Properties of the fast sodium channels in pyramidal neurones isolated from the CA1 and CA3 areas of the hippocampus of postnatal rats.

The fast sodium (Na)current of hippocampal neurones was recorded using the intracellular perfusion method. Neurones were isolated from the CA1 and CA3 hippocampal subregions separately, after treatment of the tissue with trypsin. There were no differences between the current-voltage (I-V) characteristics of CA1 and CA3 neurones. In contrast to this, the steady-state inactivation (h infinity) of both types of neurones was significantly different. Additionally, there were two subpopulations of CA1 neurones, which showed different courses of h infinity. Compared with CA1 neurones, the steady-state activation and inactivation curves of CA3 neurones overlapped much more in the potential range -80 mV to -50 mV. These results are consistent with the well-known fact that CA3 neurones show spontaneous burst activity, while CA1 cells do not. The time constant of activation (tau m) depended upon the membrane potential in the same way for all CA3 neurones investigated. However, there were two different subpopulations of CA3 cells, showing different voltage dependence of the time constant of inactivation. We conclude that these differences reflect two types of pyramidal cells within the same subregion.