THE TIME RELATIONSHIP BETWEEN OESTROGEN ADMINISTRATION AND THE SECRETION OF LUTEINIZING HORMONE IN RATS

ABSTRACT The time relationship between administration of oestrogen and secretion of luteinizing hormone (LH) has been investigated in pregnant and pseudopregnant rats. The criterion taken for LH secretion was cholesterolization of corpora lutea determined 48 hours after oestrogen administration, which could be expected to be positive within 42 hours. It was found that the stimulating effect of oestrogen could be blocked by phenobarbital given up to 32 hours after oestrogen administration. This period 0–32 hours after oestrogen administration was termed the »critical period« during which stimulation of oestrogen-sensitive regions of the CNS occurs, with activation of the anterior pituitary and consequent LH secretion. During this critical period the blocking action of phenobarbital gradually decreases, and the number of ineffective »block« rises. The first failure of phenobarbital blockade was found at 22 hours following oestrogen administration. From 22–32 hours after oestrogen instances of partial blockade occur, which support the view that the activating impulses to the anterior pituitary are not instantaneous and sharply delimited in character. Apparently anterior pituitary activation and LH secretion occur simultaneously, and there is a quantitative relationship between LH secretion and the duration of pituitary stimulation. The duration of neurogenic stimulation was approximated statistically as being about 9½ hours. The differences between our findings and those obtained with hypophysectomy or pituitary stalk section are discussed.