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

To assess to what extent the presence of an ectopic pituitary differentially affected circulating prolactin (PRL) and gonadotropin levels at different times of the year, rats kept under 12 h light, 12 h dark (12:12 LD) photoperiods and receiving a pituitary graft or a sham operation in summer or winter 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 varying from 21:53 h to 00:54 h and the mesor and amplitude higher in spring than autumn in males and higher in autumn than in spring in females. After grafting a pituitary, changes in serum PRL related to time of day were no longer observed. In pituitary-grafted male rats killed during spring, serum PRL levels were higher than controls at only a few time points throughout the 24 h cycle, whereas in rats killed during autumn, there were no significant differences in PRL levels between grafted and control rats. Pituitary-grafted female rats killed during spring showed serum PRL levels significantly higher than those of sham-operated rats, while in female rats killed in autumn, PRL levels of pituitary-grafted and sham-operated rats did not differ. Significant variations of luteinizing hormone (LH) related to time of day were found in sham-operated male rats only, with acrophases at 23:52 h and 00:24 h for spring and autumn, respectively, and the mesor and amplitude of the rhythm significantly higher in autumn. Pituitary transplants suppressed 24th variations in circulating LH and depressed its levels during the two seasons examined. As far as follicle-stimulating hormone (FSH), pituitary grafts decreased circulating levels, with the extent of decrease higher during autumn than in spring. The results indicate that some endocrine consequences of the grafting of an ectopic pituitary are dependent on time of year.