Central dysmyelination reduces the temporal fidelity of synaptic transmission and the reliability of postsynaptic firing during high-frequency stimulation

Abstract

Auditory brain stem circuits rely on fast, precise, and reliable neurotransmission to process auditory information. To determine the fundamental role of myelination in auditory brain stem function, we examined the evoked auditory brain stem response (ABR) from the Long Evans shaker (LES) rat, which lacks myelin due to a genetic deletion of myelin basic protein. In control rats, the ABR evoked by a click consisted of five well-defined waves (denoted waves I-V). In LES rats, waves I, IV, and V were present, but waves II and III were undetectable, indicating disrupted function in the earliest stages of central nervous system auditory processing. In addition, the developmental shortening of the interval between waves I and IV that normally occurs in control rats was arrested and resulted in a significant increase in the central conduction time in LES rats. In brain stem slices, action potential transmission between the calyx of Held terminals and the medial nucleus of the trapezoid body (MNTB) neurons was delayed and less reliable in LES rats, although the resting potential, threshold, input resistance, and length of the axon initial segment of the postsynaptic MNTB neurons were normal. The amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) and the degree of synaptic depression during high-frequency stimulation were not different between LES rats and controls, but LES rats exhibited a marked slow component to the EPSC decay and a much higher rate of presynaptic failures. Together, these results indicate that loss of myelin disrupts brain stem auditory processing, increasing central conduction time and reducing the reliability of neurotransmission.

Keywords: ABR; MNTB principal neuron; auditory brain stem; auditory neuropathy; calyx of Held synapse; myelin.

Fig. 1.

Auditory brain stem responses (ABR) from Long Evans shaker (LES) rats showed loss of central waves and significantly increased latency. A and B: representative ABR recordings using click stimuli in control (A) and LES rats (B) at postnatal day 20 (P20). Roman numerals identify waves I–V in control and waves I and IV in the LES rat. Red dashed lines indicate the peak of wave IV along different sound intensities. C: summary of the latencies of waves I and IV (the time from initiation of sound stimulation to peak of wave) at 90 and 70 dB in control and LES rats (**P < 0.01). Values are means ± SE. Latencies were measured the time from the initial to the peak of waves.

Fig. 2.

Higher ABR threshold in LES rats in response to high-frequency sound stimulation in LES rats. ABR thresholds at frequencies of 4, 8, 12, and 16 kHz were recorded using tone stimuli in control and LES rats (at P18 to P30). Thresholds at 8-, 12-, and 16-kHz stimuli in LES rats are significantly higher than in control (*P < 0.05). Values are means ± SE.

Fig. 3.

Arrested development of the ABR waveform in LES rats. A and B: ABR click recordings at 70 dB from control and LES rats at P18, P22, and P27. Controls show a clear reduction in the latencies of waves I and IV during postnatal development (A). LES rats show a slight developmental reduction in the latency of wave I, with no significant change in the latency of wave IV (B). Note the absence of wave II and III in LES rats. Arrows indicate the inflection on the rising phase of wave IV. C and D: summary of postnatal changes in latencies of waves I and IV in control (black) and LES rats (red; **P < 0.01). Dashed lines indicate the peaks of waves I and IV along postnatal ages. N.S. indicates statistical nonsignificance (P > 0.05).

Fig. 4.

Altered action potential (AP) waveform and pre- and postsynaptic AP failures in the medial nucleus of the trapezoid body (MNTB) from LES rats. A: postsynaptic APs evoked by afferent fiber stimulation in the MNTB neurons from control and LES rats (at P18 and P17, respectively). Top arrows indicate the peak of APs showing that AP from LES rat is strongly delayed. Bottom arrow indicates the afterdepolarization (ADP) in neurons from the MNTB of LES rat. B: AP trains recorded in response to repetitive stimulation at 300 Hz from control (black trace) and LES (red trace) rats. Recordings from LES rat showed postsynaptic AP failures caused by excitatory postsynaptic potentials (EPSPs), which did not reach threshold, indicated by the red horizontal line. C: LES rats showed frequent presynaptic AP and EPSP failures at 300 Hz (blue arrows indicate presynaptic AP and EPSP failures).

Fig. 5.

Delayed onset and failures of excitatory postsynaptic currents (EPSCs) in dysmyelinated synapses. A: representative traces of single EPSCs recorded during afferent fiber stimulation in the MNTB from control (black) and LES (red) rats. Dotted line indicates EPSC of control aligned to that of LES synapse to show the decay kinetics of EPSCs from control and LES synapses. B: representative traces of EPSCs produced by a train of stimuli (100 Hz, 500 ms) from control (P17, black) and LES (P17, red). C: normalized amplitude of EPSCs is summarized for control and LES synapses (100 Hz). D: plot of EPSCs against cumulative EPSCs in control (black) and LES (red). E: EPSC train at 300 Hz in control and LES synapses in normal external solution (2 mM CaCl₂). A number of EPSC failures are present in the late end of the train (arrows). F: EPSC train at 200 Hz with external recording solution containing 1.6 mM CaCl₂ to reduce the depression of EPSCs. A longer delayed onset and failures of EPSCs are shown in the LES synapse. Inset: expanded time scale of boxed areas of the trace (top, initial EPSCs in train; bottom, later EPSCs in train). Note the increased delay of EPSC onset and failures of EPSCs (red arrows).

Fig. 6.

Dysmyelination did not change the intrinsic firing properties of the MNTB neuron. A: single AP evoked by a single step-current injection (500 pA, 2 ms) from postsynaptic principal neurons in control (P16, black trace) and LES rats (P18, red trace). B and C: representative traces of APs evoked by depolarizing currents (−50 to 200 pA) in control (black trace; B) and LES rats (red trace; C). D–H: summaries of resting membrane potential (D), threshold of AP (E), amplitude of AP (F), half-width of AP (G), and input resistance (H).

Fig. 7.

Expression of ankyrin-G (AnkG) in the LES rat brain stem. Fixed slices of auditory brain stem containing the MNTB were stained for AnkG and vesicular glutamate transporter 1 (VgluT1). In expanded images (insets, bottom left), AnkG was located at axon initial segments of principal neurons in the MNTB (green) in control and LES auditory brain stems. VgluT1 is stained for the calyx of Held terminals in the MNTB (red). Both control and LES rats were at P23. Scale bar = 20 and 10 μm for main images and insets, respectively.