Developmental influence of glycinergic transmission: regulation of NMDA receptor-mediated EPSPs

The influence of excitatory transmission on postsynaptic structure is well established in developing animals, but little is known about the role of synaptic inhibition. We addressed this issue in developing gerbils with two manipulations designed to decrease glycinergic transmission in an auditory nucleus, the lateral superior olive (LSO), before the onset of sound-evoked activity. First, contralateral cochlear ablation functionally denervated the glycinergic pathway from the medial nucleus of the trapezoid body (MNTB) to the LSO, while leaving the excitatory pathway intact. Second, continuous release of a glycine receptor antagonist, strychnine (SN), was used to decrease transmission. The strength of excitatory and inhibitory synapses was examined with whole-cell recordings from LSO neurons in a brain-slice preparation. The percentage of LSO neurons exhibiting MNTB-evoked IPSPs was reduced in both ablated and SN-treated animals. In those neurons displaying IPSPs, the amplitude was significantly reduced. This decrease was accompanied by an 8 mV depolarization in the IPSP equilibrium potential. In contrast, the ipsilaterally evoked EPSPs were of unusually long duration in experimental animals. These long-duration EPSPs were significantly shortened by hyperpolarizing the neuron to -90 mV or exposing them to aminophosphonopentanoic acid (AP-5), an NMDA receptor antagonist. Membrane hyperpolarization and AP-5 had little effect in control neurons. In addition, LSO neurons from ablated or SN-treated animals displayed broad rebound depolarizations after membrane hyperpolarization, and these were abolished in the presence of Ni2+. Because both cochlear ablation and SN-rearing were initiated before the onset of sound-evoked activity, the results suggest that spontaneous glycinergic transmission influences the development of postsynaptic properties, including the IPSP reversal potential, NMDA receptor function, and a Ca2+ conductance.