- Email: [email protected]
P.1.c.030 Antidepressant treatment reduces fos-like immunoreactivity in different regions of periaqueductal gray matter
P.1.c Basic neuroscience – Neuropharmacology
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[3] Sokolova S., Zhang C.L., Arens J., Khodorov B., Heinemann U., 1992, Effects of 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) on different patterns of epileptiform activity in rat temporal cortex slices, Neurosci. Res. Commun., 10, 171–176.
azepines. Furthermore, cannabinoid receptor antagonists may be considered useful to treat benzodiazepine dependence. Supported by Grant from Spanish Ministry of Health (FIS 03/216) to J. Manzanares.
P.1.c.028 The administration of the cannabinoid CB1 receptor antagonist AM-251 blocked the anxiolytic and sedative actions of benzodiazepines
References
D.V. Navarro, E. Sanguino ° , J. Manzanares. Universidad Miguel Hernandez-CSIC, Instituto de Neurociencias de Alicante, San Juan de Alicante, Spain A large body of evidence suggests that the cannabinoid CB1 receptor play a key role in the regulation of emotional behaviours. Indeed, we have previously shown that CB1 cannabinoid receptor KO mice displayed anxiety-like behaviours that are also associated to increased proopiomelanocortin gene expression in the anterior pituitary, decreased corticosterone secretion and hypersensitivity to restraint stress (Urig¨uen et al., 2004). In addition, a lack of anxiolytic action was observed in CB1 receptors KO mice after acute administration of bromazepam suggesting that this cannabinoid receptor may be involved in the therapeutic activity of benzodiazepines. The purpose of this study was to examine whether the blockade of the cannabinoid CB1 receptor may alter the anxiolytic and sedative actions of benzodiazepines. To this aim, the behavioural effects of the cannabinoid CB1 receptor antagonist AM-251 on the anxiolytic (light dark test, LDT) and sedative (Neurological Severity Score, NSS) actions of alprazolam (APZ) were evaluated. Male Swiss mice (n = 7−9/group) were divided into three experiments: (1) Dose response effects of alprazolam on the LDT, (2) Effects of AM-251 on the anxiolytic (LDT) or (3) sedative (NSS) actions of alprazolam. The acute administration of APZ (20, 40 or 80 mg/Kg or its water vehicle 0.3 ml/mouse, p.o, 30 min) produced a significant increase (p < 0.001) in the time that mice explored the light area at 40 mg/Kg but was without effect at 20 or 80 mg/Kg. These results provided information on the doses of APZ producing anxiolytic or sedative effects. In the second experiment, mice were divided into four groups: (1) Control group: Vehicle 1 (0.3 ml/mouse of water, p.o., 30 min) plus Vehicle 2 (DMSO, Tween 80 and saline in a 1:1:8 ratio, 0.3 ml/mouse, i.p., 60 min), (2) APZ (40 mg/kg, p.o, 30 min), (3) AM-251 (3 mg/kg, i.p., 60 min), (4) AM-251 plus APZ (same doses and times). The results revealed that administration of APZ produced a significant increase (68%) in the time that mice spent in the light area in the LDT compared to vehicle-treated mice. The administration of AM-251 by itself was without effects but completely blocked the anxiolytic actions of APZ (p < 0.001). In the third experiment mice were also divided in the same groups as the second experiment but the dose of APZ was much higher (0.5 mg/kg, p.o, 30 min). The effects of AM-251 and APZ on sedation were examined using the NSS test. This test evaluated the motor coordination of mice crossing bars of different width (2 cm, 1 cm, 0.5 cm). The time to cross the bars was recorded. The results revealed that administration of APZ produced incapacity to walk on 2 cm, 1 cm and 0.5 cm bar in the NSS test (p < 0.001), however, previous administration of AM-251 blocked the motor coordination induced by APZ (p < 0.001). Taken together these results suggest that CB1 receptors play a pivotal role in the anxiolytic and sedative actions of benzodi-
[1] Urig¨uen L., Perez-Rial S., Ledent C.L., Palomo T., Manzanares J., 2004, Impaired action of anxiolytics in mice deficient in cannabinoid CB1 receptors, Neuropharmacology, 46, 7, 966–973.
