Circadian and cyclic LHRH variations in the organum vasculosum of the lamina terminalis of female and male rats.

The organum vasculosum of the lamina terminalis (OVLT) was investigated by immunofluorescence and quantitative electron microscopic methods, and its LHRH content was measured at intervals during the estrous cycle and in male rats. A well-defined circadian rhythm in the LHRH content was demonstrated in the OVLT. The highest value was observed on the day of proestrus with peaks at noon and in the late afternoon. The amount of LHRH was about the same in the male as in the female rats on the day of diestrus 1. The OVLT in both sexes contained LHRH-positive fibers and terminals, but less immunofluorescence was visible on the day of diestrus and in the male animals. Quantitative electron microscopic analysis shows that in the OVLT nerve terminals, abutting on the basement membrane, more granulated vesicles (80-90 nm in diameter) are present on the day of proestrus than on any other day of the cycle or in the male. Our results suggest that the OVLT may participate in some way in the control of the cyclic gonadotrophic function of the anterior pituitary and/or in the behavioral effect of sexual hormones.