Long-lasting inhibitory synaptic depression is age- and calcium-dependent

Abstract

The developmental refinement of excitatory synapses is often influenced by neuronal activity, and underlying synaptic mechanisms have been suggested. In contrast, few studies have asked whether inhibitory synapses are reorganized during development and whether this is accompanied by use-dependent changes of inhibitory synaptic strength. The topographic inhibitory projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) undergoes synapse elimination during development (Sanes and Takács, 1993). To determine whether there is an associated period of synaptic plasticity, whole-cell recordings were obtained from developing LSO neurons of gerbils in a brain slice preparation. In current-clamp recordings, low-frequency stimulation of the MNTB led to a decline in IPSP amplitude by 43%. In voltage-clamp recordings, hyperpolarized LSO neurons also exhibited a long-lasting depression of MNTB-evoked inhibitory synaptic currents (34%) after low-frequency stimulation. When LSO neurons were depolarized, low-frequency stimulation of the MNTB produced a significantly larger inhibitory synaptic depression (59%). This synaptic plasticity declined dramatically by postnatal days 17-19. Similar to well studied forms of excitatory synaptic plasticity, inhibitory depression depended on postsynaptic calcium. We propose that such activity-dependent synaptic depression may support the developmental rearrangement of inhibitory terminals as they compete with neighboring excitatory and/or inhibitory inputs.

Fig. 1.

Long-lasting depression of inhibitory synapses in the LSO. A, Schematic of the inhibitory pathway from MNTB to LSO used in the present study is shown. LSO neurons receive a direct inhibitory projection from several MNTB neurons and excitatory inputs from several ipsilateral ventral cochlear nucleus (VCN) neurons. MNTB neurons also project to the MSO. B, IPSPs were recorded from the medial limb of the LSO, while electrical stimuli were delivered to the MNTB. In this P9 LSO neuron, IPSP amplitude remained fairly stable during the first 15 min recording period (the time at which each IPSP was obtained is shown beneatheachtrace). LFS (1 Hz for 15 min;graybar) was then provided to the MNTB at a stimulus intensity that evoked a maximum amplitude IPSP. IPSPs were depressed during the 1 hr period after LFS. Thebottomdashedlineindicates the initial resting membrane potential. During the pre-LFS period, the amplitude of an IPSP recorded at a −40 mV membrane potential (+0.02 nA) was augmented by ∼3 mV, and a similar depolarization 1 hr after LFS increased the depressed IPSP by ∼1 mV.

Fig. 2.

Long-lasting depression of inhibitory synapses in the LSO at a holding potential of −55 mV or less (voltage clamp).A, Representative recordings from a P10 LSO neuron show a maximum amplitude IPSC recorded at V_HOLD = −80 mV at the beginning of the experiment (0 min) and its post-LFS depression at the end of the experiment (90 min). B, Long-lasting depression of IPSCs for all recorded LSO neurons at P7–P12 is shown. Means (± SEM) of IPSCs show that synaptic depression was significant after LFS (blackcircles). Age-matched control neurons that did not receive any LFS or intermediary MNTB stimulation (asterisks) did not get depressed. See Figure 6 for statistics.

Fig. 3.

Long-lasting depression of inhibitory synapses in the LSO at a holding potential of 0 mV. A, Long-lasting depression of inhibitory transmission for an individual P9 LSO neuron is shown. Top, Representative MNTB-evoked IPSCs are shown before (left) and after (right) LFS. Bottom, The amplitude of IPSCs in one neuron declined dramatically after LFS, and this depression persisted for 1 hr. B, Long-lasting depression of IPSCs for all recorded LSO neurons at P7–P12 is shown. Means (± SEM) of IPSCs show that synaptic depression was profound after LFS (blackcircles). Age-matched control neurons in which LFS was not delivered (whitecircles) or without LFS and intermediary MNTB stimulation (asterisks) displayed a smaller, but significant, decrease in amplitude.

Fig. 4.

Long-lasting synaptic depression was age-dependent. Top, MNTB-evoked IPSCs in a P17 neuron before LFS (left) and 60 min after LFS (right). Bottom, A comparison of summary data from P17 to P19 LSO neurons that were treated with LFS (whitesquares) with data from P7 to P12 neurons (graycircles). The magnitude of synaptic depression in the older age group is much reduced (see Fig. 6 for statistics).

Fig. 5.

Long-lasting synaptic depression was calcium-dependent. A,Bottom, Comparison of synaptic depression in P7–P12 neurons at holding potentials of less than or equal to −55 mV recorded with normal pipette solution [graycircles (from Fig. 2)] or with pipette solution containing 20 mm BAPTA (blacksquares). Synaptic depression was abolished by intracellular perfusion with BAPTA (see Fig. 6 for statistical comparisons). Top, Representative currenttraces for one BAPTA-treated P9 neuron.B,Bottom, Comparison of synaptic depression in P7–P12 neurons at a holding potential of 0 mV recorded with normal pipette solution [graycircles (from Fig. 3)] or with pipette solution containing 20 mm BAPTA (graysquares). Synaptic depression was abolished by intracellular perfusion with BAPTA (see Fig. 6 for statistical comparisons). Top, Representative currenttraces for one BAPTA-treated P9 neuron. Note that the mean size of IPSCs in BAPTA-treated neurons became larger after LFS, although the variance was high.

Fig. 6.

Comparison of percent change of IPSC amplitude at 90 min in each experimental group. At V_HOLD ≤ −55 mV, LFS produced a significant change in IPSC amplitude as compared with no stimulation (No Stim; p < 0.05; t = symbol 45 2.16; df = 14) or LFS + BAPTA (p < 0.05; t = 2.1; df = 15). At V_HOLD = 0 mV, LFS at P7–P12 produced a significant change in IPSC amplitude as compared with no LFS (p < 0.0005; t = symbol 45 3.6; df = 29), no stimulation (p < 0.05; t = symbol 45 3.6; df = 29), LFS at P17–P19 (p < 0.0005; t= symbol 45 4.14; df = 28), or LFS + BAPTA (p < 0.0001; t = 3.6; df = 31). Finally, LFS induced a significantly greater reduction in IPSC amplitude when delivered at V_HOLD = 0 as compared with V_HOLD ≤ −55 (p < 0.05; t = 2.23; df = 29). The mean percent change was calculated by comparing the average IPSC amplitude recorded at 50–60 min after LFS with the initial IPSC amplitude.