Target-specific regulation of presynaptic release properties at auditory nerve terminals in the avian cochlear nucleus

Abstract

Short-term synaptic plasticity (STP) acts as a time- and firing rate-dependent filter that mediates the transmission of information across synapses. In the auditory brain stem, the divergent pathways that encode acoustic timing and intensity information express differential STP. To investigate what factors determine the plasticity expressed at different terminals, we tested whether presynaptic release probability differed in the auditory nerve projections to the two divisions of the avian cochlear nucleus, nucleus angularis (NA) and nucleus magnocellularis (NM). Estimates of release probability were made with an open-channel blocker ofN-methyl-d-aspartate (NMDA) receptors. Activity-dependent blockade of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) with application of 20 μM (+)-MK801 maleate was more rapid in NM than in NA, indicating that release probability was significantly higher at terminals in NM. Paired-pulse ratio (PPR) was tightly correlated with the blockade rate at terminals in NA, suggesting that PPR was a reasonable proxy for relative release probability at these synapses. To test whether release probability was similar across convergent inputs onto NA neurons, PPRs of different nerve inputs onto the same postsynaptic NA target neuron were measured. The PPRs, as well as the plasticity during short trains, were tightly correlated across multiple inputs, further suggesting that release probability is coordinated at auditory nerve terminals in a target-specific manner. This highly specific regulation of STP in the auditory brain stem provides evidence that the synaptic dynamics are tuned to differentially transmit the auditory information in nerve activity into parallel ascending pathways.

Keywords: angularis; depression; magnocellularis; release probability; short-term plasticity.

Fig. 1.

Activity dependent blockade of the slow component of the excitatory postsynaptic current (EPSC) by MK801 at auditory nerve synapses in the avian cochlear nucleus angularis (NA). A: EPSCs recorded under whole cell patch clamp in the avian brain stem slice. Holding voltage (V_hold) at +50 mV while electrically stimulating the nerve inputs resulted in a large EPSC in control ACSF. Slow component was blocked by 20 μM MK801; total blockade occurred with 10 μM DNQX + 20 μM MK801. Antagonists against ionotropic GABA and glycine receptor were present in all conditions (10 μM gabazine and 3 μM strychnine). Stimulus artifact was digitally blanked for clarity. B: schematic of experimental configuration recording in A. Transverse section, dorsolateral quadrant of the brain stem. NM, nucleus magnocellularis. I_m, membrane current. Adapted from MacLeod and Carr (2005) with permission. C: time course of MK801 blockade in experiment in A. Peak EPSC (open circles) persisted in MK801, while the late EPSC (filled circles) showed activity-dependent decrement. Timing of the current measurements is illustrated by position of open and filled circles above traces in A. AMPAR, AMPA receptor; NMDAR, NMDA receptor. D: summary data of the remaining EPSC following maximal MK801 blockade in 7 neurons (mean + SD). n.s., Not significant. **P < 10⁻⁶.

Fig. 2.

MK801 blockade rates differed in the 2 cochlear nuclei. A: application of DNQX to EPSCs evoked at depolarized voltages eliminated the fast peak component for 1 NA neuron. Inset: an expanded view with peak indicated with an asterisk. Washin of MK801 (20 μM; in addition to DNQX) blocked the slow component in an activity-dependent manner [early (a) and late (b) blockade; see B]. B: activity-dependent blockade of the slow DNQX-insensitive component with MK801. MK801 is washed in for 1–3 min before the stimulus is restarted. Rapid blockade was fit with a single-exponential decay curve [solid line; time constant (τ) = 11.3 trials]. Same NA neuron as in A. C: histogram of MK801 blockade time courses in 29 NA neurons. Shaded bars indicate the subset of neurons (n = 11) that were recorded under higher glycine conditions identical to NM recordings (see methods). Mean blockade rate was nearly identical in the 2 conditions (5 μM glycine, 8.6 ± 3.0; 30 μm glycine, 9.4 ± 3.8, P = 0.54; mean ± SD). D: activity-dependent blockade of the slow DNQX-insensitive component by MK801 in 1 NM neuron. Similar experiment as in B except that in this example there was no delay in stimulation during washin of MK801. E: histogram of MK801 blockade time courses in NM neurons (n = 13). F: averaged time courses of blockade during synaptic stimulation while recording in NA (n = 29) or in NM (n = 13). Individual experiments were aligned on the onset of decline, normalized in amplitude, and averaged (data are means ± SE). Solid lines are the single-exponential fit of these declines that result in similar values as mean of individual fits reported in text. In separate experiments, the intrinsic blockade rate tested with direct glutamate stimulation showed no significant differences between the two nuclei (“glu puff,” see methods; Student's t-test, P = 0.35, n = 10 experiments in NA, n = 9 experiments in NM, solid and dashed lines respectively, no markers).

Fig. 3.

