A Comparison of Habituation Rates in Adult-Born and General Population Neurons in the Zebra Finch Caudomedial Nidopallium

Adult neurogenesis, or the addition of new neurons to the brain after development, is an incompletely understood process with potential implications for learning and memory. The zebra finch (Taeniopygia guttata) song system, which receives new neurons during adulthood and is involved in an auditory learning process that in many ways parallels that of humans, makes them a useful model for investigating these putative roles. While some suggest that new neurons are activated during learning in much the same way as the general neuronal population, others argue that new neurons have a more plastic role in learning and memory. To probe this question, we examined auditory learning in the songbird caudomedial nidopallium (NCM, analogous to the mammalian auditory association cortex), a region that plays a role in auditory processing. Neurons in the NCM express the immediate early gene zenk, a correlate of neural activity, when exposed to novel conspecific song, but the response lessens as the song is repeated. This habituation is specific and long-lasting, suggesting that the NCM is specialized for learning and forming memories of new songs quickly. To assess the degree to which new neurons participate in song learning, we compared the habituation rate in adult-born neurons and the general neuronal population. One group of birds was repeatedly exposed to a novel conspecific song to induce NCM habituation, while another group heard the song for the first time immediately prior to sacrifice. We used a triple-labeling protocol to stain for a cell birth marker (BrdU), a neuronal marker (Hu), and an activity-dependent immediate early gene protein product (ZENK). With this protocol, a triple-labeled cell was an adult-born neuron activated during song exposure. Comparing the habituation rates of new and old neurons could reveal their relative roles in song learning. If new neurons habituate at a similar rate as the general neuronal population, this would indicate that they are incorporated into existing auditory learning circuitry and play similar roles as existing neurons. If the drop in new neuron response with song repetition is significantly more than in old neurons, this could point to a more plastic role for adult-born neurons in the NCM. We hypothesized that we would see evidence for the latter possibility. Additionally, we predicted greater activation in response to novel song in the new neuronal population than in the general neuronal population, which would also indicate relatively high plasticity. Our preliminary results support these hypotheses, and also indicate that adult-born neurons have higher baseline activity than general population neurons. Further work must be done to elaborate these findings, but they are suggestive of a role for young adult-born neurons in learning and memory.