 h after injection, reached its peak in 2-3 h and gradually subsided after 5-6 h. A dose-response plot constructed from the scores of the locomotor function test, using 5-7 batches of 10 mice each for each dose, is shown in Fig. 1. A dose-response plot for L-dopa derived in a similar manner is shown for comparison. The suppression of spontaneous motor activity by reserpine in 6 control batches measured on the Animex apparatus for 96 h is shown in Fig. 2. The reversal induced by AMD, 1000 mg/kg, 4 b after reserpine is superimposed to show its magnitude and temporal course. The A M D reversal of the reserpine syndrome was further studied by administering the various agents listed in Table I, either simultaneously or 1 h before A M D (1000 mg/kg). All drugs were administered by i.p. injection. The reversal was found to
~'~.~_oI00t
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I
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,7"""
"I
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J
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~,,~ 80 I.LI ¢ ~
I
0 I0
I
40
I
I
I
I
I00 2 5 0 5 0 0 I000 DOSE, mg/kg i,p.
Fig. 1. Dose-response of AMD and of L-dopa antagonism of reserpine-induced locomotor suppression in the mouse. Brackets indicate standard deviation.
180
Q
SHORT COMMUNICATIONS
6
C
3
5
4
0 0
3
>2
U 0
24
HOURS
48
OF CONTINUOUS
72
96
OBSERVATION
Fig. 2. Graphic presentation of mean hourly Animex activity counts of 6 batches of 10 mice each monitored for 96 h. Reserpine 4 mg/kg was given at 23 h. Shown in black is the mean hourly counts of 4 batches given reserpine 4 mg/kg followed 4 h later by AMD, 1000 mg/kg.
be significantly antagonised by the centrally active decarboxylase inhibitors RO 4-4602 and NSD-1034 but not by methyldopahydrazine (MK-486). It was also antagonised by disulfuram and FLA 63. Chlorpromazine and phenoxybenzamine antagonised the A M D reversal but pimozide did not. When administration of A M D was delayed until 24 h after reserpinisation, its effect was greatly reduced. Several batches of mice were subjected to a second test of the ability of A M D to reverse the reserpine syndrome after a period of 3 weeks: the response to A M D was still markedly diminished. Finally, the ability of A M D to reverse the akinesia produced by the tyrosine hydroxylase inhibitor H 44/68 was also studied. Five hours following H 44/68 (250 mg/kg) only a few animals were active on the locomotor suppression test (mean, 3.71 ± 0.3 for 4 batches). A M D (1000 mg/kg), administered 3 h after H 44/68 failed to prevent or reverse this effect. These observations confirm previous reports that A M D can reverse the reserpine syndrome in small laboratory animals. The unexpected finding that the time interval between the administration of reserpine and the subsequent administration of A M D is of critical importance probably explains the conflicting reports mentioned above. The failure of A M D to reverse the reserpine syndrome in animals who had received an earlier dose of reserpine as long as 3 weeks previously suggests that intact amine storage granules are required for the reversal since the granules are irreversibly injured by reserpine and restoration of their function in nerve endings must await the synthesis of new granules in the perikaryon and their transport down the axons, a process which is known to require 4-6 weeks 6. Our data also confirm the report of Uretsky and Seiden that RO 4-4602 in high
181
SHORT COMMUNICATIONS TABLE I EFFECT OF VARIOUS DRUGS AND DELAY ON
Drug
Dose (mg/kg)
1. AMD alone 1000 2. AMD 1000 mg/kg plus MK-486 25 RO 4-4602 625 NSD-1034 100 FLA 63 25 Disulfuram 400 Chlorpromazine 5 Phenoxybenzamine 20 Pimozide 1 3. AMD delayed 24 h 1000 4. AMD on second trial after 3-week interval 1000
AMD
REVERSAL OF RESERPINE AKINESIA IN MICE
No. of batches
Mean ( + 1 S.E.) no. of animals protected/batch at 2 h after A M D
Significance o f difference from A M D alone (Student's t test, two-tailed)
7
7.6 ± 0.5
--
4 5 5 4 4 4 4 4 4
8.0 ± 1.1 0.6 ± 0.4 1.7 ! 0.4 5.2 -4- 0.6 1.0 ± 1.0 2.7 ± 1.1 2.5 + 0.5 7.0 + 0.8 2.7 -t- 0.2
