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Amphetamine and antidepressant drug effects on GABA- and NMDA-related seizures
Bruin
Research Bdefin, Vol. 30. pp. 607-610, Printed in the USA. All rights reserved.
1993
Copyright0
0361-9230/93 $6.00 + .OO 1993 Pergamon Press Ltd.
Amphetamine and Antidepressant Drug Effects on GABA- and NMDA-Related Seizures THOMAS
B. BOROWSKI,
R. DUNCAN
KIRKBY
AND
LARRY
KOKKINIDIS’
Department of Psychology, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0 WO, Canada Received
24 February
1992; Accepted
3 August
1992
BOROWSKI, T. B., R. D. KIRKBY AND L. KOKKINIDIS. Amphetamine and antidepressantdrug effects on GABA- and NMDA-relatedseizures. BRAIN RES BULL 30(5/6) 607-610, 1993.-Research has shown a synergistic relationship between amphetamine sensitization and limbic system kindling. To explore the role of GABA and NMDA receptor activity in modulating the positive effects of amphetamine on epileptogenesis, alterations in GABA- and NMDA-related convulsions were examined after acute and chronic amphetamine administration. A single injection of d-amphetamine (7.5 mg/kg) significantly decreased latencies to generalized motor seizures induced 12 h later by the noncompetitive GABA, receptor antagonist picrotoxin (10 mg/ kg). The increased sensitivity to clonus was specific to acute amphetamine treatment and was not evident following withdrawal from chronic drug exposure. Seizures induced by NMDLA (1,000 mg/kg), on the other hand, were not modified by acute amphetamine injection; however, the latency to clonus was reduced substantially after NMDLA injection to mice chronically preexposed to amphetamine. The short- and long-term amphetamine effects on GABA- and NMDA-associated convulsive activity were not paralleled by similar drug treatment schedules involving acute (20 mg/kg) and chronic administration of desipramine, zimelidine, and buproprion. These results suggest that amphetamine may be acting on inhibitory and excitatory amino acid systems independently of its monoaminergic properties. The implications of these findings were discussed in relation to amphetamine sensitization of mesolimbic functioning.
GABA
NMDA
Amphetamine
Desipramine
Zimelidine
Buproprion
Picrotoxin
Sensitization
of antidepressants known to inhibit the reuptake of norepinephrine (desipramine), serotonin (zimelidine), and dopamine (buproprion) on convulsions induced by systemic administration of picrotoxin and NMDLA.
RECENT work from this laboratory demonstrated a synergistic relationship between repeated amphetamine treatment and amygdaloid kindling (19,20). There are two components that characterize the facilitating effects of amphetamine on kindling of the amygdala: First, the robustness of the amphetamine/kindling interaction is predicated on the availability of the drug during the kindling process (short-term effect); second, animals with a prior history of amphetamine exposure show a further acceleration of kindling evolution (long-term effect). In the absence of drug treatment during the kindling process, however, the long-term positive actions of amphetamine on kindling rate are not evident following stimulant withdrawal. Considerable research has shown that reduced GABA inhibition and heightened NMDA excitation are involved in kindling acquisition (3,5-7,12,14,28,29). An evaluation of the acute and chronic effects of amphetamine on convulsive activity elicited by GABA antagonism and stimulation of NMDA receptors might provide evidence concerning the influence of amphetamine on GABA and NMDA processes, and by implication, furnish some insight as to the possible contribution of these mechanisms to the amphetamine/kindling cross-sensitization. A further purpose of this study was to determine whether changes in GABA- and NMDA-modulated generalized seizures after amphetamine treatment are related to this stimulant’s monoaminergic properties. This was accomplished by conducting parallel experiments assessing the short- and long-term effects
METHOD
Procedure Male Swiss mice (Animal Resources Centre, University of Saskatchewan, Saskatoon, Saskatchewan, Canada) weighing 2530 g at the initiation of the study were housed individually in standard polypropylene mouse cages with free access to food and water. Eight independent experiments were conducted involving the long- and short-term effects of d-amphetamine sulphate (7.5 mg/kg), desipramine (20 mg/kg), zimelidine (20 mg/ kg), and buproprion (20 mg/kg) on convulsions induced by IP injection of either picrotoxin (10 mg/kg) or NMDLA (1,000 mg/kg). The identical procedural design was used for each experiment, which involved two phases. For amphetamine (Experiment I), mice received either twice-daily (12 h apart) IP injections of damphetamine sulphate (7.5 mg/kg) or saline for 14 consecutive days (chronic phase). Twelve hours following the last chronic injection, one half the animals in the amphetamine group received an injection of amphetamine (7.5 mg/kg), whereas the other half was treated with saline. Similarly, one half the mice
’ To whom requests for reprints should be addressed. 607
608
BOROWSKI, i3 CHRONIC CHRONIC
SAL
KIRKBY AND KOKKINIDIS
.: CHRONIC
AMPH
SAL
CHRONIC
AMPH
SAL
ACUTE
ACUTE
SAL
ACUTE
ACUTE
AMPH
FIG. 1. Mean latency to clonus (&SEM)induced by IP injection of picrotoxin (10 mgikg) 12 h after acute administration of ~-amphetamine (7.5 mg/kg) and saline to animals chronically exposed to either to damphetamine (AMPH) or saline (SAL).
