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Effect of chronic neuroleptic or L-DOPA administration on GABA levels in the rat substantia nigra
Life Sciencaa, Vol . 21, pp . 1489-1496 Priatad is tha II .8 .A.
Pergamoa Praea
EFFECT OF CHRONIC NEUROLEPTIC OR L-DOPA ADMINISTRATION ON GABA LEVELS IN THE RAT SUBSTANTIA NIGRA Kenneth G . L1oyd l and Oleh Hornykiewicz 2 Department of Psychopharmacology, Clarke Institute of Psychiatry, 250 College Street, Toronto, Canada, M5T IR8 and Department of Pharmacology, University of Toronto, Toronto, Canada 1 . Present address : Neupharmacology Unit, Synthêlabo, L .E .R .S ., 31 Avenue Paul Vaillant Couturier, F 92220 BAGNEUX, France . 2 . Present address : Institute of Biochemical Pharmacology, University of Vienna, Waehringer Strasse 17, A-1090 Vienna, Austria . (Becaived is fuel form October 3, 1977) Summ a~ Male albino rats were administered daily with haloperidol, clozapine or L-DOPA and sacrificed 18 hours after the last dose of the drug . Acutely haloperidol (5mg/kg, i .p .) greatly lowered nigral GABA levels whereas after 167 daily doses the nigral GAGA levels were not significantly different from controls, but were significantly increased as compared with the acutely treated animals . In contrast, acute L-DOPA (2 x 100mg, p .o .) greatly raised nigral GAGA levels whereas after chronic L-DOPA (167 days) nigral GABA levels were not significantly different from controls and were significantly lower as compared with the animals receiving the acute treatment . Clozapine (20 mg/kg, i .p . either acutely or chronically) did 5~ot have as marked an effect on nigral GABA levels as did haloperidol . Of these various drug regimens only chronic L-DOPA significantly affected nigral GAD activity, producing a moderate decrease . It is well known that the substantia nigra (specifically the pars compacta) contains cell bodies of dopamine (DA) containing neurons which project to the neostriatum (1) . These neurons degenerate in Parkinson's disease (2) . There is evidence suggesting that GABA neurons inhibit the nigra-striatal DA neurons (3-7) . 3H-GABA binding is decreased in the substantia nigra (but not caudate or putamen) of patients with Parkinson's disease indicating the presence of GABA receptors on nigral DA cell bodies or dendrites (8) and a GAGA-pathway from the striatum to the substantia nigra seems to be well established (9-13) . These striato-nigral GABA neurons are influenced by alterations in dopaminer-
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gic activity as indicated by increased striatal or nigral GABA levels following DA agonists and decreased GABA levels (and increased turnover) following neuroleptics (14-16) . However, the influence of DA agonists and antagonists on GABA levels has been examined only upon acute administration of the drugs in question .As schizophrenic patients receive chronic neuroleptic therapy, and as GABA as well as DA have been implicated in schizophrenia (cf . 17,18)we have studied the effects of prolonged neuroleptics or L-DOPA administration on GABf1ergic parameters in the rats substantia nigra . Materials and Methods Male albino (Wistar)rats (starting fight 100 g) were used and received either haloperidol (5mg/kg, i .p ., Haldol~ for injection) daily at 16 : 00 hours on days 1-167, or clopazine (20 mg/kg, p ., dissolved in a small volume of 1% lactic acid, made up to volume and adjusted to pH 5 .5 - 6 .0 with solid sodium bicarbonate) daily at 16 .00 hours on days 1-100 or L-DOPA via gastric tube on the following schedule : days 1-33, 50 mg/rat/day ; days 101-167,2 x 100 mg/rat/ day . On days 1-100 L-DOPA was given at 16 :00 hours ; on days 101-167, L-DOPA was administrated at 9 :00 and 16 : 00 hours . Acutely treated animals received a single injection of either haloperidol or clopazine, 18 hours prior to sacrifice , or L-DOPA, 100 mg, 18 and 24 hours prior co sacrifice . Control animals received 1 .0 ml water via gastric tube on the same schedule as L-DOPA administration as this appeared to be the most stressful route of administration . The time of sacrifice at 18 hours post-drug was chosen in order to examine the possible long-acting alterations in the levels of GABA and GAD in response to the various drugs rather than the disturbances which follow immediately (2-6 hrs) after drug administration . Animals were decapitated 18 hours after the last drug administration, the brains removed and frozen in dry-ice-butanol within a strictly controlled 3040 second time interval . The substantia nigra was dissected from coronal slices of frozen brains