Not P < P < P < P < P < P < Not P <
4
2.5 -q- 0.3
P < 0.001
significant 0.001 0.001 0.025 0.001 0.005 0.001 significant 0.001
dosages blocks the AMD reversal of the reserpine syndrome. Thus, the observed action of AMD must be mediated by one of its decarboxylation products. The present finding that both disulfuram and FLA 63 - - potent inhibitors of dopamine-fl-hydroxylase - - attentuate the AMD reversal, indicates that it depends on the formation of a-methylnoradrenaline (AMNA). Our finding that pimozide, a selective blocker of d o p a m i n e r e c e p t o r s 1, failed to a n t a g o n i s e the A M D reversal whereas p h e n o x y b e n z amine, which selectively blocks n o r a d r e n a l i n e receptors, b l o c k e d it effectively, f u r t h e r indicates t h a t the o b s e r v e d reversal o f the reserpine s y n d r o m e requires s t i m u l a t i o n o f n o r a d r e n a l i n e receptors, either directly by A M N A o r by n o r a d r e n a l i n e released by A M N A . The l a t t e r is suggested by the a p p a r e n t r e q u i r e m e n t for f u n c t i o n a l granules a n d by the failure o f A M D to reverse a k i n e s i a p r o d u c e d by the tyrosine h y d r o x y l a s e i n h i b i t o r H 44/68. Thus, one is led to conclude t h a t the a n t a g o n i s m o f the reserpine s y n d r o m e by A M D does n o t necessarily m e a n t h a t a - m e t h y l a t e d amines f u n c t i o n as s u r r o g a t e n e u r o t r a n s m i t t e r s in the brain. One is also led to w o n d e r w h e t h e r the effect o f l e v o d o p a on the reserpine s y n d r o m e m a y n o t also d e p e n d o n s t i m u l a t i o n o f n o r a d r e n a l i n e receptors. Clearly f u r t h e r study is necessary to resolve these points.
This work was supported by the Medical Research Council. 1 ANDI~N,N. E., BUTCHER, S. F., CORRODI, H., FUXE, K., AND UNGERSTEDT,U., Receptor activity and turnover of dopamine and noradrenaline after neuroleptics, Europ, J. Pharmacol., 11 (1970)
303-314. 2 BARBEAU,A., SOURKES,T. L., ET MURPHY, G. F., Les cat6cholamines dans la maladie de [Parkinson. In J. DE AJURIAGUERRA(Ed.), Monoamines et Systdme Nerveux Central, Masson, Paris, 1962, pp. 247-262. 3 CARLSSON,A., AND LINDQVIST,M., ln-vivo decarboxylation of a-methyldopa and a-methyl metatyrosine, Acta physiol, scand., 54 (1962) 87-94.
182
SHORT COMMUNICATIONS
4 DAY, M. D., AND RAND, M. J., Awakening from reserpine sedation by a-methyldopa, J. Pharm. Pharmacol., 15 0963) 631-632. 5 FERMAGLICH, J., AND O'DOHERTY, D. S., Synergism of levodopa by alpha methyldopa, Trans. Amer. neurol. Ass., 96 (1971) 231-234. 6 H.KGGENDAL,J., AND DAHLSTR()M,A., The recovery of noradrenaline in adrenergic nerve terminals of the rat after reserpine treatment, J. Pharm. Pharrnacol., 23 (1971) 81-89. 7 HANSON, L. C. F., AND HENNING, M., Effects of a-methyl-dopa on conditioned behaviour in the cat, Psychopharmacologia (Berl.), l l (1967) 1-7. 8 LOTTI, V. J., AND PORTER, C. C., Potentiation and inhibition of some central actions of L (--)dopa by decarboxylase inhibitors, J. Pharmacol. exp. Ther., 172 (1970) 406~,15. 9 MARKHAM,C. H., CLARK, W. G., AND WINTERS, W. D., Effect of alpha-methyl dopa and reserpine in Huntington's chorea, Parkinson's disease and other movement disorders, Life Sci., 9 (1963) 697-705. l0 MARSH, D. O., SCHNEIDEN, H., AND MARSHALL, J., A controlled clinical trial of aplha methyl dopa in Parkinsonian tremor, J. Neurol. Neurosurg. Psychiat., 26 (1963) 505-510. I l SVENSSON T. H., AND THEIME, G., An investigation of a new instrument to measure motor activity of small animals, Psychopharmacologia (BerL), 14 (1969) 157-163. 12 SWEET, R. D., LEE, J. E., AND McDOWELL, F. H., Methyldopa as an adjunct to levodopa treatment of Parkinson's disease, Clin. PharmacoL Ther., 13 (1972) 23-27. 13 URETSKY, N. J., AND SEIDEN, L. S., Effect of a-methyl dopa on the reserpine-induced suppression of motor activity and the conditioned avoidance response, J. Pharmacol. exp. Ther., 168 0969) 153-162.
