Influence of age, strain and season on circadian periodicity of pituitary, gonadal and adrenal hormones in the serum of male laboratory rats.

The influence of age, strain and season on the circadian pattern of serum levels of LH, FSH, prolactin androgens and corticosterone was studied in five groups of male laboratory rats. Significant 24-hour periodicity was observed for serum levels of corticosterone in all five groups, for androgen levels in four, for prolactin levels in three, for LH levels in two and for FSH levels in one group of rats. There were significant influences of age, strain and season on the temporal patterns and/or on 24-hour mean serum hormone levels. The results indicate that some of the disagreements on existence or nonexistence of circadian rhythms and on rhythm patterns in serum hormone levels may be explained by the fact that animals of different ages or strains had been used or that experiments were performed at different times of the year.