The Will to Sing
Adult neurogenesis, or the phenomenon where new neurons are born and incorporated into brain circuitry after an organism has fully matured, is a topic that is both hotly debated yet inadequately addressed. The zebra finch (Taeniopygia guttata) song system is a useful model to investigate the role of adult-born neurons in modulating neural circuitry and affecting robust behavior as zebra finch song development has many parallels to human vocal development and auditory processing. Adult neurogenesis occurs in the zebra finch HVC (proper name, high vocal center), a brain area that governs song motor production and song perception. Half of adult-born HVC neurons (HVC NNs) project to the robust nucleus of the arcopallium (RA) which implicates a role for adult neurogenesis in motor production of song. Interestingly, there is a subpopulation of HVC NNs that do not project to this region and are also active during song. We have previously demonstrated that this subpopulation of HVC NNs express the phosphoprotein DARPP-32, are active during song by measuring immediate early genes and are contacted by tyrosine hydroxylase fibers. These preliminary findings suggest that DARPP-32 HVC NNs receive inputs from the periaqueductal gray (PAG), a region that receives signals from other brain regions involved in appetitive and consummatory aspects of reproductive behavior and is active during mating behavior including in courtship dance and directed song. Lesioning PAG cell bodies abolishes these mating behaviors, including directed song. To this end, we hypothesize that DARPP-32 HVC NNs are excitatory interneurons that are preferentially active during directed song and serves to integrate the social salience of song, contributing to the motor output of directed song that is more precise and faster than undirected song. To compare DARPP-32 HVC NN activity between directed and undirected song, we measured immediate early gene induction (zenk) during song behavioral monitoring. Birds were split into three conditions: control where birds did not sing, directed song, and undirected song. Birds were also injected with fluorogold retrograde tracer to ensure a significant population of DARPP32 HVC NNs were not RA projecting. In addition, bird songs were analyzed to assess possible roles for DARPP32 HVC NNs in modulating bioacoustic features of song. Supporting our hypothesis, these preliminary results suggest that DARPP32 HVC NNs initially have variable firing rates irrespective of song they sang, but as they become older and incorporate within the circuit, DARPP32 HVC NNs are preferentially active during directed song. Although further work must be done to fully characterize DARPP32 HVC NNs, this provides a putative role for adult-born neurons in encoding and integrating the social salience of song into motor behavior.