N 6-Cyclohexyladenosine and 3-( 2-Carboxypiperazine-4-yl )-1-propenyl-1-phosphonic acid enhance the effect of antiepileptic drugs against induced seizures in mice

Purpose: The influence of N6-Cyclohexyladenosine (CHA), an adenosine A1 agonist and 3-(2Carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid (CPPene), a selective N-methyl-D-aspartate (NMDA) antagonist upon the anticonvulsant activity of diazepam (DA), sodium valproate (VP), diphenylhydantoin (DPH), phenobarbital (PB) and carbamazepine (CAZ) was investigated in mice. All agents were administered intraperitoneally. Methods: Convulsive seizures were induced by the use of electro shocks and pentylenetetrazole (PTZ). Results: CHA (2 mg/kg, i.p.) and CPPene (2.5 mg/kg, i.p.) were found to enhance the anticonvulsant activity of the tested antiepileptic drugs against both electro convulsions and PTZ-induced convulsions. Both CHA and CPPene significantly decreased the ED50 values of these drugs against both electro convulsions and PTZ-induced convulsions, and increased the convulsive threshold. CHA (2 mg/kg, i.p.) and CPPene (2.5 mg/kg, i.p.) did not affect the plasma level of any of the tested antiepileptic drugs, indicating no pharmacokinetic interactions at the systemic administration. CHA (2 mg/kg, i.p.) or CPPene (2.5 mg/kg, i.p.), alone or in combination with the tested antiepileptic drugs produced no significant changes in their effects on the heart rate, blood pressure, body temperature, gross behavior or on the locomotor activity of experimental animals. Combinations of the antiepileptic drugs with CHA (2 mg/kg, i.p.) or CPPene (2.5 mg/kg, i.p.) were also devoid of significant effects on the motor performance and long-term memory in mice demonstrated by the Chimney test and passive avoidance task. CHA (5 mg/kg, i.p.) alone or in combination with the tested antiepileptic drugs produced inhibition of locomotor activity and motor coordination, sedation and hypothermia as well as impairing of long-term memory. Conclusion: Adenosine A1 agonists and NMDA antagonists enhance the efficacy of common antiepileptic drugs, indicating the involvement of adenosine and NMDA receptors in the convulsive pathway. The potential therapeutic benefits of such interactions may be taken into consideration and merit further investigations in animals and humans. INTRODUCTION Epilepsies are common disorder with an overall point prevalence of 4-10 per 1000 individuals (1). Therapy is symptomatic in that available drugs inhibit seizures but neither effective prophylaxis nor cure is available. Compliance with medication is a problem because of the need for long-term therapy together with many unwanted effects. Many patients remain seizure-free on prophylactic drug therapy, however, existing antiepileptic drugs have considerable potential for concentration-dependent and idiosyncratic toxicity (2). Their use in combination is plagued by pharmacokinetic and pharmacodynamic interactions, which result in a decrease of their therapeutic benefit. Thus new compounds and combinations are urgently needed. During the past decade, the nucleoside, adenosine, has become widely accepted as having a neuromodulatory role in both the peripheral and central nervous system primarily by actions resulting in reduction of neuronal excitability (3,4). Early studies suggest that the extracellular adenosine, derived in part from the breakdown of adenosine triphosphate (ATP), may play an important role throughout the brain as an endogenous anticonvulsant, serving to reduce the severity of seizures or to prevent their recurrences (5-7). Systemic administration or intracranial injections (8-15) of adenosine analogs have been shown to inhibit the expression of seizures in animals. Adenosine antagonists, such as theophylline or caffeine produce the opposite effect Corresponding Author: Abdel-Azim Assi, Department of Pharmacology, Faculty of Medicine, Assiut University, Assiut, Egypt, assi@wpo.nccu. edu (16-20). Moreover, rapid elevations in brain levels of adenosine have been documented after experimental seizures (21) and post-seizures in epileptic patients (4). In order to explore any possible interaction between the commonly used antiepileptic drugs and adenosine analogues, the present investigation was carried out. N6-Cyclohexyladenosine, (CHA) an adenosine A1 agonist, was selected because many studies have reported that the anticonvulsant action of adenosine and its analogues is predominantly mediated via an interaction with adenosine receptors of the A1 subtype (5, 9, 10). The role of glutamate receptors in the modulation of seizure activity has been intensively investigated and NMDA and AMPA receptor antagonists have been reported to be powerful anticonvulsants in a wide range of animal models of epilepsy (22-29). Because NMDA receptors are involved in both seizure initiation and propagation in the CNS, it is also of interest to investigate the role of such agents in epilepsy and their influence on the anticonvulsant effect of antiepileptic drugs. In the present study, the influence of co-administration of CHA and 3-(2-Carboxypiperazine-4-yl)-1-propenyl1-phosphonic acid (CPPene) with each of sodium valproate, diazepam, diphenylhydantoin, phenobarbital and carbamazepine on their effectiveness against seizures induced by electric shock and pentylenetetrazole as well as on their blood concentrations were investigated. Moreover, the effect of CHA, CPPene and their combinations on some physiological and behavioral parameters was evaluated.