Organization and development of zebra finch HVC and paraHVC based on expression of zRalDH, an enzyme associated with retinoic acid production.

The zRalDH gene encodes an aldehyde dehydrogenase associated with the conversion of retinaldehyde (the main vitamin A metabolite) into retinoic acid and its expression is highly enriched in the song control system of adult zebra finches (Taeniopygia guttata). Within song control nucleus HVC, zRalDH is specifically expressed in the neurons that project to area X of the striatum. It is also expressed in paraHVC, commonly considered a medial extension of HVC that is closely associated with auditory areas in the caudomedial telencephalon. Here we used in situ hybridization to generate a detailed analysis of HVC and paraHVC based on expression of zRalDH for adult zebra finches of both sexes and for males during the song-learning period. We demonstrate that the distribution of zRalDH-positive cells can be used for accurate assessments of HVC and paraHVC in adult and juvenile males. We describe marked developmental changes in the numbers of zRalDH-expressing cells in HVC and paraHVC, reaching a peak at day 50 posthatch, an effect potentially due to dynamic changes in the population of X-projecting cells in HVC. We also show that zRalDH-expressing cells in adult females, although much less numerous than in males, have a surprisingly broad distribution along the medial-to-lateral extent of HVC, but are lacking where paraHVC is found in adult males. Our study thus contributes to our understanding of the nuclear organization of the song system and the dynamics of its developmental changes during the song-learning period.