P.1.c.029 A comparative pharmacokinetic study in healthy volunteers of the effect of carbamazepine and oxcarbazepine on CYP 3A4 A.H. Andreasen1 ° , K. Brøsen1 , P. Damkier2 . 1 University of Southern Denmark, Research Unit of Clinical Pharmacology, 5000 Odense C, Denmark; 2 Odense University Hospital, Department of Biochemistry, Pharmacology & genetics, 5000 Odense C, Denmark Purpose: Carbamazepine and oxcarbazepine are well-known inducers of drug metabolism via CYP3A4. There is some clinical experience suggesting that carbamazepine has a stronger potential for enzyme induction than oxcarbazepine. However this has never been documented in a head-to-head comparative study. Thus we performed a study in healthy volunteers to investigate the relative inductive effect of carbamazepine and oxcarbazepine, respectively, with the metabolism of quinidine as a marker for the CYP3A4 activity. Methods: Ten healthy, male volunteers participated in an open, cross-over, parallel-group study consisting of two periods separated by a 4-week wash-out period. They were randomised into group A and B; group A referring to 1200 mg oral oxcarbazepine daily for 17 days and group B to 800 mg oral carbamazepine for 17 days and vice versa in the 2nd period. A 200 mg oral quinidine full kinetics of plasma and urine was performed on day 17 in each period. Results: Formation clearance of 3-hydroxyquinidine was increased by 89% (CI: 1.36−2.64; p = 0.0022) and 181% (CI: 2.20−3.60, p < 0.0001) after treatment with oxcarbazepine and carbamazepine, respectively, compared to baseline. This inductive effect was also reflected in the AUCratio of 3-hydroxyquinidine to quinidine. T 12 and Cmax were reduced statistically significantly. tmax was not changed in either period. Conclusion: We confirm a clinically significant inductive effect of both oxcarbazepine and carbamazepine. The inductive effect of carbamazepine was about 50% higher than that of oxcarbazepine. P.1.c.030 Antidepressant treatment reduces fos-like immunoreactivity in different regions of periaqueductal gray matter R.M.W. Oliveira1 ° , C. Lino-de-Oliveira2 , A.P. Carobrez2 , T.C.M. De Lima2 , E.A. Del Bel3 , F.S. Guimar˜aes4 . 1 UEM, Department of Pharmacology, Maring´a, Brazil; 2 UFSC, Department of Pharmacology, Florian´opolis, Brazil; 3 FORP-USP, Department of Physiology, Ribeir˜ao Preto, Brazil; 4 FMRP-USP, Department of Pharmacology, Ribeir˜ao Preto, Brazil Purpose of the Study: Uncontrollable stress is proposed to play a role in the development of affective disorders in humans. In
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laboratory animals, the uncontrollable stress usually induces passive emotional coping characterized by conservation-withdrawal strategies. Most of the animal models of depression are based on behavioural changes induced by this kind of stress that are attenuated by antidepressant treatment. Although very much is known about the acute pharmacological effects of the antidepressant drugs (e.g., blockade of serotonin and/or noradrenaline re-uptake and/or destruction, blockade of their receptors, the mechanisms responsible for their behavioural effects in animal models remain controversial. In the brain, an extensive circuitry controls the behavioral changes induced by stress. Antidepressant drugs seem to act on that circuitry impairing behavioral consequences of stress. Functional studies suggest the periaqueductal gray matter (PAG) as a pivotal structure in the expression of emotional behavior, including stress response. Thus, the aim of this work was to study c-fos expression in the PAG of rats submitted to the forced swimming stress previously treated with antidepressant drugs. Methods: Male Wistar rats were submitted to 15 min of swimming and 24 h