in the chronic saline condition were injected with amphetamine,
while the remaining half were treated with saline (acute phase). Twelve hours Iater, mice in the four groups fn = 12) were injected IP with 10 mg/kg picrotoxin, placed in aclear Plexiglas chamber (30 X 30 X 30 cm), and the latency to clonus was recorded. Clonus was defined as a generalized seizure in which animals lost postural control. In some cases, a complete generalized convulsion was not evident, and in this eventuality mice were assigned the maximum latency score of 600 s. Experiment 2 evaluated the effects of amphetamine preexposure on seizure latencies after an IP injection of 1,000 mg/kg NMDLA using the identical procedure described for Experiment I. The same two (chronic drug treatment) X two (acute drug treatment) design was employed to assess the long- and shortterm effects of desipramine (Experiments 3 and 4), buproprion ~Expe~ments 5 and 6) and zimelidine (Ex~~ments 7 and 8) on seizures induced by picrotoxin and NMDLA.
AMPH
FIG. 2. Mean latency to clonus (ISEM) induced by fl? injection of NMDLA (1,000 mg/kg) 12 h after acute administmtio~ of d-amphetamine (7.5 mg/kg) and saline to animals chronically exposed to either amphetamine (AMPH) or saline(SAL).
received an acute injection of amphetamine 12 h prior to NMDLA challenge. The effects of chronic and acute exposure of desipramine. buprop~on, and zimelidine on picrotoxin- and NMDL,A~licited clonus activity are shown in Table 1.ANOVA of the desipramine
TABLE 1 MEAN LATENCY TO CLONUS (+-SEM)INDUCED BY Peripheral INJECTION OF PICROTOXtN (10 mg/kg) AND NMDLA (1,000 mg/kg) 12 h AFTER ACIJTE ADMINISTRATIONOF SALINE. DESIPRAMINE (20 mg/kg), BUPROPRION(20 mglkg), AND VMELIDINE (20 mdkg) TO MICE CHRONICALLYEXPOSED TO EITHER SALINE (SAL), DESIPRAMINE(DES), BUPROPRION(BUP), OR 2lMELIDlNE (Z&I) Picrotoxin
NMDLA Chronic Treatment
Acute Treatment
SAL
DES
SAL
DES
384.3 (14.7) 441.8 (29.1)
366.7 (38.6) 411.2 (29.5)
238.8 (37. I) 254.3 (36.3)
270.3 (15.9) 223.3 (15.2)
BUP
SAL
BLIP
438.3 (19.9) 432.4 (21.7)
425. I (23.8) 397.3 (27.7)
259.0 (33.2) 280.7 (36.9)
285.2 (30.6) 231.9 ( 13.4)
SAL
ZIM
SAL
ZIM
337.3 (11.5) 383.4 (23.1)
409.3 (20.0) 389.8 (24.8)
327.0 (29.9) 347.1 (42.5)
308.6 (32.5) 305.5 (32.5)
RESULTS
The mean latency to clonus (tSEM) following IP administration of 10 mg/kg picrotoxin as a function of chronic and acute administration of amphetamine is depicted in Fig. 1. Analysis of variance (ANOVA) of the clonus data yielded a significant main effect for short-term drug treatment, F( I. 44) = 7.86, p < 0.01. As can be seen in Fig. 1. withdrawa from chronic amphetamine treatment resulted in a nonsignificant decrease in the seizure effectiveness of picrotoxin, Regardiess of this tendency, Newman-Keuls multiple comparisons (LY= 0.05) revealed that amphetamine injected 12 h prior to picrotoxin challenge signi~cantly reduced clonus latencies in mice chronically exposed to either amphetamine or saline. The mean (?SEM) latency to clonus induced by NMDLA treatment following short- and long-term amphetamine treatment is shown in Fig. 2. A substantial decrease in the latency to clonus was evident after withdrawal from chronic amphetamine treatment, F( 1. 44) = 17.00, p < 0.00 I. This increase in seizure susceptibility was evident irrespective of whether mice
SAL DES
SAL
SAL BUP
SAL ZIM
GABA, NMDA, AND AMPHETAMINE and buproprion data found no significant main or interaction effects. The analysis of the picrotoxin results revealed a marginally significant main effect for chronic zimelidine treatment, F( 1, 44) = 3.37, p < 0.075. There was a tendency for long-term zimelidine adminstration to increase clonus latencies after picrotoxin injection. The reduced seizure sensitivity following zimelidine preexposure was specific to GABA functioning, and neither chronic nor acute zimelidine treatment significantly influenced the latency to clonus after NMDLA challenge. DISCUSSION
The principal aim of this study was to evaluate the effects of acute and chronic amphetamine treatment on GABA and NMDA functioning by assessing changes in the seizure-promoting properties of picrotoxin and NMDLA. The results showed that a single injection of amphetamine shortened the latency to clonus after administration of the noncompetitive GABA* receptor antagonist picrotoxin. Earlier research demonstrated a decrease in pentylenetetrazol (PTZ) seizure thresholds after withdrawal from chronic administration of d-amphetamine that was dose dependent (3 I). According to our data, the chronicity of the drug schedule does not appear to be a critical factor in determining drug-induced reductions in GABAergic activity, and it is entirely possible that the PTZ threshold effect might have been observed with an acute administration of the drug. Bourdelais and Kalivas (2) found peripheral amphetamine injection to lower extracellular GABA concentrations in the ventral palladium. The amphetamine-induced decrease in GABA levels peaked 40 min after drug injection and was maintained throughout the duration of the experiment (3 h). While our results cannot be attributed to a specific neural region, the finding that amphetamine facilitated the development of picrotoxin-elicited convulsions 12 h after injection suggests that stimulant-induced changes in the central inhibitory properties of GABA endure for some time after acute drug exposure. The effects of amphetamine on picrotoxin-associated seizures has some explanatory value concerning the positive actions of this stimulant on amygdaloid kindling. It will be recalled that amphetamine treatment together with electrical stimulation of the amygdala facilitates seizure acquisition (19,20). Given the documented bidirectional transfer between electrical kindling and chemical kindling induced by picrotoxin (4,15), our data raise the possibility that the amphetamine-related reduction of GABA-mediated inhibition of epileptiform activity might play a role in the positive consequences of daily amphetamine treatment on amygdaloid kindling. A recent report found long-term amphetamine treatment to produce a small but significant increase in the convulsive threshold of bicuculline administration ( 17). The