14 VAN ROSSUM, J. M., AND HURKMANS,J. A., Reversal of the effect of a-methyldopa by monoamine oxidase inhibitors, J. Pharm. Pharmacol., 15 (1963) 493-499.
Brain Research, 71 (1974) 178-182 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Reversal of reserpine-induced bradykinesia by a-methyldopa - new light on its modus operandi
R. C. DUVOISIN* ANDC. D. MARSDEN University Department of Neurology, King's College Hospital and Institute of Psychiatry, London S.E. 5 (Great Britain)
(Accepted January 25th, 1974)
a-Methyldopa (AMD) is used to treat hypertension but recently it has been found to enhance the effect of levodopa in the therapy of Parkinson's diseaseS,1L It has been postulated that due to its ability to inhibit dopa decarboxylase, A M D may prevent the extracerebral metabolism of levodopa, thereby allowing a larger portion of the administered dose to reach the striatum where it may then be decarboxylated to form dopamine. Certainly its hydrazine analogue, a-methyldopa hydrazine (MK-486), acts in this manner, selectively inhibiting extracerebral decarboxylase s. However, this may not be the only possible way in which AMD may potentiate levodopa therapy. Another might be the 'correction' of the striatal dopamine deficit by a-methylated amines derived from AMD. If these could assume physiological function in the brain as Carlsson and Lindqvist 3 suggested and effectively substitute for the missing dopamine, then one would anticipate that AMD alone might exert a beneficial effect in Parkinson's disease. Since clinical experiments have been uncertain on this point, A M D having been variously found to benefit 10, exacerbate 2 or simply to fail altogether to modify the Parkinsonian state 9, pharmacological experiments on the effects of A M D on reserpine induced bradykinesia in small laboratory animals should be of interest. The evidence here, however, has also been limited and conflicting. Day and Rand 4 reported that 250 mg/kg A M D given intraperitoneally (i.p.) 17 h after reserpine yielded a gradual reversal of 'reserpine sedation' and restored locomotor activity. Subsequently, Uretsky and Seiden 18, in a more extensive study, found that 200 mg/kg A M D given 15 h after reserpine reversed the suppression of motor activity and conditioned avoidance responding in mice. They also noted that the decarboxylase inhibitor RO 4-4062, given in doses known to suppress cerebral decarboxylation, prevented the reversal. However, van Rossum and Hurkmans 14 reported that AMD combined with a monoamine oxidase inhibitor had no effect on the motor activity of mice pretreated with reserpine, 1 mg/kg daily for 3 days, and Hanson and Henning 7 found that A M D * Present address: Department of Neurology, The Mount Sinai School of Medicine, Fifth Avenue at 100th Street, New York, N.Y. 10029, U.S.A.
SHORT COMMUNICATIONS
179
failed to reverse the suppression of conditioned avoidance responses induced in the cat by reserpine given 22.5-25.75 h previously. In the course of an investigation of the reserpine syndrome in the mouse, we have recently encountered some unexpected findings which place the interaction of reserpine and A M D in a new perspective and provide an explanation of these apparently conflicting reports. Reserpine, 4 mg/kg, was given by intraperitoneal injection to male, white, 'P' strain mice and the effects studied by frequent, repeated direct observation and quantitative analysis of spontaneous motor activity using the Animex counter 11. Locomotor function was scored by counting the number of mice per batch of l0 who moved off a specifically demarcated 8 in. × l0 in. area in 15 sec, testing each animal individually. Normal animals walked briskly off the area within the time limit. Two hours after receiving reserpine the mice exhibited little spontaneous motor activity and lay quietly huddled together; none walked off the demarcated area. Several features analogous to human Parkinsonism were noted including a kyphotic posture, a resting tremor o f the paws noticeable when the animals were placed in a sitting-up position, a plastic rigidity detectable by manipulation of the limbs and a marked slowness and poverty of movement. All these features of the reserpine syndrome were reversed by A M D given 4 h after reserpine by i.p. injection in doses ranging from 100 to 1000 mg/kg. The reversal was first evident about 1 h after injection, reached its peak in 2-3 h and gradually subsided after 5-6 h. A dose-response plot constructed from the scores of the locomotor function test, using 5-7 batches of 10 mice each for each dose, is shown in Fig. 1. A dose-response plot for L-dopa derived in a similar manner is shown for comparison. The suppression of spontaneous motor activity by reserpine in 6 control batches measured on the Animex apparatus for 96 h is shown in Fig. 2. The reversal induced by AMD, 1000 mg/kg, 4 b after reserpine is superimposed to show its magnitude and temporal course. The A M D reversal of the reserpine syndrome was further studied by administering the various agents listed in Table I, either simultaneously or 1 h before A M D (1000 mg/kg). All drugs were administered by i.p. injection. The reversal was found to
~'~.~_oI00t
I
I
~,~, 6o ~1---
i ooo"
,7"""
"I
4O
w~ ,,',8 2o
NJ
J
L-dora
~,,~ 80 I.LI ¢ ~
I
0 I0
I
40
I
I
I
I
I00 2 5 0 5 0 0 I000 DOSE, mg/kg i,p.