later to an additional 5 min swimming session. Rats received an antidepressant (Desipramine, DMI, 10 mg/Kg, n = 7; Clormipramine, CLM, 10 mg/kg, n = 7) or saline injection (SAL, n = 6) at 1, 9 and 24 hour before the 5 min swimming session and were sacrificed 2 hours after the second stress session. Brains were removed, sectioned and stained to detect Fos-like immunoreactivity (FLI). The dorsomedial (dmPAG), dorsolateral (dlPAG), lateral (lPAG) divisions were analyzed at the rostral (R), intermediate (I), and caudal (C) stereotaxic levels. The ventrolateral PAG (vlPAG) was analyzed only in the caudal portion of PAG. Data are expressed as the overall number (mean + SEM) of FLI recorded in each region. Statistical differences between each drug treatment (DMI or CLM) and saline were evaluated by the Mann-Whitney U test. Results: DMI significantly reduced (p < 0.05) FLI in dmPAG/R (SAL: 26.8+6.7×DMI: 6.8+1.8), dmPAG/C (SAL: 16.9+4.2×DMI: 2.8+0.8), dlPAG/I (SAL: 18+6.8×DMI: 2.9+0.8), lPAG/R (SAL: 19+5.8×DMI: 3+0.6), lPAG/C (SAL: 20.7+6.3× DMI: 2+0.5), and vlPAG (SAL: 17+7×DMI: 3+0.6). CLM injection significantly decreased (p < 0.05) FLI in dlPAG/I (SAL: 18+6.8×CLM: 3.2+1.4) and tended to do the same (p = 0.051) in the dmPAG/R (SAL: 26.8+6.7×CLM: 10+4.4) and lPAG/R (SAL: 19+5.8×CLM: 2.8+0.5). Conclusions: The present data indicate that, under swimming stress, antidepressant drugs inhibit different divisions of the PAG. The pattern of FLI in the PAG depends on the drug administered. However, reduced cellular activity in the intermediate portion of dlPAG along with reduction of activity in the rostral portions of dmPAG and lPAG may reflect a common neural substrate to antidepressant effects.
P.1.c.031 Effect of acute and chronic light/dark cycle alteration on seizure threshold in mice: modulation by melatonin N. Yahyavi-Firouz-Abadi ° , P. Tahsili-Fahadan, K. Riazi, A.R. Dehpour. Tehran University of Medical Sciences (TUMS), School of Medicine, Department of Pharmacology, Tehran, Islamic Republic of Iran Purposes: Epileptic mechanisms in the brain are subject to longduration, time-ordered neuromodulatory processes controlled by endogenous oscillators which are responsible for appropriately phased modulation of various normal physiological processes,
including the 24-h sleep/wakefulness cycle. Changes in seasonal and circadian rhythms have been shown to alter the anticonvulsant properties of various drugs. It is shown that pharmacological doses of exogenous melatonin are capable of synchronizing the circadian rhythms of locomotor activity and melatonin synthesis. The present study attempts to elucidate the effect of light/dark (LD) cycle alteration on pentylentetrazol (PTZ)-induced clonic seizure threshold in male NMRI mice. Methods: Experiment was designed in two acute and chronic alteration of LD cycle, each consisted three sets of animals. In acute experiment, 24h before clonic seizure threshold (CST) determination, subjects of the first group were maintained on 12h on 12h off LD cycle; while animals of the second and third sets experienced abrupt 6h phase shifts resulting in 6h on 18h off and 18h on 6h off LD cycles, respectively. CST was determined at zeitgeber time (ZT) = 4 of the next day. In chronic experiments, three distinct sets of mice were maintained on 12/12, 8/16 and 16/8 LD cycles for two weeks prior to testing. The effect of administration of melatonin as a modulator of cycle was also assessed using ip administration of melatonin (5, 10, 20 mg/kg). Clonic seizure threshold was determined by iv infusion of PTZ into tail vein of freely moving mice. Results: Acute change of 12/12 LD cycle to 16/8 LD cycle resulted in significant effect on seizure threshold [One-way ANOVA, F(2.17) = 23.129, p < 0.001]. Mice