A Subtype of Adult-Born Neuron in Male Zebra Finch HVC Expresses DARPP-32 and Does Not Project to RA as part of the Vocal Motor Pathway

Adult male zebra finches (Taeniopygia guttata) continually incorporate adult-born neurons into HVC, a telencephalic brain region necessary for the production of learned song. These neurons express immediate early genes following song production, suggesting a role for neurogenesis in song production throughout the lifespan. Half of these adult-born HVC neurons (HVC NNs) send their axons to RA as part of the vocal motor pathway underlying learned song production, but the other half do not, and their identity remains unknown. Here we used cell birth-dating, retrograde tract tracing, and immunofluorescence to demonstrate that half of all HVC NNs express the neuron-specific phosphoprotein DARPP-32, a protein associated with DA receptor expression. We also demonstrate that DARPP-32+ HVC NNs are contacted by tyrosine hydroxylase immunoreactive fibers suggesting that they receive catecholaminergic input, are transiently larger than DARPP-32-negative HVC NNs, and do not send axons to RA as part of the vocal motor pathway. Taken together, these findings indicate the existence of a class of HVC NNs that do not project to RA and may constitute the aforementioned unknown half of all HVC NNs.