Contribution of the hyperpolarization-activated current to the resting membrane potential of rat nodose sensory neurons

1. The voltage- and time-dependent characteristics of the hyperpolarization -activated current (I-H) and its contribution to the resting membrane poten tial of neonatal rat nodose sensory neurons were investigated using the who le-cell tight seal method of voltage and current damp recording. 2. I-H was found in all neonatal nodose neurons in vitro, contrary to previ ous reports where its presence was particular for A-type neurons, me used t he presence of both tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resista nt (TTX-R) sodium currents to distinguish C-from A-type neurons (TTX-S only ). We obtained further support for the presence of I-H in C-type neurons wi th experiments in which I-H was demonstrated in a subset of neurons sensiti ve to capsaicin. 3. In both groups I-H activated at potentials negative to -50 mV, developed slowly with time and was inhibited by 1-5 mM extracellular caesium. At -12 0 mV, I-H activated with a fast time constant of 73 +/- 3 ms in A-type neur ons and 163 +/- 37 ms in C-type neurons (P < 0.05). A second, slower time c onstant of 682 +/- 83 ms was observed in A-type neurons and 957 +/- 122 ms in C-type neurons. 4. A- and C-type neurons differed in the amplitude of I-H. The mean magnitu de of I-H at -110 mV was -2338 +/- 258 pA in A-type neurons but only -241 /- 40 pA (P < 0.001) in C-type neurons. This disparity persisted when curre nts were normalized for capacitance. The reversal potentials for I-H were - 39 +/- 4 mV for A-type neurons and -37 +/- 5 mV for C-type neurons (P > 0.0 5). 5. During current clamp recording I-H caused time-dependent rectification i n response to hyperpolarizing current injections from the resting membrane potential. CsCl abolished the rectification and hyperpolarized the resting potential of A-type neurons from -55 +/- 3 mV to -61 +/- 4 mV and C-type ne urons from -62 +/- 2 mV to -71 +/- 3 mV. Taken together, the results in the se studies indicate that I-H contributes to the resting membrane potential in all nodose neurons.