European Journal of Pharmacology, 82 (1982) 195-197 Elsevier Biomedical Press Short communication A PURINERGIC COMPONENT IN THE ANTICONVULSANT ACTION OF CARBAMAZEPINE? JOHN H. SKERRITT, LES P. DAVIES and GRAHAM A.R. JOHNSTON * Department of Pharmacology', University of Sydney, NSW, 2006, Australia Received 9 June 1982, accepted 5 July 1982 195 J.H. SKERITT, L.P. DAVIES and G.A.R. JOHNSTON, A purinergic component in the anticonvulsant action of carbamazepine?, European J. Pharmacol. 82 (1982) 195-197. The tricyclic anticonvulsant carbamazepine inhibited the binding of [3H]phenylisopropyladenosine (PIA) to rat brain membranes with a K i value of 44 #M, which is in the range of therapeutic levels of this drug in brain and serum. Other anticonvulsants and tricyclic antidepressants had little effect on PIA binding. Carbamazepine weakly inhibited adenosine uptake into rat brain slices at 400 ~M. An effect on adenosine receptors may be an important component of the anticonvulsant action of carbamazepine. Carbamazepine Adenosine receptors Anticonvulsant action 1. Introduction Evidence is accumulating to suggest a role for brain adenosine in seizures and seizure protection. Adenosine and related purines protect against audiogenic and pentylenetetrazole induced seizures in mice (Maitre et al., 1974), while an increase in brain levels of adenosine has been observed fol- lowing seizure induction (Winn et al., 1980). Re- cently, binding sites for radiolabelled metaboli- cally stable adenosine analogues, such as N 6- cyclohexyladenosine, N 6-phenylisopropyladeno - sine (PIA) and 2-chloroadenosine have been char- acterised in mammalian brain (Schwabe, 1981). Rapid changes in adenosine receptor density in brain have been reported following chemically in- duced seizures (Wybenga et al., 1981). We have investigated the in vitro effects of some major anticonvulsant drugs upon adenosine receptor binding and uptake, and report that the tricyclic anticonvulsant, carbamazepine displaces PIA binding at concentrations relevant to the ther- apeutic levels of this drug. * To whom all correspondence should be addressed. 0014-2999/82/0000-0000/$02.75 ~ 1982 Elsevier Biomedical Press 2. Materials and methods Binding studies were carried out on washed synaptosomal membranes prepared from whole rat brain as earlier described (Skerritt et al., 1982). The membranes were stored frozen for 1-14 days. On the day of use, they were thawed, centrifuged (48000 × g, 20 min), resuspended in distilled water and recentrifuged before final suspension in 50 mM Tris-HC1 pH 7.1 buffer containing 1 mM MgC12. Prior to assay, the membranes were in- cubated (10 min, 37°C) with adenosine deaminase (1 #g/ml) to remove endogenous adenosine. The membranes (0.6-1.0 mg protein) were incubated with 5.5 nM [3H]PIA (Amersham International, specific activity 20 Ci/mmol) in a total vol of 1 ml, in the presence or absence of various drugs for 20 min at 37°C. Nonsaturable binding was de- termined using 100 ~M unlabelled PIA, and in- cubations were terminated by filtration (Whatman GF/B filters). Tubes and filters were rapidly washed with 7.5 ml ice-cold incubation buffer. Scatchard analysis of binding data was performed using six concentrations of PIA (0.625-20 nM). Adenosine uptake was performed using prisms (approx. 0.1 × 0.1 × 1 mm) of rat cerebral cortex