Electrophysiological properties of dentate granule cells in mouse hippocampal slices maintained in vitro

Intracellular recordings were obtained from granule cells of the dentate gyrus in mouse hippocampal slices maintained in vitro. All the spikes observed in standard Krebs solution had a short duration and were tetrodotoxin (TTX)-sensitive. When elicited by synaptic activation or by direct electrical stimulation of the cells, these fast sodium spikes were followed by a brief spike afterhyperpolarization. In contrast, antidromic spikes elicited by electrical stimulation of the hilum as well as spikes arising at the end of hyperpolarizing current pulses passed through the recording microelectrode were followed by depolarizing afterpotentials (DAPs). These DAPs were reversed into brief spike afterhyperpolarizations by depolarization of the cells. After substitution of calcium (Ca) by barium (Ba) or after introduction of tetraethylammonium (TEA) in the bath, the fast spike repolarization became slower and the brief spike afterhyperpolarizations were abolished, suggesting that they involved fast K conductances. Slow spikes and long-lasting depolarizations were also elicited in granule cells in the presence of Ba or TEA. Since these slow events were left unaffected by TTX and were selectively abolished by the Ca channel blockers cobalt or cadmium, they are likely to be mediated by voltage-dependent Ca conductances, unmasked by the reduction of the fast K conductances.