Age-dependent effect of Freund's adjuvant on 24-hour rhythms in plasma prolactin, growth hormone, thyrotropin, insulin, follicle-stimulating hormone,luteinizing hormone and testosterone in rats

The effect of Freund's adjuvant administration on 24-hour changes of plasma prolactin, growth hormone (GH), thyrotropin (TSH), insulin, follicle-stimu lating hormone (FSH), luteinizing hormone (LH) and testosterone were studie d in young (2 months) and aged (18 months) male Wistar rats. Rats were inje cted s.c. with Freund's adjuvant or adjuvant's vehicle and, 18 days later, they were killed at 6 different time intervals throughout a 24-hour cycle t o measure circulating hormone levels by specific RIAs. Young rats receiving adjuvant's vehicle exhibited significant rime-of-day-dependent variations in plasma TSH, LH and testosterone, with maximal levels at 1300 h, 0100 h a nd 1700 h, respectively. Prolactin and insulin levels, analyzed globally in a factorial ANOVA, showed significant time-of-day changes with maximal lev els at 1300 - 1700 h and 2100 h, respectively. The daily rhythms in plasma LH and testosterone found in young rats were not longer observed in Freund' s adjuvant-injected rats, while as far as TSH, a second peak was observed a t 0100 h after Freund's adjuvant administration. Twenty-four hour rhythms i n circulating TSH, LH and testosterone were blunted in old rats receiving e ither Freund's adjuvant or its vehicle. Aged rats exhibited significantly h igher circulating levels of prolactin, and lower levels of GH, TSH, FSH and testosterone. The results indicate that secretion of prolactin, GH, TSH, F SH and testosterone are age-dependent, as are the responses of TSH, LH and testosterone to Freund's adjuvant administration.