Circadian rhythm of fluctuations in the level of gonadoliberin in the hypothalamus of rats and the influence on it of various xenobiotics.

There are no regular patterns in the fluctuations of the content of gonadoliberin in relation to the time of day in the preoptic region and the median eminence in females and males. However, the existence of a well-marked circadian rhythm of the fluctuations of this neurohormone has been identified in females in the vascular organ of the lamina terminalis. Its level is extremely low in the morning; its level increases five-fold immediately after noon, and ten-fold at 1700-1800 h. This rhythm does not depend on the stage of the estrous cycle and is manifested identically both in diestrus and in proestrus. Circadian fluctuations in the content of gonadoliberin in the vascular organ of the lamina terminalis were not observed in males. The chronic exposure to various xenobiotics in low concentrations (toloul, benzol, dioxan, styrol, and lead) leads to a break in the circadian rhythm of the gonadoliberin content in the vascular organ of the lamina terminalis in females; this is expressed first and foremost in a significant increase in the level of this neurohormone in the morning hours. This is apparently associated with attenuation of inhibitory tone; this is indirectly confirmed by a simultaneous increase in the level of prolactin in the blood.