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

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 clamp recording. I H was found in all neonatal nodose neurons in vitro , contrary to previous reports where its presence was particular for A-type neurons. We 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. In both groups I H activated at potentials negative to −50 mV, developed slowly with time and was inhibited by 1–5 m m 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. 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). 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.