Hyperpolarization-activated cation channels confer tonotopic specialization for temporal encoding of sound frequency...

Sensory neurons are equipped with physiological properties vital for accurate signal processing. The functional importance of such properties is exemplified in auditory circuits where intrinsic excitability is optimized to detect frequency-specific features. In birds, the neurons of nucleus magnocellularis (NM) receive primary auditory input (Rubel and Parks, 1975a; Parks and Rubel, 1978; Jackson et al., 1982) and are arranged tonotopically. NM comprises a superficially homogenous neural population, but several physiological properties vary systematically along its tonotopic frequency axis. In particular, expression of voltage-gated conductances plays a pivotal role in creating selectivity that enables temporal precision. Here, we identify a previously undescribed gradient of hyperpolarization-activated cation channels (IH). Whole cell patch clamp techniques and immunostaining for HCN1, an IH channel subunit, demonstrated an expression gradient corresponding to NM's tonotopic axis. To investigate the function of tonotopic IH expression in NM, we applied a depolarizing ramp injection protocol to measure the impact of pharmacologically blocking IH on neural active properties (Ferragamo and Oertel, 2002; McGinley and Oertel, 2006; Oline et al. 2016). Next, we investigated whether this tonotopic patterning of HCN facilitates encoding of temporally patterned inputs. We injected depolarizing current pulse trains before and during HCN channel block. During pharmacological block, there was a reduction of NM spike entrainment to input pulses suggesting a key contribution of HCN channels to NM's ability to encode its synaptic drive. Results show that there is tonotopic distribution of HCN channels in NM which provides a novel mechanism that enables NM neurons to encode temporally patterned excitatory input.

This study is the first to describe a tonotopic gradient of IH channels in a vertebrate cochlear nucleus. Physiological and computational model assays suggest that the tonotopic expression pattern of HCN channels enables improved neural encoding of high frequency, temporally patterned input. Temporal response fidelity enables precise sound localization computations.