Action of serotonin on the hyperpolarization-activated cation current (I-h) in rat CA1 hippocampal neurons

We studied the effects of serotonin (5-HT) on hippocampal CA1 pyramidal neu rons. In current-clamp mode, 5-HT induced a hyperpolarization and reduction of excitability due to the opening of inward rectifier K+ channels, follow ed by a late depolarization and partial restoration of excitability. These two components could be dissociated, as in the presence of BaCl2 to block K + channels, 5-HT induced a depolarization accompanied by a reduction of mem brane resistance, whereas in the presence of ZD 7288 [4-(N-ethyl-N-phenylam ino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride], a selective block er of the hyperpolarization-activated cation current (I-h), 5-HT only hyper polarized neurons. We then studied the action of 5-HT on Ih in voltage-clam p conditions. 5-HT increased I-h at -90 mV by 29.1 +/- 2.9% and decreased t he time constant of activation by 20.1 +/- 1.7% (n = 16), suggesting a shif t in the voltage dependence of the current towards more positive potentials ; however, the fully activated current measured at -140 mV also increased ( by 14.1 +/- 1.7%, n = 14); this increase was blocked by ZD 7288, implying a n effect of 5-HT on the maximal conductance of I-h. Both the shift of activ ation curve and the increase in maximal conductance were confirmed by data obtained with ramp protocols. Perfusion with the membrane-permeable analogu e of cAMP, 8-bromoadenosine 3'5'-cyclic monophosphate (8-Br-cAMP), increase d I-h both at -90 and -140 mV, although the changes induced were smaller th an those due to 5-HT. Our data indicate that 5-HT modulates I-h by shifting its activation curve to more positive voltages and by increasing its maxim al conductance, and that this action is likely to contribute to the 5-HT mo dulation of excitability of CA1 cells.