幼若ラットの脳内gonadotropin分泌調節機構の発達と性分化に関する電気生理学的研究
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In order to elucidate the process of the development and sex differentiation of the brain systems controlling gonadotropin secretion, electrophysiological studies were performed using immature intact rats of both sexes after endocrinological manipulations. Medial preoptic (MPO) unit responses to the medial amygdala (AMYG) or the dorsal hippocampus (HPC) stimulation (0.1-2.0mA, 0.2msec, 2/3Hz) and hypothalamic arcuate (ARC) unit reponses to the MPO stimulation were examined by means of post-stimulus time histograms, and percentages of neurons responding in a facilitatory or in an inhibitory manner and threshold currents for the induction of the unit response were calculated. Furthermore, the effects of gonadectomy and estrogen administration on serum gonadotropin concentrations were examined in immature female rats of various ages.<BR>Intact rats were classified into 4 groups : i.e., 21-25, 26-30, 31-35 and 36-40 days of age. Neither responses of ARC neurons to the MPO stimulation nor thresholds of the MPO stimulation for the induction of the ARC unit responses showed any significant differences during prepuberal growth nor between sexes. In male rats, the percentages of MPO neurons which were facilitated by the AMYG stimulation decreased during the development (x<SUP>2</SUP>=11.963), while the change in percentages of MPO units responding to the stimulation in female rats was rather small (x<SUP>2</SUP>=4.053). Significant developmental changes were observed in the percentages of MPO units responding to the HPC stimulation in both males (x<SUP>2</SUP> = 39.957, p<0.001) and females (x<SUP>2</SUP>=15.628, p<0.02). Moreover, differences in the number of MPO units responding to the HPC stimulation were observed between intact 21-25-day-old male and female rats (p<0.02), between neonatally (at 2 or 3 days of age) gonadectomized 21-25-day-old males and females (p<0.005), and between neonatally gonadectomized and intact 21-25-day-old males (p<0.05), though responsive patterns of the MPO units to the AMYG stimulation were not significantly different between both sexes except at the age of 31-35 days (p<0.05).<BR>Thresholds of the AMYG stimulation for the induction of the MPO unit responses were lowered and those of the HPC stimulation elevated during development in both sexes. Estrogen injection at weaning lowered the threshold of the AMYG, and elevated that of the HPC in female rats. Testosterone injection at weaning in male rats lowered the HPC threshold. In rats which had been gonadectomized at weaning, thresholds of the HPC stimulation were significantly lower and those of the AMYG stimulation higher than in intact rats of the same age.<BR>Endocrinologically, a simultaneous injection of estrogen with ovariectomy caused a significant and transient increase in serum gonadotropin concentrations 1 day after the treatment in 40-day-old female rats, 2 days after in 30-day-old rats, and 3 days after in 25day-old rats.<BR>These results showed the existence of sexual differences and developmental changes in the limbic-preoptic neural transmissions. It was suggested that the AMYG-MPO system might develop after weaning, presumably under the influence of gonadal hormones, and the lowering in thresholds of the AMYG stimulation during maturation or following estrogen treatment in female rats might be related to the development of stimulatory feedback mechanisms of estrogen which induce phasic gonadotropin release indispensable for ovulation in mature female rats. The HPC-MPO system might have a sexual dimorphism before the age of 20 days, and testes at very early periods of life as well as during postnatal growth might have an important role in the development of HPC-MPO mechanisms in the male. In female rats, estrogen might reorganize the role of the HPC, which had been a trigger for the initial FSH release for ovarian growth in the early prepuberal period,
- 一般社団法人 日本内分泌学会の論文