Testosterone-dependent plasticity of avian forebrain neurons is not restricted to the song control system

Testosterone is acting on brain areas involved in the control of sexual behaviour, for example the preoptic area and the song system. We now demonstrate that it also affects other avian brain areas, as exemplified here by measurement of spine densities. Depletion of testosterone by castration or application of cyproterone acetate leads to a decrease in spine density in secondary sensory areas like lateral neo- and hyperstriatum and hyperstriatum accessorium and dorsale, or in associative areas such as the caudal archi- and neostriatum. We conclude that testosterone is acting directly on the spines, and suggest that the mechanism of spine density control by hormones may have arisen because of energy demands.