Effect of cAMP and ATP on the hyperpolarization-activated current in mouse dorsal root ganglion neurons

In mouse dorsal root ganglion (DRG) neurons the activation curve of the hyperpolarization-activated current (I-h) shifted towards depolarized potentials when cAMP was present in the pipette. The relation between the midpoint potential and cAMP concentration could be described by a Hill function with a half-maximal concentration of 0.55 mu M cAMP, reflecting a direct action of cAMP on the channel. With 5 mM ATP and a saturating concentration of cAMP an additional shift of the midpoint potential is observed which can be explained by phosphorylation. Application of Rp-cAMPS and Sp-cAMPS support the hypothesis of both a phosphorylation pathway and a direct effect exhibited by these molecules. The bell-shaped curves, relating the time constants for the slow and fast current components to the voltage, shifted towards positive membrane potentials when cAMP and ATP were in the pipette. The fully activated I-h/voltage relation and the reversal potential were not dependent on the presence of cAMP or ATP in the pipette. The mean resting membrane potential of -59 mV, using the perforated-patch configuration, hyperpolarized in the presence of extracellular CsCl. In the whole-cell configuration the resting membrane potential was significantly more negative at 0 mu M cAMP (-61 mV) than at 100 mu M cAMP (-57 mV). Thus, the activation of I-h, regulated by both the intracellular cAMP and the ATP concentration, may influence the excitability of DRG neurons.