The distribution of substance P and met-enkephalin in vocal control nuclei among oscine species and its relation to song complexity

Abstract

Substance P (SP) and methionine-enkephalin (ENK) have been reported to appear in song control nuclei of oscine species. However, it remains unknown whether or not SP and ENK location in song control nuclei is correlated with song behavior. To address this issue, the present study first measured two variables for song complexity, i.e., song repertoire sizes, and syllable repertoire sizes in 11 oscine species. Then, we examined the distribution of SP and ENK in four control nuclei, two in the motor pathway, i.e., HVC and the robust nucleus of arcopallium (RA), and the other two in the forebrain pathway, i.e., Area X and the lateral magnocellular nucleus of the anterior nidopallium (LMAN). Finally, we measured the relative amounts of immunoreactivity for SP and ENK in song control nuclei, and tested whether they were correlated with song complexity. Our results showed that: (1) SP and ENK were broadly distributed in the song control nuclei of studied species. However, SP immunohistochemistry was more robust in comparison with ENK, and SP is generally more abundant in the two song learning nuclei than those in the two song producing ones; (2) SP and ENK staining patterns in song control nuclei did not show any obvious phylogenetic relationship among studied oscine species; (3) there was a significant correlation between the relative amounts of immunoreactivity for SP and the song and syllable repertoire sizes. Our results suggest that SP or ENK might be involved in song behavior, such as birdsong learning or memory.