threshold effect may involve compensatory changes in GABA pre- and postsynaptic mechanisms to the stimulant-induced decreases in GABA activity related to acute amphetamine treatment. Of the reuptake inhibitors, only chronic zimelidine preexposure marginally increased clonus latencies following picrotoxin injection. There is reason to believe that serotonin has an inhibitory function with respect to epileptogenesis (30), and the possibility exists that the reported increase in seizure threshold seen during amphetamine withdrawal ( 17) might be regulated by amphetamine-induced alterations of serotonergic mechanisms. Together, the results concerning the specific reuptake inhibitors indicate that the enhanced sensitivity to picrotoxin seizures following acute amphetamine injection does not involve changes in monoaminergic functioning. The desipramine data, however, have implications concerning its chronic effects on epileptogenesis. Specifically, antidepressants can induce epileptic convul-
609
sions (lo), and McIntyre et al. (27) found that repeated exposure to desipramine increased amygdaloid kindling rates in rats, an effect attributed to the downregulation of B-adrenoreceptors. Recent research concerning other possible mechanisms of action of antidepressants has focused on GABA functioning (24,25). GABAB receptors have been implicated in the antidepressant effects of these drugs, whereas GABA* receptors appear to modulate seizure susceptibility (26). The observation that picrotoxininduced seizures were not influenced by desipramine preexposure suggests that the proconvulsant properties of long-term desipramine treatment do not involve changes in this inhibitory amino acid system. The second major finding of this study is that chronic amphetamine administration resulted in a marked decrease in the latency to clonus following injection of NMDLA. This result is in agreement with a recent report showing a substantial lowering of the convulsive threshold of NMDLA after long-term exposure to d-amphetamine (17). The profile for amphetamine was not duplicated by chronic administration of either desipramine, zimelidine, or buproprion, suggesting that the long-term consequences of amphetamine on NMDLA-elicited seizures may evolve independently of its monoamine+ consequences. Given the positive actions of repeated amphetamine exposure on NMDLA seizures, the possibility needs to be considered that changes in the NMDA component of excitatory amino acid systems might modulate the long-term element of the previously reported amphetamine/kindling interaction. It is noteworthy, however, that this proposed mechanism is not critical for the genesis of the amphetamine/kindling synergism, that is, chronic amphetamine treatment does not influence epileptogenesis if kindling proceeds during drug withdrawal in the absence of daily amphetamine treatment (19,20). It might be the case that the enhanced GABA inhibition seen after chronic amphetamine treatment, as demonstrated by increased bicuculline convulsive thresholds ( 17), counteracts the positive actions of NMDA excitation on seizure activity. With continued amphetamine exposure during kindling, however, the disinhibitory properties of reduced GABA activity resulting from acute drug treatment would have a permissive role in allowing for the long-term sensitizing actions of NMDA excitation to be expressed. Recent research involving the relationship between amygdaloid kindling and mesolimbic brain-stimulation reward showed that, as is the case for chronic stimulant administration (2 1,23), amygdaloid kindling enhances ventral tegmental selfstimulation responding and decreases reward thresholds after amphetamine challenge (22). Thus, the mechanisms underlying kindling acquisition, as well as the synergistic effects of amphetamine on amygdaloid kindling and on picrotoxin and NMDLA-elicited seizure activity, might also have an interactive role in modulating dopamine-mediated behavioral sensitization. Consistent with this conclusion is the recent demonstration that the NMDA receptor antagonists MK-80 1 and ketamine prevent the development of stimulant-induced sensitization ( 16,17). It is known that several nuclei of the amygdala, a hmbic structure that is in particular sensitive to the evolution of kindled epileptiform activity (9) and amphetamine sensitization of epileptogenesis ( 18), send glutamergic and/or aspartergic (Glu/Asp) projections to the nucleus accumbens (11). In addition, ventral tegmental area (VTA) neurons receive inhibitory GABA synaptic inputs (34) and the nucleus accumbens appears to be one of the sources for the GABA projections to the A 10 cell grouping of the VTA (33,35). With respect to the latter point, bicuculline administration into the VTA results in generalized convulsions (1). The nucleus accumbens also receives Glu/Asp projections from several cortical regions, including the medial prefrontal cortex (1 l), and the VTA is known to contain considerable con-
610
BOROWSKI,
centrations of Glu/Asp (13). As well, recent evidence suggests a direct descending aspartergic projection from the medial prefrontal cortex to the VTA (8), and in vitro recordings from dopamine cells in the VTA found the application of NMDA to activate all neurons tested in a concentration-dependent manner (32). Within this framework, it is tempting to speculate that stimulant- and kindling-induced sensitization of central reward processes (21-23) might involve the interactive effects of GABA and Glu/Asp on mesolimbic dopamine activity. Although the precise mechanisms of amphetamine-elicited effects on inhibi-
KIRKBY
AND
KOKKINIDIS
tory and excitatory amino acid processes need to be elucidated, and the involvement of these changes in influencing the dopamine-modulation of reward and motivational behavior remains to be defined, it would appear that alterations in these systems contribute to the cascade of neuronal events underlying amphetamine sensitization development. ACKNOWLEDGEMENTS
This research was supported by Grant A7042 from the Natural Sciences and Engineering Research Council of Canada. Appreciation is extended to Smith, Kline. and French for their gift of d-amphetamine.
