Development and elimination of endbulb synapses in the chick cochlear nucleus

The development of synaptic function was examined at auditory nerve synapses in the rostromedial region of the cochlear nucleus magnocellularis of the chick. EPSCs were studied beginning at embryonic day 12 (E12), when synaptic transmission was first observed, through E19. The amplitude of evoked EPSCs produced by AMPA receptor (AMPA-R) increased 30-fold over this age range, whereas NMDA receptor (NMDA-R)-mediated transmission peaked at E14 and then declined almost completely. At E12, >80% of the miniature EPSCs exhibited both receptor components, and <10% were NMDA-R only. With age, the contribution of NMDA-R to miniature EPSCs steadily declined, suggesting that NMDA-R number is gradually reduced at individual postsynaptic sites. Between E12 and E16, the number of axonal inputs to each cell reduced by half. In simultaneous recordings from adjacent neurons, synchronous EPSCs were observed that resulted from spontaneous firing of the same presynaptic fiber. The difference in amplitude of the EPSCs in the two cells was greater in E14 than E12, whereas at E16 synchronous events were no longer observed, suggesting that the weaker input was destined for elimination. The relative amplitude of the NMDA-R component, compared with the AMPA-R component, was larger for the weaker inputs. When elimination was underway, AMPA-R quantal size was much reduced for the weakest terminals. Thus, elimination of auditory nerve terminals and pruning of axonal branches is preceded by a reduction in quantal efficacy.