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

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 reorgan ized during development and whether this is accompanied by use-dependent ch anges of inhibitory synaptic strength. The topographic inhibitory projectio n from the medial nucleus of the trapezoid body (MNTB) to the lateral super ior olive (LSO) undergoes synapse elimination during development (Sanes and Taka'cs, 1993). To determine whether there is an associated period of syna ptic plasticity, whole-cell recordings were obtained from developing LSO ne urons of gerbils in a brain slice preparation. In current-clamp recordings, low-frequency stimulation of the MNTB led to a decline in IPSP amplitude b y 43%. In voltage-clamp recordings, hyperpolarized LSO neurons also exhibit ed a long-lasting depression of MNTB-evoked inhibitory synaptic currents (3 4%) after low-frequency stimulation. When LSO neurons were depolarized, low -frequency stimulation of the MNTB produced a significantly larger inhibito ry synaptic depression (59%). This synaptic plasticity declined dramaticall y by postnatal days 17-19. Similar to well studied forms of excitatory syna ptic plasticity, inhibitory depression depended on postsynaptic calcium. We propose that such activity-dependent synaptic depression may support the d evelopmental rearrangement of inhibitory terminals as they compete with nei ghboring excitatory and/or inhibitory inputs.