Results of experiments using steroid injections or implants have indicated that pituitary gonadotrophin secretion is not only modified by a change in perfusion rate of the releasing factors (RF) of luteinizing hormone (LH) and follicle-stimulating hormone, but that steroid feedback directly at the pituitary level may be involved in the cyclic regulation of gonadotrophin secretion (Ramirez, Abrams & McCann, 1964; Harris & Campbell, 1966; Arimura & Schally, 1971). Consequently, it has been suggested that pituitary responsiveness to LH-RF might change during the oestrous cycle as a result of variation in steroid secretion (Antunes-Rodrigues, Dhariwal & McCann, 1966). This experiment investigates this possibility.
Female Sprague—Dawley rats weighing 220–250 g and maintained under a lighting régime of 14 h light: 10 h darkness at a temperature of 21 ± 2 °C were used. Only animals showing at least two consecutive 4-day oestrous cycles were studied. LH-RF-evoked stimulation of LH release was examined