experienced 16/8 LD cycle had lower CST, while keeping animals under longer (16/8 LD) photoperiod did not produce a significant pro-convulsant effect (25.63±2.34 for 16/8, 32.72±2.37 for 8/16 LD, 32.41±1.67 for 12/12 LD cycle). This pro-convulsant effect of acute increase in light period was reversed by single i.p. administration of melatonin (10 mg/kg) at ZT = 10 of the first day. In chronic experiments, mice maintained on both shorter (6/18 LD) and longer (18/6 LD) photoperiods showed a significant decrease in seizure threshold compared to animals under 12/12 LD cycle [One-way ANOVA, F(2.17) = 14.20, p < 0.001] (26.57±2.14 for 16/8 LD, 26.69±4.33 for 8/16 LD, 34.52±1.80 for 12/12 LD cycle). Chronic i.p. administration of melatonin (10 mg/kg) at ZT = 10 in both photoperiods reversed the effect of LD cycle alteration on seizure threshold. Conclusions: The results obtained demonstrate that acute increase and chronic changes in the LD cycle, by either increasing or decreasing the photoperiod, are able to decrease the seizure threshold in mice. Moreover, the effect of LD cycle alteration could be reversed by administration of exogenous melatonin. The underlying mechanisms for the interaction of seizure susceptibility, circadian rhythms and melatonin and warrant further investigation.
P.1.c.032 Regulation of NA and GABA systems on DOI (agonist 5-HT2A/2C) anxiolytic-like effect in the four-plate test and cerebral localization of target receptors F. Masse ° , M. Hascoet, M. Bourin. Faculty of Medicine, Pharmacology, Nantes, France Introduction: The purpose of the present study was to analyse the possible interactions between the noradrenaline (NA)/serotonin (5-HT) systems and GABA/5-HT systems, in the mediation of the anxiolytic-like effect of DOI (5-HT2A/2C agonist) in the fourplate test (FPT) and to localize the cerebral structure where the DOI acts.
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[3] Sokolova S., Zhang C.L., Arens J., Khodorov B., Heinemann U., 1992, Effects of 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) on different patterns of epileptiform activity in rat temporal cortex slices, Neurosci. Res. Commun., 10, 171–176.
azepines. Furthermore, cannabinoid receptor antagonists may be considered useful to treat benzodiazepine dependence. Supported by Grant from Spanish Ministry of Health (FIS 03/216) to J. Manzanares.
P.1.c.028 The administration of the cannabinoid CB1 receptor antagonist AM-251 blocked the anxiolytic and sedative actions of benzodiazepines
References
D.V. Navarro, E. Sanguino ° , J. Manzanares. Universidad Miguel Hernandez-CSIC, Instituto de Neurociencias de Alicante, San Juan de Alicante, Spain A large body of evidence suggests that the cannabinoid CB1 receptor play a key role in the regulation of emotional behaviours. Indeed, we have previously shown that CB1 cannabinoid receptor KO mice displayed anxiety-like behaviours that are also associated to increased proopiomelanocortin gene expression in the anterior pituitary, decreased corticosterone secretion and hypersensitivity to restraint stress (Urig¨uen et al., 2004). In addition, a lack of anxiolytic action was observed in CB1 receptors KO mice after acute administration of bromazepam suggesting that this cannabinoid receptor may be involved in the therapeutic activity of benzodiazepines. The purpose of this study was to examine whether the blockade of the cannabinoid CB1 receptor may alter the anxiolytic and sedative actions of benzodiazepines. To this aim, the behavioural effects of the cannabinoid CB1 receptor antagonist AM-251 on the anxiolytic (light dark test, LDT) and sedative (Neurological Severity Score, NSS) actions of alprazolam (APZ) were evaluated. Male Swiss mice (n = 7−9/group) were divided into