Seasonally dependent effect of ectopic pituitary grafts on 24-hour rhythmsin serum prolactin and gonadotropins in rats

To assess to what extent the presence of an ectopic pituitary differentiall y affected circulating prolactin (PRL) and gonadotropin levels at different times of the year, rats kept under 12h light, 12h dark (12:12 LD) photoper iods and receiving a pituitary graft or a sham operation in summer or winte r were examined 3 months later. In both male and female sham-operated rats, a circadian variation in serum PRL levels was found, with an acrophase var ying from 21:53h to 00:54h and the mesor and amplitude higher in spring tha n autumn in males and higher in autumn than in spring in females. After gra fting a pituitary, changes in serum PRL related to time of day were no long er observed. In pituitary-grafted male rats killed during spring, serum PRL levels were higher than controls at only a few time points throughout the 24h cycle, whereas in rats killed during autumn, there were no significant differences in PRL levels between grafted and control rats. Pituitary-graft ed female rats killed during spring showed serum PRL levels significantly h igher than those of sham-operated rats, while in female rats killed in autu mn, PRL levels of pituitary-grafted and sham-operated rats did not differ. Significant variations of luteinizing hormone (LH) related to time of day w ere found in sham-operated male rats only, with acrophases at 23:52h and 00 :24h for spring and autumn, respectively, and the mesor and amplitude of th e rhythm significantly higher in autumn. Pituitary transplants suppressed 2 4h variations in circulating LH and depressed its levels during the two sea sons examined. As far as follicle-stimulating hormone (FSH), pituitary graf ts decreased circulating levels, with the extent of decrease higher during autumn than in spring. The results indicate that some endocrine consequence s of the grafting of an ectopic pituitary are dependent on time of year.