Coexpression of auxiliary Kvβ2 subunits with Kv1.1 channels is required for developmental acquisition of unique firing properties of zebrafish Mauthner cells

Each neuron possesses a unique firing property, which is largely attributed to heterogeneity in the composition of voltage-gated ion channel complexes. The zebrafish Mauthner (M) cells, which are bilaterally paired giant reticulospinal neurons (RSNs) in the hindbrain and induce rapid escape behavior, generate only a single spike at the onset of depolarization. This single spiking is in contrast with the repetitive firing of the M-cell's morphologically homologous RSNs, MiD2cm and MiD3cm, which are also involved in escapes. However, how the unique firing property of the M-cells is established and the underlying molecular mechanisms remain unclear. In the present study, we first demonstrated that the single-spiking property of the M-cells was acquired at 4 days post-fertilization (dpf), accompanied with an increase in dendrotoxin-I (DTX)-sensitive low-threshold K+ currents, prior to which the M-cell repetitively fires as its homologs. Second, in situ hybridization showed that among DTX-sensitive Kv1 channel α-subunits, zKv1.1a was unexpectedly expressed in the homologs and the bursting M-cells at 2 dpf. In contrast, zKvβ2b, an auxiliary β-subunit of Kv1 channels, was expressed only in the single-spiking M-cells. Third, zKv1.1a expressed in Xenopus oocytes functioned as low-threshold K+ channels, and its currents were enhanced by coexpression of zKvβ2b subunits. Finally, knockdown of zKvβ2b expression in zebrafish larvae resulted in repetitive firing of the M-cells at 4 dpf. Taken together, these results suggest that associative expression of Kvβ2 subunits with Kv1.1 channels is crucial for developmental acquisition of the unique firing properties of the M-cells among homologous neurons.