Afferent input is necessary for seasonal growth and maintenance of adult avian song control circuits

The neural circuits that regulate song behavior in adult songbirds undergo pronounced seasonal changes in morphology, primarily in response to changes in plasma testosterone (T). Most song nuclei have T receptors. We asked wh ether seasonal growth and maintenance of nuclei within these circuits are d irect responses to the effects of T or its metabolites or are mediated indi rectly via the effects of T on afferent nuclei. Photosensitive white-crowne d sparrows were exposed to one of three treatments. (1) The neostriatal nuc leus HVc (also known as the "high vocal center") was lesioned unilaterally, and the birds were exposed to long-day (LD) photoperiods and breeding leve ls of T for 30 d. (2) Birds were exposed to LD plus T (LD+T) for 30 d; then HVc was lesioned, and the birds were killed after an additional 30 d expos ure to LD+T. (3) HVc was lesioned, and the sparrows were housed on short-da y (SD) photoperiods in the absence of T treatment for 30 d. In both LD+T gr oups, the direct efferent targets of HVc, the robust nucleus of the archist riatum (RA) and area X, were smaller ipsilateral to the lesion. The lesion did not prevent growth of the hypoglossal motor nucleus, which does not rec eive direct afferent input from HVc. RA and area X were also smaller ipsila teral to the lesion in the SD birds. These results indicate that afferent i nput is required both for the growth of adult song circuits in response to typical breeding photoperiod and hormone conditions and for the maintenance of efferent nuclei in either their regressed or enlarged states.