REFERENCES I. Arnt, J.; Scheel-Kruger, J. GABA in the ventral tegmental area: differential regional effects on locomotion, aggression and food intake after microinjections of GABA agonists and antagonists. Life Sci. 25:1351-1360; 1979. 2. Bourdelais. A.: Kalivas, P. W. Amphetamine lowers extracellular GABA concentration in the ventral pallidum. Brain Res. 5 16:I32-
136; 1990. 3. Bumham, W. M. The GABA hypothesis 4.
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of kindling: Recent assay studies. Neurosci. Biobehav. Rev. I3:28 I-288; 1989. Cain, D. Kindling by repeated intraperitoneal or intracerebral injection of picrotoxin transfers to electrical kindling. Exp. Neural. 97~243-254; 1987. Cain, D. P. Excitatory neurotransmitters in kindling: Excitatory amino acid, choline& and opiate mechanisms. Neurosci. Biobehav. Rev. 13:269-276; 1989. Cain, D. P.; Desborough, K. A.; McKitrick, D. J. Retardation of amygdala kindling by antagonism of NMD-aspartate and muscarinic cholinergic receptors: Evidence for the summation of excitatory mechanisms in kindling. Exp. Neural. 100: 179- 187; 1988. Callahan, P. M.; Paris, J. M.; Cunningham, K. A.; Shinnick-Gallagher. P. Decrease of GABA-immunoreactive neurons in the amygdala after electrical kindling in the rat. Brain Res. 555:335-339: 1991. Christie, M. J.; Bridge, S.: James, L. B.; Beart, P. M. Excitotoxin lesions suggest an aspartergic projection from rat medial prefrontal cortex to ventral tegmental area. Brain Res. 333: 169-172; 1985. Corcoran, M. E. Characteristics and mechanisms of kindling. In: Kalivas, P. W.; Barnes, C. D., eds. Sensitization ofthe nervous system. Caldwell. NJ: Telford Press: 1988:8 l-1 16. Dallas, V. Iatrgenic epilepsy due to antidepressant drugs. Br. Med. J. 4:80-82; 1969. Fuller, T. A.; Russchen, F. T.; Price, J. L. Sources of presynaptive glutamergic/aspartergic afferents to the ventral striatopallidal region. J. Comp. Neural. 258:3 17-338; 1987. Gilbert, M. E. The NMDA-receptor antagonist, MK-80 1, suppresses limbic kindling and kindled seizures. Brain Res. 463:90-99: 1988. Gundlach, A. L.; Beart, P. M. Neurochemical studies of the mesolimbic dopaminergic pathway: Glycinergic mechanisms and glycinergic-dopaminergic interactions in the rat ventral tegmentum. J. Neurochem. 38:574-581; 1982. Holmes, K. H.; Bilkey, D. K.; Laverty, R.; Goddard, G. V. The Nmethyl-Daspartate antagonists aminophosphonovalerate and carboxypiperazinephosphonate retard the development and expression of kindled seizures. Brain Res. 506:227-235; 1990. Kalichman, M. W. Pharmacological investigation of convulsant aminobutyric acid (GABA) antagonists in amygdala-kindled rats. Epilepsia 23:163-171; 1982. Karler, R.; Calder, L. D; Chaudhry, I. A.; Turkanis, S. A. Blockade of “reverse tolerance” to cocaine and amphetamine by MK-801. Life Sci. 45:599-606; 1989. Karler, R.; Chaudhry, I. A.; Calder, L. D.; Turkanis, S. A. Amphetamine behavioral sensitization and excitatory amino acids. Brain Res. 537176-82; 1990. Kirkby, R. D.; Gelowitz, D. L.; Kokkinidis, L. The effects of amphetamine preexposure on electrical kindling of the hippocampus and related transfer phenomena. Brain Res. 550: 16 I-164: 1990.
