- Email: [email protected]
Anticonvulsant effects of acute and repeated fluoxetine treatment in unstressed and stressed mice
Brain Research 1033 (2005) 90 – 95 www.elsevier.com/locate/brainres
Research report
Anticonvulsant effects of acute and repeated fluoxetine treatment in unstressed and stressed mice Danka Pericˇic´*, Josipa Lazic´, Dubravka Sˇvob Sˇtrac Laboratory for Molecular Neuropharmacology, Division of Molecular Medicine, Rud¯er Bosˇkovic´ Institute, Bijenicka cesta 54, P.O. Box 180, 10002 Zagreb, Croatia Accepted 19 November 2004 Available online 30 December 2004
Abstract Comorbidity of epilepsy and depression is not rare. Stress can affect both depression and seizures. Therefore, it is important to know whether an antidepressant drug has pro- or anticonvulsant properties and whether these properties will be modified by stress. We tested the effects of the antidepressant drug fluoxetine on the seizure threshold for picrotoxin in unstressed and swim-stressed mice. The mice were, prior to exposure to swim stress and the intravenous infusion of picrotoxin (a non-competitive GABAA receptor antagonist), pretreated with fluoxetine (a selective serotonin reuptake inhibitor), either acutely or repeatedly (5 days), and the latency to the onset of two convulsant signs and death was registered. The convulsant signs were running/bouncing clonus and tonic hindlimb extension. As expected, swim stress enhanced the seizure threshold for picrotoxin. Fluoxetine (20 mg/kg ip) given acutely increased in unstressed and swim-stressed mice the dose of picrotoxin producing tonic hindlimb extension and in unstressed mice the dose of picrotoxin producing death. Neither 10 nor 20 mg/ kg of fluoxetine affected doses of picrotoxin needed to produce running bouncing/clonus. Repeated treatment with fluoxetine (20 mg/kg ip) enhanced significantly in unstressed and swim-stressed mice doses of picrotoxin needed to produce tonic hindlimb extension and death, and in stressed mice also the dose of picrotoxin producing running/bouncing clonus. The results demonstrate that the antidepressant drug fluoxetine, given acutely or repeatedly, shows anticonvulsant properties against convulsions induced in unstressed and swim-stressed mice by antagonist of GABAA receptors, picrotoxin. Swim stress failed to modify the anticonvulsant properties of fluoxetine. D 2004 Elsevier B.V. All rights reserved. Theme: Neurotransmitters, modulators, transporters, and receptors, Topic: Serotonin Keywords: Fluoxetine; Acute and repeated treatment; Picrotoxin-induced convulsions; Swim stress
1. Introduction Selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors, including fluoxetine, are among the most widely used antidepressant drugs. Since depression and epilepsy often appear in the same patient, it is important to know whether the drug used to treat depression has pro- or anticonvulsant properties. Some clinical studies suggested that caution should be exerted when using fluoxetine as an
* Corresponding author. Fax: +385 1 4561010. E-mail address: [email protected] (D. Pericˇic´). 0006-8993/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2004.11.025
antidepressive treatment in epileptic patients [9,27]. Therefore, several groups of authors studied the effect of fluoxetine in different animal models of epilepsy. Anticonvulsant activity of this drug has been demonstrated on focally evoked limbic motor seizures in rats [28], in a rat model of focally evoked complex partial seizures [21], in an in vitro picrotoxin-induced model of epilepsy [18], in the pilocarpine model of temporal lobe epilepsy [12], and in genetically epilepsy-prone rats, in which this effect was explained by an increase in extracellular serotonin [4,6,36]. However, some more recent studies do not suggest the anticonvulsant properties of fluoxetine, or at least not in a genetic rat model of absence epilepsy [15]. All these studies,
D. Pericˇic´ et al. / Brain Research 1033 (2005) 90–95
except one [12], refer to the effects of acute fluoxetine treatment. It is known that many central neuronal systems, including the serotonergic system, are sensitive to different stressors [5,10,17]. Both serotonin and stress have been implicated in affective disorders, especially depression, and swim stress is often used as a biological stressor and as an animal model with predictive value for antidepressant drugs. Hence, the aim of this study was to explore whether acute and repeated fluoxetine treatments are able to affect seizures produced in unstressed and swim-stressed mice by picrotoxin, a non-competitive antagonist of GABAA receptors, and at the same time to assess whether swim stress modifies the possible effect of fluoxetine on seizure threshold. Convulsions were produced by intravenous (iv) infusion of picrotoxin. This convulsant has been used extensively to manipulate the GABA system and to investigate its involvement in epileptic activity. Picrotoxin-induced seizures represent a model of generalized convulsive epilepsy [19]. As shown previously [24], among the GABA related convulsants, swim stress was most effective against convulsions produced by this convulsant. We measured drug and swim stress-induced changes in doses of picrotoxin needed to produce running/ bouncing clonus and tonic hindlimb extension, two convulsant signs whose onset in mice can very precisely be determined.
2. Materials and methods 2.1. Animals Male CBA mice (25–30 g), 3 months old and bred in our institute, were used. They were housed at a constant temperature (228C) and with a light cycle of 12-h light/ 12-h darkness (lights on at 7.00 a.m.). They were caged in groups of ten. The size of the cages was 41 cm [length] 26 cm [width] 15 cm [height]. Food and water were freely available. Prior to experiment, the animals were not habituated to iv drug administration. All animal care and experimental procedures were carried out in accordance with the National Institute of Health Guide for the Care and Use of Laboratory Animals (NIH publication No. 8023) revised 1996, and with the Croatian law on animal welfare.
91
2.3. Drugs Fluoxetine hydrochloride and picrotoxin, both from Sigma (St. Louis, MO), were used. Fluoxetine was dissolved in saline and picrotoxin was dissolved in warm saline. Picrotoxin was given by constant iv infusion into a tail vein, while fluoxetine was administered intraperitoneally (ip) in a volume 1 ml per 100 g body weight. Fluoxetine was administered acutely (65 min prior to picrotoxin and 40 min prior to swim stress) or repeatedly (once daily for five consecutive days, with the last injection given 65 min prior to picrotoxin). Control animals received ip injection of saline. The time of drug administration in the acute experiment is within the literature data (30–120 min prior to different tests) and corresponds to almost maximum enhancement of 5-HT in dialysate of several brain regions following fluoxetine administration. 2.4. Convulsive activity For determination of convulsive activity, the animal was taken from its home cage and placed in a glass cylinder (20 7 cm2) with numerous holes for ventilation. The tail of the animal was drawn through a hole of the plastic cover and warmed for 1 min under a tensor lamp. A butterfly infusion needle (0.3 mm) was inserted into the tail vein and correct placement was verified by the appearance of blood in the infusion tubing. During the infusion the animal was held lightly by the tip of the tail to allow free movement. The concentration of picrotoxin was 0.75 mg/ml and the infusion rate controlled by a microinfusion pump was 1.1 ml/min. The animal was observed throughout infusion and the time between the start of infusion and the onset of convulsive signs was measured, mainly as described by Kosobud and Crabbe [16], by an observer unaware of the treatment. The convulsive signs were running/bouncing clonus (RB clonus, violent whole-body clonus, including running and explosive jumps) and tonic hindlimb extension (THE, characterized by extreme rigidity, with forelimbs and hindlimbs extended caudally). For each animal, the dose of convulsant (mg/kg of body weight) required to elicit a particular convulsant sign was calculated from the time of infusion, the infusion rate, the concentration of picrotoxin, and body weight. The time to death was also measured. All experiments were carried out between 9:00 and 13:00 h.
