Rapid seasonal-like regression of the adult avian song control system

Abstract

We analyzed how rapidly avian song control nuclei regress after testosterone (T) withdrawal. Regression of neuronal attributes resulting from T withdrawal has been observed in several animal models. The time course over which regression occurs is not known, however. To address this issue, we castrated adult male white-crowned sparrows and rapidly shifted them to short-day photoperiods after being held under breeding conditions (longday photoperiod and systemic T exposure) for 3 weeks. We found that the volume of one song nucleus, HVC, regressed 22% within 12 h after T withdrawal. Changes in HVC neuron density after T withdrawal were dynamic; density increased at 12 h and then decreased by 4 days. HVC neuron number was reduced by 26% by 4 days. The volumes of Area X and the robust nucleus of the arcopallium (RA) were significantly regressed by 7 and 20 days, respectively. RA somatic area and neuronal spacing were significantly reduced by 2 days. The rapidity of HVC regression is unprecedented among vertebrate models of hormone-sensitive neural circuits. These results reveal that the rapid regression of the song control system provides a model for the important role sex steroid hormones play in mediating adult neural plasticity and in neuroprotection.birdsong ͉ neurodegeneration ͉ neuroprotection ͉ plasticity ͉ testosterone

Seasonal-like growth and regression of the avian song control system: Neural and behavioral plasticity in adult male Gambel’s white-crowned sparrows

“…Acute withdrawal of circulating T induces a significant decrease in HVC volume within twelve hours (Thompson et al, 2007). This decrease occurs before the birds experience a change in photoperiod, which demonstrates that, at the very least, the initial regression of HVC volume is driven largely by the withdrawal of circulating sex steroids and not by physiological changes driven by photoperiod.…”

“…The volumes of Area X and RA are not significantly regressed until seven and 20 days, respectively, following the transition to non-breeding conditions (Thompson et al, 2007). RA soma area and neuron density significantly regress by two days after the transition to non-breeding conditions and continue to regress for at least 20 days (Thompson et al, 2007).…”

“…HVC essentially collapses in on itself as the space between neurons decreases. Over the next four days, neuron number and soma area (a measure of neuron size) significantly decrease (Thompson et al, 2007). At this point, the rate of neuron loss offsets the decrease in neuron spacing, and neuron density significantly decreases.…”

“…The initial decrease in HVC volume within twelve hours of T withdrawal, however, results from an increase in neuron density (Figure 2A, Thompson et al, 2007). HVC essentially collapses in on itself as the space between neurons decreases.…”

“…It is not yet known if changes in photoperiod are sufficient to induce steroid-independent regression of the song control system in photostimulated males with high levels of circulating T. Despite the fact that the onset of photorefractoriness is not dependent upon a transition to SD photoperiod, we typically expose birds to SD photoperiod at the same time circulating T is withdrawn to ensure that any stimulatory signal LD photoperiod may contribute to maintaining a breeding-state song control system is removed (Thompson et al, 2007). This manipulation is not the same as a transition to photorefractoriness, because male whitecrowned sparrows are probably still photostimulated after just 3-4 weeks of LD and high levels of circulating T. Yet the manipulation abruptly removes two factors known to promote enlarged song control system nuclei; high levels of circulating T and LD photoperiod.…”