with the aid of the atlas of Konig and Klippel (19) and homoyenized (400 :1) in ice-cold de-ionized water . GABA concentrations were deternined by the method of Enna and Snyder (20) with the following modifications : before use in the assay the GABA-binding membrane preparation (from rat brain) was incubated for .60 minutes at 37°C in 0 .1 M potassium phosphate buffer (pH 7 .2) containing 0 .05 percent Triton-X-100, centrifuged at 48,00 x g x 20 minutes and then resuspended in 0 .1 M potassium phosphate buffer (pH 7 .2) . This procedure increases the capacity of the membranes to bind 3H-GABA (21,22) . The buffer used in the assay was 0 .1 M potassium phosphate (pH 7 .2) rather than the Tris buffer utilized by Enna and Snyder (20) . In our hands, the former buffer yields greater consitency in the assay . L-Glutamate L-Carboxy-lose (E~ 4 .1 .1 .15 .,glutamic acid decarboxylase, GAD) activity was estimated as described by Lloyd et al (23) . Proteins were determined by the method of Lowry et al (24) . Results At 100 days the clopazine animals weighed 315 .3 + 20 .5 g . and the controls 450 .2 + 9 .1 grams . At 167 days the haloperidol anima s weighed 378 .2 ~ 25 .6 g, the L ~DPA animals 444 .4 + 10 .2 g and the controls 514 .1 +9 .5 g . Thesé lower weights of the treated animals may indicate a slight degree of toxicity upon chronic administration of the drugs at these doses . However this toxicity likely is not important to the outcome as in most instances the neurochemical parameters of the chronically treated groups were not different from the controls (see below and Ref . 25) . The effects of acute or chronic administration of neuroleptic or L-DOPA on
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Chronic Druga on GABA Levnla
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GABA levels in the substantia nigra are shown in Table I . Acutely, the neuroleptics clozapine and haloperidol significantly decreased GABA levels in the substantia nigra by 49 and 74 percent, respectively . After 100 daily injections of clozapine (20 mg/kg,i .p .) the nigral GABA levels were not significantly different from controls altough a decrease was still observed . After 167 days of haloperidol (5 mg/kg,i .p .) the nigral GABA concentrations were not significantly different from controls, although a tendency towards increased levels was noted. GABA levels measured after chronic administration were significantly higher than those after acute haloperidol administration (513 percent, p<0 .001) . In contrast to the neuroleptics, acute administration of L-DOPA resulted in increased nigral GABA levels (to 168 percent of control) . Upon chronic administration of L-DOPA GABA levels were not different fray those of controls animals-but were greatly lowered as compared with acute L-DOPA administration . The effect of these treatment regimens on nigral GAD activity is shown in Table 2 . GAD activity in tM substantia nigra was not significantly altered by either acute or chronic neuroleptic administration . Small, decreases of similar magnitudé in nigral GAD were observed following either acute or chronic L-DOPA administration . Also, in the striatum GAD activity was similar for both control rats (66 .7 + y~ mol C /mg protein/30 min .,n~4) and rats chronically treated with L-DOPA (58.0 +~ .5 N mol COZ/mg protein/30 min ., n~8) . In control animals nigral GABA concentrations and GAD activity showed a weak but significant correlation (x=0 .563, pß .05) . No significant correlation was evident in the haloperidol (r=0 .257, p~0 .10) or L-DOPA (r~0 .121, X0 .10) treated animals . Discussion The most significant finding of this study was that following chronic administration of haloperidol (daily injections of 5 mg/kg for 167 days) or L-DOPA (on a progressive dose schedule for 167 days) there was a~marked attenuation of their respective acute effects (Table 1) . Thus, GABA levels in the substantia nigra were not significantly different from controls after chronic haloperidol (as compared with a large decrease after acute administration) or chronic L-DOPA (as compared with an increase after acute treatment), That these changes (development of tolerance) were not due to a generalized increase in drug metabolism is seen by the following : (i) for haloperidol, alterations of choline acetytransferase activity appear during prolonged haloperidol treatment, and are specifically correlated with alterations in the cataleptogenic effect of the drug (25) ; and (ii) there is across tolerance between neuroleptics on some biochemical parameters (26-27), The present finding