2.2. Stress procedure 2.5. Statistical analysis Mice were subjected to swim stress (10-min swimming) at 18–198C. After swimming the animals were dried with a towel and placed near the heater. The iv injection of convulsant started 15 min after termination of stress. Control unstressed animals were taken for comparison.
Results are expressed as mean values F SEM. Statistical analysis of the results was by one-way analysis of variance (ANOVA) followed by Newman–Keuls test and by two-way ANOVA, since in the same experiment, the factors stress and drug were studied. The analyses were performed using
92
D. Pericˇic´ et al. / Brain Research 1033 (2005) 90–95
GraphPad Prism version 4.00 for Windows, GraphPad Software, San Diego, California, USA. P values of b0.05 were considered significant.
3. Results 3.1. Effect of acute fluoxetine treatment on picrotoxin-induced convulsions in unstressed and swim-stressed mice As shown in Fig. 1, in accordance with our results published previously [21], 15 min after exposure to swim stress, the threshold doses of picrotoxin producing running/ bouncing clonus, tonic hindlimb extension, and death were enhanced by 85%, 108%, and 108%, respectively, i.e., swim stress, as indicated by two-way ANOVA produced a highly significant ( P b 0.0001) anticonvulsive effect [running/ bouncing clonus: F(1,57) = 317.85; tonic hindlimb extension: F(1,56) = 394.58; death: F(1,54) = 430.40]. Fluoxetine at 10 and 20 mg/kg failed to produce a significant effect on doses of picrotoxin needed to produce running/ bouncing clonus, but the effects on tonic hindlimb extension [ F(2,56) = 13.68] and death [ F(2,54) = 12.17] were highly significant ( P b 0.0001). In neither of these cases was the interaction drug stress significant. Planned comparisons using the Newman–Keuls test indicated that fluoxetine 10 mg/kg enhanced only in stressed animals the dose of picrotoxin producing tonic hindlimb extension (12%; P b 0.05), while fluoxetine 20 mg/kg enhanced in unstressed animals doses of picrotoxin needed to produce tonic hindlimb extension (42%; P b 0.001) and death (39%; P b 0.001) and in stressed animals the dose of picrotoxin producing tonic hindlimb extension (11%; P b 0.05). To assess the possible involvement of hypothermia in the anticonvulsant effect of swim stress, we tested the convulsant potency of picrotoxin 15 min after exposure of mice for 10 min to an ambient temperature of 48C. Unlike swim stress, this stress decreased the threshold doses of picrotoxin producing convulsions. Doses of picrotoxin needed to produce tonic hindlimb extension were 25.07 F 1.1 (N = 8) in the control and 22.12 F 0.71 mg/kg (N = 8) in cold stressed group ( P b 0.05, Student’s t test). 3.2. Effect of repeated fluoxetine treatment on picrotoxin-induced convulsions in unstressed and swim-stressed mice Since acute administration of fluoxetine 20 mg/kg exerted an anticonvulsant effect in both unstressed and stressed animals, this dose was chosen for studying the possible anticonvulsant effect of repeated fluoxetine treatment. As shown in Fig. 2 and indicated by two-way ANOVA, swim stress had again a pronounced ( P b 0.0001) anticonvulsant effect, as evidenced by enhancements of 56.6%, 70.3%, and
Fig. 1. Effect of fluoxetine and swim stress on the dose of picrotoxin needed to produce running/bouncing clonus (RB clonus), tonic hindlimb extension (THE), and death in male CBA mice. Swim stress (10-min swimming in water at 18–198C) ended 15 min before iv infusion of convulsant. Fluoxetine (10 and 20 mg/kg ip) was administered 65 min prior to picrotoxin and 40 min prior to swim stress. Bars represent means F SEM from 7 to 16 animals per group. aP b 0.001 versus dose of picrotoxin needed to produce the same sign in saline-treated group; bP b 0.001 versus dose of picrotoxin needed to produce the same convulsive sign in the corresponding unstressed group; cP b 0.05 as compared to dose of picrotoxin needed to produce the same convulsive sign in saline-treated stressed group (ANOVA followed by Newman–Keuls test).
69% of doses of picrotoxin needed to produce running/ bouncing clonus [ F(1,27) = 137.57], tonic hindlimb extension [ F(1,27) = 141.05], and death [ F(1,27) = 88.65], respectively. Fluoxetine (20 mg/kg) given repeatedly for 5 days produced a significant effect on doses of picrotoxin producing running/bouncing clonus [ F(1,27) = 9.32; P b 0.005], tonic hindlimb extension [ F(1,27) = 47.35; P b 0.0001], and death [ F(1,27) = 17.90; P b 0.0002). In
D. Pericˇic´ et al. / Brain Research 1033 (2005) 90–95
93
running/bouncing clonus, tonic hindlimb extension, and death, respectively. Similar effects of repeated fluoxetine treatment on the seizure threshold for picrotoxin and death in unstressed and swim-stressed mice were confirmed by insignificant drug stress interaction in the two-way ANOVA.
4. Discussion
Fig. 2. Effect of repeated fluoxetine treatment on the dose of picrotoxin needed to produce running/bouncing clonus (RB clonus), tonic hindlimb extension (THE), and death in unstressed and stressed male CBA mice. Fluoxetine (20 mg/kg ip) was administered once daily for five consecutive days. The last injection was given 65 min prior to picrotoxin and 40 min prior to swim stress. Swim stress (10-min swimming in water at 18–198C) ended 15 min before iv infusion of convulsant. Bars represent means F SEM from 7 to 8 animals per group. aP b 0.001 and P b 0.05 versus dose of picrotoxin needed to produce tonic hindlimb extension and death in salinetreated group; bP b 0.001 versus dose of picrotoxin needed to produce the same convulsive sign in the corresponding unstressed group; cP b 0.01 or P b 0.001 as compared to dose of picrotoxin needed to produce the same convulsive sign in saline-treated stressed group and in fluoxetine-treated unstressed group (ANOVA followed by Newman–Keuls test).
