Pineal arginine vasotocin activity increases 200-fold during August in adult rats and hamsters.

In a previous study, measurements of arginine vasotocin-immunoactivity (iAVT) in immature rats over a period of 14 months, led to the discovery of a significant yearly variation, with peak levels of iAVT in August. In the present study, iAVT was measured in pineals obtained from mature male and female rats and hamsters once or twice weekly from July until early September 1982. For all groups, mean pineal AVT-immunoactivity was less that 7 pg/gland in early July, but then increased significantly by August 11-13. For hamsters, maximum values of 1,272 +/- 49 (mean +/- S.E.: n = 3) and 1,065 +/- 62 pg/gland were recorded for males and females, respectively. For rats, peak values measured were 940 +/- 12 pg/gland for males and 1,040 +/- 34 for females. The AVT-activity levels then decreased to less than 100 pg/gland by early September. Thus, a dramatic August elevation of pineal iAVT is characteristic of hamsters as well as rats, and of mature as well as immature animals.