that acute haloperidol administration decreases and acute L-DOPA increases nigral GABA levels is in agreement with similar observations by other investigators, possibly indicating an opposite effect of these drugs on GABA turnover (14-16) . In the present study acute clozapine administration also decreased GABA concentrations in the substantia nigra (although to a lesser extent than haloperidol) . Mc Geer et al (15) using a larger dose of clozapine (50 mg/kg) did not observe a similar change . A. possible reason for this discrepancy is that in the present study sacrifice was 18 hours after drug administration whereas in the previous report sacrifice was 2 hours after clozapine. The opposite effects on nigral GABA levels of acute administration of DA mimetics (for a corresponding effect of apomorphine, cf .24) or haloperidol imply that these changes are likely mediated via a canmon (DA) receptor . This mediation may be through a direct DA :GABA connection (28-29) or via a cholinergic interneuron in the striatum (linking the DA neu-
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Chronic Drugs on GABA Levels
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rons with the GABA neuron?) (15) . An interesting observation is that clozapine was less effective than haloperidol in lowering GABA levels in the susbtantia nigra, but that with chronic administration the "nornialization" of levels was less aparent for clozapine than hal.operidol (although in both chronic groups the GABA levels were not significantly different from controls) . A possible explanation is that haloperidol activates the feed back and compensatory mechanism to a much greater extent than does clozapine . The finding that the changes in nigral GAGA levels were not paralleled by corresponding changes in GAD levels (Table 2)seems to imply that GAD activity was not the step responsible for the developement of tolerance to the acute changes in GABA levels elicited by neuroleptics or L-DOPA ; possibly, alterations of GAGA release regulate these long-term drug effects . In our previous report (30) GAD activity in the striatum of rats was found to be increased upon prolonged (109 days)L-DOPA administration ; in the present study, both acute and chronic administration of L-DOPA resulted in an apparent lowering of nigral (Table 2) and unaltered striatal GAD activity . We do not have a plausible explanation for this discrepâncy ; the only procedural difference between these two studies is the considerably longer period of chronic L-DOPA administration in the present group . L-DOPA is known to inhibit GAD activity "in vitro" (31) and has previously been reported to decrease GAD activity in animal brain for a least 24 hours after acute administration . (32) . The present findings may have a direct clinical correlate . Thus, it has been hypothesized that in Parkinson's disease a decrease in striato-nigral GABA neuron activity releases nigral DA cell bodies from a tonic GABA inhibition (33) . The results of the present study are in agreement with such a possibility insofar as the initial increase of GABA turnover seen with acute treatment was greatly attenuated upon chronic administration of neuroleptics . References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10 . 11 . 12 . 13 . 14 . 15 .
N .-E. ANDEN, A. CARLSSON, A . DAHLSTROM, F. FOXE, N .-A. HILLARP and K. LARSSON, Life Sci . 3 523-530 (1964) . 0 . HORNYKiEWICZ,~Ft$rniac . Revs . 18 925-964 (1966) . A . R. CROSS~MN, R:-J~IALKE1~and~ N . WOODRUFF, grit, - J . Pharmac. 49 696-698 (1974) . A . DRAY and T. J . GONYE, J . Physiol . 246 88-89 (1975) A . DRAY and D. W. STRAUGHAN, TFFârm~harmac . 28 400-405 (1976) . P . FELTZ, Can. J . P~h~siol,Pharmac~T lII3Z1î3 jî971) . W . PRECHT and M- YOSRIQI~, ra n es~2 229-233 (1971) . K . G. LLOYD, L . SHEMEN andÛ~1DR1~YfCIEWICZ, Brain Res . 127, 269-278 (1977) . J . S. KIM, I . J . BAK, R. HASSLER and Y . OKADA;E3i~Bratff-Res . 14 95-104 (1971) . P . L. McGEER, E . G . McGEER, J . P. WADA and E . JUNG, Brain Res . 32 425-431 (1971) . T . HATTORI, P . L . McGEER, H . C . FIBIGER and E . G . McGEER, Brain Res . 54 103-114 (1973) . F . FONNUM, I . GROFOVA, E . RINViK, J . STORM-MATHISEN and F . WALBERG, Brain Res . 71 77-92 (1974) . K.G. TLDYD, H . MOHLER,G . BARTHOLINI and 0 . HORNYKIEWICZ, Advances in Parkins~ism (W . Birkmayer and 0. Hornykiewicz, eds) 186-192,~ïta~h~Bä'S81 1 J . 5. KIM and R . HASSLER, Brain Res . 88 150-153 (1975) . P .L . McGEER,D .S . GREWAAL anTE -.~G:~fcGEER, Advances in ~arkinsonism (W . Birkmayer and 0 . Hornukiewicz, eds) 132-140, on s~o '('1977) .
