Serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels were measured by radioimmunoassay in groups of Wistar rats at 6-hourly intervals during the course of a 4-day oestrous cycle. In addition to a surge at about 18.00 h on the day of pro-oestrus, a circadian rhythm in LH levels was observed which was accentuated during metoestrus. FSH levels showed a protracted periovulatory rise which reached peak levels at about 03.00 h on the day of oestrus. A daily rhythm was not observed.
The incorporation of [35S]methionine (injected subcutaneously 1 h before death) into the trichloroacetic acid-precipitable proteins of the anterior pituitary and of discrete brain areas implicated in the control of gonadotrophin release, was also measured. An increase in protein synthetic activity was observed in the anterior pituitary and the area of the median eminence on the evening of pro-oestrus with peak levels coinciding with the LH surge at 18.00 h on the day of pro-oestrus. An increase in protein synthetic activity relative to that in a 'control' area (the putamen) was observed in the preoptic area and in the amygdala 15–18 h before the LH surge. It is suggested that these changes in protein synthetic activity are associated with the train of neural and humoral events leading to ovulation.
After ovariectomy, protein synthetic activity in all areas investigated was at the low levels observed during the oestrous cycle. Daily injections of 20 μg oestradiol benzoate into intact females led to levels of protein synthesis as great as, or greater than, the maximal levels observed during the oestrous cycle.
Measurements of variations in total protein concentration/unit wet weight of pituitary or brain tissue showed that they were apparently unrelated to variations in protein synthetic activity. Protein concentration appeared to be high in all the brain samples taken at oestrus and low in those taken at metoestrus. Furthermore, superimposed upon a marked daily rhythm, there was a peak of protein concentration on the evening of dioestrus both in the preoptic area and in the area of the median eminence.
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