three experiments: (1) Dose response effects of alprazolam on the LDT, (2) Effects of AM-251 on the anxiolytic (LDT) or (3) sedative (NSS) actions of alprazolam. The acute administration of APZ (20, 40 or 80 mg/Kg or its water vehicle 0.3 ml/mouse, p.o, 30 min) produced a significant increase (p < 0.001) in the time that mice explored the light area at 40 mg/Kg but was without effect at 20 or 80 mg/Kg. These results provided information on the doses of APZ producing anxiolytic or sedative effects. In the second experiment, mice were divided into four groups: (1) Control group: Vehicle 1 (0.3 ml/mouse of water, p.o., 30 min) plus Vehicle 2 (DMSO, Tween 80 and saline in a 1:1:8 ratio, 0.3 ml/mouse, i.p., 60 min), (2) APZ (40 mg/kg, p.o, 30 min), (3) AM-251 (3 mg/kg, i.p., 60 min), (4) AM-251 plus APZ (same doses and times). The results revealed that administration of APZ produced a significant increase (68%) in the time that mice spent in the light area in the LDT compared to vehicle-treated mice. The administration of AM-251 by itself was without effects but completely blocked the anxiolytic actions of APZ (p < 0.001). In the third experiment mice were also divided in the same groups as the second experiment but the dose of APZ was much higher (0.5 mg/kg, p.o, 30 min). The effects of AM-251 and APZ on sedation were examined using the NSS test. This test evaluated the motor coordination of mice crossing bars of different width (2 cm, 1 cm, 0.5 cm). The time to cross the bars was recorded. The results revealed that administration of APZ produced incapacity to walk on 2 cm, 1 cm and 0.5 cm bar in the NSS test (p < 0.001), however, previous administration of AM-251 blocked the motor coordination induced by APZ (p < 0.001). Taken together these results suggest that CB1 receptors play a pivotal role in the anxiolytic and sedative actions of benzodi-
[1] Urig¨uen L., Perez-Rial S., Ledent C.L., Palomo T., Manzanares J., 2004, Impaired action of anxiolytics in mice deficient in cannabinoid CB1 receptors, Neuropharmacology, 46, 7, 966–973.
P.1.c.029 A comparative pharmacokinetic study in healthy volunteers of the effect of carbamazepine and oxcarbazepine on CYP 3A4 A.H. Andreasen1 ° , K. Brøsen1 , P. Damkier2 . 1 University of Southern Denmark, Research Unit of Clinical Pharmacology, 5000 Odense C, Denmark; 2 Odense University Hospital, Department of Biochemistry, Pharmacology & genetics, 5000 Odense C, Denmark Purpose: Carbamazepine and oxcarbazepine are well-known inducers of drug metabolism via CYP3A4. There is some clinical experience suggesting that carbamazepine has a stronger potential for enzyme induction than oxcarbazepine. However this has never been documented in a head-to-head comparative study. Thus we performed a study in healthy volunteers to investigate the relative inductive effect of carbamazepine and oxcarbazepine, respectively, with the metabolism of quinidine as a marker for the CYP3A4 activity. Methods: Ten healthy, male volunteers participated in an open, cross-over, parallel-group study consisting of two periods separated by a 4-week wash-out period. They were randomised into group A and B; group A referring to 1200 mg oral oxcarbazepine daily for 17 days and group B to 800 mg oral carbamazepine for 17 days and vice versa in the 2nd period. A 200 mg oral quinidine full kinetics of plasma and urine was performed on day 17 in each period. Results: Formation clearance of 3-hydroxyquinidine was increased by 89% (CI: 1.36−2.64; p = 0.0022) and 181% (CI: 2.20−3.60, p < 0.0001) after treatment with oxcarbazepine and carbamazepine, respectively, compared to baseline. This inductive effect was also reflected in the AUCratio of 3-hydroxyquinidine to quinidine. T 12 and Cmax were reduced statistically significantly. tmax was not changed in either period. Conclusion: We confirm a clinically significant inductive effect of both oxcarbazepine and carbamazepine. The inductive effect of carbamazepine was about 50% higher than that of oxcarbazepine. P.1.c.030 Antidepressant treatment reduces fos-like immunoreactivity in different regions of periaqueductal gray matter R.M.W. Oliveira1 ° , C. Lino-de-Oliveira2 , A.P. Carobrez2 , T.C.M. De Lima2 , E.A. Del Bel3 , F.S. Guimar˜aes4 . 