Vol . 21, No . 10, 1977
16 . 17 . 18 . 19 . 20 . 21 . 22 . 23 . 24 . 25 . 26 . 27 . 28 . 29 . 30 . 31 . 32 . 33 .
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Chronic Drugs on GABA Levels
G. G. S . COLLINS, Biochem . Pharmac . 22 101-111 (1913) . K. G. LLOYD, Monoam n~~c-~o ~ ne r ~_ Interactions _in _the Brain (L . L . ewA YorTc. BUTCHER, ed) n press, ca emu E. ROBERTS, Neurosci . Res . Pro . Bull . . 10 468-482 (1976) . J . R. F . KONI a~ïf . 7~KL ,%~e _R~ Brain , Krieger, Huntington, N. Y . (1975) . S. J . ENNA and S . H . SNYDER, J . Neurochem. 26 . 221-224 (1976) . S. J . ENNA and S . H . SNYDER, Molec~TiarmacT . 13 442-453 (1977) . D. T . WONG and J . S . HORNG, L~~ciencés ~t~44~452 (1977) . K . G . LLOYD, H . MOHLER, Ph . an . ARYHOLINI, J . Neurochem . _25 789-795 (1975) . 0 . H . LOWRY, N . J . ROSENBROUCH, A . L . FARR and R. J . RANDALL, J . Biol . chem . 193 265-275 (1951) . K.~ . LT6YD,M. SHIBUYA, L. DAVIDSONand 0 . HORNYKIEWICZ, Advances in . eds) Biochemical Ps cho harmacolo , Vol . 16 (E . Costa and GT p~ltaven ress, ew or c 197Tj. H . ASPER, M, BAGGLIOLINI, H . R. BURKI, H . LAUENER, W. RUCH and G. STILLE, Euro . J . Pharmacol . 22 287-294 (1973) . osium on Do amine ~AYTON,~6E6ÉK ând J . KORF, Satellite S (G . N . Woodruff, P . J . Roberts and3e s n ress~ress New York (1977) . M. PEREZ DE LA MORA, K . FOXE, T . HOKFELT and A . LJUNDAHL, Neurosci . Letts . 1 109-114 (1975) . 1~~UXE, T. HOKFELT, A . LJUNDAHL, L . AGNOSTI, 0 . JOHANSON and M. PEREZ DE LA NX)RA, Med . Biol . 53 177-183 (1975) . K. G. LLOYD ~ O~f6RNYKIEWICZ, Nature, Lond . 243 521-523 (1973) . P .~ HOLTZ and E . WESTERMANN, Arch .~pa~23T'311 332 (1957) . D . S. KURTZ and J . N. KANFER~ NérocTiem. 1S -f235-2236 (1971) . 0. HORNYKIEWICZ, K. G. LLOYD and L~ÀVi6~ON,GABA in Nervous S stem Function (E . Roberts, T . N . Chase and D . B . Tower, ids p. , aven reP New York (1976) .