unstressed animals, fluoxetine given for 5 days enhanced by 36% and 27% doses of picrotoxin needed to produce tonic hindlimb extension and death ( P b 0.001; Newman–Keuls test). In stressed animals, the same treatment enhanced by 14% ( P b 0.01), 35%, and 26% ( P b 0.001 versus salinetreated stressed group) doses of picrotoxin needed to produce
The results of the present study demonstrated the anticonvulsant properties of acute and repeated fluoxetine treatment against convulsions induced in mice by picrotoxin, a non-competitive GABAA receptor antagonist. The anticonvulsant effect was present in unstressed and swimstressed mice. Further, our results confirmed and extended the data of previous studies [1,24,25,29,33] demonstrating a pronounced anticonvulsant effect of swim stress against convulsions induced by GABA-related convulsants. As in our previous studies, the anticonvulsant effect of swim stress observed in the present study was very pronounced, e.g., swim stress more than doubled the seizure threshold for picrotoxin. In mice subjected to ip injections for 5 days, the anticonvulsant effect of swim stress appeared to be smaller than in mice that received only one ip injection before swim stress, suggesting that tolerance to the anticonvulsant effect might develop not only after repeated swim stress [24], but also after repeated stressing of animals with different stressors. However, a direct comparison of these effects was not possible, since the results were from two different experiments. The mechanism of the anticonvulsant effect of swim stress is not quite clear, although the involvement of a2-adrenoceptors [25] and neurosteroids [29] has been proposed. We have shown previously [24] that swimming at room temperature also reduced the convulsant potency of picrotoxin, although the effect was smaller than that observed at 18–198C. It is possible that a more intensive exercise (swimming) due to a lower temperature was responsible for a more intensive anticonvulsant effect of swim stress. The effect of a lower temperature by itself might presumably be excluded since cold stress (10 min at 48C) failed to produce an anticonvulsive effect. Besides, the fact that some other forms of stress such as immobilization stress (lasting for 15 min) prolong the latency of convulsions produced by ip administration of picrotoxin (our unpublished results) suggests that exposure to a lower temperature does not appear to be substantial for the described stress-induced changes in response to convulsants. It has recently been shown that swim stress produces a profound inhibition of 5-HT2A receptor-mediated head twitch behavior [22], but it has been suggested that inhibition of this behavior does not appear to be related to swim stress-induced anticonvulsant effect. Acute treatment with fluoxetine (20 mg/kg) enhanced in unstressed and swim-stressed mice doses of picrotoxin
94
D. Pericˇic´ et al. / Brain Research 1033 (2005) 90–95
needed to produce tonic hindlimb extension. Although the effect of drug in stressed appeared to be lesser than in unstressed mice, suggesting the possibility that swim stress has changed the effect of this drug, this was not confirmed by a significant drug stress interaction in two-way ANOVA. Fluoxetine (20 mg/kg) given repeatedly for 5 days produced an anticonvulsant effect that was equally pronounced in unstressed and stressed mice, at least in relation to doses of picrotoxin needed to produce tonic hindlimb extension and death. Both acute and repeated fluoxetine treatment had a greater effect on picrotoxin-induced tonic hindlimb extension and death than on running/bouncing clonus, which is in accordance with the suggestion that the convulsive responses which appear following the administration of convulsants represent qualitatively distinct seizure components mediated by separable and independent anatomical circuits located in the forebrain and hindbrain [7]. Anticonvulsant activity of acute fluoxetine treatment has already been demonstrated in several, although not all [15], animal models of epilepsy [4,6,18,21,28,36]. In stress naive genetically epilepsy-prone rats, this effect was explained by an increase in extracellular serotonin [4,6,36]. Similarly, the anticonvulsant action of fluoxetine in substantia nigra was explained by an enhancement of the synaptic action of endogenous 5-HT in substantia nigra [21]. It is well known that serotonin may exert its effects via at least 14 different receptor subtypes, but only a few of them (5-HT1A, 5-HT2C, 5-HT7) have been mentioned in relation to the control of seizures [2,8,11,15,18,32]. It has been demonstrated that inhibition of epileptiform bursts by serotonina may be mediated by activation of the 5-HT1A receptor subtype [32], while the antagonism of this receptor subtype may increase or augment seizure severity [35]. Lu and Gean [18] suggested the involvement of 5-HT1A receptors in the effects of fluoxetine, since inhibition of epileptiform activity induced by this drug was blocked by 5-HT1A receptor antagonist. On the contrary, concomitant administration of 5-HT1A receptor antagonists enhanced the anticonvulsant effect of fluoxetine in genetically epilepsyprone rats [4] and in rats with focal hippocampal seizures [34], which can be explained by a greater increase in extracellular 5-HT due to prevention of a negative feedback of serotonin at somatodendritic level [3,30]. Mutant mice that lack 5-HT1A receptors also exhibit higher 5-HT concentrations and enhanced fluoxetine-induced serotonergic neurotransmission [20]. Chronic treatment with fluoxetine desensitizes presynaptic 5-HT1A receptors and therefore the effect on cortical extracellular 5-HT is greater than after acute treatment [14]. Perhaps, this might explain the fact that in stressed animals pretreated repeatedly with fluoxetine, the anticonvulsant effect appeared to be greater than in stressed animals pretreated acutely with fluoxetine. At variance with the data mentioned previously, another group of authors [8,15] reported that fluoxetine and 5-HT1A
receptor agonists increase the number of spike-wave discharges in a genetic rat model of absence epilepsy. Pretreatment with 5-HT1A receptor antagonists significantly attenuated while pretreatment with a selective 5-HT2C antagonist increased the effect of fluoxetine [15]. The authors concluded that an increase in endogenous 5-HT produces a dual effect on spike-wave discharges, and the inhibition is mediated by 5-HT2C receptors. A positive role of this receptor subtype in the control of seizures has also been suggested by Applegate and Tecott [2]. Unlike 5-HT2C receptors, 5-HT7 receptors appear to play a negative role in regulating epileptiform activity in the rat model of absence epilepsy [11]. Although fluoxetine enhances primarily the concentrations of extracellular serotonin by inhibition of its reuptake, it has been reported that this drug may also enhance extracellular noradrenaline in the frontal [13] and extracellular dopamine in prefrontal cortex by a mechanism not dependent on serotonin [26]. Since inhibitors of reuptake of noradrenaline also affect picrotoxin-induced seizures [23], the possible involvement of other monoamines, besides serotonin, in the observed effects of fluoxetine on picrotoxin-induced convulsions cannot be excluded. However, unlike fluoxetine, desipramine, the inhibitor of reuptake of noradrenaline into adrenergic axons [31], enhanced the seizure threshold for picrotoxin only in stressed animals. In conclusion, our results demonstrate that the antidepressant drug fluoxetine, given acutely or repeatedly, exerts anticonvulsant properties against convulsions induced in unstressed and swim-stressed mice by antagonist of GABAA receptors, picrotoxin. Swim stress does not appear to modify the anticonvulsant properties of fluoxetine. Further studies are needed to determine the mechanism of the anticonvulsant effect of fluoxetine observed in this study and to evaluate the possible clinical significance of the results obtained from different animal models of epilepsy.
Acknowledgments This study was supported by the Croatian Ministry of Science, Education and Sport. The skilful technical assistance of Mrs. Zlatica Tonsˇetic´ is gratefully acknowledged.
References [1] A.L. Abel, R.F. Berman, Effects of water immersion stress on convulsions induced by pentylenetetrazol, Pharmacol. Biochem. Behav. 45 (1993) 823 – 825. [2] C.D. Applegate, L.H. Tecott, Global increases in seizure susceptibility in mice lacking 5-HT2C receptors: a behavioural analysis, Exp. Neurol. 154 (1998) 522 – 530. [3] A. Bortolozzi, M. Amargos-Bosch, M. Toth, F. Artigas, A. Adell, In vivo efflux of serotonin in the dorsal raphe nucleus of 5-HT1A receptor knockout mice, J. Neurochem. 88 (2004) 1373 – 1379.
