Primordial rhythmic bursting in embryonic cochlear ganglion cells

This study examined the nature of spontaneous discharge patterns in cochlea r ganglion cells in embryonic day 13 (E13) to early E17 chicken embryos (st ages 39-43). Neural recordings were made with glass micropipettes. No sound -driven activity was seen for the youngest embryos (maximum intensity 107 d B sound pressure level). Ganglion cells were labeled with biotinylated dext ran amine in four embryos. In two animals, primary afferents projected to h air cells in the middle region along the length of the basilar papilla in w hich, in one cell, the terminals occupied a neural transverse position and, in the other, a more abneural location. Statoacoustic ganglion cells showi ng no spontaneous activity were seen for the first time in the chicken. The proportion of "silent" cells was largest at the youngest stages (stage 39, 67%). In active cells, mean spontaneous discharge rates [9.4 +/- 10.4 spik es (Sp)/sec; n = 44] were lower than rates for older embryos (19 +/- 17 Sp/ sec) (Jones and Jones, 2000). Embryos at stages 39-41 evidenced even lower rates (4.2 +/- 5.0 Sp/sec). The most salient feature of spontaneous activit y for stages 39-43 was a bursting discharge pattern in >75% of active neuro ns (33 of 44). Moreover, in 55% of these cells, there was a clear, slow, rh ythmic bursting pattern. The proportion of cells showing rhythmic bursting was greatest at the youngest stages (39-42) and decreased to <30% at stage 43. Rate of bursting ranged from 1 to 54 bursts per minute. The presence of rhythmic bursting in cochlear ganglion cells at E13-E17 provides an explan ation for the existence of such patterns in central auditory relays. The bu rsting patterns may serve as a patterning signal for central synaptic refin ements in the auditory system during development.