Activity-dependent refinement of inhibitory connections.

Several lines of evidence suggest that excitatory synaptic transmission contributes to the maturation of precise neuronal connections. In the present study we determined whether the specific innervation pattern of single inhibitory arborizations was dependent upon neuronal activity during development. A homogeneous group of glycinergic inhibitory neurons in the central auditory system, the medial nucleus of the trapezoid body (MNTB), was functionally denervated in neonatal gerbils. The anatomical specificity of single MNTB terminal arborizations was subsequently measured along the tonotopic axis of a postsynaptic target, the lateral superior olive. Here we demonstrate that inhibitory terminal boutons spread a significantly greater distance along the frequency axis of the postsynaptic target following functional denervation. Although total arbor length remained unchanged, there was a significant increase in the number of branch points, suggesting de novo sprouting. The results indicate that normal inhibitory synaptic activity contributes to the developmental refinement of specific neuronal connections.