D. Pericˇic´ et al. / Brain Research 1033 (2005) 90–95 [4] R.A. Browning, A.V. Wood, M.A. Merill, J.W. Dailey, P.C. Jobe, Enhancement of the anticonvulsant effect of fluoxetine following blockade of 5-HT1A receptors, Eur. J. Pharmacol. 336 (1997) 1 – 6. [5] F. Chaouloff, O. Berton, P. Mormede, Serotonin and stress, Neuropsychopharmacology 21 (1999) 28S – 32S. [6] J.W. Dailey, Q.S. Yan, L.E. Adams-Curtis, J.R. Ryu, K.H. Ko, P.K. Mishra, P.C. Jobe, Neurochemical correlates of antiepileptic drugs in the genetically epilepsy-prone rat (GEPR), Life Sci. 58 (1996) 259 – 266. [7] K. Gale, Progression and generalization of seizure discharge: anatomical and neurochemical substrates, Epilepsia 29 (Suppl. 2) (1988) S15 – S34. [8] K. Gerber, P. Filakovszky, P. Halasz, G. Bagdy, The 5-HT1A agonist 8-OH-DPAT increases the number of spike-wave discharges in a genetic rat model of absence epilepsy, Brain Res. 807 (1998) 243 – 245. [9] G.L. Gigli, M. Diomedi, A. Troisi, F. Baldinetti, M.G. Marciani, E. Girolami, A. Pasini, Lack of potentiation of anticonvulsant effect by fluoxetine in drug-resistant epilepsy, Seizure 3 (1994) 221 – 224. [10] F.G. Graeff F.S. Guimara¯es, T.G.C.S. De Andrade, J.F.W. Deakin, Role of 5-HT in stress, anxiety, and depression, Pharmacol. Biochem. Behav. 54 (1996) 129 – 141. [11] M. Graf, R. Jakus, S. Kantor, G. Levay, G. Bagdy, Selective 5-HT1A and 5-HT7 antagonists decrease epileptic activity in the WAG/Rij rat model of absence epilepsy, Neurosci. Lett. 359 (2004) 45 – 48. [12] E.J. Hernandez, P.A. Williams, F.E. Dudek, Effects of fluoxetine and TFMPP on spontaneous seizures in rats with pilocarpine-induced epilepsy, Epilepsia 43 (2002) 1337 – 1345. [13] Z.A. Hughes, S.C. Stanford, Increased noradrenaline efflux induced by local infusion of fluoxetine in the rat frontal cortex, Eur. J. Pharmacol. 317 (1996) 83 – 90. [14] R. Invernizzi, M. Bramante, R. Samanin, Role of 5-HT1A receptors in the effects of acute and chronic fluoxetine on extracellular serotonin in the frontal cortex, Pharmacol. Biochem. Behav. 54 (1996) 143 – 147. [15] R. Jakus, M. Graf, G. Juhasz, K. Gerber, G. Levay, P. Halasz, G. Bagdy, 5-HT2C receptors inhibit and 5-HT1A receptors activate the generation of spike-wave discharges in a genetic rat model of absence epilepsy, Exp. Neurol. 184 (2003) 964 – 972. [16] A.E. Kosobud, J.C. Crabbe, Genetic correlations among inbred strain sensitivities to convulsions induced by 9 convulsant drugs, Brain Res. 526 (1990) 8 – 16. [17] J.F. Lo´pez, H. Akil, S.J. Watson, Neural circuits mediating stress, Biol. Psychiatry 46 (1999) 1461 – 1471. [18] K.T. Lu, P.W. Gean, Endogenous serotonin inhibits epileptiform activity in rat hippocampal CA1 neurons via 5-hydroxytryptamine1A receptor activation, Neuroscience 86 (1998) 729 – 737. [19] L. Mackenzie, A. Medvedev, J.J. Hiscock, K.J. Pope, J.O. Willoughby, Picrotoxin-induced generalised convulsive seizure in rat: changes in regional distribution and frequency of the power of electroencephalogram rhythms, Clin. Neurophysiol. 113 (2002) 586 – 596. [20] L.H. Parsons, T.M. Kerr, L.H. Tecott, 5-HT1A receptor mutant mice exhibit enhanced tonic, stress-induced and fluoxetine-induced serotonergic neurotransmission, J. Neurochem. 77 (2001) 607 – 617.
95
[21] A. Pasini, A. Tortorella, K. Gale, The anticonvulsant action of fluoxetine in substantia nigra is dependent upon endogenous serotonin, Brain Res. 724 (1996) 84 – 88. [22] D. Pericˇic´, Swim stress inhibits 5-HT2A receptor-mediated head twitch behaviour in mice, Psychopharmacology 167 (2003) 373 – 379. [23] D. Pericˇic´, D. Sˇvob, Interaction of stress and noradrenergic drugs in the control of picrotoxin-induced seizures, Epilepsy Res. 51 (2002) 179 – 187. [24] D. Pericˇic´, M. Jazvinsˇc´ak, D. Sˇvob, K. Mirkovic´, Swim stress alters the behavioural response of mice to GABA-related and some GABAunrelated convulsants, Epilepsy Res. 43 (2001) 145 – 152. [25] D. Pericˇic´, D. Sˇvob, M. Jazvinsˇc´ak Jembrek, K. Mirkovic´ Kos, The involvement of a2-adrenoceptors in the anticonvulsant effect of swim stress in mice, Psychopharmacology 158 (2001) 87 – 93. [26] L. Pozzi, R. Invernizzi, C. Garavaglia, R. Samanin, Fluoxetine increases extracellular dopamine in the prefrontal cortex by a mechanism not dependent on serotonin: a comparison with citalopram, J. Neurochem. 73 (1999) 1051 – 1057. [27] V.P. Prasher, Seizures associated with fluoxetine therapy, Seizure 2 (1993) 315 – 317. [28] S. Prendiville, K. Gale, Anticonvulsant effect of fluoxetine on focally evoked limbic motor seizures in rats, Epilepsia 34 (1993) 381 – 384. [29] D.S. Reddy, M.A. Rogawski, Stress-induced deoxycorticosteronederived neurosteroids modulate GABAA receptor function and seizure susceptibility, J. Neurosci. 22 (2002) 3795 – 3805. [30] L. Romero, F. Artigas, Preferential potentiation of the effects of serotonin uptake inhibitors by 5-HT1A receptor antagonists in the dorsal raphe pathway: role of somatodendritic autoreceptors, J. Neurochem. 68 (1997) 2593 – 2603. [31] S.B. Ross, A.L. Renyi, Tricyclic antidepressant agents, II, effect of oral administration on the uptake of 3-H-noradrenaline and 14-C-5hydroxytryptamine in slices of the midbrain–hypothalamus region of the rat, Acta Pharmacol. Toxicol. 36 (1975) 395 – 408. [32] D. Salgado-Commissariat, K.A. Alkadhi, Serotonin inhibits epileptiform discharge by activation of 5-HT1A receptors in CA1 pyramidal neurons, Neuropharmacology 36 (1997) 1705 – 1712. [33] P. Soubrie, M.H. Thiebot, A. Jobert, J.L. Montastruc, F. Hery, M. Hamon, Decreased convulsant potency of picrotoxin and pentetrazol and enhanced [3H]flunitrazepam cortical binding following stressful manipulations in rats, Brain Res. 189 (1980) 505 – 517. [34] Y. Wada, N. Hirao, J. Shiraishi, M. Nakamura, Y. Koshino, Pindolol potentiates the effect of fluoxetine on hippocampal seizures in rats, Neurosci. Lett. 267 (1999) 61 – 64. [35] K. Watanabe, C.R. Ashby, H. Katsumori, Y. Minabe, The effect of acute administration of various selective 5-HT receptor antagonists on focal hippocampal seizures in freely-moving rats, Eur. J. Pharmacol. 398 (2000) 239 – 246. [36] Q.S. Yan, P.C. Jobe, J.W. Dailey, Evidence that a serotonergic mechanism is involved in the anticonvulsant effect of fluoxetine in genetically epilepsy-prone rats, Eur. J. Pharmacol. 252 (1994) 105 – 112.
