A critical period for estrogen action on neurons of the song control system in the zebra finch.

Abstract

The song nuclei of the male zebra rinch (Poephila guttata) contain larger neurons than those of the female. This gender difference arises after hatching as a result of cell atrophy in the female and cell growth in the male. Implantation of estrogen in female chicks induces masculine differentiation of neurons in their song nuclei. The effects of estrogen on neuron size decline steeply after posthatching day 35 when neuronal atrophy begins. Estrogen loses its masculinizing effects completely after day 45 when the adult level of neuronal atrophy is reached. Thus, the end and the intensity of hormone action appear to be correlated with the timing of neuronal atrophy. and day 45. We chose for study two of the forebrain song nuclei, the ventral nucleus of hyperstriatum caudal section (HVc) and the robust nucleus of archistriatum (RA). The cross-sectional area of neuronal somata serves as a sensitive measure of their growth and atrophy. Birds were perfused with saline and 10% formalin. Brains were cut into parasagittal sections of 30 ttm in thickness and stained with creyslviolet. For the measurement of cross-sectional area, the perikaryal outlines of 50 randomly chosen neurons per bird were drawn with a Zeiss camera lucida and the area within the outlines was calculated by a computer-aided planimetric method.Administration of gonadal steroids early in life can induce the development of brain anatomical and behavioral attributes unique to one or the other gender. The inductive action of hormone is generally limited to a short period before or after birth (1). However, the factors controlling the length of the critical period and variation in the intensity of hormone action during the period are not known in most cases. This lack of knowledge is partly due to reliance on descriptive behavioral criteria for hormone action. Sexually dimorphic neurons and neuronal populations are convenient for the establishment of quantitative relationships between the timing and effect of hormone administration (2, 3). The brain nuclei that control song in the zebra finch (Poephila guttata) show marked sexual dimorphism (4). The gender differences do not exist at birth but arise later from a decrease in cell size and cell death in the female nuclei as well as from an increase in cell size in the male (5). An increase in cell number in one of the male nuclei may also contribute to the differences (6). Administration of estrogen to a female chick induces the development of masculine cell size in her song control system (7,8). This report describes how the time of estrogen implantation affects neuron size in two of the female song nuclei. MATERIALS AND METHODSZebra finch eggs were collected from our own breeding stock and incubated in an incubator so that the exact dates of their hatching could be determined. The chicks were raised by Bengalese finches as foster parents. Estrogen (50 ,ug of 17,8-estradiol) was administered as a slow-release (Silastic) subcutenous implant. Removal of the implant after various lengths of time showed ...

Zebra finch sexual differentiation: The aromatization hypothesis revisited

“…For example, HVC and lMAN soma size were completely masculinized with E2 treatment on the day of hatching (Jacobs et al, 1995), and similar results were detected for RA with E2 treatment during the first week of hatching combined with adult T (Adkins- Regan et al, 1994). Soma sizes in HVC and RA in females treated with E2 beginning on post-hatching day 15 or 30 were comparable to male values (Konishi and Akutagawa, 1988). With treatment beginning on the day of hatching, E2 substantially increased the diameter of RA neurons, and DHT had a small, but statistically significant effect on this measure (Gurney, 1981).…”

Mismatch in sexual dimorphism of developing song and song control system in blue-capped cordon-bleus, a songbird species with singing females and males

“…In zebra finches, most RA neurons are born before/around hatching and post-hatching neurogenesis seems to exclude the arcopallium (in which RA forms) of songbirds (Konishi and Akutagawa, 1990;Vellema et al, 2010). Since neuron numbers of cordon-bleus' RA increased after 20 dph, a large number of these neurons must be born much later as compared to the zebra finch, which might be plausible in light of the much longer ontogeny of the cordonbleus (this study) as compared to the zebra finch (Immelmann, 1969;Konishi and Akutagawa, 1988). Alternatively, early born neurons might differentiate into RA neurons at a late time point.…”

Two separate areas of the brain differentially guide the development of a song control nucleus in the zebra finch.

“…Neuron number was calculated by multiplying the cell density by the volume of the RA. A more detailed explanation of such measurement protocols has been published (4,12).…”