Posthearing Ca²⁺Currents and Their Roles in Shaping the Different Modes of Firing of Spiral Ganglion Neurons

Abstract

Whereas pre-hearing spiral ganglion neurons (SGNs) rely faithfully on outputs from spontaneously active developing hair cells, the electrical phenotypes of post-hearing neurons are shaped by distinct rapid and graded receptor potentials from hair cells. To date, technical difficulties in isolation of fragile post-hearing neurons from the rigid bony labyrinth of the inner ear have hindered analyses of the electrical phenotype of SGNs. Therefore, we have recently developed new strategies to isolate post-hearing mouse SGNs for functional analyses. Here, we describe the coarse and fine properties of Ca2+ currents, which sculpt the firing properties of post-hearing SGNs. Murine SGNs express multiple Ca2+ channel currents to confer diverse functions. We have demonstrated that suppression of Ca2+ currents results in significant hyperpolarization of the resting membrane potential (rmp) of basal SGNs, suggesting that Ca2+ influx primes rmp for excitation. In contrast, removal of external Ca2+ has modest effects on rmp of apical SGNs. The blockade of Ca2+ currents with a cocktail of specific blockers attenuates spontaneously active SGNs. Paradoxically, different subtypes of Ca2+ currents, such as R-type currents, may activate resting outward conductances since blockage of the current results in depolarization of rmp. In keeping with whole-cell current data, single channel records revealed multiple diverse Ca2+ channels in SGNs. Additionally, there were differential expressions of distinct Ca2+ current-densities in the apico-basal contour of the adult cochlea. This report provides invaluable insights into Ca2+-dependent processes in adult SGNs.

Genetic, Cellular, and Functional Evidence for Ca2+Inflow through Cav1.2 and Cav1.3 Channels in Murine Spiral Ganglion Neurons

“…We used 1-to 4-month- Isolation of SGNs. SGNs were isolated from male and female mouse inner ears as described previously (Lv et al, 2010. Briefly, mice were killed and the temporal bones were removed in a solution containing Minimum Essential Medium with Hank's salt (Invitrogen), 0.2 g/L kynurenic acid, 10 mM MgCl 2 , 2% FBS (v/v), and 6 g/L glucose.…”

“…The SGN tissue was dissected and split into three equal segments: apical, middle, and basal across the modiolar axis. We used the apical and basal thirds to ensure an adequate, viable neuronal yield for the experiments (Lv et al, 2010). Additionally, we pooled tissue from three mice into each SGN culture.…”

“…We cultured SGNs for ϳ24 -48 h to allow detachment of Schwann cells from neuronal membrane surfaces. We used strict electrophysiological criteria, described previously (Lv et al, 2010), to govern the quality of acceptable data. All electrophysiological experiments were performed at room temperature (21Ϫ22°C).…”

“…Similarly, inflowing Ca 2ϩ ions through multiple Ca v channels in auditory neurons represent the prevailing trigger of neurotransmitter release at the cochlear nucleus and spiral ganglion neuron (SGN) firing phenotypes (Chen et al, 2011;Lv et al, 2012), thereby orchestrating peripheral auditory information coding (Trussell, 1997;Cao and Oertel, 2010;Howard and Rubel, 2010;Rutherford et al, 2012). For long-term structural changes and growth, Ca 2ϩ influx through L-type channels promotes SGN survival in vitro (Hegarty et al, 1997).…”

“…For long-term structural changes and growth, Ca 2ϩ influx through L-type channels promotes SGN survival in vitro (Hegarty et al, 1997). Of significant application and pragmatic ramifications, in vivo, is the finding that direct electrical stimulation of SGNs increases SGN survival (Mitchell et al, 1997;Leake et al, 1999). Conversely, excessive Ca 2ϩ inhibits neural outgrowth (Roehm et al, 2008), raising the possibility that a window of intracellular Ca 2ϩ activity is required for long-term plasticity in auditory neurons.…”