Minimizing synaptic depression by control of release probability

Abstract

Transmission at the end-bulb synapse formed by auditory nerve terminals onto the soma of neurons in the avian nucleus magnocellularis is characterized by high transmitter release probability and strong synaptic depression. Activation of presynaptic GABA(B) receptors minimizes depression at this synapse and significantly enhances synaptic strength during high-frequency activity. Here we investigate synaptic mechanisms underlying this phenomenon. EPSC amplitudes evoked by 200 Hz trains increased more than twofold when release probability was reduced with Cd(2+) or baclofen. This effect was not exhibited by a transmitter depletion model of presynaptic depression, which predicts that EPSC amplitudes reach a common steady-state amplitude during high-frequency trains, despite alterations of initial release probability. However, an additional source of postsynaptic depression was sufficient to explain our findings. Aniracetam, a modulator of AMPA receptors that reduces desensitization, decreased the amount of synaptic depression during trains, indicating that desensitization occurred during trains of stimuli. However, this effect of aniracetam was absent when release probability was lowered with baclofen or Cd(2+). No effect of aniracetam on the NMDA component of the EPSC was seen, confirming a postsynaptic site of action of aniracetam. When desensitization was reduced with aniracetam, steady-state EPSC amplitudes during trains were found to converge over a wide range of release probabilities, as predicted by the depletion model. Additional evidence of AMPA receptor desensitization was provided by direct measurement of quantal amplitudes immediately after stimulus trains. Thus, presynaptic modulation by GABA(B) receptors regulates the extent of AMPA receptor desensitization and controls synaptic strength, thereby modulating the flow of information at an auditory synapse.

Fig. 1.

Enhancement of synaptic strength by lowering release probability. Ai, Trains of 10 EPSCs were evoked at 200 Hz under control conditions (bottom trace) or in the presence of 50 μm baclofen (top trace). The first peak in baclofen was reduced to 19.3% of control. V_m = −30 mV. Averages of 5–10 trials are shown. Stimulus artifacts have been removed.Aii, EPSCs 8–10 have been enlarged to illustrate the increase in EPSC amplitudes when initial release probability was reduced with baclofen (control, black trace; baclofen,gray trace). Bi, Same as inA, but in the presence of Cd²⁺ (20 μm) (top trace). Peak of EPSC₁in Cd²⁺ was reduced to 20.1% of control.Bii, EPSCs 8–10 have been enlarged to illustrate enhancement of EPSC_SS in the presence of Cd²⁺ (control, black trace; Cd²⁺, gray trace). C,Filled circles show amplitude ratio of EPSCs in baclofen to controls for each stimulus. As the control EPSCs depress during the train, this value increases. Average enhancement of EPSCs for stimuli 8–10 in this cell was 223%. Open circles show ratios of EPSCs in Cd²⁺ relative to controls. Average steady-state enhancement of EPSCs in this cell was 186%. Data inA–C are from two different neurons. D, Bar graph comparing results for Cd²⁺(n = 6) and baclofen (n = 6).Black bars show ratios of EPSC₁ amplitude in 50 μm baclofen or 20 μmCd²⁺ relative to control (EPSC_{1, BAC (or Cd)}/EPSC_{1, CON}). Gray barsshow ratios of steady-state EPSC amplitude (average of EPSCs 8–10 of 200 Hz trains) in baclofen or Cd²⁺ relative to control (EPSC_{SS, BAC (or Cd)}/EPSC_{SS, CON}).

Fig. 2.

Relationship between EPSC_SSand P_R. Ai, Trains (200 Hz) of 10 stimuli were recorded in levels of Cd²⁺ranging from 0 to 100 μm. The first EPSC in each train is shown.Aii, Superimposed traces in high Cd²⁺(gray trace) and in 0 Cd²⁺(black trace). B, EPSC_SS(averages of EPSCs 8–10) are plotted versus amplitude of the first EPSC for each train. C, Pooled results from six neurons. Data from each neuron were grouped in bins associated with EPSC₁ values from 0–1, 1–2 nA, etc. Means and SEs were calculated for each bin. A linear fit to the data points (excluding the leftmost point associated with the smallest value of EPSC₁) indicated a significant negative correlation between EPSC₁ and EPSC_SS (r² = 0.968;p < 0.0001).

Fig. 3.

Effect of baclofen on EPSC_SS.Ai, Trains (200 Hz) were delivered in baclofen concentrations ranging from 0 to 100 μm. The first EPSC of the train is shown. Aii, Responses in 0 and 100 μm baclofen are shown. After the second stimulus of the train, EPSCs in baclofen are larger than controls. B, EPSC_SS is plotted against EPSC₁ of each train. As P_R is lowered with baclofen, steady-state EPSCs increase in size, reaching a plateau as saturating levels of baclofen are reached. In this cell, enhancement of EPSC_SS relative to control was 361% when EPSC₁ was reduced with baclofen to between 5.1 and 8.1% of its control value. C, Pooled data from six neurons. Data were binned as in Figure 2C. A linear fit to theeight rightmost data points was highly significant (r² = 0.903;p < 0.001).

Fig. 4.