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Pergamoa Praea
EFFECT OF CHRONIC NEUROLEPTIC OR L-DOPA ADMINISTRATION ON GABA LEVELS IN THE RAT SUBSTANTIA NIGRA Kenneth G . L1oyd l and Oleh Hornykiewicz 2 Department of Psychopharmacology, Clarke Institute of Psychiatry, 250 College Street, Toronto, Canada, M5T IR8 and Department of Pharmacology, University of Toronto, Toronto, Canada 1 . Present address : Neupharmacology Unit, Synthêlabo, L .E .R .S ., 31 Avenue Paul Vaillant Couturier, F 92220 BAGNEUX, France . 2 . Present address : Institute of Biochemical Pharmacology, University of Vienna, Waehringer Strasse 17, A-1090 Vienna, Austria . (Becaived is fuel form October 3, 1977) Summ a~ Male albino rats were administered daily with haloperidol, clozapine or L-DOPA and sacrificed 18 hours after the last dose of the drug . Acutely haloperidol (5mg/kg, i .p .) greatly lowered nigral GABA levels whereas after 167 daily doses the nigral GAGA levels were not significantly different from controls, but were significantly increased as compared with the acutely treated animals . In contrast, acute L-DOPA (2 x 100mg, p .o .) greatly raised nigral GAGA levels whereas after chronic L-DOPA (167 days) nigral GABA levels were not significantly different from controls and were significantly lower as compared with the animals receiving the acute treatment . Clozapine (20 mg/kg, i .p . either acutely or chronically) did 5~ot have as marked an effect on nigral GABA levels as did haloperidol . Of these various drug regimens only chronic L-DOPA significantly affected nigral GAD activity, producing a moderate decrease . It is well known that the substantia nigra (specifically the pars compacta) contains cell bodies of dopamine (DA) containing neurons which project to the neostriatum (1) . These neurons degenerate in Parkinson's disease (2) . There is evidence suggesting that GABA neurons inhibit the nigra-striatal DA neurons (3-7) . 3H-GABA binding is decreased in the substantia nigra (but not caudate or putamen) of patients with Parkinson's disease indicating the presence of GABA receptors on nigral DA cell bodies or dendrites (8) and a GAGA-pathway from the striatum to the substantia nigra seems to be well established (9-13) . These striato-nigral GABA neurons are influenced by alterations in dopaminer-
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gic activity as indicated by increased striatal or nigral GABA levels following DA agonists and decreased GABA levels (and increased turnover) following neuroleptics (14-16) . However, the influence of DA agonists and antagonists on GABA levels has been examined only upon acute administration of the drugs in question .As schizophrenic patients receive chronic neuroleptic therapy, and as GABA as well as DA have been implicated in schizophrenia (cf . 17,18)we have studied the effects of prolonged neuroleptics or L-DOPA administration on GABf1ergic parameters in the rats substantia nigra . Materials and Methods Male albino (Wistar)rats (starting fight 100 g) were used and received either haloperidol (5mg/kg, i .p ., Haldol~ for injection) daily at 16 : 00 hours on days 1-167, or clopazine (20 mg/kg, p ., dissolved in a small volume of 1% lactic acid, made up to volume and adjusted to pH 5 .5 - 6 .0 with solid sodium bicarbonate) daily at 16 .00 hours on days 1-100 or L-DOPA via gastric tube on the following schedule : days 1-33, 50 mg/rat/day ; days 101-167,2 x 100 mg/rat/ day . On days 1-100 L-DOPA was given at 16 :00 hours ; on days 101-167, L-DOPA was administrated at 9 :00 and 16 : 00 hours . Acutely treated animals received a single injection of either haloperidol or clopazine, 18 hours prior to sacrifice , or L-DOPA, 100 mg, 18 and 24 hours prior co sacrifice . Control animals received 1 .0 ml water via gastric tube on the same schedule as L-DOPA administration as this appeared to be the most stressful route of administration . The time of sacrifice at 18 hours post-drug was chosen in order to examine the possible long-acting alterations in the levels of GABA and GAD in response to the various drugs rather than the disturbances which follow immediately (2-6 hrs) after drug administration . Animals were decapitated 18 hours after the last drug administration, the brains removed and frozen in dry-ice-butanol within a strictly controlled 3040 second time interval . The substantia nigra was dissected from coronal slices of frozen brains with the aid of the atlas of Konig and Klippel (19) and homoyenized (400 :1) in ice-cold de-ionized water . GABA concentrations were deternined by the method of Enna and