19 Kirkby. R. D.; Kokkinidis, L. Amphetamine sensitization and amygdala kindling: Pharmacological evaluation of catecholaminergic and cholinergic mechanisms. Brain Res. Bull. 26:357-364; 1990. 20. Kirkby. R. D.; Kokkinidis. L. Evidence for a relationship between amphetamine sensitization and electrical kindling of the amygdala. Exp. Neurol. 97:270-279; 1987. 21. Kokkinidis. L. Neurochemical correlates of postamphetamine depression and sensitization in animals: Implications for behavioral pathology. In: Simon, P.; Soubrie, P.: Widlocher, D.. eds. Animal models of psychiatric disorders. 2. An inquiry into schizophrenia and depression. Base]: Karger; 1988: 148- 173. 22. Kokkinidis, L.; Borowski, T. B. Sensitization of mesolimbic brain stimulation reward after electrical kindling of the amygdala. Brain Res. Bull. 27:79 l-796: 199 I. 23. Kokkinidis, L.: McCarter, B. D. Postcocaine depression and sensitization of brain stimulation reward: Analysis of reinforcement and performance effects. Pharmacol. Biochem. Behav. 36:463-47 I; 1990. 24. Lloyd, K. G.: Pile, A. Chronic antidepressants and GABA synapses. Neuropharmacology 23:841-842: 1984. of -aminobutyric 25. Lloyd, K. G.; Thuret. F.: Pile, A. Upregulation acid (GABA) B binding sites in rat frontal cortex: A common action of repeated administration of different classes of antidepressants and electroshock. J. Pharmacol. Exp. Ther. 235:191-199; 1985. 26. Matsumoto, R. R. GABA receptors: Are cellular differences related to function? Brain Res. Rev. 14:203-225; 1989. 27. McIntyre, D. C.: Edson, N.; Chao, G.: Knowles, V. Differential effect of acute vs chronic desmethylimipramine on the rate of amygdala kindling in rats. Exp. Neural. 28: 158-166; 1982. 28. McNamara. J. 0.; Russell, R. D.: Rigsbee, L.: Bonhaus, D. W. Anticonvulsant and antiepileptogenic actions of MK-801 in the kindling and electroshock models. Neuropharmacology 27:563-568; 1988. 29. Morimoto, K. Seizure-triggering mechanisms in the kindling model of epilepsy: Collapse of GABA-mediated inhibition and activation of NMDA receptors. Neurosci. Biobehav. Rev. 13:253-260; 1989. 30. Racine, R.; Coscina, D. V. Effects of midbrain raphe lesions or systemic pchlorophenylalanine on the development of kindled seizures in rats. Brain Res. Bull. 4: I-7: 1979. 31. Riffee, W. H.; Gerald. M. C. The effects of chronic administration and withdrawal of (+)-amphetamine on seizure threshold and endogenous catecholamine concentrations and their rates of biosynthesis in mice. Psychopharmacology (Berl.) 5 1: 17% 179; 1977. 32. Seutin, V.; Verbanck. P.: Massotte. L.; Dresse, A. Evidence for the presence of N-methyl-Daspartate receptors in the ventral tegmental area of the rat: An electrophysiological in vitro study. Brain Res. 514:147-150; 1990. 33. Walaas, I.; Fonnum, F. Biochemical evidence for -aminobutyrate containing fibres from the nucleus accumbens to the substantia nigra and ventral tegmental area in the rat. Neuroscience 5:63-72; 1980. 34. Wolfe, P.; Olpe, H. R.; Avrith, D. Haas, H. L. GABAergic inhibition of neurons in the ventral tegmental area. Experientia 3473-74; 1978. 35. Yim, C.; Mogenson, G. J. Effect of picrotoxin and nipecotic acid on the inhibitory response of dopaminergic neurons in the ventral tegmental area to stimulation of the nucleus accumbens. Brain Res. 1991466-472: 1980.
Research Bdefin, Vol. 30. pp. 607-610, Printed in the USA. All rights reserved.
1993
Copyright0
0361-9230/93 $6.00 + .OO 1993 Pergamon Press Ltd.
Amphetamine and Antidepressant Drug Effects on GABA- and NMDA-Related Seizures THOMAS
B. BOROWSKI,
R. DUNCAN
KIRKBY
AND
LARRY
KOKKINIDIS’
Department of Psychology, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0 WO, Canada Received
24 February
1992; Accepted
3 August
1992
BOROWSKI, T. B., R. D. KIRKBY AND L. KOKKINIDIS. Amphetamine and antidepressantdrug effects on GABA- and NMDA-relatedseizures. BRAIN RES BULL 30(5/6) 607-610, 1993.-Research has shown a synergistic relationship between amphetamine sensitization and limbic system kindling. To explore the role of GABA and NMDA receptor activity in modulating the positive effects of amphetamine on epileptogenesis, alterations in GABA- and NMDA-related convulsions were examined after acute and chronic amphetamine administration. A single injection of d-amphetamine (7.5 mg/kg) significantly decreased latencies to generalized motor seizures induced 12 h later by the noncompetitive GABA, receptor antagonist picrotoxin (10 mg/ kg). The increased sensitivity to clonus was specific to acute amphetamine treatment and was not evident following withdrawal from chronic drug exposure. Seizures induced by NMDLA (1,000 mg/kg), on the other hand, were not modified by acute amphetamine injection; however, the latency to clonus was reduced substantially after NMDLA injection to mice chronically preexposed to amphetamine. The short- and long-term amphetamine effects on GABA- and NMDA-associated convulsive activity were not paralleled by similar drug treatment schedules involving acute (20 mg/kg) and chronic administration of desipramine, zimelidine, and buproprion. These results suggest that amphetamine may be acting on inhibitory and excitatory amino acid systems independently of its monoaminergic properties. The implications of these findings were discussed in relation to amphetamine sensitization of mesolimbic functioning.
