Modulation by PKA of the hyperpolarization-activated current (Ih) in cultured rat olfactory receptor neurons

The hyperpolarization-activated I-h channel is modulated by neurotransmitters acting through the cAMP messenger system. In rat olfactory receptor neurons (ORNs), dopamine, by inhibition of adenylyl cyclase, shifts the voltage of half-maximal activation (V-1/2) of I-h to more negative potentials and decreases I-h maximal relative conductance. Whether these effects result from a phosphorylation-dependent mechanism is unclear. Therefore, we used whole-cell patch-clamp recording techniques to study cAMP-dependent phosphorylation via PKA on I-h in rat ORNs. General protein kinase inhibition (50 nm K252a) produced a hyperpolarizing shift in I-h V-1/2 and decreased I-h maximal conductance. Specific inhibition of PKA with H-89 (500 nm) also shifted the V-1/2 of I-h to more negative potentials, and, in some cells, decreased I-h maximal conductance. PKA-mediated phosphorylation (cBIMPS, 50 muM) shifted I-h V-1/2 more positive, modulated the kinetics of I-h channel activation and increased I-h peak current amplitude. Internal perfusion of the catalytic subunit of PKA (84 nm) also shifted I-h V-1/2 positive and this shift was blocked by co-perfusion with PKI (50 nm). These results show that in rat ORNs, the voltage dependence of I-h activation can be modulated by PKA-dependent phosphorylation. We also show that PKA and other protein kinases may be involved in the regulation of I-h maximal conductance. Our findings suggest that changes in the phosphorylation state of ORNs may affect resting properties as well as modulate odor sensitivity.