1 UEM, Department of Pharmacology, Maring´a, Brazil; 2 UFSC, Department of Pharmacology, Florian´opolis, Brazil; 3 FORP-USP, Department of Physiology, Ribeir˜ao Preto, Brazil; 4 FMRP-USP, Department of Pharmacology, Ribeir˜ao Preto, Brazil Purpose of the Study: Uncontrollable stress is proposed to play a role in the development of affective disorders in humans. In
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laboratory animals, the uncontrollable stress usually induces passive emotional coping characterized by conservation-withdrawal strategies. Most of the animal models of depression are based on behavioural changes induced by this kind of stress that are attenuated by antidepressant treatment. Although very much is known about the acute pharmacological effects of the antidepressant drugs (e.g., blockade of serotonin and/or noradrenaline re-uptake and/or destruction, blockade of their receptors, the mechanisms responsible for their behavioural effects in animal models remain controversial. In the brain, an extensive circuitry controls the behavioral changes induced by stress. Antidepressant drugs seem to act on that circuitry impairing behavioral consequences of stress. Functional studies suggest the periaqueductal gray matter (PAG) as a pivotal structure in the expression of emotional behavior, including stress response. Thus, the aim of this work was to study c-fos expression in the PAG of rats submitted to the forced swimming stress previously treated with antidepressant drugs. Methods: Male Wistar rats were submitted to 15 min of swimming and 24 h later to an additional 5 min swimming session. Rats received an antidepressant (Desipramine, DMI, 10 mg/Kg, n = 7; Clormipramine, CLM, 10 mg/kg, n = 7) or saline injection (SAL, n = 6) at 1, 9 and 24 hour before the 5 min swimming session and were sacrificed 2 hours after the second stress session. Brains were removed, sectioned and stained to detect Fos-like immunoreactivity (FLI). The dorsomedial (dmPAG), dorsolateral (dlPAG), lateral (lPAG) divisions were analyzed at the rostral (R), intermediate (I), and caudal (C) stereotaxic levels. The ventrolateral PAG (vlPAG) was analyzed only in the caudal portion of PAG. Data are expressed as the overall number (mean + SEM) of FLI recorded in each region. Statistical differences between each drug treatment (DMI or CLM) and saline were evaluated by the Mann-Whitney U test. Results: DMI significantly reduced (p < 0.05) FLI in dmPAG/R (SAL: 26.8+6.7×DMI: 6.8+1.8), dmPAG/C (SAL: 16.9+4.2×DMI: 2.8+0.8), dlPAG/I (SAL: 18+6.8×DMI: 2.9+0.8), lPAG/R (SAL: 19+5.8×DMI: 3+0.6), lPAG/C (SAL: 20.7+6.3× DMI: 2+0.5), and vlPAG (SAL: 17+7×DMI: 3+0.6). CLM injection significantly decreased (p < 0.05) FLI in dlPAG/I (SAL: 18+6.8×CLM: 3.2+1.4) and tended to do the same (p = 0.051) in the dmPAG/R (SAL: 26.8+6.7×CLM: 10+4.4) and lPAG/R (SAL: 19+5.8×CLM: 2.8+0.5). Conclusions: The present data indicate that, under swimming stress, antidepressant drugs inhibit different divisions of the PAG. The pattern of FLI in the PAG depends on the drug administered. However, reduced cellular activity in the intermediate portion of dlPAG along with reduction of activity in the rostral portions of dmPAG and lPAG may reflect a common neural substrate to antidepressant effects.
