Limits...
Functional contributions of HCN channels in the primary auditory neurons of the mouse inner ear.

Kim YH, Holt JR - J. Gen. Physiol. (2013)

Bottom Line: We found that HCN1 is the most prominent subunit contributing to Ih in SGNs.Deletion of Hcn1 resulted in reduced conductance (Gh), slower activation kinetics (τfast), and hyperpolarized half-activation (V1/2) potentials.Together, our data indicate that Ih contributes to SGN membrane properties and plays a role in temporal aspects of signal transmission between the cochlea and the brain, which are critical for normal auditory function.

View Article: PubMed Central - HTML - PubMed

Affiliation: Neuroscience Graduate Program, University of Virginia School of Medicine, Charlottesville, VA 22908, USA.

ABSTRACT
The hyperpolarization-activated current, Ih, is carried by members of the Hcn channel family and contributes to resting potential and firing properties in excitable cells of various systems, including the auditory system. Ih has been identified in spiral ganglion neurons (SGNs); however, its molecular correlates and their functional contributions have not been well characterized. To investigate the molecular composition of the channels that carry Ih in SGNs, we examined Hcn mRNA harvested from spiral ganglia of neonatal and adult mice using quantitative RT-PCR. The data indicate expression of Hcn1, Hcn2, and Hcn4 subunits in SGNs, with Hcn1 being the most highly expressed at both stages. To investigate the functional contributions of HCN subunits, we used the whole-cell, tight-seal technique to record from wild-type SGNs and those deficient in Hcn1, Hcn2, or both. We found that HCN1 is the most prominent subunit contributing to Ih in SGNs. Deletion of Hcn1 resulted in reduced conductance (Gh), slower activation kinetics (τfast), and hyperpolarized half-activation (V1/2) potentials. We demonstrate that Ih contributes to SGN function with depolarized resting potentials, depolarized sag and rebound potentials, accelerated rebound spikes after hyperpolarization, and minimized jitter in spike latency for small depolarizing stimuli. Auditory brainstem responses of Hcn1-deficient mice showed longer latencies, suggesting that HCN1-mediated Ih is critical for synchronized spike timing in SGNs. Together, our data indicate that Ih contributes to SGN membrane properties and plays a role in temporal aspects of signal transmission between the cochlea and the brain, which are critical for normal auditory function.

Show MeSH

Related in: MedlinePlus

Biophysical characterization of Ih in neonatal (P1–4) WT and Hcn-deficient SGNs. (A) Family of representative Ih from WT, WT + 100 µM ZD7288, Hcn1−/−, Hcn2−/−, and Hcn1,2−/− SGNs as indicated. Currents were evoked using the same voltage-clamp protocol shown in Fig. 2. Residual currents in Hcn1,2−/− SGNs were blocked by 100 µM ZD7288. (B) Representative Gh activation curves for Hcn1−/−, Hcn2−/−, and Hcn1,2−/− SGNs fitted with Boltzmann equations. V1/2 and s are indicated on the graphs. (C) Summary of mean maximal Gh. (D) Summary of mean half-activation voltage. (E) Mean slope factor. Number of samples and genotype are indicated below. Error bars equal +1 SD. *, P < 0.05; **, P < 0.01; ***, P < 0.001.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
getmorefigures.php?uid=PMC3753603&req=5

fig4: Biophysical characterization of Ih in neonatal (P1–4) WT and Hcn-deficient SGNs. (A) Family of representative Ih from WT, WT + 100 µM ZD7288, Hcn1−/−, Hcn2−/−, and Hcn1,2−/− SGNs as indicated. Currents were evoked using the same voltage-clamp protocol shown in Fig. 2. Residual currents in Hcn1,2−/− SGNs were blocked by 100 µM ZD7288. (B) Representative Gh activation curves for Hcn1−/−, Hcn2−/−, and Hcn1,2−/− SGNs fitted with Boltzmann equations. V1/2 and s are indicated on the graphs. (C) Summary of mean maximal Gh. (D) Summary of mean half-activation voltage. (E) Mean slope factor. Number of samples and genotype are indicated below. Error bars equal +1 SD. *, P < 0.05; **, P < 0.01; ***, P < 0.001.

Mentions: To determine whether the Hcn mRNA expression patterns (Fig. 1) were translated into physiological expression of Ih, we recorded from SGNs excised from mice deficient in Hcn1, Hcn2, or both. Because our data and several in situ hybridization studies indicate very low expression levels of Hcn3 mRNA (Ludwig et al., 1998; Moosmang et al., 1999), Hcn3−/− mice were not investigated. Hcn4 is expressed in SGNs; however, Hcn4−/− mice are embryonic lethal (Stieber et al., 2003), and conditional Hcn4−/− mice are not available. Representative families of Ih recorded from SGNs of WT, Hcn1−/−, Hcn2−/−, and Hcn1,2−/− mice are shown in Fig. 4 A. Inward currents indicative of Ih were present in SGNs harvested from mice of all four genotypes. Current amplitudes and activation kinetics were noticeably diminished in Hcn1−/− SGNs relative to WT. Hcn2−/− SGNs showed reduced current amplitudes; however, their current activation kinetics was not significantly different from WT. In Hcn1,2−/− SGNs, in which both Hcn1 and Hcn2 were deficient, Ih was significantly reduced.