Research report
Anticonvulsant effects of acute and repeated fluoxetine treatment in unstressed and stressed mice Danka Pericˇic´*, Josipa Lazic´, Dubravka Sˇvob Sˇtrac Laboratory for Molecular Neuropharmacology, Division of Molecular Medicine, Rud¯er Bosˇkovic´ Institute, Bijenicka cesta 54, P.O. Box 180, 10002 Zagreb, Croatia Accepted 19 November 2004 Available online 30 December 2004
Abstract Comorbidity of epilepsy and depression is not rare. Stress can affect both depression and seizures. Therefore, it is important to know whether an antidepressant drug has pro- or anticonvulsant properties and whether these properties will be modified by stress. We tested the effects of the antidepressant drug fluoxetine on the seizure threshold for picrotoxin in unstressed and swim-stressed mice. The mice were, prior to exposure to swim stress and the intravenous infusion of picrotoxin (a non-competitive GABAA receptor antagonist), pretreated with fluoxetine (a selective serotonin reuptake inhibitor), either acutely or repeatedly (5 days), and the latency to the onset of two convulsant signs and death was registered. The convulsant signs were running/bouncing clonus and tonic hindlimb extension. As expected, swim stress enhanced the seizure threshold for picrotoxin. Fluoxetine (20 mg/kg ip) given acutely increased in unstressed and swim-stressed mice the dose of picrotoxin producing tonic hindlimb extension and in unstressed mice the dose of picrotoxin producing death. Neither 10 nor 20 mg/ kg of fluoxetine affected doses of picrotoxin needed to produce running bouncing/clonus. Repeated treatment with fluoxetine (20 mg/kg ip) enhanced significantly in unstressed and swim-stressed mice doses of picrotoxin needed to produce tonic hindlimb extension and death, and in stressed mice also the dose of picrotoxin producing running/bouncing clonus. The results demonstrate that the antidepressant drug fluoxetine, given acutely or repeatedly, shows anticonvulsant properties against convulsions induced in unstressed and swim-stressed mice by antagonist of GABAA receptors, picrotoxin. Swim stress failed to modify the anticonvulsant properties of fluoxetine. D 2004 Elsevier B.V. All rights reserved. Theme: Neurotransmitters, modulators, transporters, and receptors, Topic: Serotonin Keywords: Fluoxetine; Acute and repeated treatment; Picrotoxin-induced convulsions; Swim stress
1. Introduction Selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors, including fluoxetine, are among the most widely used antidepressant drugs. Since depression and epilepsy often appear in the same patient, it is important to know whether the drug used to treat depression has pro- or anticonvulsant properties. Some clinical studies suggested that caution should be exerted when using fluoxetine as an
* Corresponding author. Fax: +385 1 4561010. E-mail address: [email protected] (D. Pericˇic´). 0006-8993/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2004.11.025
antidepressive treatment in epileptic patients [9,27]. Therefore, several groups of authors studied the effect of fluoxetine in different animal models of epilepsy. Anticonvulsant activity of this drug has been demonstrated on focally evoked limbic motor seizures in rats [28], in a rat model of focally evoked complex partial seizures [21], in an in vitro picrotoxin-induced model of epilepsy [18], in the pilocarpine model of temporal lobe epilepsy [12], and in genetically epilepsy-prone rats, in which this effect was explained by an increase in extracellular serotonin [4,6,36]. However, some more recent studies do not suggest the anticonvulsant properties of fluoxetine, or at least not in a genetic rat model of absence epilepsy [15]. All these studies,
D. Pericˇic´ et al. / Brain Research 1033 (2005) 90–95
except one [12], refer to the effects of acute fluoxetine treatment. It is known that many central neuronal systems, including the serotonergic system, are sensitive to different stressors [5,10,17]. Both serotonin and stress have been implicated in affective disorders, especially depression, and swim stress is often used as a biological stressor and as an animal model with predictive value for antidepressant drugs. Hence, the aim of this study was to explore whether acute and repeated fluoxetine treatments are able to affect seizures produced in unstressed and swim-stressed mice by picrotoxin, a non-competitive antagonist of GABAA receptors, and at the same time to assess whether swim stress modifies the possible effect of fluoxetine on seizure threshold. Convulsions were produced by intravenous (iv) infusion of picrotoxin. This convulsant has been used extensively to manipulate the GABA system and to investigate its involvement in epileptic activity. Picrotoxin-induced seizures represent a model of generalized convulsive epilepsy [19]. As shown previously [24], among the GABA related convulsants, swim stress was most effective against convulsions produced by this convulsant. We measured drug and swim stress-induced changes in doses of picrotoxin needed to produce running/ bouncing clonus and tonic hindlimb extension, two convulsant signs whose onset in mice can very precisely be determined.
2. Materials and methods 2.1. Animals Male CBA mice (25–30 g), 3 months old and bred in our institute, were used. They were housed at a constant temperature (228C) and with a light cycle of 12-h light/ 12-h darkness (lights on at 7.00 a.m.). They were caged in groups of ten. The size of the cages was 41 cm [length] 26 cm [width] 15 cm [height]. Food and water were freely available. Prior to experiment, the animals were not habituated to iv drug administration. All animal care and experimental procedures were carried out in accordance with the National Institute of Health Guide for the Care and Use of Laboratory Animals (NIH publication No. 8023) revised 1996, and with the Croatian law on animal welfare.
91
2.3. Drugs Fluoxetine hydrochloride and picrotoxin, both from Sigma (St. Louis, MO), were used. Fluoxetine was dissolved in saline and picrotoxin was dissolved in warm saline. Picrotoxin was given by constant iv infusion into a tail vein, while fluoxetine was administered intraperitoneally (ip) in a volume 1 ml per 100 g body weight. Fluoxetine was administered acutely (65 min prior to picrotoxin and 40 min prior to swim stress) or repeatedly (once daily for five consecutive days, with the last injection given 65 min prior to picrotoxin). Control animals received ip injection of saline. The time of drug administration in the acute experiment is within the literature data (30–120 min prior to different tests) and corresponds to almost maximum enhancement of 5-HT in dialysate of several brain regions following fluoxetine administration. 2.4. Convulsive activity For determination of convulsive activity, the animal was taken from its home cage and placed in a glass cylinder (20 7 cm2) with numerous holes for ventilation. The tail of the animal was drawn through a hole of the plastic cover and warmed for 1 min under a tensor lamp. A butterfly infusion needle (0.3 mm) was inserted into the tail vein and correct placement was verified by the appearance of blood in the infusion tubing. During the infusion the animal was held lightly by the tip of the tail to allow free movement. The concentration of picrotoxin was 0.75 mg/ml and the infusion rate controlled by a microinfusion pump was 1.1 ml/min. The animal was observed throughout infusion and the time between the start of infusion and the onset of convulsive signs was measured, mainly as described by Kosobud and Crabbe [16], by an observer unaware of the treatment. The convulsive signs were running/bouncing clonus (RB clonus, violent whole-body clonus, including running and explosive jumps) and tonic hindlimb extension (THE, characterized by extreme rigidity, with forelimbs and hindlimbs extended caudally). For each animal, the dose of convulsant (mg/kg of body weight) required to elicit a particular convulsant sign was calculated from the time of infusion, the infusion rate, the concentration of picrotoxin, and body weight. The time to death was also measured. All experiments were carried out between 9:00 and 13:00 h.
