Adenosine is released during seizure activity where it has been described as an endogenous anticonvulsant through its ability to inhibit glutamate release via presynaptic adenosine A1 receptors. In contrast, activation of adenosine A2A receptors facilitates glutamate release. We have investigated the relative contributions of adenosine A1 and A2A receptors to seizure activity in the CA1 region of the rat.
Hippocampal slices, 600 mm thick, were prepared from rat pups of either sex aged between 17 and 22 days. Rats were killed by cervical dislocation in accordance with Schedule 1 of the UK Animals (Scientific Procedures) Act, 1986. Slices were maintained in nominally Mg2+-free ACSF, bubbled with 95 % O2 and 5 % CO2. Field excitatory postsynaptic potentials (fEPSPs) were recorded from area CA1 with an ACSF-filled glass microelectrode in response to stimulation of stratum radiatum at 15 s intervals. Seizures were induced at 10 min intervals using a 2 s 60 Hz train of stimuli. The A1 receptor antagonist CPT (1 mM) and the A2A receptor antagonist ZM241385 (100 nM) were applied via the bath solution for at least 10 min before the first train was given. Experiments were performed at 33Ð34°C. Data are presented as means ± S.E.M. and n = the number of slices.
Up to six trains of stimuli were given to each slice. Under control conditions, the mean duration of the first three seizures elicited by the trains (28 ± 4 s) did not differ (paired t test; P > 0.05; n = 6) from the mean duration of the second three train-induced seizures (28 ± 5 s). Seizure activity depressed the concurrently recorded fEPSP by 62 ± 7 %. In contrast, the mean duration of the seizures elicited in the presence of CPT (37 ± 8 s; n = 7) was significantly greater (P < 0.02) than the corresponding control seizures (20 ± 4 s). CPT also greatly attenuated (P < 0.001) the seizure-induced depression of the fEPSP (24 ± 3 % depression) compared with controls (57 ± 5 % depression). Furthermore, the occurrence of spontaneous seizures was greater in CPT-treated slices than controls (73 vs. 22 % of slices, respectively). ZM241385 slightly reduced the duration of seizure activity (23 ± 4 s; P < 0.01; n = 11) compared with corresponding controls (28 ± 4 s) but had no appreciable influence on the seizure-induced depression of the fEPSP (control 75 ± 3% ZM241385 71 ± 5 %).
These results confirm the powerful role of endogenous adenosine, acting via A1 receptors, in the termination of epileptiform activity in area CA1 of the hippocampus. Our data also show a small contribution to the promotion of seizure activity by adenosine A2A receptors. This reduced role may reflect the lower numbers of A2A receptors in area CA1.
L.V.D. is supported by the MRC.
All procedures accord with current UK legislation.