Auditory and hormonal stimulation interact to produce neural growth in adult canaries

Gonadal hormones can produce striking behavioral and neural plasticity in adult organisms. For example, systemic administration of testosterone to adult female canaries induces the development of male-typical song behavior and results in a striking increase in the size of brain nuclei that are known to be involved with song control. The mechanism whereby androgens produce such neural plasticity is not known, although it has seemed likely that growth-promoting effects of androgens are due to a direct induction of protein synthesis in cells containing hormone receptors (following activation of specific genes by the hormone-receptor complex). In this experiment we have examined the trophic effect of testosterone in the song-control nucleus HVc (caudal nucleus of the ventral hyperstriatum), which has been shown to contain androgen-concentrating cells as well as neurons that are especially responsive to conspecific song. We report here that testosterone administration increases the volume of HVc in hearing adult female canaries only; testosterone-induced growth of HVc is greatly attenuated in birds that are deprived of auditory stimulation via deafening. Thus, testosterone treatment alone is not a sufficient stimulus for neural growth in HVc. This result suggests that testosterone does not stimulate growth solely via a direct action on hormone receptors in HVc, but rather that testosterone and sensory stimulation can act synergistically to produce structural plasticity in the adult brain.