Persistent Nav1.6 current at axon initial segments tunes spike timing of cerebellar granule cells

Cerebellar granule (CG) cells generate high-frequency action potentials that have been proposed to depend on the unique properties of their voltage-gated ion channels. To address the in vivo function of Nav1.6 channels in developing and mature CG cells, we combined the study of the developmental expression of Nav subunits with recording of acute cerebellar slices from young and adult granule-specific Scn8a KO mice. Nav1.2 accumulated rapidly at early-formed axon initial segments (AISs). In contrast, Nav1.6 was absent at early postnatal stages but accumulated at AISs of CG cells from P21 to P40. By P40–P65, both Nav1.6 and Nav1.2 co-localized at CG cell AISs. By comparing Na + currents in mature CG cells (P66–P74) from wild-type and CG-specific Scn8a KO mice, we found that transient and resurgent Na + currents were not modified in the absence of Nav1.6 whereas persistent Na + current was strongly reduced. Action potentials in conditional Scn8a KO CG cells showed no alteration in threshold and overshoot, but had a faster repolarization phase and larger post-spike hyperpolarization. In addition, although Scn8a KO CG cells kept their ability to fire action potentials at very high frequency, they displayed increased interspike-interval variability and firing irregularity in response to sustained depolarization. We conclude that Nav1.6 channels at axon initial segments contribute to persistent Na + current and ensure a high degree of temporal precision in repetitive firing of CG cells.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78706/1/jphysiol.2010.183798.pd

Elementary properties of CaV1.3 Ca2+ channels expressed in mouse cochlear inner hair cells

Mammalian cochlear inner hair cells (IHCs) are specialized to process developmental signals during immature stages and sound stimuli in adult animals. These signals are conveyed onto auditory afferent nerve fibres. Neurotransmitter release at IHC ribbon synapses is controlled by L-type CaV1.3 Ca2+ channels, the biophysics of which are still unknown in native mammalian cells. We have investigated the localization and elementary properties of Ca2+ channels in immature mouse IHCs under near-physiological recording conditions. CaV1.3 Ca2+ channels at the cell pre-synaptic site co-localize with about half of the total number of ribbons present in immature IHCs. These channels activated at about −70 mV, showed a relatively short first latency and weak inactivation, which would allow IHCs to generate and accurately encode spontaneous Ca2+ action potential activity characteristic of these immature cells. The CaV1.3 Ca2+ channels showed a very low open probability (about 0.15 at −20 mV: near the peak of an action potential). Comparison of elementary and macroscopic Ca2+ currents indicated that very few Ca2+ channels are associated with each docked vesicle at IHC ribbon synapses. Finally, we found that the open probability of Ca2+ channels, but not their opening time, was voltage dependent. This finding provides a possible correlation between presynaptic Ca2+ channel properties and the characteristic frequency/amplitude of EPSCs in auditory afferent fibres

Fine Tuning of Ca(V)1.3 Ca2+ Channel Properties in Adult Inner Hair Cells Positioned in the Most Sensitive Region of the Gerbil Cochlea

Hearing relies on faithful signal transmission by cochlear inner hair cells (IHCs) onto auditory fibres over a wide frequency and intensity range. Exocytosis at IHC ribbon synapses is triggered by Ca2+ inflow through CaV1.3 (L-type) Ca2+ channels. We investigated the macroscopic (whole-cell) and elementary (cell-attached) properties of Ca2+ currents in IHCs positioned at the middle turn (frequency ,2 kHz) of the adult gerbil cochlea, which is their most sensitive hearing region. Using near physiological recordings conditions (body temperature and a Na+ based extracellular solution), we found that the macroscopic Ca2+ current activates and deactivates very rapidly (time constant below 1 ms) and inactivates slowly and only partially. Single-channel recordings showed an elementary conductance of 15 pS, a sub-ms latency to first opening, and a very low steady-state open probability (Po: 0.024 in response to 500-ms depolarizing steps at ,218 mV). The value of Po was significantly larger (0.06) in the first 40 ms of membrane depolarization, which corresponds to the time when most Ca2+ channel openings occurred clustered in bursts (mean burst duration: 19 ms). Both the Po and the mean burst duration were smaller than those previously reported in high-frequency basal IHCs. Finally, we found that middle turn IHCs are likely to express about 4 times more Ca2+ channels per ribbon than basal cells. We propose that middle-turn IHCs finely-tune CaV1.3 Ca2+ channel gating in order to provide reliable information upon timing and intensity of lower-frequency sounds

Distinct developmental patterns in the expression of transient, persistent, and resurgent Na+ currents in entorhinal cortex layer-II neurons

Sub- and near-threshold voltage-dependent Na+ currents (VDSCs) are of major importance in determining the electrical properties of medial entorhinal cortex (mEC) layer-II neurons. Developmental changes in the ability of mEC layer-II stellate cells (SCs) to generate Na+-dependent, subthreshold electrical events have been reported between P14 and P18. In this study we examined the modifications occurring in the various components of VDSCs during postnatal development of mEC SCs. The transient, resurgent, and persistent Na+ currents (I-NaT, I-NaR and I-Nap, respectively) showed distinct patterns of developmental expression in the time window considered (PS to P24-27). All three currents prominently and steeply increased in absolute amplitude and conductance from P5 to at least P16. However, capacitive charge accumulation, an index of membrane surface area, also markedly increased in the same time window, and in the case of I-NaT the specific conductance per unit of accumulated capacitive charge remained relatively constant. By contrast, specific I-NaR and I-Nap conductances showed a significant tendency to increase, especially from P5 to P18. Neither I-NaR nor I-Nap represented a constant fraction of the total Na+ current at all developmental ages. Indeed, detectable levels of I-NaR and I-Nap were present in only similar to 20% and similar to 70%, respectively, of the cells on P5, and were observed in all cells only from P10 onwards. Moreover, the average I-NaR-to-I-NaT conductance ratio increased steadily from similar to 0.004 (P5) up to a plateau level of similar to 0.05 (P22+), whereas the I-Nap-to-I-NaT conductance ratio increased only from similar to 0.009 on P5 to similar to 0.02 on P22+. The relative increase in conductance ratio from P5 to P22 was significantly greater for I-NaR than for I-Nap, indicating that I-NaR expression starts later than that of I-Nap. These findings show that in mEC layer-II SCs the single functional components of the VDSC are regulated differentially from each other as far as their developmental expression is concerned