Effect of aniracetam on synaptic depression.Ai, A single EPSC in 5 mm aniracetam (gray trace). Control peak (black trace) is indicated with an arrow. In this cell, aniracetam increased the peak amplitude by 60% (from −6.19 to −10.24 nA). Aii, A 200 Hz train in control (black trace) and aniracetam (gray trace) is shown. Traces have been normalized to the amplitude of the first peak in each train. Averages of five traces are shown. Bi, Normalized peak amplitudes during 200 Hz stimulus trains (control,black circles; aniracetam, gray circles). Data from 13 cells. Depression is significantly reduced by aniracetam for stimuli 2–10 of trains (p < 0.01; paired t test). Bii, Relative enhancement of EPSC amplitudes by aniracetam throughout the train, calculated as 100% · (EPSC_ANI − EPSC_CON)/EPSC_CON. C, Effect of aniracetam on synaptic depression in the continuous presence of 20 μm Cd²⁺. Ci, EPSC trains at 200 Hz in 20 μm Cd²⁺(black trace) and 20 μmCd²⁺ plus 5 mm aniracetam (gray trace). In this example, aniracetam increased the peak of the first EPSC by 55.8% (from 0.84 to 1.30 nA).Cii, Responses during trains in the presence of Cd²⁺ (black circles) and Cd²⁺ plus aniracetam (gray circles) were normalized to the first response amplitude and plotted versus stimulus number. No effect of aniracetam on synaptic depression was seen when P_R was lowered with 20 μm Cd²⁺ (n = 12).

Fig. 5.

Convergence of EPSC_SS in the presence of aniracetam A, In the continuous presence of 5 mm aniracetam, 200 Hz trains of 10 stimuli were delivered in various concentrations of Cd²⁺ ranging from 0 to 100 μm. Ai, The first EPSC of each train is shown. Aii, Superimposed traces in high Cd²⁺ (gray trace) and low Cd²⁺ (black trace) are shown.B, EPSC_SS is plotted versus EPSC₁ for each train. C, Data from seven cells in aniracetam (filled circles). As in Figure 2, data from each neuron were grouped in bins associated with EPSC₁ values from 0–1, 1–2 nA, etc. Means and SEs were calculated for each bin. The slope of a linear fit to the data points (excluding the 2 leftmost pointsassociated with EPSC₁ values from 0 to 2 nA) was not significantly different from 0 (r² = 0.26;p = 0.20). Control data from Figure2C have been scaled up by a factor of 1.37 to account for the effect of aniracetam on the amplitude of a single EPSC (see Fig. 4A) and are shown for comparison (open circles).

Fig. 6.

NMDA component of EPSC is not affected by aniracetam. A, Pairs of EPSCs at 20 msec intervals were recorded at +50 mV in GYKI-52466 and DNQX under control conditions (Ai) and in 5 mm aniracetam (Aii). Average of five traces. Peak 2 (bottom traces) in Ai and Aii was obtained by subtraction of the averaged single EPSC (data not shown).B, Paired-pulse ratios (EPSC₂/EPSC₁) for responses in control solutions and in 5 mm aniracetam (n = 9). C, Average values of EPSC₂/EPSC₁ for NMDA component of EPSC (control and aniracetam) and AMPA component (control;n = 9).

Fig. 7.

Depression of quantal size during stimulus trains.Ai, Averaged EPSC recorded in 3 mmSr²⁺. Aii, Twenty superimposed traces after a single stimulus. Asynchronously released quanta are visible arising from the decay of the preceding EPSC. Dotted line indicates zero-current baseline. Aiii, Means and SE of amplitude and latency for quantal events binned in groups of 30. For this synapse, the amplitude of the first bin was depressed by 49% relative to the steady-state amplitude at longer latencies. A single-exponential fit to the data gave a recovery time course of 27 msec. Bi, Averaged traces from a train of five stimuli at 200 Hz in 3 mm Sr²⁺.Bii, Twenty-five superimposed traces after the fifth EPSC of the train shown in Bi. Biii, Mean amplitude and latency for quantal events binned in groups of 30. In this synapse, the amplitude of the first bin was depressed by 38% relative to steady-state amplitude at longer latencies. Recovery time course of quantal amplitudes was 11 msec.

Fig. 8.

Depletion and desensitization model of synaptic depression. A, Simulations based on a presynaptic depletion model that does not include receptor desensitization. Trains of 10 stimuli at 200 Hz with P_R values ranging from 0.02 to 0.75. Smaller responses to stimulus #1 correspond to lower P_R values. Time constant of recovery from depression was 100 msec. EPSCs reached similar steady-state values when P_R ranged from 0.15 to 0.75. Inset on right shows the 10th response on an expanded vertical scale. Note that data points are superimposed from trains withP_R of 0.15 to 0.75. Y values (normalized to EPSC₁) for these four data points ranged from 0.075 to 0.079. B, Desensitization was included in the model as described in Materials and Methods. P_R was varied as inA. Steady-state EPSCs were maximal atP_R = 0.15. Inset shows 10th response on an expanded vertical scale. C, EPSC_SS is plotted versus EPSC₁. Results of the presynaptic model are shown by filled circles; results of simulations incorporating postsynaptic desensitization are shown with open circles.