2.2. Stress procedure 2.5. Statistical analysis Mice were subjected to swim stress (10-min swimming) at 18–198C. After swimming the animals were dried with a towel and placed near the heater. The iv injection of convulsant started 15 min after termination of stress. Control unstressed animals were taken for comparison.
Results are expressed as mean values F SEM. Statistical analysis of the results was by one-way analysis of variance (ANOVA) followed by Newman–Keuls test and by two-way ANOVA, since in the same experiment, the factors stress and drug were studied. The analyses were performed using
92
D. Pericˇic´ et al. / Brain Research 1033 (2005) 90–95
GraphPad Prism version 4.00 for Windows, GraphPad Software, San Diego, California, USA. P values of b0.05 were considered significant.
3. Results 3.1. Effect of acute fluoxetine treatment on picrotoxin-induced convulsions in unstressed and swim-stressed mice As shown in Fig. 1, in accordance with our results published previously [21], 15 min after exposure to swim stress, the threshold doses of picrotoxin producing running/ bouncing clonus, tonic hindlimb extension, and death were enhanced by 85%, 108%, and 108%, respectively, i.e., swim stress, as indicated by two-way ANOVA produced a highly significant ( P b 0.0001) anticonvulsive effect [running/ bouncing clonus: F(1,57) = 317.85; tonic hindlimb extension: F(1,56) = 394.58; death: F(1,54) = 430.40]. Fluoxetine at 10 and 20 mg/kg failed to produce a significant effect on doses of picrotoxin needed to produce running/ bouncing clonus, but the effects on tonic hindlimb extension [ F(2,56) = 13.68] and death [ F(2,54) = 12.17] were highly significant ( P b 0.0001). In neither of these cases was the interaction drug stress significant. Planned comparisons using the Newman–Keuls test indicated that fluoxetine 10 mg/kg enhanced only in stressed animals the dose of picrotoxin producing tonic hindlimb extension (12%; P b 0.05), while fluoxetine 20 mg/kg enhanced in unstressed animals doses of picrotoxin needed to produce tonic hindlimb extension (42%; P b 0.001) and death (39%; P b 0.001) and in stressed animals the dose of picrotoxin producing tonic hindlimb extension (11%; P b 0.05). To assess the possible involvement of hypothermia in the anticonvulsant effect of swim stress, we tested the convulsant potency of picrotoxin 15 min after exposure of mice for 10 min to an ambient temperature of 48C. Unlike swim stress, this stress decreased the threshold doses of picrotoxin producing convulsions. Doses of picrotoxin needed to produce tonic hindlimb extension were 25.07 F 1.1 (N = 8) in the control and 22.12 F 0.71 mg/kg (N = 8) in cold stressed group ( P b 0.05, Student’s t test). 3.2. Effect of repeated fluoxetine treatment on picrotoxin-induced convulsions in unstressed and swim-stressed mice Since acute administration of fluoxetine 20 mg/kg exerted an anticonvulsant effect in both unstressed and stressed animals, this dose was chosen for studying the possible anticonvulsant effect of repeated fluoxetine treatment. As shown in Fig. 2 and indicated by two-way ANOVA, swim stress had again a pronounced ( P b 0.0001) anticonvulsant effect, as evidenced by enhancements of 56.6%, 70.3%, and
Fig. 1. Effect of fluoxetine and swim stress on the dose of picrotoxin needed to produce running/bouncing clonus (RB clonus), tonic hindlimb extension (THE), and death in male CBA mice. Swim stress (10-min swimming in water at 18–198C) ended 15 min before iv infusion of convulsant. Fluoxetine (10 and 20 mg/kg ip) was administered 65 min prior to picrotoxin and 40 min prior to swim stress. Bars represent means F SEM from 7 to 16 animals per group. aP b 0.001 versus dose of picrotoxin needed to produce the same sign in saline-treated group; bP b 0.001 versus dose of picrotoxin needed to produce the same convulsive sign in the corresponding unstressed group; cP b 0.05 as compared to dose of picrotoxin needed to produce the same convulsive sign in saline-treated stressed group (ANOVA followed by Newman–Keuls test).
69% of doses of picrotoxin needed to produce running/ bouncing clonus [ F(1,27) = 137.57], tonic hindlimb extension [ F(1,27) = 141.05], and death [ F(1,27) = 88.65], respectively. Fluoxetine (20 mg/kg) given repeatedly for 5 days produced a significant effect on doses of picrotoxin producing running/bouncing clonus [ F(1,27) = 9.32; P b 0.005], tonic hindlimb extension [ F(1,27) = 47.35; P b 0.0001], and death [ F(1,27) = 17.90; P b 0.0002). In
D. Pericˇic´ et al. / Brain Research 1033 (2005) 90–95
93
running/bouncing clonus, tonic hindlimb extension, and death, respectively. Similar effects of repeated fluoxetine treatment on the seizure threshold for picrotoxin and death in unstressed and swim-stressed mice were confirmed by insignificant drug stress interaction in the two-way ANOVA.
4. Discussion
Fig. 2. Effect of repeated fluoxetine treatment on the dose of picrotoxin needed to produce running/bouncing clonus (RB clonus), tonic hindlimb extension (THE), and death in unstressed and stressed male CBA mice. Fluoxetine (20 mg/kg ip) was administered once daily for five consecutive days. The last injection was given 65 min prior to picrotoxin and 40 min prior to swim stress. Swim stress (10-min swimming in water at 18–198C) ended 15 min before iv infusion of convulsant. Bars represent means F SEM from 7 to 8 animals per group. aP b 0.001 and P b 0.05 versus dose of picrotoxin needed to produce tonic hindlimb extension and death in salinetreated group; bP b 0.001 versus dose of picrotoxin needed to produce the same convulsive sign in the corresponding unstressed group; cP b 0.01 or P b 0.001 as compared to dose of picrotoxin needed to produce the same convulsive sign in saline-treated stressed group and in fluoxetine-treated unstressed group (ANOVA followed by Newman–Keuls test).
