Hyperpolarization-activated inward currents contribute to spontaneous electrical activity and CO2/H+ sensitivity of cultivated neurons of fetal rat medulla

Neurons growing out from cultivated fetal medullary slices that exhibited spontaneous electrical activity after blockade of synaptic transmission were investigated by the patch-clamp technique for their response to decreases in the extracellular pH. Increases in the [H+] induced by increases in PCO2, resulted in a decrease in spike frequency associated with a decrease in the rate of depolarization preceeding each action potential. The type of ion channel, contributing to interspike depolarization, and which may therefore be the site of CO2/H+ action, was identified by application of agents that inhibited the hyperpolarization-activated cation, I-H, channel (Cs+ and ZD7288). Application of Cs+ and ZD7288 slightly hyperpolarized the cell membrane, decreased the interspike slope and inhibited CO2/H+-induced modulations of spike frequency in one group of CO2-inhibited medullary neurons, suggesting that I-H contributes to spontaneous neuronal activity and to CO2/H+-sensitivity. CO2/H+ effects on I-H were further confirmed in voltage-clamp experiments. Increasing the bath CO2 from 2% to 9% reduced the I-H amplitude, shifted the mean E-H from -54 to -60 mV, lengthened the voltage-dependent delay of current activation and increased the time-constants of activation at all potentials studied. It is concluded that depolarizing inward currents through I-H channels participate in the gradual ramp-like change in membrane potential which depolarizes the cell up to the threshold of Na+ spike generation. CO2/H+-induced inhibition of I-H reduces the contribution of this ion current to the interspike depolarization and accounts for the CO2/H+-induced decrease in spike frequency in one type of CO2/H+-inhibited medullary cells. (C) 1998 IBRO. Published by Elsevier Science Ltd.