Development of functional innervation in the second and third order auditory nuclei of the chick

The neurones that constitute the auditory nuclei of the brainstem in the chick (nuclei magnocellularis, NM, and laminaris, NL) are generated between days 2 and 4 of incubation. These neurones migrate towards the dorsal surface of the brainstem over the next few days and reach their final destination at about day 9 of incubation. We have examined the development of functional connections between the auditory nerve and neurones in NM and between neurones in NM and NL in embryos from stage 34 (day 8 of incubation) using electrophysiological techniques and electron and light microscopy. The earliest extracellular recordings of electrically evoked field and spike potentials were made in NM with stimulation of the ipsilateral auditory nerve and in NL with stimulation of the ipsilateral NM at stage 35 (day 9). No activity could be recorded in NL with stimulation of the auditory nerve at this stage. By stage 37 (day 11), direct stimulation of the contralateral NM evoked responses in NL and by stage 38 (day 12) stimulation of the auditory nerves evoked stable field potentials in NL. These potentials changed polarity as the electrode penetrated NL in a direction that was perpendicular to the laminar arrangement of neurone somas and parallel to the dendritic axes of these neurones. In 18 of 26 analyses of current-source density in NL of 12 preparations between stages 38 and 40 there was a sink of current associated with synaptic activity at levels both above and below the source of current (cell somas) following stimulation of the ipsilateral auditory nerve. In the remaining analyses, and in all 15 analyses from preparations older than stage 40, stimulation of the ipsilateral input evoked only a single sink of current above the level of the cell somas. In all preparations from embryos at stage 38 and older, stimulation of the contralateral auditory nerve was associated with a single sink of current below the level of the cell somas. The axon projections to the ipsilateral NL from neurones in NM were examined using HRP labelling between stages 38 and 40. The presence of terminal fields of single axons in both the dorsal and ventral dendritic regions of the ipsilateral NL at these ages was confirmed. Furthermore, dense vesicles within synaptic terminals in both the dorsal and ventral dendritic fields could be identified in preparations at stage 36 following injection of HRP into NL and stimulation of the ipsilateral NM.(ABSTRACT TRUNCATED AT 400 WORDS)