GABA
NMDA
Amphetamine
Desipramine
Zimelidine
Buproprion
Picrotoxin
Sensitization
of antidepressants known to inhibit the reuptake of norepinephrine (desipramine), serotonin (zimelidine), and dopamine (buproprion) on convulsions induced by systemic administration of picrotoxin and NMDLA.
RECENT work from this laboratory demonstrated a synergistic relationship between repeated amphetamine treatment and amygdaloid kindling (19,20). There are two components that characterize the facilitating effects of amphetamine on kindling of the amygdala: First, the robustness of the amphetamine/kindling interaction is predicated on the availability of the drug during the kindling process (short-term effect); second, animals with a prior history of amphetamine exposure show a further acceleration of kindling evolution (long-term effect). In the absence of drug treatment during the kindling process, however, the long-term positive actions of amphetamine on kindling rate are not evident following stimulant withdrawal. Considerable research has shown that reduced GABA inhibition and heightened NMDA excitation are involved in kindling acquisition (3,5-7,12,14,28,29). An evaluation of the acute and chronic effects of amphetamine on convulsive activity elicited by GABA antagonism and stimulation of NMDA receptors might provide evidence concerning the influence of amphetamine on GABA and NMDA processes, and by implication, furnish some insight as to the possible contribution of these mechanisms to the amphetamine/kindling cross-sensitization. A further purpose of this study was to determine whether changes in GABA- and NMDA-modulated generalized seizures after amphetamine treatment are related to this stimulant’s monoaminergic properties. This was accomplished by conducting parallel experiments assessing the short- and long-term effects
METHOD
Procedure Male Swiss mice (Animal Resources Centre, University of Saskatchewan, Saskatoon, Saskatchewan, Canada) weighing 2530 g at the initiation of the study were housed individually in standard polypropylene mouse cages with free access to food and water. Eight independent experiments were conducted involving the long- and short-term effects of d-amphetamine sulphate (7.5 mg/kg), desipramine (20 mg/kg), zimelidine (20 mg/ kg), and buproprion (20 mg/kg) on convulsions induced by IP injection of either picrotoxin (10 mg/kg) or NMDLA (1,000 mg/kg). The identical procedural design was used for each experiment, which involved two phases. For amphetamine (Experiment I), mice received either twice-daily (12 h apart) IP injections of damphetamine sulphate (7.5 mg/kg) or saline for 14 consecutive days (chronic phase). Twelve hours following the last chronic injection, one half the animals in the amphetamine group received an injection of amphetamine (7.5 mg/kg), whereas the other half was treated with saline. Similarly, one half the mice
’ To whom requests for reprints should be addressed. 607
608
BOROWSKI, i3 CHRONIC CHRONIC
SAL
KIRKBY AND KOKKINIDIS
.: CHRONIC
AMPH
SAL
CHRONIC
AMPH
SAL
ACUTE
ACUTE
SAL
ACUTE
ACUTE
AMPH
FIG. 1. Mean latency to clonus (&SEM)induced by IP injection of picrotoxin (10 mgikg) 12 h after acute administration of ~-amphetamine (7.5 mg/kg) and saline to animals chronically exposed to either to damphetamine (AMPH) or saline (SAL).
in the chronic saline condition were injected with amphetamine,
while the remaining half were treated with saline (acute phase). Twelve hours Iater, mice in the four groups fn = 12) were injected IP with 10 mg/kg picrotoxin, placed in aclear Plexiglas chamber (30 X 30 X 30 cm), and the latency to clonus was recorded. Clonus was defined as a generalized seizure in which animals lost postural control. In some cases, a complete generalized convulsion was not evident, and in this eventuality mice were assigned the maximum latency score of 600 s. Experiment 2 evaluated the effects of amphetamine preexposure on seizure latencies after an IP injection of 1,000 mg/kg NMDLA using the identical procedure described for Experiment I. The same two (chronic drug treatment) X two (acute drug treatment) design was employed to assess the long- and shortterm effects of desipramine (Experiments 3 and 4), buproprion ~Expe~ments 5 and 6) and zimelidine (Ex~~ments 7 and 8) on seizures induced by picrotoxin and NMDLA.
