Subthreshold sodium current underlies essential functional specializations at primary auditory afferents

Primary auditory afferents are generally perceived as passive, timing-preserving lines of communication. Contrasting this view, identifiable auditory afferents to the goldfish Mauthner cell undergo potentiation of their mixed electrical and chemical-synapses in response to high-frequency bursts of activity. This property likely represents a mechanism of input sensitization because they provide the Mauthner cell with essential information for the initiation of an escape response. Consistent with this synaptic specialization, we show here that these afferents exhibit an intrinsic ability to respond with bursts of 200-600 Hz and this property critically relies on the activation of a persistent sodium current, which is counterbalanced by the delayed activation of an A-type potassium current. Furthermore, the interaction between these conductances with the membrane passive properties supports the presence of electrical resonance, whose frequency preference is consistent with both the effective range of hearing in goldfish and the firing frequencies required for synaptic facilitation, an obligatory requisite for the induction of activity-dependent changes. Thus our data show that the presence of a persistent sodium current is functionally essential and allows these afferents to translate behaviorally relevant auditory signals into patterns of activity that match the requirements of their fast and highly modifiable synapses. The functional specializations of these neurons suggest that auditory afferents might be capable of more sophisticated contributions to auditory processing than has been generally recognized.