Clin Exp Otorhinolaryngol.  2019 Aug;12(3):279-286. 10.21053/ceo.2018.01431.

Abrupt Change in Electrophysiological Properties Begins From Postnatal Day 7 Before Hearing Onset in the Developing Mice Auditory Cortical Layer II/III Neurons

• Ahn SC

• KMID: 2455369
• DOI: http://doi.org/10.21053/ceo.2018.01431

Abstract

OBJECTIVES
In the developing auditory cortex, maturation of electrophysiological properties and cell types before and after hearing onset has been reported previously. However, the exact timing of firing pattern change has not been reported. In this study, firing pattern change was investigated from postnatal day 3 (P3) to P12 in auditory cortical layer II/III neurons to investigate whether firing pattern changes dramatically after a specific point during development.
METHODS
ICR mice pups aged from P3 to P12 were sacrificed to obtain 300-mm-thick brain slices containing the primary auditory cortex. From cortical layer II/III neurons, the patterns of action potential firing generated by current injection were examined using whole cell current clamp technique and the characteristics of Na⁺ currents involved in action potential firing were investigated using whole cell voltage clamp technique.
RESULTS
From P3 to P6, most cells did not show action potential firing (29 of 46 cells), and some cells responding to current injection showed a single action potential at the initial depolarizing current step (17 of 46 cells). This firing pattern changes from P7. From P7 to P9, cells begin to show regular spiking to current injection. The spiking frequency increased after P10. In studying Na⁺ current with whole cell voltage clamp, Na⁺ current densities increased gradually (32.0±2.0 pA/pF [P3-P6, n=7], 51.2±2.0 pA/pF [P7-P9, n=13], and 69.5±3.7 pA/pF [P10-P12, n=13]) in low external [Na⁺] condition. Na⁺ current recovery was accelerated and inactivation curves shifted to hyperpolarization with age.
CONCLUSION
As regular spiking cells were observed from P7 but never from P3 to P6, P7 might be regarded as an important milestone in the development of auditory cortical layer II/III neurons. This change might mainly result from the increase in Na⁺ current density.

Keyword

ICR Mice; Auditory Cortex; Action Potential

MeSH Terms

Action Potentials
Animals
Auditory Cortex
Brain
Fires
Hearing*
Mice*
Mice, Inbred ICR
Neurons*

Figure

• 

  Fig. 1. Intrinsic neuronal properties from P4 (A), P8 (B), and P11 (C) mice. The current protocol is shown above figures. Filled squares below figures (A-C) indicate voltage measuring points for the current-voltage relationship in (D-F). Current-voltage relations in neurons of different ages are shown in (D-F: P3–P6 [D], P7–P9 [E], P10–P12 [F]). Fitting lines obtained from linear regression are drawn over the mean values. A graph of input resistances against age is shown in (G). Asterisks over columns indicate the statistical significance. (H) The relation between the number of spikes and injected currents is shown. (I) A graph of resting membrane potential against age is shown. P, postnatal day.

• 

  Fig. 2. Na+ currents recorded from neurons of different ages. The current protocol is shown above figures. In (A-C), the external [Na+] was reduced from 150 mM to 88 mM. Na+ currents recorded when [Na+] was 150 mM are shown in (D). Asterisks shown in figures indicate delayed response due to space clamp error. Current-voltage relations recorded in the low [Na+] condition are shown in (E). The peak current densities obtained from (E) are shown in (F). P, postnatal day; aCSF, artificial cerebrospinal fluid.

• 

  Fig. 3. Inactivation of Na+ currents from neurons of different ages. Inset: Na+ currents in a P5 neuron. The voltage protocol is shown above the inset. (A) The fraction of peak currents (I/Imax) is plotted against the inactivation voltage. The smooth curve is the best fit to a modified first order Boltzmann equation. Half inactivation voltages obtained from (A) are shown in (B). Asterisks over columns indicate the statistical significance. P, postnatal day.

• 

  Fig. 4. Na+ current recovery from inactivation. The first Na+ current was elicited by step depolarization from –71.2 to –31.2 mV. After a 10-millisecond initial interval, 1 millisecond was added to each interval. A total of 25 sweeps were performed (A–C). The relative current (I/Imax) plotted as a function of interpulse interval was shown in (D), where a single exponential function was fitted to the data. (E) The time constants were 4.7±0.5 (P3–P6, n=13), 2.8±0.2 (P7–P9, n=3), and 2.3±0.1 (P10–P12, n=21). Asterisks over columns indicate the statistical significance. P, postnatal day.

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