The purine nucleoside adenosine is released during seizure activity and exerts an anticonvulsant influence through inhibition of glutamate release and hyperpolarization of neurons via adenosine A(1) receptors. However, activation of adenosine A(2A) and A(3) receptors may counteract the inhibitory effects of A(1) receptors. We have therefore examined the extent to which endogenous adenosine released during seizure activity activates the different adenosine receptor subtypes and the implications for seizure activity in the rat hippocampus in vitro. Brief trains of high-frequency stimulation in nominally Mg(2+)-free artificial cerebrospinal fluid evoked epileptiform activity and resulted in a transient depression of the simultaneously recorded CA1 field excitatory postsynaptic potential. In the presence of 8-cyclopentyl-1,3-dimethylxanthine (CPT), an adenosine A(1) receptor antagonist, the occurrence of spontaneous seizure activity was greatly increased as was the duration and intensity of evoked seizures, whilst the postictal depression of basal synaptic transmission was greatly attenuated. Application of ZM 241385, an adenosine A(2A) receptor antagonist, shortened the duration of epileptiform activity, whereas administration of MRS 1191, an adenosine A(3) receptor antagonist, both decreased the duration and intensity of seizures. Combined application of the A(2A) and A(3) receptor antagonists also resulted in a reduction in seizure duration and intensity. However, no evidence was found for a role for protein kinase C in the regulation of seizure activity by endogenous adenosine. Our data confirm the dominant anticonvulsant role that endogenous and tonic adenosine play via the A(1) receptor, and suggest that the additional adenosine receptor subtypes may compromise this anticonvulsant property through promotion of seizure activity.