FEEDBACK OF FOLLICLE-STIMULATING-HORMONE TO INHIBIT LUTEINIZING-HORMONE AND STIMULATE FOLLICLE-STIMULATING-HORMONE RELEASE IN OVARIECTOMIZED RATS

Injection of ovine follicle-stimulating hormone (o-FSH) into the third cerebral ventricle significantly suppressed plasma luteinizing hormone (LH) release in conscious, unrestrained ovariectomized rats. Intraventricular injection of o-FSH (0.5-mu-g) significantly lowered the plasma levels of LH by 70 min and decreased pulse frequency, whereas the pulsatile release of FSH was unchanged during the 180-min duration of the experiment. In a 12-hour experiment, the suppression of LH release reached the maximum within 3 h (p < 0.001), followed by a return of plasma LH level to initial values 6-8 h after intraventricular injection of o-FSH at doses of 0.5, 4 and 8-mu-g. Plasma levels of FSH were not significantly changed by the low dose of o-FSH (0.5-mu-g), but there was a delayed elevation of plasma FSH during the 8-12 h after intraventricular injection of o-FSH at higher doses (4 and 8-mu-g). On the other hand, plasma prolactin was unchanged after intraventricular injection of o-FSH. Since the pituitary responsiveness to synthetic LH-releasing hormone (LHRH) was unimpaired at the end of the sampling and incubation with o-FSH suppressed the release of LHRH from median eminence terminals, these data indicate that FSH acts intrahypothalamically to suppress LHRH release. The late elevation of plasma FSH may be the result of enhanced release of FSH-releasing factor or of a delayed, direct effect of the hormone on the gonadotropes to increase release of FSH. The positive feedback effects of FSH may help maintain FSH release during the follicular phase of the cycle and aid follicular development, while the suppression of LH release may forestall the preovulatory release of LH.