Developmental changes in short-term plasticity at the rat calyx of Held synapse

Abstract

The calyx of Held synapse of the medial nucleus of the trapezoid body functions as a relay synapse in the auditory brainstem. In vivo recordings have shown that this synapse displays low release probability and that the average size of synaptic potentials does not depend on recent history. We used a ventral approach to make in vivo extracellular recordings from the calyx of Held synapse in rats aged postnatal day 4 (P4) to P29 to study the developmental changes that allow this synapse to function as a relay. Between P4 and P8, we observed evidence for the presence of large short-term depression, which was counteracted by short-term facilitation at short intervals. Major changes occurred in the last few days before the onset of hearing for air-borne sounds, which happened at P13. The bursting pattern changed into a primary-like pattern, the amount of depression and facilitation decreased strongly, and the decay of facilitation became much faster. Whereas short-term plasticity was the most important cause of variability in the size of the synaptic potentials in immature animals, its role became minor around hearing onset and afterward. Similar developmental changes were observed during stimulation experiments both in brain slices and in vivo following cochlear ablation. Our data suggest that the strong reduction in release probability and the speedup of the decay of synaptic facilitation that happen just before hearing onset are important events in the transformation of the calyx of Held synapse into an auditory relay synapse.

Figure 1.

Developmental changes in firing pattern. A, Example of bursting firing pattern of a principal neuron from the MNTB of a P5 rat during an in vivo juxtacellular recording. Bottom, Complex waveforms at higher time resolution, consisting of a prespike, and followed by an eEPSP and the postsynaptic action potential (eAP). The bottom panel shows the interevent interval histogram. Preferred intervals cluster around 10 ms, a few hundred milliseconds, and 10 s. B, As in A, except that the age of the rat was P11. The interval histogram shows the lack of very long intervals. C, As in A, except that the age of the rat was P28. Middle, Three complex waveforms; the middle one lacks an eAP. The bottom shows the interval histogram, showing smaller and less variable intervals than at the younger ages. D, Surface plot showing the age dependence of the event interval distribution in a total of 142 units. Sixty-three cells from Tritsch et al. (2010) within the P4–P8 range have been included. In each cell, the histogram is normalized to its largest value. Dashed gray line and arrow indicate hearing onset at P13. E, Developmental decrease in CV of event intervals. F, Developmental increase in average firing rate.

Figure 2.

Developmental changes in timing and reliability of synaptic transmission. A, The first complex waveform in the middle panels of Figure 1, A and C, are aligned on the prespike and scaled to the same peak amplitude to illustrate the difference in the time course of the complex waveform at P5 and at P28. B, Developmental changes in eAP half-width. The solid line is an exponential fit with time constant 4 d. C, Developmental changes in latency between prespike and eAP. The solid line is an exponential fit on the data starting at P5, with time constant 8 d. D, Depression of eAP size as a function of eAP interval. eAP sizes from individual recordings (n = 57) were log binned and normalized to the average of the 3 bins with the longest intervals before averaging within the different age groups. Inset shows example of depression of eAP size at short intervals in a P5 rat. E, Relation between eAP size and log interval. Green trace shows the binned average with SD. Black trace shows model fit with single recovery time constant of 320 ms. F, Percentage of subthreshold eEPSPs per cell as a function of age. Red line shows linear regression (r = −0.1; p > 0.4).

Figure 3.

Short-term plasticity changes during development. A, Illustration of complex waveforms from a P7 rat showing synaptic facilitation at short intervals (left), depression at intermediate intervals leading to a postsynaptic failure (middle), and recovery from depression at long intervals (right). Symbols above the waveforms refer to events shown in B. B, Dependence of eEPSP size on interevent interval. Red dots show individual suprathreshold eEPSP amplitudes, purple dots show subthreshold eEPSP amplitudes, and individual squares, triangles, and inverted triangles refer to examples shown in A. Green symbols show binned averages with SD, and black circles indicate the fit with an STP model with both facilitation and depression. Fit parameters were as follows: amplitude 2.0 mV; 95% facilitation per AP, decaying with a time constant of 125 ms; and 38% depression per AP, decaying with a time constant of 1.0 s. The fit could account for 71% of the variance in the eEPSP amplitudes. C, As in B, except the recording was from a P11 animal. Fit parameters were as follows: amplitude 0.6 mV; 62% facilitation per AP, decaying with a time constant of 12.7 ms; and 2.9% depression per AP, decaying with a time constant of 4.4 s. The fit could account for 32% of the variance in the eEPSP amplitudes. D, As in B, except the recording was from a P26 animal. Fit parameters were as follows: amplitude 1.25 mV; 11% facilitation per AP, decaying with a time constant of 11.5 ms. There was no depression. The fit could account for 5.2% of the variance in the eEPSP amplitudes.

Figure 4.

Developmental changes in STP. A, Developmental decrease in the time constant of facilitation, as obtained from the fit with an STP model. Only cells that showed significant facilitation are included (45 of 65 cells). Gray symbols show binned averages with SDs. B, As in A, showing the developmental changes in the amount of facilitation (53 of 65 cells). All cells are included, except when they showed only depression. Cells without significant facilitation are included as 0%. C, Age dependence of the time constant for recovery from depression as obtained from the fit in cells with significant depression (42 of 65 cells). D, Developmental decrease in the amount of depression. Cells without significant depression are included as 0%. E, Developmental decrease in the percentage of variance that can be explained by the model fit (coefficient of determination). Cells without both significant facilitation and depression are included as 0%.

