Expression of NGF and trkA mRNA in song control and other regions of the zebra finch brain

We used in situ hybridization to measure the expression of NGF and trkA mRNA in the zebra finch brain at posthatch day 11 (P11), P25, and in adulthood. Expression of NGF and trkA was restricted to specific areas of the telencephalon in the adult zebra finch brain. Interestingly the expression of NGF and trkA overlapped in most brain regions, suggesting that NGF acts at sites close to cells that synthesize it. In song regions of adults, both NGF and trkA were clearly expressed in lMAN, HVC, and RA in males and in lMAN and RA in females. At P11, NGF and trkA mRNA were detected only in RA in both sexes. At P25, when sex differences in lMAN and RA begin to emerge, NGF mRNA was expressed in lMAN and RA in both sexes and trkA was detected at low levels in lMAN in both sexes. Whereas the level of trkA expression in RA of males at P25 was consistently low but detectable, expression in females was not detected. The volume of RA defined by NGF was significantly larger in males than females at P25. We also found a tendency for the intensity of NGF in RA to be higher in males than in females at P25, although the difference was not statistically significant. The presence of NGF and trkA mRNA in RA and lMAN at P25 suggests that they may participate in sexually dimorphic neural development of RA and lMAN, possibly by participating in sex-specific cell survival.