Fig. 1. Dose-response of AMD and of L-dopa antagonism of reserpine-induced locomotor suppression in the mouse. Brackets indicate standard deviation.
180
Q
SHORT COMMUNICATIONS
6
C
3
5
4
0 0
3
>2
U 0
24
HOURS
48
OF CONTINUOUS
72
96
OBSERVATION
Fig. 2. Graphic presentation of mean hourly Animex activity counts of 6 batches of 10 mice each monitored for 96 h. Reserpine 4 mg/kg was given at 23 h. Shown in black is the mean hourly counts of 4 batches given reserpine 4 mg/kg followed 4 h later by AMD, 1000 mg/kg.
be significantly antagonised by the centrally active decarboxylase inhibitors RO 4-4602 and NSD-1034 but not by methyldopahydrazine (MK-486). It was also antagonised by disulfuram and FLA 63. Chlorpromazine and phenoxybenzamine antagonised the A M D reversal but pimozide did not. When administration of A M D was delayed until 24 h after reserpinisation, its effect was greatly reduced. Several batches of mice were subjected to a second test of the ability of A M D to reverse the reserpine syndrome after a period of 3 weeks: the response to A M D was still markedly diminished. Finally, the ability of A M D to reverse the akinesia produced by the tyrosine hydroxylase inhibitor H 44/68 was also studied. Five hours following H 44/68 (250 mg/kg) only a few animals were active on the locomotor suppression test (mean, 3.71 ± 0.3 for 4 batches). A M D (1000 mg/kg), administered 3 h after H 44/68 failed to prevent or reverse this effect. These observations confirm previous reports that A M D can reverse the reserpine syndrome in small laboratory animals. The unexpected finding that the time interval between the administration of reserpine and the subsequent administration of A M D is of critical importance probably explains the conflicting reports mentioned above. The failure of A M D to reverse the reserpine syndrome in animals who had received an earlier dose of reserpine as long as 3 weeks previously suggests that intact amine storage granules are required for the reversal since the granules are irreversibly injured by reserpine and restoration of their function in nerve endings must await the synthesis of new granules in the perikaryon and their transport down the axons, a process which is known to require 4-6 weeks 6. Our data also confirm the report of Uretsky and Seiden that RO 4-4602 in high
181
SHORT COMMUNICATIONS TABLE I EFFECT OF VARIOUS DRUGS AND DELAY ON
Drug
Dose (mg/kg)
1. AMD alone 1000 2. AMD 1000 mg/kg plus MK-486 25 RO 4-4602 625 NSD-1034 100 FLA 63 25 Disulfuram 400 Chlorpromazine 5 Phenoxybenzamine 20 Pimozide 1 3. AMD delayed 24 h 1000 4. AMD on second trial after 3-week interval 1000
AMD
REVERSAL OF RESERPINE AKINESIA IN MICE
No. of batches
Mean ( + 1 S.E.) no. of animals protected/batch at 2 h after A M D
Significance o f difference from A M D alone (Student's t test, two-tailed)
7
7.6 ± 0.5
--
4 5 5 4 4 4 4 4 4
8.0 ± 1.1 0.6 ± 0.4 1.7 ! 0.4 5.2 -4- 0.6 1.0 ± 1.0 2.7 ± 1.1 2.5 + 0.5 7.0 + 0.8 2.7 -t- 0.2
Not P < P < P < P < P < P < Not P <
4
2.5 -q- 0.3
P < 0.001
significant 0.001 0.001 0.025 0.001 0.005 0.001 significant 0.001
dosages blocks the AMD reversal of the reserpine syndrome. Thus, the observed action of AMD must be mediated by one of its decarboxylation products. The present finding that both disulfuram and FLA 63 - - potent inhibitors of dopamine-fl-hydroxylase - - attentuate the AMD reversal, indicates that it depends on the formation of a-methylnoradrenaline (AMNA). Our finding that pimozide, a selective blocker of d o p a m i n e r e c e p t o r s 1, failed to a n t a g o n i s e the A M D reversal whereas p h e n o x y b e n z amine, which selectively blocks n o r a d r e n a l i n e receptors, b l o c k e d it effectively, f u r t h e r indicates t h a t the o b s e r v e d reversal o f the reserpine s y n d r o m e requires s t i m u l a t i o n o f n o r a d r e n a l i n e receptors, either directly by A M N A o r by n o r a d r e n a l i n e released by A M N A . The l a t t e r is suggested by the a p p a r e n t r e q u i r e m e n t for f u n c t i o n a l granules a n d by the failure o f A M D to reverse a k i n e s i a p r o d u c e d by the tyrosine h y d r o x y l a s e i n h i b i t o r H 44/68. Thus, one is led to conclude t h a t the a n t a g o n i s m o f the reserpine s y n d r o m e by A M D does n o t necessarily m e a n t h a t a - m e t h y l a t e d amines f u n c t i o n as s u r r o g a t e n e u r o t r a n s m i t t e r s in the brain. One is also led to w o n d e r w h e t h e r the effect o f l e v o d o p a on the reserpine s y n d r o m e m a y n o t also d e p e n d o n s t i m u l a t i o n o f n o r a d r e n a l i n e receptors. Clearly f u r t h e r study is necessary to resolve these points.
