Differential effects of elevated extracellular calcium concentrations on field potentials in dentate gyrus and CA1 of the rat hippocampal slice preparation.

The effect of a short-lasting elevation of the extracellular Ca2+ concentration (from 2 to 6 mM) on the field potentials in CA1 of the rat hippocampal slice was an increase of both the CA1 population spike (PS) and the excitatory postsynaptic potential (EPSP); this effect persisted after returning to 2 mM Ca2+ Ringer, and thus can be considered as a Ca2+-induced long-term potentiation (LTP). In the dentate gyrus (DG) a quite different effect was encountered; here the PS decreased and the EPSP increased only slightly during the perfusion with 6 mM Ca2+, and no reproducible long-term effect was induced. The results indicate that substantial differences exist in the balance between inhibitory and excitatory processes in the neural networks of the two hippocampal subregions; these differences are enhanced during perfusion with high Ca2+, which induces LTP in CA1 but not in the DG.