Modulatory effects of nitric oxide-active drugs on the anticonvulsant activity of lamotrigine in an experimental model of partial complex epilepsy in the rat

Abstract

Background: The effects induced by administering the anticonvulsant lamotrigine, the preferential inhibitor of neuronal nitric oxide synthase 7-nitroindazole and the precursor of NO synthesis L-arginine, alone or in combination, on an experimental model of partial complex seizures (maximal dentate gyrus activation) were studied in urethane anaesthetized rats. The epileptic activity of the dentate gyrus was obtained through the repetitive stimulation of the angular bundle and maximal dentate gyrus activation latency, duration and post-stimulus afterdischarge duration were evaluated.

Results: Either Lamotrigine (10 mg kg-1) or 7-nitroindazole (75 mg kg-1) i.p. administration had an anticonvulsant effect, significantly reducing the number of animals responding to angular bundle stimulation. On the contrary, i.p. injection of L-arginine (1 g kg-1) induced an aggravation of the epileptiform phenomena, demonstrated by the significant augmentation of the duration of both maximal dentate activation and afterdischarge. Furthermore, the injection of lamotrigine and 7-nitroindazole in combination significantly increased the anticonvulsant effects induced by the same drugs separately, either reducing the number of responding animals or decreasing both maximal dentate gyrus activation and afterdischarge durations. On the contrary, the combined treatment with L-arginine and lamotrigine did not modify the maximal dentate gyrus activation parameters suggesting an adversative effect of L-arginine-increased nitric oxide levels on the lamotrigine-induced anticonvulsant action.

Conclusion: The present results indicate that the nitrergic neurotransmission exerts a significant modulatory role in the control of the development of paroxystic phenomena in the maximal dentate gyrus activation model of epilepsy. Finally, our data suggest a functional relationship between the nitric oxide system and the anticonvulsant effect of lamotrigine which could be enhanced by reducing nitric oxide levels and, conversely, dampened by an increased nitrergic activity.

Figure 1

Maximal dentate gyrus activation and evoked population spike parameters. A: Measurements of latency and duration of maximal dentate gyrus activation (MDA) and afterdischarge (AD) during and after a 400 μA, 20 Hz stimulus train of the angular bundle (AB) for 10 sec. The recordings were amplified using low level DC (A) and a wide band AC (B) pre-amplifiers respectively. B: An average of five evoked responses in the dentate gyrus to the stimulation of the ipsilateral angular bundle. The amplitude of the population spikes (PS amp) was measured as illustrated in the figure, considering the mid-point between the dashed lines as the top of the PS. Calibration are indicated on the figure.

Figure 2

Effects of 7-Nitroindazole, L-arginine and Lamotrigine administration alone and in combination on the number of responding animals. The bars indicate the number of animals responding with a typical MDA activity to the angular bundle stimulation after each drug treatment. In abscissa the progressive number of train of stimuli applied every 10 min is indicated. The chi-square test was used to compare the animals responding and not responding to the electrical stimulation. The differences were considered marginally and highly significant at the level of P < 0.05 (*) and P < 0.005 (**) respectively.

Figure 3

Effects of 7-Nitroindazole, L-arginine and Lamotrigine administration alone and in combination on MDA latency. Effects of 7-Nitroindazole (75 mg kg^-1 i.p.), L-arginine (1 g kg^-1 i.p.) and Lamotrigine (10 mg kg^-1 i.p.) administration alone and in combination on MDA latency (n = 10 rats for each treatment). In abscissa the progressive number of train of stimuli applied every 10 min is indicated. Asterisks between lines (-*-) indicate a significant difference between the two treatments (P < 0.05).

Figure 4

Effects of 7-Nitroindazole, L-arginine and Lamotrigine administration alone and in combination on MDA duration. Effects of 7-Nitroindazole (75 mg kg^-1 i.p.), L-arginine (1 g kg^-1 i.p.) and Lamotrigine (10 mg kg^-1 i.p.) administration alone and in combination on MDA duration (n = 10 rats for each treatment). In abscissa the progressive number of train of stimuli applied every 10 min is indicated. Asterisks along graph lines (*) indicate a significant difference versus control values (P < 0.05). Asterisks between lines (-*-) indicate a significant difference between the two treatments (P < 0.05).

Figure 5

Effects of 7-Nitroindazole, L-arginine and Lamotrigine administration alone and in combination on AD duration. Effects of 7-Nitroindazole (75 mg kg^-1 i.p.), L-arginine (1 g kg^-1 i.p.) and Lamotrigine (10 mg kg^-1 i.p.) administration alone and in combination on AD duration (n = 10 rats for each treatment). In abscissa the progressive number of train of stimuli applied every 10 min is indicated. Asterisks along graph lines (*) indicate a significant difference versus control values (P < 0.05). Asterisks between lines (-*-) indicate a significant difference between the two treatments (P < 0.05).