Ontogeny and Patterning

Dynamics between neuronal birth, addition, and survival as the song circuit grows to breeding state

Gambel’s white-crowned sparrow is capable of adding more than 68,000 neurons to HVC within four days of transition to breeding conditions. Along with this addition in new neurons, these sparrows also increase song production and song quality. To identify the dynamics between neurogenesis, the addition of new HVC neurons, and neuronal turnover (or death and replacement) during rapid seasonal growth, we labeled dividing neural stem cells and their progeny and quantified the incorporation of these progeny into HVC. Throughout the course of the experiment, we recorded individual bird’s song and analyzed the spectral features and stereotypy of whole songs and individual syllables. We assessed correlations between song stereotypy and rate, neuronal changes within HVC, and neural stem cell proliferation in the nearby ventricular zone during seasonal rapid growth. Characterizing the interactions between stem cell proliferation, neuronal addition and survival, and a biologically relevant behavior will allow for further identification and testing of molecular and physiological mechanisms underlying adult neurogenesis.

How neuronal birth and death change across seasons to allow growth and regression of the song circuit
Model of turnover in HVC

Neuronal birth and death, and the balance between the two, are fundamental processes of adult neural plasticity. Songbirds are an excellent model for exploring the dynamics of neuronal turnover and their effects on behavior, as seasonal production of song is under the control of a discrete but plastic neural circuit. This circuit includes the avian song control nucleus HVC and its target, the robust nucleus of the arcopallium. Seasonal plasticity of HVC in Gambel’s white-crowned sparrows involves pronounced changes in neuron number; full growth and regression of HVC occur within 7 days of a rapid transition between breeding and non-breeding conditions, and neuron number changes by around 68,000 neurons. This dramatic differences in neuron number between seasons suggest that balance between neuronal birth and death differ between seasons as well. To determine if there are seasonal differences in neuronal turnover, we labeled two cohorts of new HVC neurons separated varying lengths of time in both breeding and nonbreeding condition birds. We determined turnover rate by quantifying the number of new neurons from each cohort that persist in HVC through the different time intervals. In nonbreeding condition birds the number of new neurons remaining in HVC is higher from the second cohort of neurons than the first cohort, whereas the number of new neurons persisting in HVC is higher from the first cohort in breeding conditions. Our study suggests that the seasonal changes in HVC neuron number results from a dynamic shifting of the balance between neuronal birth and death.