Enhanced activation of HCN channels reduces excitability and spike-timing regularity in maturing vestibular afferent neurons.

Vestibular ganglion neurons (VGN) transmit information along parallel neuronal pathways whose signature distinction is variability in spike-timing; some fire at regular intervals while others fire at irregular intervals. The mechanisms driving timing differences aren't fully understood but two opposing (but not mutually exclusive) hypotheses have emerged. In the first, regular-spiking is inversely correlated to the density of low-voltage gated potassium currents ( I KL ). In the second, regular spiking is directly correlated to the density of hyperpolarization-activated cyclic nucleotide sensitive currents ( I H ). Supporting the idea that variations in ion channel composition shape spike-timing, VGN from the first post-natal week respond to synaptic-noise-like current injections with irregular-firing patterns if they have I KL and with more regular firing patterns if they don't. However, in vitro firing patterns are not as regular as those in vivo. Here we considered whether highly-regular spiking requires I H currents and if this dependence emerges later in development after channel expression matures. We recorded from rat VGN somata of either sex aged postnatal day nine through 21. Counter to expectation, in vitro firing patterns were less diverse, more transient-spiking, and more irregular at older ages than at younger ages. Resting potentials hyperpolarized and resting conductance increased, consistent with developmental upregulation of I KL Activation of I H (by increasing intracellular cAMP) increased spike rates but not spike-timing regularity. In a model, we found that activating I H counter-intuitively suppressed regularity by recruiting I KL Developmental upregulation in I KL appears to overwhelm I H These results counter previous hypotheses about how I H shapes vestibular afferent responses. SIGNIFICANCE STATEMENT Vestibular sensory information is conveyed on parallel neuronal pathways with irregularly-firing neurons encoding information using a temporal code and regularly-firing neurons using a rate code. This is a striking example of spike-timing statistics influencing information coding. Previous studies from immature vestibular ganglion neurons (VGN) identified hyperpolarization-activated mixed cationic currents ( I H ) as driving highly-regular spiking and proposed that this influence grows with the current during maturation. We found that I H becomes less influential, likely because maturing VGN also acquire low-voltage gated potassium currents ( I KL ), whose inhibitory influence opposes I H Since efferent activity can partly close I KL , VGN firing patterns may become more receptive to extrinsic control. Spike-timing regularity likely relies on dynamic ion channel properties and complementary specializations in synaptic connectivity.