Dopaminergic modulation of cultured rat olfactory receptor neurons

The presence of dopamine D 2 receptors in olfactory receptor neurons (ORNs) suggests that odor sensitivity may be modulated by neurotransmitters at the level of primary sensory neurons. Because the hyperpolarization-activated, I h , channel is present in ORNs, controls cell excitability and is modulated by dopamine in other systems, I used standard patch-clamp techniques to study the effects of dopamine on I h in cultured rat ORNs. Application of 1 ?M dopamine reversibly shifted I h activation to more negative potentials and decreased I h , maximal relative conductance. Sulpiride, a dopamine D 2 receptor antagonist, blocked the effects of dopamine; while, quinpirole (selective D 2 agonist) mimicked them. External application of an adenylyl cyclase inhibitor (SQ 22536) produced a reversible decrease in I h peak currents, but had no effect OR I h voltage dependence of activation. However, internal application of cAMP shifted I h activation to more depolarized potentials. These findings showed that dopamine regulates I h by activation of D 2 receptors and subsequent inhibition of adenylyl cyclase. Whether this modulation resulted from a direct interaction of cAMP with the channel or a phosphorylation-dependent mechanism was unclear. I found that both cAMP (through a direct interaction with the channel) and PKA-mediated phosphorylation regulated the voltage dependence of I h activation. In addition, PKA modulated Ih maximal relative conductance; cAMP had no effect. These results showed that, in rat ORNs, the voltage dependence of I h activation is dually regulated by a direct action of cAMP on the channel and by PKA; while, the maximal conductance is regulated by phosphorylation alone. Since I h contributes to the modulation of cell excitability, these findings suggest a mechanism by which dopaminergic reduction of cAMP levels and the phosphorylation state in ORNs would inhibit I h and thereby alter resting properties as well as modulate odor sensitivity.