Snyder (20) with the following modifications : before use in the assay the GABA-binding membrane preparation (from rat brain) was incubated for .60 minutes at 37°C in 0 .1 M potassium phosphate buffer (pH 7 .2) containing 0 .05 percent Triton-X-100, centrifuged at 48,00 x g x 20 minutes and then resuspended in 0 .1 M potassium phosphate buffer (pH 7 .2) . This procedure increases the capacity of the membranes to bind 3H-GABA (21,22) . The buffer used in the assay was 0 .1 M potassium phosphate (pH 7 .2) rather than the Tris buffer utilized by Enna and Snyder (20) . In our hands, the former buffer yields greater consitency in the assay . L-Glutamate L-Carboxy-lose (E~ 4 .1 .1 .15 .,glutamic acid decarboxylase, GAD) activity was estimated as described by Lloyd et al (23) . Proteins were determined by the method of Lowry et al (24) . Results At 100 days the clopazine animals weighed 315 .3 + 20 .5 g . and the controls 450 .2 + 9 .1 grams . At 167 days the haloperidol anima s weighed 378 .2 ~ 25 .6 g, the L ~DPA animals 444 .4 + 10 .2 g and the controls 514 .1 +9 .5 g . Thesé lower weights of the treated animals may indicate a slight degree of toxicity upon chronic administration of the drugs at these doses . However this toxicity likely is not important to the outcome as in most instances the neurochemical parameters of the chronically treated groups were not different from the controls (see below and Ref . 25) . The effects of acute or chronic administration of neuroleptic or L-DOPA on
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Vol. 21, No . 10, 1977
Chronic Druga on GABA Levnla
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GABA levels in the substantia nigra are shown in Table I . Acutely, the neuroleptics clozapine and haloperidol significantly decreased GABA levels in the substantia nigra by 49 and 74 percent, respectively . After 100 daily injections of clozapine (20 mg/kg,i .p .) the nigral GABA levels were not significantly different from controls altough a decrease was still observed . After 167 days of haloperidol (5 mg/kg,i .p .) the nigral GABA concentrations were not significantly different from controls, although a tendency towards increased levels was noted. GABA levels measured after chronic administration were significantly higher than those after acute haloperidol administration (513 percent, p<0 .001) . In contrast to the neuroleptics, acute administration of L-DOPA resulted in increased nigral GABA levels (to 168 percent of control) . Upon chronic administration of L-DOPA GABA levels were not different fray those of controls animals-but were greatly lowered as compared with acute L-DOPA administration . The effect of these treatment regimens on nigral GAD activity is shown in Table 2 . GAD activity in tM substantia nigra was not significantly altered by either acute or chronic neuroleptic administration . Small, decreases of similar magnitudé in nigral GAD were observed following either acute or chronic L-DOPA administration . Also, in the striatum GAD activity was similar for both control rats (66 .7 + y~ mol C /mg protein/30 min .,n~4) and rats chronically treated with L-DOPA (58.0 +~ .5 N mol COZ/mg protein/30 min ., n~8) . In control animals nigral GABA concentrations and GAD activity showed a weak but significant correlation (x=0 .563, pß .05) . No significant correlation was evident in the haloperidol (r=0 .257, p~0 .10) or L-DOPA (r~0 .121, X0 .10) treated animals . Discussion The most significant finding of this study was that following chronic administration of haloperidol (daily injections of 5 mg/kg for 167 days) or L-DOPA (on a progressive dose schedule for 167 days) there was a~marked attenuation of their respective acute effects (Table 1) . Thus, GABA levels in the substantia nigra were not significantly different from controls after chronic haloperidol (as compared with a large decrease after acute administration) or chronic L-DOPA (as compared with an increase after acute treatment), That these changes (development of tolerance) were not due to a generalized increase in drug metabolism is seen by the following : (i) for haloperidol, alterations of choline acetytransferase activity appear during prolonged haloperidol treatment, and are specifically correlated with alterations in the cataleptogenic effect of the drug (25) ; and (ii) there is across tolerance between neuroleptics on some biochemical parameters (26-27), The present finding that acute haloperidol administration decreases and acute L-DOPA increases nigral GABA levels is in agreement with similar observations by other