unstressed animals, fluoxetine given for 5 days enhanced by 36% and 27% doses of picrotoxin needed to produce tonic hindlimb extension and death ( P b 0.001; Newman–Keuls test). In stressed animals, the same treatment enhanced by 14% ( P b 0.01), 35%, and 26% ( P b 0.001 versus salinetreated stressed group) doses of picrotoxin needed to produce
The results of the present study demonstrated the anticonvulsant properties of acute and repeated fluoxetine treatment against convulsions induced in mice by picrotoxin, a non-competitive GABAA receptor antagonist. The anticonvulsant effect was present in unstressed and swimstressed mice. Further, our results confirmed and extended the data of previous studies [1,24,25,29,33] demonstrating a pronounced anticonvulsant effect of swim stress against convulsions induced by GABA-related convulsants. As in our previous studies, the anticonvulsant effect of swim stress observed in the present study was very pronounced, e.g., swim stress more than doubled the seizure threshold for picrotoxin. In mice subjected to ip injections for 5 days, the anticonvulsant effect of swim stress appeared to be smaller than in mice that received only one ip injection before swim stress, suggesting that tolerance to the anticonvulsant effect might develop not only after repeated swim stress [24], but also after repeated stressing of animals with different stressors. However, a direct comparison of these effects was not possible, since the results were from two different experiments. The mechanism of the anticonvulsant effect of swim stress is not quite clear, although the involvement of a2-adrenoceptors [25] and neurosteroids [29] has been proposed. We have shown previously [24] that swimming at room temperature also reduced the convulsant potency of picrotoxin, although the effect was smaller than that observed at 18–198C. It is possible that a more intensive exercise (swimming) due to a lower temperature was responsible for a more intensive anticonvulsant effect of swim stress. The effect of a lower temperature by itself might presumably be excluded since cold stress (10 min at 48C) failed to produce an anticonvulsive effect. Besides, the fact that some other forms of stress such as immobilization stress (lasting for 15 min) prolong the latency of convulsions produced by ip administration of picrotoxin (our unpublished results) suggests that exposure to a lower temperature does not appear to be substantial for the described stress-induced changes in response to convulsants. It has recently been shown that swim stress produces a profound inhibition of 5-HT2A receptor-mediated head twitch behavior [22], but it has been suggested that inhibition of this behavior does not appear to be related to swim stress-induced anticonvulsant effect. Acute treatment with fluoxetine (20 mg/kg) enhanced in unstressed and swim-stressed mice doses of picrotoxin
94
D. Pericˇic´ et al. / Brain Research 1033 (2005) 90–95
needed to produce tonic hindlimb extension. Although the effect of drug in stressed appeared to be lesser than in unstressed mice, suggesting the possibility that swim stress has changed the effect of this drug, this was not confirmed by a significant drug stress interaction in two-way ANOVA. Fluoxetine (20 mg/kg) given repeatedly for 5 days produced an anticonvulsant effect that was equally pronounced in unstressed and stressed mice, at least in relation to doses of picrotoxin needed to produce tonic hindlimb extension and death. Both acute and repeated fluoxetine treatment had a greater effect on picrotoxin-induced tonic hindlimb extension and death than on running/bouncing clonus, which is in accordance with the suggestion that the convulsive responses which appear following the administration of convulsants represent qualitatively distinct seizure components mediated by separable and independent anatomical circuits located in the forebrain and hindbrain [7]. Anticonvulsant activity of acute fluoxetine treatment has already been demonstrated in several, although not all [15], animal models of epilepsy [4,6,18,21,28,36]. In stress naive genetically epilepsy-prone rats, this effect was explained by an increase in extracellular serotonin [4,6,36]. Similarly, the anticonvulsant action of fluoxetine in substantia nigra was explained by an enhancement of the synaptic action of endogenous 5-HT in substantia nigra [21]. It is well known that serotonin may exert its effects via at least 14 different receptor subtypes, but only a few of them (5-HT1A, 5-HT2C, 5-HT7) have been mentioned in relation to the control of seizures [2,8,11,15,18,32]. It has been demonstrated that inhibition of epileptiform bursts by serotonina may be mediated by activation of the 5-HT1A receptor subtype [32], while the antagonism of this receptor subtype may increase or augment seizure severity [35]. Lu and Gean [18] suggested the involvement of 5-HT1A receptors in the effects of fluoxetine, since inhibition of epileptiform activity induced by this drug was blocked by 5-HT1A receptor antagonist. On the contrary, concomitant administration of 5-HT1A receptor antagonists enhanced the anticonvulsant effect of fluoxetine in genetically epilepsyprone rats [4] and in rats with focal hippocampal seizures [34], which can be explained by a greater increase in extracellular 5-HT due to prevention of a negative feedback of serotonin at somatodendritic level [3,30]. Mutant mice that lack 5-HT1A receptors also exhibit higher 5-HT concentrations and enhanced fluoxetine-induced serotonergic neurotransmission [20]. Chronic treatment with fluoxetine desensitizes presynaptic 5-HT1A receptors and therefore the effect on cortical extracellular 5-HT is greater than after acute treatment [14]. Perhaps, this might explain the fact that in stressed animals pretreated repeatedly with fluoxetine, the anticonvulsant effect appeared to be greater than in stressed animals pretreated acutely with fluoxetine. At variance with the data mentioned previously, another group of authors [8,15] reported that fluoxetine and 5-HT1A
receptor agonists increase the number of spike-wave discharges in a genetic rat model of absence epilepsy. Pretreatment with 5-HT1A receptor antagonists significantly attenuated while pretreatment with a selective 5-HT2C antagonist increased the effect of fluoxetine [15]. The authors concluded that an increase in endogenous 5-HT produces a dual effect on spike-wave discharges, and the inhibition is mediated by 5-HT2C receptors. A positive role of this receptor subtype in the control of seizures has also been suggested by Applegate and Tecott [2]. Unlike 5-HT2C receptors, 5-HT7 receptors appear to play a negative role in regulating epileptiform activity in the rat model of absence epilepsy [11]. Although fluoxetine enhances primarily the concentrations of extracellular serotonin by inhibition of its reuptake, it has been reported that this drug may also enhance extracellular noradrenaline in the frontal [13] and extracellular dopamine in prefrontal cortex by a mechanism not dependent on serotonin [26]. Since inhibitors of reuptake of noradrenaline also affect picrotoxin-induced seizures [23], the possible involvement of other monoamines, besides serotonin, in the observed effects of fluoxetine on picrotoxin-induced convulsions cannot be excluded. However, unlike fluoxetine, desipramine, the inhibitor of reuptake of noradrenaline into adrenergic axons [31], enhanced the seizure threshold for picrotoxin only in stressed animals. In conclusion, our results demonstrate that the antidepressant drug fluoxetine, given acutely or repeatedly, exerts anticonvulsant properties against convulsions induced in unstressed and swim-stressed mice by antagonist of GABAA receptors, picrotoxin. Swim stress does not appear to modify the anticonvulsant properties of fluoxetine. Further studies are needed to determine the mechanism of the anticonvulsant effect of fluoxetine observed in this study and to evaluate the possible clinical significance of the results obtained from different animal models of epilepsy.
Acknowledgments This study was supported by the Croatian Ministry of Science, Education and Sport. The skilful technical assistance of Mrs. Zlatica Tonsˇetic´ is gratefully acknowledged.
References [1] A.L. Abel, R.F. Berman, Effects of water immersion stress on convulsions induced by pentylenetetrazol, Pharmacol. Biochem. Behav. 45 (1993) 823 – 825. [2] C.D. Applegate, L.H. Tecott, Global increases in seizure susceptibility in mice lacking 5-HT2C receptors: a behavioural analysis, Exp. Neurol. 154 (1998) 522 – 530. [3] A. Bortolozzi, M. Amargos-Bosch, M. Toth, F. Artigas, A. Adell, In vivo efflux of serotonin in the dorsal raphe nucleus of 5-HT1A receptor knockout mice, J. Neurochem. 88 (2004) 1373 – 1379.
