The effect of calcium removal on the suppression by adenosine of epileptiform activity in the hippocampus: demonstration of desensitization.

1. Previous work has suggested that presynaptic effects of adenosine may be dependent on divalent cations. The present study was undertaken to determine whether a similar requirement existed at postsynaptic sites. 2. Extracellular recordings were made in the CA1 pyramidal cell layer of rat hippocampal slices following orthodromic stimulation of Schaffer collateral fibres in stratum radiatum or antidromic stimulation of the alveus. In antidromic stimulation experiments, CaCl2 was omitted (calcium-free medium) or reduced to 0.24 mM (low calcium medium) and in some experiments MgSO4 was increased to 2 mM. Kynurenic acid at concentrations of 1 and 5 mM in calcium-free medium and 1 mM in low calcium medium had no effect on secondary spike size. 3. Adenosine and baclofen induced a concentration-dependent reduction in the amplitude of orthodromic potentials with maximum effects at 20 and 5 microM respectively. 4. In nominally calcium-free medium, bursts of multiple population spikes were obtained in response to antidromic stimulation. Adenosine had little effect in reducing the secondary spike amplitude. At high concentration (2 mM) an initial depression was seen which declined within 3-5 min. 5. Sensitivity to adenosine was restored in low calcium medium or by raising magnesium. Although raising the divalent cation concentration increased the inhibitory effect of adenosine, desensitization was still seen. 6. 2-Chloroadenosine (100-500 microM) and R-PIA (50 microM), which are not substrates for either the nucleoside transporters or adenosine deaminase, were inactive in the absence of calcium. S-(2-hydroxy-5 nitrobenzyl)-6-thioinosine, an adenosine uptake blocker, at a concentration 100 MicroM had no effect on secondary potential size and did not restore adenosine sensitivity in calcium-free medium.7. Thapsigargin, which discharges intracellular calcium stores, had no significant effect at 1 MicroM on the bursts of action potentials and did not change the effect of 0.5 mM adenosine in calcium-free medium.8. Unlike adenosine, baclofen concentration-dependently reduced the secondary spike size in calcium free medium and no sign of recovery was observed during maintained superfusion for up to 45 min. No cross-desensitization was seen between baclofen and adenosine.9. Applications of adenosine locally by pressure to neuronal somata or dendrites still resulted in desensitized responses in calcium-free medium.10. It is concluded that the postsynaptic sensitivity to adenosine is dependent on the concentration of divalent cations in the extracellular space implying an effect of cations on adenosine receptor activation or transduction processes.