Figure 5.

Developmental changes in STP during paired-pulse stimulation in slice recordings. A, Top, Example EPSCs obtained at the indicated interval in a P5 whole-cell voltage-clamp experiment at 1.2 mm calcium. Bottom, Red dots show individual EPSC amplitudes, green symbols show binned averages with SD, and black circles show fit with an STP model with both facilitation and depression. Fit parameters were as follows: amplitude 5.2 nA; 74% facilitation per AP, decaying with a time constant of 72 ms; and 25% depression per AP, decaying with a time constant of 6.5 s. The fit could account for 68% of the variance in the EPSC amplitudes. B, As in A, except the calcium concentration was lowered to 0.6 mm. Fit parameters were amplitude 0.6 nA, 87% facilitation per AP, decaying with a time constant of 47 ms. The fit could account for 43% of the variance in the EPSC amplitudes. C, As in A, except the recording was from a P15 neuron. Fit parameters were as follows: amplitude 3.4 nA; 104% facilitation per AP, decaying with a time constant of 7.3 ms; and 9% depression per AP, decaying with a time constant of 10.9 s. The fit could account for 57% of the variance in the EPSC amplitudes. D, As in C, except that calcium concentration was lowered to 0.6 mm. Fit parameters were as follows: amplitude 1.4 nA; 136% facilitation per AP, decaying with a time constant of 4.6 ms. The fit could account for 42% of the variance in the EPSC amplitudes. The scale bar for EPSCs in C also pertains to EPSCs shown in A, B, and D.

Figure 6.

Developmental changes in STP in slices during in vivo-like stimulation. A, Histogram of intervals for stimulus protocol. B, Time derivative of whole-cell current-clamp responses of a P5 cell to in vivo-like stimulus protocol. Note the large range and random character of intervals between stimuli. C, Small segment of the responses shown in B. D, Responses shown in C aligned at the rising phase of the iEPSP. Colors correspond to markers above the trace in C. E, Interval dependence of the iEPSP maximal amplitude in a P5 cell at 1.2 mm calcium. The green curve indicates binned average. Error bars show the SD. The black circles are the result of the fit with an STP model, with amplitude 91.4 V/s; 74% facilitation per AP, decaying with a time constant of 95 ms; and 33% depression per AP, decaying with a time constant of 0.5 s. The fit could account for 50% of the variance in the iEPSP amplitudes. F, As in E, except that the calcium concentration was 0.6 mm. Red circles are suprathreshold EPSPs, blue subthreshold. The fit parameters were as follows: amplitude 21 V/s, 70% facilitation per AP, decaying with a time constant of 35 ms. The fit accounted for 28% of the amplitude variance. G, As in E, except that the recording was from a P15 animal. Fit parameters were as follows: amplitude 103 V/s; 47% facilitation per AP, decaying with a time constant of 10.2 ms; 1% depression per AP, decaying with a time constant of 1.7 s. The fit accounted for 18% of the amplitude variance. H, As in G, except that the calcium concentration was 0.6 mm. Fit parameters were as follows: amplitude 42 V/s, 74% facilitation per AP, decaying with a time constant of 5.4 ms. Fit accounted for 4% of the variance.

Figure 7.

Developmental in vivo changes in STP during electrical stimulation following cochlear ablation. A, Illustration of electrically evoked complex waveforms from a P6 rat showing synaptic facilitation at short intervals (left), depression at intermediate intervals leading to a postsynaptic failure (middle), and recovery from depression at long intervals (right) animal. Symbols above the waveforms refer to events shown in B. B, Dependence of eEPSP size on stimulus interval. Same in vivo-like protocol was used as in Figure 6. Red dots show individual suprathreshold eEPSP amplitudes, purple dots show subthreshold eEPSP amplitudes, and individual squares, triangles, and inverted triangles refer to examples shown in A. Green symbols show binned averages with SD, and black circles indicate fit with an STP model with both facilitation and depression. Fit parameters were as follows: amplitude 0.5 mV; 130% facilitation per AP, decaying with a time constant of 97 ms; and 29% depression per AP, decaying with a time constant of 725 ms. The fit could account for 50% of the variance in the eEPSP amplitudes. C, As in B, except that the recording was from a P28 animal. Fit parameters were as follows: amplitude 0.8 mV; 64% facilitation per AP, decaying with a time constant of 6.8 ms; there was no depression. The fit could account for 15% of the variance in the eEPSP amplitudes.

Figure 8.

Comparison of developmental changes in STP in vivo and in vitro. The following three types of experiments are included: spontaneous in vivo data (Fig. 4, open circles; same data), slice data (open gray triangle), and electrically evoked in vivo data (closed squares). Symbols show binned averages with SDs. If multiple protocols from the same cell were applicable, the average from the different protocols was used. A, Developmental decrease in time constant of facilitation, as obtained from the fit with an STP model. Only fits that showed significant facilitation are included. B, As in A, showing the developmental changes in the amount of facilitation. Fits showing only depression are not included. C, Age dependence of the time constant for recovery from depression as obtained from the fit in cells with significant depression. Fits for slice experiments performed at 0.6 mm were not included to calculate the amount or time constant of depression. D, Developmental decrease in the amount of depression. Fits without significant depression were included as 0%. E, Developmental decrease in the coefficient of determination, the percentage of variance that can be explained by the model fit. For stimulation experiments, only the in vivo-like protocols were included. Slice experiments performed at 0.6 mm were not included. Fits without both significant facilitation and depression were included as 0%.