RECEPTORS UNDERLYING EXCITATORY SYNAPTIC TRANSMISSION IN SLICES OF THE RAT ANTEROVENTRAL COCHLEAR NUCLEUS

1. The anteroventral cochlear nucleus (AVCN) contains two principal ce ll types that receive input from the auditory nerve. Stellate cells re ceive 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 techniqu es to study excitatory postsynaptic currents (EPSCs) in these two prin cipal cells of the rat AVCN. 2. EPSCs evoked in stellate cells by stim ulation of the auditory nerve were graded with stimulus strength, indi cating a high degree;ee of convergence of input to these cells. At dep olarized membrane potentials, EPSCs evoked in stellate neurons had a d ual-component time course. The slow component was blocked by the N-met hyl-D-aspartate(NMDA) receptor antagonist DL-2-amino-5-phosphonovaleri c acid (APV), and the fast component was abolished by the non-NMDA rec eptor antagonist 6-cyano-7-nitroquinoxaiine-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 c onstant of decay 700 mu s at room temperature). Membrane depolarizatio n 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 pote ntial was very rapid, suggesting that the NMDA component arises from s ites on or close to the soma. 4. Miniature EPSCs, recorded in the pres ence of tetrodotoxin (?TX) at depolarized potentials, also had a dual- component time course. The fast and slow components of the miniature E PSCs were blocked by CNQX and APV, respectively. This result indicates that NMDA and non-NMDA receptors can be co-localized at the same, pre sumably end bulb, release sites. 5. The relative contribution of the s low, NMDA component to the end bulb EPSC declined significantly with a ge (postnatal days 11-22). 6. These results indicate that both NMDA an d 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 receptor s can also mediate synaptic transmission at the end bulb synapse, but their contribution becomes less as the auditory system matures. This f inding suggests that NMDA receptors may play an important role in the development of this synapse.