A: summary diagram of the tonotopic axis location of NA recordings for MK801 decay experiments. Dorsoventral axis was divided into half-quartile bands that were assigned an approximate sound frequency (see methods). Each recording was assigned a tonotopic percentile band for the analysis in B. B: decay time constant during MK801 blockade plotted against tonotopic location for NA and NM recordings. Linear regression analysis showed no significant effect of location on decay times for either nucleus (NA, n = 26, R² = 0.119, P = 0.085; NM, n = 13, R² = 0. 195, P = 0.13). Two-way ANOVA did, however, show a significant difference in decay times between the 2 nuclei (see text).

Fig. 4.

A: time course of MK801 blockade was correlated with short-term plasticity in NA (n = 26, linear regression, R² = 0.29, P = 0.0043). Insets: EPSC traces in response to a paired-pulse stimulus from 2 different example neurons (a and b). B: time course of MK801 blockade was uncorrelated with EPSC amplitude (n = 26, linear regression, R² = 0.094, P = 0.128).

Fig. 5.

Increasing the amplitude of the electrical stimulation in the auditory nerve tract providing input to NA neurons led to recruitment of additional fibers and larger EPSC amplitudes but no change in the paired-pulse ratio (PPR). A, left: schematic of stimulation paradigm illustrated with the inner circle stimulating a single fiber (level 1), while increased stimulus strength activated a larger circle stimulating the 1st fiber and 1 additional fiber (level 2). Right: paired-pulse stimulation (top) evoked a pair of EPSCs (bottom 2 traces), with larger EPSCs for level 2. Traces were averaged from multiple trials at each plateau indicated in B–E. Stimulus illustrates timing but not amplitude. B: amplitude of EPSC1 of the pair increased with stimulus level, showing transition point at threshold for second level. C: amplitude of EPSC2 of the pair also increased with stimulus level with nearly the same threshold point as EPSC1. D: mean (±SD) amplitudes for EPSCs for neuron shown in B and C. Filled circles, EPSC1; open circles, EPSC2. Level plateaus were selected to avoid threshold transitions. E: PPR (mean EPSC2/mean EPSC1 for each stimulus level point) is relatively stable over the range of stimulus levels, excluding threshold transitions.

Fig. 6.

Correlated short-term synaptic plasticity across the range of stimulus strength levels: PPRs. A: summary scatterplot of the PPR measured as the plateau level 1 (x-axis) and the PPR measured at the next higher stimulus plateau (y-axis). For a subset of neurons the PPR at a third level could be measured, and these were also plotted against the PPR at the corresponding level 1 (level 3). The points clustered along the diagonal (thin line), and linear regression (thick line) showed a strong correlation in the PPR across stimulus levels (n = 33 levels measured in 28 neurons, R² = 0.797, P < 0.0001). B: PPR showed no dependence on the amplitude of EPSC1 at any level (n = 61 levels, R² = 0.052, P = 0.066). C: to estimate the amplitude of the synaptic recruitment at higher levels (independent EPSClev2), the raw amplitudes of the EPSCs at level 1 were subtracted from the raw amplitudes of the EPSCs at level 2. To estimate the amplitude of fibers recruited to level 3, the raw amplitudes of the EPSCs at level 2 were subtracted from the raw amplitudes of the EPSCs at level 3 (not shown). D: PPR measurements based on independent EPSC estimates of level 2 (or 3) inputs were also clustered around the diagonal and correlated with the PPR measured for level 1 EPSCs (n = 33 levels, R² = 0.292, P = 0.0012). E: similar plot as B but for the subtracted EPSC amplitudes showed that PPR for each level had a weak negative correlation with EPSC amplitudes (n = 61, R² = 0.062, P = 0.043).

Fig. 7.

Correlated short-term synaptic plasticity across the range of stimulus strength levels: 5-pulse trains. A: in 1 NA neuron, 50-Hz trains evoked at 3 different levels all showed depression. Top: unscaled overlay of 3 average synaptic current traces evoked at different levels. Bottom: same traces but scaled to the initial EPSC amplitude: amplitude of the final scaled EPSCs in the trains for lowest level/weakest stimulus (1), middle (2), and highest level/strongest stimulus (3). B: second example NA neuron, in which 50-Hz trains evoked at 2 different levels both showed mild facilitation: unscaled (top) and scaled (bottom) traces; amplitudes of the final scaled EPSCs in the trains for lower level/weakest stimulus (1) and higher level/strongest stimulus (2). C: train steady-state ratios (SSR = EPSC5/EPSC1) measured for raw EPSC amplitudes at multiple stimulus levels were strongly correlated (n = 36 levels across 33 NA neurons, R² = 0.499, P < 0.0001). D: train ratios (EPSC5/EPSC1) measured for independent EPSC amplitudes are also correlated, although more weakly (n = 36, R² = 0.161, P = 0.015).