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 potential of neonatal rat nodose sensory neurons were investigated using the whole-cell tight seal method of voltage and current damp recording. 2. I-H was found in all neonatal nodose neurons in vitro, contrary to previous reports where its presence was particular for A-type neurons, me used the presence of both tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (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 with experiments in which I-H was demonstrated in a subset of neurons sensitive 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 -120 mV, I-H activated with a fast time constant of 73 +/- 3 ms in A-type neurons and 163 +/- 37 ms in C-type neurons (P < 0.05). A second, slower time constant 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 magnitude 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 currents 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 > 0.05). 5. During current clamp recording I-H caused time-dependent rectification in 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 neurons from -62 +/- 2 mV to -71 +/- 3 mV. Taken together, the results in these studies indicate that I-H contributes to the resting membrane potential in all nodose neurons.