P.1.c.031 Effect of acute and chronic light/dark cycle alteration on seizure threshold in mice: modulation by melatonin N. Yahyavi-Firouz-Abadi ° , P. Tahsili-Fahadan, K. Riazi, A.R. Dehpour. Tehran University of Medical Sciences (TUMS), School of Medicine, Department of Pharmacology, Tehran, Islamic Republic of Iran Purposes: Epileptic mechanisms in the brain are subject to longduration, time-ordered neuromodulatory processes controlled by endogenous oscillators which are responsible for appropriately phased modulation of various normal physiological processes,
including the 24-h sleep/wakefulness cycle. Changes in seasonal and circadian rhythms have been shown to alter the anticonvulsant properties of various drugs. It is shown that pharmacological doses of exogenous melatonin are capable of synchronizing the circadian rhythms of locomotor activity and melatonin synthesis. The present study attempts to elucidate the effect of light/dark (LD) cycle alteration on pentylentetrazol (PTZ)-induced clonic seizure threshold in male NMRI mice. Methods: Experiment was designed in two acute and chronic alteration of LD cycle, each consisted three sets of animals. In acute experiment, 24h before clonic seizure threshold (CST) determination, subjects of the first group were maintained on 12h on 12h off LD cycle; while animals of the second and third sets experienced abrupt 6h phase shifts resulting in 6h on 18h off and 18h on 6h off LD cycles, respectively. CST was determined at zeitgeber time (ZT) = 4 of the next day. In chronic experiments, three distinct sets of mice were maintained on 12/12, 8/16 and 16/8 LD cycles for two weeks prior to testing. The effect of administration of melatonin as a modulator of cycle was also assessed using ip administration of melatonin (5, 10, 20 mg/kg). Clonic seizure threshold was determined by iv infusion of PTZ into tail vein of freely moving mice. Results: Acute change of 12/12 LD cycle to 16/8 LD cycle resulted in significant effect on seizure threshold [One-way ANOVA, F(2.17) = 23.129, p < 0.001]. Mice experienced 16/8 LD cycle had lower CST, while keeping animals under longer (16/8 LD) photoperiod did not produce a significant pro-convulsant effect (25.63±2.34 for 16/8, 32.72±2.37 for 8/16 LD, 32.41±1.67 for 12/12 LD cycle). This pro-convulsant effect of acute increase in light period was reversed by single i.p. administration of melatonin (10 mg/kg) at ZT = 10 of the first day. In chronic experiments, mice maintained on both shorter (6/18 LD) and longer (18/6 LD) photoperiods showed a significant decrease in seizure threshold compared to animals under 12/12 LD cycle [One-way ANOVA, F(2.17) = 14.20, p < 0.001] (26.57±2.14 for 16/8 LD, 26.69±4.33 for 8/16 LD, 34.52±1.80 for 12/12 LD cycle). Chronic i.p. administration of melatonin (10 mg/kg) at ZT = 10 in both photoperiods reversed the effect of LD cycle alteration on seizure threshold. Conclusions: The results obtained demonstrate that acute increase and chronic changes in the LD cycle, by either increasing or decreasing the photoperiod, are able to decrease the seizure threshold in mice. Moreover, the effect of LD cycle alteration could be reversed by administration of exogenous melatonin. The underlying mechanisms for the interaction of seizure susceptibility, circadian rhythms and melatonin and warrant further investigation.
P.1.c.032 Regulation of NA and GABA systems on DOI (agonist 5-HT2A/2C) anxiolytic-like effect in the four-plate test and cerebral localization of target receptors F. Masse ° , M. Hascoet, M. Bourin. Faculty of Medicine, Pharmacology, Nantes, France Introduction: The purpose of the present study was to analyse the possible interactions between the noradrenaline (NA)/serotonin (5-HT) systems and GABA/5-HT systems, in the mediation of the anxiolytic-like effect of DOI (5-HT2A/2C agonist) in the fourplate test (FPT) and to localize the cerebral structure where the DOI acts.