Hormonal control of cell form and number in the zebra finch song system

Abstract

Administration of testosterone (T), 17 beta-estradiol (E2), or 5 alpha-dihydrotestosterone (DHT) to female zebra finch chicks (Poephila guttata) at hatching exerts effects on brain sexual differentiation. Within a telencephalic station (the nucleus robustus archistriatalis, RA) of the neural pathway which participates in the efferent control of song, masculinization of several indices of neuronal size is induced by exposure to T or E2. Within RA, a sensitive assay of a single neuron's sexually differentiated state is the diameter of its soma. By this criteria, all of the neurons within RA can be masculinized with a sufficient dose of T. As the dose of T is progressively decreased, the proportion of RA neurons which undergo the transition from female to male falls, while the magnitude of the change in soma size remains basically unaltered. Administration of T or DHT masculinizes the number of neurons in RA.

Zebra finch sexual differentiation: The aromatization hypothesis revisited

“…Area X cannot be seen in female zebra finches with a standard Nissl stain (Nordeen and Nordeen, 1989). In the brainstem, nXIIts is larger in volume in males than in females, primarily due to enhanced spacing between the neurons in males (Gurney, 1981;Nottebohm and Arnold, 1976). Overall, the syrinx is heavier in males than in females, which is at least in part caused by an increased size of fibers in the ventralis and dorsalis muscles (Wade and Buhlman, 2000).…”

“…For example, the forebrain regions HVC and RA are larger in volume in males than in females; they contain more cells in males and the cells are larger in males (Gurney, 1981;Nottebohm and Arnold, 1976) (Fig. 1).…”

“…In contrast, some masculinizing effects of adult androgen on the motor nucleus nXIIts have been reported in females that had not received prior treatment. For example, administrating 5␣-dihydrotestosterone (DHT) to adult zebra finch females increases the volume of nXIIts, due to a change in the spacing of the neurons (Gurney, 1981(Gurney, , 1982, and treatment with T increases the volume of the entire hypoglossal nucleus in adult females (Arnold, 1980) although it is still smaller than that of males. In our hands, however, treatments of females with T and males with the anti-androgen flutamide did not significantly modify the volume of the nXIIts (Wade and Buhlman, 2000).…”

Neurophysiological and behavioral development in birds: Song learning as a model system

“…During their studies Nottebohm and Arnold [6] noticed that HVc and RA volumes were at least three times greater in male canaries which sing long, complex songs than in female canaries which sing short, simple songs. The sex difference in these song nuclei was not just one of volume: Males have larger numbers of neurons with more dendrites and more dendritic spines (e.g., [7]). These sex differences in the brain are not specific to canaries but seem to apply generally to those species in which the male does most, if not all, of the singing (e.g., [2]).…”

“…In addition to the obvious genetic differences, differentiation into male and female patterns of song control nuclei is determined by the presence of sex hormones [32]. Testosterone injections in newly hatched female zebra finch chicks result in the development of a malelike song system, with enlarged song nuclei volumes, a larger complexity of dendrites, an enhanced number of spines, and a greater amount of androgen receptors [7,33]. Since these effects can be created by both testosterone and the female hormone estradiol it is not clear whether testosterone is metabolized to estradiol in the brain or whether males do have high estradiol titers during early development.…”