AMPH
FIG. 2. Mean latency to clonus (ISEM) induced by fl? injection of NMDLA (1,000 mg/kg) 12 h after acute administmtio~ of d-amphetamine (7.5 mg/kg) and saline to animals chronically exposed to either amphetamine (AMPH) or saline(SAL).
received an acute injection of amphetamine 12 h prior to NMDLA challenge. The effects of chronic and acute exposure of desipramine. buprop~on, and zimelidine on picrotoxin- and NMDL,A~licited clonus activity are shown in Table 1.ANOVA of the desipramine
TABLE 1 MEAN LATENCY TO CLONUS (+-SEM)INDUCED BY Peripheral INJECTION OF PICROTOXtN (10 mg/kg) AND NMDLA (1,000 mg/kg) 12 h AFTER ACIJTE ADMINISTRATIONOF SALINE. DESIPRAMINE (20 mg/kg), BUPROPRION(20 mglkg), AND VMELIDINE (20 mdkg) TO MICE CHRONICALLYEXPOSED TO EITHER SALINE (SAL), DESIPRAMINE(DES), BUPROPRION(BUP), OR 2lMELIDlNE (Z&I) Picrotoxin
NMDLA Chronic Treatment
Acute Treatment
SAL
DES
SAL
DES
384.3 (14.7) 441.8 (29.1)
366.7 (38.6) 411.2 (29.5)
238.8 (37. I) 254.3 (36.3)
270.3 (15.9) 223.3 (15.2)
BUP
SAL
BLIP
438.3 (19.9) 432.4 (21.7)
425. I (23.8) 397.3 (27.7)
259.0 (33.2) 280.7 (36.9)
285.2 (30.6) 231.9 ( 13.4)
SAL
ZIM
SAL
ZIM
337.3 (11.5) 383.4 (23.1)
409.3 (20.0) 389.8 (24.8)
327.0 (29.9) 347.1 (42.5)
308.6 (32.5) 305.5 (32.5)
RESULTS
The mean latency to clonus (tSEM) following IP administration of 10 mg/kg picrotoxin as a function of chronic and acute administration of amphetamine is depicted in Fig. 1. Analysis of variance (ANOVA) of the clonus data yielded a significant main effect for short-term drug treatment, F( I. 44) = 7.86, p < 0.01. As can be seen in Fig. 1. withdrawa from chronic amphetamine treatment resulted in a nonsignificant decrease in the seizure effectiveness of picrotoxin, Regardiess of this tendency, Newman-Keuls multiple comparisons (LY= 0.05) revealed that amphetamine injected 12 h prior to picrotoxin challenge signi~cantly reduced clonus latencies in mice chronically exposed to either amphetamine or saline. The mean (?SEM) latency to clonus induced by NMDLA treatment following short- and long-term amphetamine treatment is shown in Fig. 2. A substantial decrease in the latency to clonus was evident after withdrawal from chronic amphetamine treatment, F( 1. 44) = 17.00, p < 0.00 I. This increase in seizure susceptibility was evident irrespective of whether mice
SAL DES
SAL
SAL BUP
SAL ZIM
GABA, NMDA, AND AMPHETAMINE and buproprion data found no significant main or interaction effects. The analysis of the picrotoxin results revealed a marginally significant main effect for chronic zimelidine treatment, F( 1, 44) = 3.37, p < 0.075. There was a tendency for long-term zimelidine adminstration to increase clonus latencies after picrotoxin injection. The reduced seizure sensitivity following zimelidine preexposure was specific to GABA functioning, and neither chronic nor acute zimelidine treatment significantly influenced the latency to clonus after NMDLA challenge. DISCUSSION
The principal aim of this study was to evaluate the effects of acute and chronic amphetamine treatment on GABA and NMDA functioning by assessing changes in the seizure-promoting properties of picrotoxin and NMDLA. The results showed that a single injection of amphetamine shortened the latency to clonus after administration of the noncompetitive GABA* receptor antagonist picrotoxin. Earlier research demonstrated a decrease in pentylenetetrazol (PTZ) seizure thresholds after withdrawal from chronic administration of d-amphetamine that was dose dependent (3 I). According to our data, the chronicity of the drug schedule does not appear to be a critical factor in determining drug-induced reductions in GABAergic activity, and it is entirely possible that the PTZ threshold effect might have been observed with an acute administration of the drug. Bourdelais and Kalivas (2) found peripheral amphetamine injection to lower extracellular GABA concentrations in the ventral palladium. The amphetamine-induced decrease in GABA levels peaked 40 min after drug injection and was maintained throughout the duration of the experiment (3 h). While our results cannot be attributed to a specific neural region, the finding that amphetamine facilitated the development of picrotoxin-elicited convulsions 12 h after injection suggests that stimulant-induced changes in the central inhibitory properties of GABA endure for some time after acute drug exposure. The effects of amphetamine on picrotoxin-associated seizures has some explanatory value concerning the positive actions of this stimulant on amygdaloid kindling. It will be recalled that amphetamine treatment together with electrical stimulation of the amygdala facilitates seizure acquisition (19,20). Given the documented bidirectional transfer between electrical kindling and chemical kindling induced by picrotoxin (4,15), our data raise the possibility that the amphetamine-related reduction of GABA-mediated inhibition of epileptiform activity might play a role in the positive consequences of daily amphetamine treatment on amygdaloid kindling. A recent report found long-term amphetamine treatment to produce a small but significant increase in the convulsive threshold of bicuculline administration ( 17). The threshold effect may involve compensatory changes in GABA pre- and postsynaptic mechanisms to the stimulant-induced decreases in GABA activity related to acute amphetamine treatment. Of the reuptake inhibitors, only chronic zimelidine preexposure marginally increased clonus latencies following picrotoxin injection. There is reason to believe that serotonin has an inhibitory function with respect to epileptogenesis (30), and the possibility exists that the reported increase in seizure threshold seen during amphetamine withdrawal ( 17) might be regulated by amphetamine-induced alterations of serotonergic mechanisms. Together, the results concerning the specific reuptake inhibitors indicate that the enhanced sensitivity to picrotoxin seizures following acute amphetamine injection does not involve changes in monoaminergic functioning. The desipramine data, however, have implications concerning its chronic effects on epileptogenesis. Specifically, antidepressants can induce epileptic convul-