This work was supported by the Medical Research Council. 1 ANDI~N,N. E., BUTCHER, S. F., CORRODI, H., FUXE, K., AND UNGERSTEDT,U., Receptor activity and turnover of dopamine and noradrenaline after neuroleptics, Europ, J. Pharmacol., 11 (1970)
303-314. 2 BARBEAU,A., SOURKES,T. L., ET MURPHY, G. F., Les cat6cholamines dans la maladie de [Parkinson. In J. DE AJURIAGUERRA(Ed.), Monoamines et Systdme Nerveux Central, Masson, Paris, 1962, pp. 247-262. 3 CARLSSON,A., AND LINDQVIST,M., ln-vivo decarboxylation of a-methyldopa and a-methyl metatyrosine, Acta physiol, scand., 54 (1962) 87-94.
182
SHORT COMMUNICATIONS
4 DAY, M. D., AND RAND, M. J., Awakening from reserpine sedation by a-methyldopa, J. Pharm. Pharmacol., 15 0963) 631-632. 5 FERMAGLICH, J., AND O'DOHERTY, D. S., Synergism of levodopa by alpha methyldopa, Trans. Amer. neurol. Ass., 96 (1971) 231-234. 6 H.KGGENDAL,J., AND DAHLSTR()M,A., The recovery of noradrenaline in adrenergic nerve terminals of the rat after reserpine treatment, J. Pharm. Pharrnacol., 23 (1971) 81-89. 7 HANSON, L. C. F., AND HENNING, M., Effects of a-methyl-dopa on conditioned behaviour in the cat, Psychopharmacologia (Berl.), l l (1967) 1-7. 8 LOTTI, V. J., AND PORTER, C. C., Potentiation and inhibition of some central actions of L (--)dopa by decarboxylase inhibitors, J. Pharmacol. exp. Ther., 172 (1970) 406~,15. 9 MARKHAM,C. H., CLARK, W. G., AND WINTERS, W. D., Effect of alpha-methyl dopa and reserpine in Huntington's chorea, Parkinson's disease and other movement disorders, Life Sci., 9 (1963) 697-705. l0 MARSH, D. O., SCHNEIDEN, H., AND MARSHALL, J., A controlled clinical trial of aplha methyl dopa in Parkinsonian tremor, J. Neurol. Neurosurg. Psychiat., 26 (1963) 505-510. I l SVENSSON T. H., AND THEIME, G., An investigation of a new instrument to measure motor activity of small animals, Psychopharmacologia (BerL), 14 (1969) 157-163. 12 SWEET, R. D., LEE, J. E., AND McDOWELL, F. H., Methyldopa as an adjunct to levodopa treatment of Parkinson's disease, Clin. PharmacoL Ther., 13 (1972) 23-27. 13 URETSKY, N. J., AND SEIDEN, L. S., Effect of a-methyl dopa on the reserpine-induced suppression of motor activity and the conditioned avoidance response, J. Pharmacol. exp. Ther., 168 0969) 153-162.
14 VAN ROSSUM, J. M., AND HURKMANS,J. A., Reversal of the effect of a-methyldopa by monoamine oxidase inhibitors, J. Pharm. Pharmacol., 15 (1963) 493-499.