Non-random nature of spontaneous mIPSCs in mouse auditory brainstem neurons revealed by recurrence quantification analysis

Abstract

A change in the spontaneous release of neurotransmitter is a useful indicator of processes occurring within presynaptic terminals. Linear techniques (e.g. Fourier transform) have been used to analyse spontaneous synaptic events in previous studies, but such methods are inappropriate if the timing pattern is complex. We have investigated spontaneous glycinergic miniature synaptic currents (mIPSCs) in principal cells of the medial nucleus of the trapezoid body. The random versus deterministic (or periodic) nature of mIPSCs was assessed using recurrence quantification analysis. Nonlinear methods were then used to quantify any detected determinism in spontaneous release, and to test for chaotic or fractal patterns. Modelling demonstrated that this procedure is much more sensitive in detecting periodicities than conventional techniques. mIPSCs were found to exhibit periodicities that were abolished by blockade of internal calcium stores with ryanodine, suggesting calcium oscillations in the presynaptic inhibitory terminals. Analysis indicated that mIPSC occurrences were chaotic in nature. Furthermore, periodicities were less evident in congenitally deaf mice than in normal mice, indicating that appropriate neural activity during development is necessary for the expression of deterministic chaos in mIPSC patterns. We suggest that chaotic oscillations of mIPSC occurrences play a physiological role in signal processing in the auditory brainstem.

Figure 1

Recurrence plots of modelled data reveal embedded periodicity in mIPSC IeIs. mIPSC IeIs (top) and the respective recurrence plots (bottom). Global recurrence quantification analysis variable values are shown below each plot. (a) Purely random release. (b) Random plus two low-frequency (LF) sinusoids (0.05 and 0.01 Hz). (c) Random plus two ‘high-frequency’ (HF) sinusoids (0.1 and 0.05 Hz). (d) Random plus three sinusoids (0.1, 0.05 and 0.01 Hz).

Figure 2

Glycinergic mIPSC frequency, amplitude and kinetics are not altered by ryanodine. (a) Voltage-clamp recordings showing mIPSC frequency does not change after addition of ryanodine (left). mIPSCs from normal and deaf mice (right). Datasets with similar frequencies in the two groups were used in this study. (b) Overlaid mIPSCs before and after application of ryanodine (top), and overlaid mIPSCs from normal and deaf mice (bottom). mIPSC kinetics and mean amplitude are not affected by ryanodine; however, decay times are slower in deaf mice in comparison to normal mice. (c) Example of a recurrence plot of mIPSC IeIs from a normal mouse. IeIs are shown at the top.

Figure 3

mIPSC IeI histograms. (a) mIPSC IeI histogram from normal mice (13 448 mIPSCs, n=9 cells). Inset: cumulative density function (CDF) of IeI's (solid line, normal mice) and an exponential distribution with the same mean (*; dashed line). (b) mIPSC IeI histograms from normal mice: control (white bars, 8913 mIPSCs, n=9 cells) and following ryanodine (black bars, 8347 mIPSCs, n=9 cells). Inset: CDFs of IeI from control and following ryanodine (*). (c) mIPSC IeI histogram from deaf mice (13 937 mIPSCs, n=9 cells). Inset: cumulative density of IeIs (solid line, deaf mice) and an exponential distribution with the same mean (*; dashed line). (d) Power spectra (db). (i) Power spectra of one normal mouse cell before (1215 mIPSCs, thick solid line) and after ryanodine addition (1285 mIPSCs, thin solid line) and a deaf mouse cell with similar mIPSC frequency (1195 mIPSCs, dashed line). (ii) Modelled mIPSCs power spectra; purely random release (thick solid line), random release plus two periodic oscillators (0.1 and 0.01 Hz sinusoids) and random plus three sinusoids (0.1, 0.05 and 0.01 Hz; see text for explanation). (e) Examples of semilogarithmic plots of IeI histograms (solid lines) and the fit obtained by a gamma distribution (dashed lines; see text for details).

Figure 4

Recurrence quantification analysis summary for mIPSCs from normal and deaf mice, and following ryanodine. (a) Mean %recurrence, (b) mean %determinism, (c) mean entropy and (d) mean maximal Lyapunov exponent for normal mice control (black bars, left) and following addition of ryanodine (white bars, right); normal (black bars, left) versus deaf mice (white bars, right). (e) Logarithmic Allan factor (A(T) versus T—see text for details) plots of a normal mouse cell before (mIPSC frequency=1.4 Hz; top) and after ryanodine addition (mIPSC frequency=1.4 Hz; bottom). Grey traces represent Allan factors calculated from 100 sets of surrogate data. (f) Same as (e), but the top graph shows a normal mouse cell (mIPSC frequency=1.8 Hz) while the bottom shows a deaf mouse cell A(T) versus T plot (mIPSC frequency=1.9 Hz). (*) denotes significant difference (p<0.05).