Development of the delay lines in the nucleus laminaris of the chicken embryo revealed by optical imaging

One strategy in localizing a sound source in the azimuthal plane is the comparison of arrival times of sound stimuli at the two ears. The processing of interaural time differences (ITDs) in the auditory brainstem was suggested by the Jeffress model in 1948. In chicks, binaural neurons in the nucleus laminaris (NL) receive input from both ipsilateral and contralateral nucleus magnocellularis (NM) neurons, with the axons of the latter acting as delay lines. A given neuron in the NL responds maximally to coinciding input from both NM neurons. To achieve maximum resolution of sound localization in the NL, the conduction velocity along these delay lines must be precisely tuned. Here, we examined the development of this velocity between embryonic days (E)12 and E18. Our optical imaging approach visualizes the contralateral delay lines along almost the complete NL of the chicken embryo. Optical imaging with the voltage-sensitive dye RH 795 showed no significant differences in the velocity between E12 and E15, but a significant increase from E15 to E18, at both 21 °C and 35 °C. Surprisingly, at 21 °C the conduction velocity in the dorso–lateral part of the NL was significantly higher compared to the situation in the ventro–medial part. The observed development in contralateral conduction velocity may be due to a developmental increase in myelination of the NM axons. Indeed, antibody staining against myelin-associated glycoprotein (α-MAG) showed no myelination of the NM axon branches within the NL at E12 and E15. On the other hand, a clear α-MAG immunoreactivity occurred at E18. Our results therefore describe the developmental physiological properties of the delay line in the chicken embryo.