609
sions (lo), and McIntyre et al. (27) found that repeated exposure to desipramine increased amygdaloid kindling rates in rats, an effect attributed to the downregulation of B-adrenoreceptors. Recent research concerning other possible mechanisms of action of antidepressants has focused on GABA functioning (24,25). GABAB receptors have been implicated in the antidepressant effects of these drugs, whereas GABA* receptors appear to modulate seizure susceptibility (26). The observation that picrotoxininduced seizures were not influenced by desipramine preexposure suggests that the proconvulsant properties of long-term desipramine treatment do not involve changes in this inhibitory amino acid system. The second major finding of this study is that chronic amphetamine administration resulted in a marked decrease in the latency to clonus following injection of NMDLA. This result is in agreement with a recent report showing a substantial lowering of the convulsive threshold of NMDLA after long-term exposure to d-amphetamine (17). The profile for amphetamine was not duplicated by chronic administration of either desipramine, zimelidine, or buproprion, suggesting that the long-term consequences of amphetamine on NMDLA-elicited seizures may evolve independently of its monoamine+ consequences. Given the positive actions of repeated amphetamine exposure on NMDLA seizures, the possibility needs to be considered that changes in the NMDA component of excitatory amino acid systems might modulate the long-term element of the previously reported amphetamine/kindling interaction. It is noteworthy, however, that this proposed mechanism is not critical for the genesis of the amphetamine/kindling synergism, that is, chronic amphetamine treatment does not influence epileptogenesis if kindling proceeds during drug withdrawal in the absence of daily amphetamine treatment (19,20). It might be the case that the enhanced GABA inhibition seen after chronic amphetamine treatment, as demonstrated by increased bicuculline convulsive thresholds ( 17), counteracts the positive actions of NMDA excitation on seizure activity. With continued amphetamine exposure during kindling, however, the disinhibitory properties of reduced GABA activity resulting from acute drug treatment would have a permissive role in allowing for the long-term sensitizing actions of NMDA excitation to be expressed. Recent research involving the relationship between amygdaloid kindling and mesolimbic brain-stimulation reward showed that, as is the case for chronic stimulant administration (2 1,23), amygdaloid kindling enhances ventral tegmental selfstimulation responding and decreases reward thresholds after amphetamine challenge (22). Thus, the mechanisms underlying kindling acquisition, as well as the synergistic effects of amphetamine on amygdaloid kindling and on picrotoxin and NMDLA-elicited seizure activity, might also have an interactive role in modulating dopamine-mediated behavioral sensitization. Consistent with this conclusion is the recent demonstration that the NMDA receptor antagonists MK-80 1 and ketamine prevent the development of stimulant-induced sensitization ( 16,17). It is known that several nuclei of the amygdala, a hmbic structure that is in particular sensitive to the evolution of kindled epileptiform activity (9) and amphetamine sensitization of epileptogenesis ( 18), send glutamergic and/or aspartergic (Glu/Asp) projections to the nucleus accumbens (11). In addition, ventral tegmental area (VTA) neurons receive inhibitory GABA synaptic inputs (34) and the nucleus accumbens appears to be one of the sources for the GABA projections to the A 10 cell grouping of the VTA (33,35). With respect to the latter point, bicuculline administration into the VTA results in generalized convulsions (1). The nucleus accumbens also receives Glu/Asp projections from several cortical regions, including the medial prefrontal cortex (1 l), and the VTA is known to contain considerable con-
610
BOROWSKI,
centrations of Glu/Asp (13). As well, recent evidence suggests a direct descending aspartergic projection from the medial prefrontal cortex to the VTA (8), and in vitro recordings from dopamine cells in the VTA found the application of NMDA to activate all neurons tested in a concentration-dependent manner (32). Within this framework, it is tempting to speculate that stimulant- and kindling-induced sensitization of central reward processes (21-23) might involve the interactive effects of GABA and Glu/Asp on mesolimbic dopamine activity. Although the precise mechanisms of amphetamine-elicited effects on inhibi-
KIRKBY
AND
KOKKINIDIS
tory and excitatory amino acid processes need to be elucidated, and the involvement of these changes in influencing the dopamine-modulation of reward and motivational behavior remains to be defined, it would appear that alterations in these systems contribute to the cascade of neuronal events underlying amphetamine sensitization development. ACKNOWLEDGEMENTS
This research was supported by Grant A7042 from the Natural Sciences and Engineering Research Council of Canada. Appreciation is extended to Smith, Kline. and French for their gift of d-amphetamine.
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