The anteroventral cochlear nucleus (AVCN) contains two principal cell types that receive input from the auditory nerve. Stellate cells receive conventional synapses on their dendrites, and bushy cells of the AVCN receive axosomatic input via large, calyceal terminals (the end bulbs of Held). We have used whole cell patch-clamp recording techniques to study excitatory postsynaptic currents (EPSCs) in these two principal cells of the rat AVCN. 2. EPSCs evoked in stellate cells by stimulation of the auditory nerve were graded with stimulus strength, indicating a high degree of convergence of input to these cells. At depolarized membrane potentials, EPSCs evoked in stellate neurons had a dual-component time course. The slow component was blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV), and the fast component was abolished by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 3. EPSCs evoked in bushy cells by auditory nerve stimulation were large (50 nS average conductance) and all-or-none at the threshold stimulus level. At -70 mV, the time course of the EPSC was very brief (average time constant of decay 700 microseconds at room temperature). Membrane depolarization revealed a slow component to the EPSC. The fast and slow components were mediated by non-NMDA and NMDA receptors, respectively. The switch-off of end bulb NMDA EPSCs by voltage jumps to the EPSC reversal potential was very rapid, suggesting that the NMDA component arises from sites on or close to the soma. 4. Miniature EPSCs, recorded in the presence of tetrodotoxin (TTX) at depolarized potentials, also had a dual-component time course. The fast and slow components of the miniature EPSCs were blocked by CNQX and APV, respectively. This result indicates that NMDA and non-NMDA receptors can be co-localized at the same, presumably end bulb, release sites. 5. The relative contribution of the slow, NMDA component to the end bulb EPSC declined significantly with age (postnatal days 11-22). 6. These results indicate that both NMDA and non-NMDA receptors underlie excitatory synaptic transmission in the AVCN of young rats. The end bulb synapse onto bushy cells generates a non-NMDA receptor-mediated EPSC with very fast kinetics. NMDA receptors can also mediate synaptic transmission at the end bulb synapse, but their contribution becomes less as the auditory system matures. This finding suggests that NMDA receptors may play an important role in the development of this synapse.