investigators, possibly indicating an opposite effect of these drugs on GABA turnover (14-16) . In the present study acute clozapine administration also decreased GABA concentrations in the substantia nigra (although to a lesser extent than haloperidol) . Mc Geer et al (15) using a larger dose of clozapine (50 mg/kg) did not observe a similar change . A. possible reason for this discrepancy is that in the present study sacrifice was 18 hours after drug administration whereas in the previous report sacrifice was 2 hours after clozapine. The opposite effects on nigral GABA levels of acute administration of DA mimetics (for a corresponding effect of apomorphine, cf .24) or haloperidol imply that these changes are likely mediated via a canmon (DA) receptor . This mediation may be through a direct DA :GABA connection (28-29) or via a cholinergic interneuron in the striatum (linking the DA neu-
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Chronic Drugs on GABA Levels
Vol . 21, No . 10, 1977
rons with the GABA neuron?) (15) . An interesting observation is that clozapine was less effective than haloperidol in lowering GABA levels in the susbtantia nigra, but that with chronic administration the "nornialization" of levels was less aparent for clozapine than hal.operidol (although in both chronic groups the GABA levels were not significantly different from controls) . A possible explanation is that haloperidol activates the feed back and compensatory mechanism to a much greater extent than does clozapine . The finding that the changes in nigral GAGA levels were not paralleled by corresponding changes in GAD levels (Table 2)seems to imply that GAD activity was not the step responsible for the developement of tolerance to the acute changes in GABA levels elicited by neuroleptics or L-DOPA ; possibly, alterations of GAGA release regulate these long-term drug effects . In our previous report (30) GAD activity in the striatum of rats was found to be increased upon prolonged (109 days)L-DOPA administration ; in the present study, both acute and chronic administration of L-DOPA resulted in an apparent lowering of nigral (Table 2) and unaltered striatal GAD activity . We do not have a plausible explanation for this discrepâncy ; the only procedural difference between these two studies is the considerably longer period of chronic L-DOPA administration in the present group . L-DOPA is known to inhibit GAD activity "in vitro" (31) and has previously been reported to decrease GAD activity in animal brain for a least 24 hours after acute administration . (32) . The present findings may have a direct clinical correlate . Thus, it has been hypothesized that in Parkinson's disease a decrease in striato-nigral GABA neuron activity releases nigral DA cell bodies from a tonic GABA inhibition (33) . The results of the present study are in agreement with such a possibility insofar as the initial increase of GABA turnover seen with acute treatment was greatly attenuated upon chronic administration of neuroleptics . References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10 . 11 . 12 . 13 . 14 . 15 .
N .-E. ANDEN, A. CARLSSON, A . DAHLSTROM, F. FOXE, N .-A. HILLARP and K. LARSSON, Life Sci . 3 523-530 (1964) . 0 . HORNYKiEWICZ,~Ft$rniac . Revs . 18 925-964 (1966) . A . R. CROSS~MN, R:-J~IALKE1~and~ N . WOODRUFF, grit, - J . Pharmac. 49 696-698 (1974) . A . DRAY and T. J . GONYE, J . Physiol . 246 88-89 (1975) A . DRAY and D. W. STRAUGHAN, TFFârm~harmac . 28 400-405 (1976) . P . FELTZ, Can. J . P~h~siol,Pharmac~T lII3Z1î3 jî971) . W . PRECHT and M- YOSRIQI~, ra n es~2 229-233 (1971) . K . G. LLOYD, L . SHEMEN andÛ~1DR1~YfCIEWICZ, Brain Res . 127, 269-278 (1977) . J . S. KIM, I . J . BAK, R. HASSLER and Y . OKADA;E3i~Bratff-Res . 14 95-104 (1971) . P . L. McGEER, E . G . McGEER, J . P. WADA and E . JUNG, Brain Res . 32 425-431 (1971) . T . HATTORI, P . L . McGEER, H . C . FIBIGER and E . G . McGEER, Brain Res . 54 103-114 (1973) . F . FONNUM, I . GROFOVA, E . RINViK, J . STORM-MATHISEN and F . WALBERG, Brain Res . 71 77-92 (1974) . K.G. TLDYD, H . MOHLER,G . BARTHOLINI and 0 . HORNYKIEWICZ, Advances in Parkins~ism (W . Birkmayer and 0. Hornykiewicz, eds) 186-192,~ïta~h~Bä'S81 1 J . 5. KIM and R . HASSLER, Brain Res . 88 150-153 (1975) . P .L . McGEER,D .S . GREWAAL anTE -.~G:~fcGEER, Advances in ~arkinsonism (W . Birkmayer and 0 . Hornukiewicz, eds) 132-140, on s~o '('1977) .
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16 . 17 . 18 . 19 . 20 . 21 . 22 . 23 . 24 . 25 . 26 . 27 . 28 . 29 . 30 . 31 . 32 . 33 .
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