Functional contributions of HCN channels in the primary auditory neurons of the mouse inner ear.

Kim YH, Holt JR - J. Gen. Physiol. (2013)

Biophysical characterization of Ih in neonatal (P1–4) WT and Hcn-deficient SGNs. (A) Family of representative Ih from WT, WT + 100 µM ZD7288, Hcn1−/−, Hcn2−/−, and Hcn1,2−/− SGNs as indicated. Currents were evoked using the same voltage-clamp protocol shown in Fig. 2. Residual currents in Hcn1,2−/− SGNs were blocked by 100 µM ZD7288. (B) Representative Gh activation curves for Hcn1−/−, Hcn2−/−, and Hcn1,2−/− SGNs fitted with Boltzmann equations. V1/2 and s are indicated on the graphs. (C) Summary of mean maximal Gh. (D) Summary of mean half-activation voltage. (E) Mean slope factor. Number of samples and genotype are indicated below. Error bars equal +1 SD. *, P < 0.05; **, P < 0.01; ***, P < 0.001.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC3753603&req=5

fig4: Biophysical characterization of Ih in neonatal (P1–4) WT and Hcn-deficient SGNs. (A) Family of representative Ih from WT, WT + 100 µM ZD7288, Hcn1−/−, Hcn2−/−, and Hcn1,2−/− SGNs as indicated. Currents were evoked using the same voltage-clamp protocol shown in Fig. 2. Residual currents in Hcn1,2−/− SGNs were blocked by 100 µM ZD7288. (B) Representative Gh activation curves for Hcn1−/−, Hcn2−/−, and Hcn1,2−/− SGNs fitted with Boltzmann equations. V1/2 and s are indicated on the graphs. (C) Summary of mean maximal Gh. (D) Summary of mean half-activation voltage. (E) Mean slope factor. Number of samples and genotype are indicated below. Error bars equal +1 SD. *, P < 0.05; **, P < 0.01; ***, P < 0.001.
Mentions: To determine whether the Hcn mRNA expression patterns (Fig. 1) were translated into physiological expression of Ih, we recorded from SGNs excised from mice deficient in Hcn1, Hcn2, or both. Because our data and several in situ hybridization studies indicate very low expression levels of Hcn3 mRNA (Ludwig et al., 1998; Moosmang et al., 1999), Hcn3−/− mice were not investigated. Hcn4 is expressed in SGNs; however, Hcn4−/− mice are embryonic lethal (Stieber et al., 2003), and conditional Hcn4−/− mice are not available. Representative families of Ih recorded from SGNs of WT, Hcn1−/−, Hcn2−/−, and Hcn1,2−/− mice are shown in Fig. 4 A. Inward currents indicative of Ih were present in SGNs harvested from mice of all four genotypes. Current amplitudes and activation kinetics were noticeably diminished in Hcn1−/− SGNs relative to WT. Hcn2−/− SGNs showed reduced current amplitudes; however, their current activation kinetics was not significantly different from WT. In Hcn1,2−/− SGNs, in which both Hcn1 and Hcn2 were deficient, Ih was significantly reduced.

Bottom Line: We found that HCN1 is the most prominent subunit contributing to Ih in SGNs.Deletion of Hcn1 resulted in reduced conductance (Gh), slower activation kinetics (τfast), and hyperpolarized half-activation (V1/2) potentials.Together, our data indicate that Ih contributes to SGN membrane properties and plays a role in temporal aspects of signal transmission between the cochlea and the brain, which are critical for normal auditory function.

View Article: PubMed Central - HTML - PubMed

Affiliation: Neuroscience Graduate Program, University of Virginia School of Medicine, Charlottesville, VA 22908, USA.

ABSTRACT
The hyperpolarization-activated current, Ih, is carried by members of the Hcn channel family and contributes to resting potential and firing properties in excitable cells of various systems, including the auditory system. Ih has been identified in spiral ganglion neurons (SGNs); however, its molecular correlates and their functional contributions have not been well characterized. To investigate the molecular composition of the channels that carry Ih in SGNs, we examined Hcn mRNA harvested from spiral ganglia of neonatal and adult mice using quantitative RT-PCR. The data indicate expression of Hcn1, Hcn2, and Hcn4 subunits in SGNs, with Hcn1 being the most highly expressed at both stages. To investigate the functional contributions of HCN subunits, we used the whole-cell, tight-seal technique to record from wild-type SGNs and those deficient in Hcn1, Hcn2, or both. We found that HCN1 is the most prominent subunit contributing to Ih in SGNs. Deletion of Hcn1 resulted in reduced conductance (Gh), slower activation kinetics (τfast), and hyperpolarized half-activation (V1/2) potentials. We demonstrate that Ih contributes to SGN function with depolarized resting potentials, depolarized sag and rebound potentials, accelerated rebound spikes after hyperpolarization, and minimized jitter in spike latency for small depolarizing stimuli. Auditory brainstem responses of Hcn1-deficient mice showed longer latencies, suggesting that HCN1-mediated Ih is critical for synchronized spike timing in SGNs. Together, our data indicate that Ih contributes to SGN membrane properties and plays a role in temporal aspects of signal transmission between the cochlea and the brain, which are critical for normal auditory function.

Show MeSH
Related in: MedlinePlus