D. Pericˇic´ et al. / Brain Research 1033 (2005) 90–95 [4] R.A. Browning, A.V. Wood, M.A. Merill, J.W. Dailey, P.C. Jobe, Enhancement of the anticonvulsant effect of fluoxetine following blockade of 5-HT1A receptors, Eur. J. Pharmacol. 336 (1997) 1 – 6. [5] F. Chaouloff, O. Berton, P. Mormede, Serotonin and stress, Neuropsychopharmacology 21 (1999) 28S – 32S. [6] J.W. Dailey, Q.S. Yan, L.E. Adams-Curtis, J.R. Ryu, K.H. Ko, P.K. Mishra, P.C. Jobe, Neurochemical correlates of antiepileptic drugs in the genetically epilepsy-prone rat (GEPR), Life Sci. 58 (1996) 259 – 266. [7] K. Gale, Progression and generalization of seizure discharge: anatomical and neurochemical substrates, Epilepsia 29 (Suppl. 2) (1988) S15 – S34. [8] K. Gerber, P. Filakovszky, P. Halasz, G. Bagdy, The 5-HT1A agonist 8-OH-DPAT increases the number of spike-wave discharges in a genetic rat model of absence epilepsy, Brain Res. 807 (1998) 243 – 245. [9] G.L. Gigli, M. Diomedi, A. Troisi, F. Baldinetti, M.G. Marciani, E. Girolami, A. Pasini, Lack of potentiation of anticonvulsant effect by fluoxetine in drug-resistant epilepsy, Seizure 3 (1994) 221 – 224. [10] F.G. Graeff F.S. Guimara¯es, T.G.C.S. De Andrade, J.F.W. Deakin, Role of 5-HT in stress, anxiety, and depression, Pharmacol. Biochem. Behav. 54 (1996) 129 – 141. [11] M. Graf, R. Jakus, S. Kantor, G. Levay, G. Bagdy, Selective 5-HT1A and 5-HT7 antagonists decrease epileptic activity in the WAG/Rij rat model of absence epilepsy, Neurosci. Lett. 359 (2004) 45 – 48. [12] E.J. Hernandez, P.A. Williams, F.E. Dudek, Effects of fluoxetine and TFMPP on spontaneous seizures in rats with pilocarpine-induced epilepsy, Epilepsia 43 (2002) 1337 – 1345. [13] Z.A. Hughes, S.C. Stanford, Increased noradrenaline efflux induced by local infusion of fluoxetine in the rat frontal cortex, Eur. J. Pharmacol. 317 (1996) 83 – 90. [14] R. Invernizzi, M. Bramante, R. Samanin, Role of 5-HT1A receptors in the effects of acute and chronic fluoxetine on extracellular serotonin in the frontal cortex, Pharmacol. Biochem. Behav. 54 (1996) 143 – 147. [15] R. Jakus, M. Graf, G. Juhasz, K. Gerber, G. Levay, P. Halasz, G. Bagdy, 5-HT2C receptors inhibit and 5-HT1A receptors activate the generation of spike-wave discharges in a genetic rat model of absence epilepsy, Exp. Neurol. 184 (2003) 964 – 972. [16] A.E. Kosobud, J.C. Crabbe, Genetic correlations among inbred strain sensitivities to convulsions induced by 9 convulsant drugs, Brain Res. 526 (1990) 8 – 16. [17] J.F. Lo´pez, H. Akil, S.J. Watson, Neural circuits mediating stress, Biol. Psychiatry 46 (1999) 1461 – 1471. [18] K.T. Lu, P.W. Gean, Endogenous serotonin inhibits epileptiform activity in rat hippocampal CA1 neurons via 5-hydroxytryptamine1A receptor activation, Neuroscience 86 (1998) 729 – 737. [19] L. Mackenzie, A. Medvedev, J.J. Hiscock, K.J. Pope, J.O. Willoughby, Picrotoxin-induced generalised convulsive seizure in rat: changes in regional distribution and frequency of the power of electroencephalogram rhythms, Clin. Neurophysiol. 113 (2002) 586 – 596. [20] L.H. Parsons, T.M. Kerr, L.H. Tecott, 5-HT1A receptor mutant mice exhibit enhanced tonic, stress-induced and fluoxetine-induced serotonergic neurotransmission, J. Neurochem. 77 (2001) 607 – 617.
95
[21] A. Pasini, A. Tortorella, K. Gale, The anticonvulsant action of fluoxetine in substantia nigra is dependent upon endogenous serotonin, Brain Res. 724 (1996) 84 – 88. [22] D. Pericˇic´, Swim stress inhibits 5-HT2A receptor-mediated head twitch behaviour in mice, Psychopharmacology 167 (2003) 373 – 379. [23] D. Pericˇic´, D. Sˇvob, Interaction of stress and noradrenergic drugs in the control of picrotoxin-induced seizures, Epilepsy Res. 51 (2002) 179 – 187. [24] D. Pericˇic´, M. Jazvinsˇc´ak, D. Sˇvob, K. Mirkovic´, Swim stress alters the behavioural response of mice to GABA-related and some GABAunrelated convulsants, Epilepsy Res. 43 (2001) 145 – 152. [25] D. Pericˇic´, D. Sˇvob, M. Jazvinsˇc´ak Jembrek, K. Mirkovic´ Kos, The involvement of a2-adrenoceptors in the anticonvulsant effect of swim stress in mice, Psychopharmacology 158 (2001) 87 – 93. [26] L. Pozzi, R. Invernizzi, C. Garavaglia, R. Samanin, Fluoxetine increases extracellular dopamine in the prefrontal cortex by a mechanism not dependent on serotonin: a comparison with citalopram, J. Neurochem. 73 (1999) 1051 – 1057. [27] V.P. Prasher, Seizures associated with fluoxetine therapy, Seizure 2 (1993) 315 – 317. [28] S. Prendiville, K. Gale, Anticonvulsant effect of fluoxetine on focally evoked limbic motor seizures in rats, Epilepsia 34 (1993) 381 – 384. [29] D.S. Reddy, M.A. Rogawski, Stress-induced deoxycorticosteronederived neurosteroids modulate GABAA receptor function and seizure susceptibility, J. Neurosci. 22 (2002) 3795 – 3805. [30] L. Romero, F. Artigas, Preferential potentiation of the effects of serotonin uptake inhibitors by 5-HT1A receptor antagonists in the dorsal raphe pathway: role of somatodendritic autoreceptors, J. Neurochem. 68 (1997) 2593 – 2603. [31] S.B. Ross, A.L. Renyi, Tricyclic antidepressant agents, II, effect of oral administration on the uptake of 3-H-noradrenaline and 14-C-5hydroxytryptamine in slices of the midbrain–hypothalamus region of the rat, Acta Pharmacol. Toxicol. 36 (1975) 395 – 408. [32] D. Salgado-Commissariat, K.A. Alkadhi, Serotonin inhibits epileptiform discharge by activation of 5-HT1A receptors in CA1 pyramidal neurons, Neuropharmacology 36 (1997) 1705 – 1712. [33] P. Soubrie, M.H. Thiebot, A. Jobert, J.L. Montastruc, F. Hery, M. Hamon, Decreased convulsant potency of picrotoxin and pentetrazol and enhanced [3H]flunitrazepam cortical binding following stressful manipulations in rats, Brain Res. 189 (1980) 505 – 517. [34] Y. Wada, N. Hirao, J. Shiraishi, M. Nakamura, Y. Koshino, Pindolol potentiates the effect of fluoxetine on hippocampal seizures in rats, Neurosci. Lett. 267 (1999) 61 – 64. [35] K. Watanabe, C.R. Ashby, H. Katsumori, Y. Minabe, The effect of acute administration of various selective 5-HT receptor antagonists on focal hippocampal seizures in freely-moving rats, Eur. J. Pharmacol. 398 (2000) 239 – 246. [36] Q.S. Yan, P.C. Jobe, J.W. Dailey, Evidence that a serotonergic mechanism is involved in the anticonvulsant effect of fluoxetine in genetically epilepsy-prone rats, Eur. J. Pharmacol. 252 (1994) 105 – 112.