Mode of Gonadotropin Secretion in Infantile Female Rats and the Role of Estrogen in Feedback Regulation*

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Abstract
The temporal aspects of gonadotropin release were studied in infantile (14-day-old) female rats. Blood samples were collected using vena cava cannulae at 10, 15, 20, or 30 min intervals and assayed for either LH alone or for both LH and FSH. Within individual animals, blood LH levels exhibited a degree of variability suggestive of a pulsatile release of the hormone. Wide fluctuations in circulating FSH were also observed, indicating that FSH might also be secreted in a pulsatile manner. Peak values for both gonadotropins were 2 to 5-fold higher than baseline levels. The majority of pups exhibited a nonrhythmic release of hormones. In some animals, LH and FSH appeared to be secreted in a temporally coincident manner. Administration of LHRH to cannulated pups evoked a simultaneous discharge of both gonadotropins. To determine whether endogenous 17β-estradiol (E2) plays a physiological role in the control of LH secretion in infantile rats, circulating LH levels were monitored in cannulated pups which had been treated twice daily with anti-E2 serum for 4 days. Passive immunization to E2 did not affect pulsatile LH secretion. Therefore, the results of this study demonstrate that in infantile female rats, LH is secreted in a pulsatile manner which is independent of E2 feedback regulation.