Expression of intracellular progesterone receptors in rat brain during different reproductive states, and involvement in maternal behavior

Progesterone is one of a complex of hormones which influences the occurrenc e of maternal behavior in rats. The present study provides information on p rogesterone's mechanism and possible neural site(s) of action with respect to maternal responsiveness. Progesterone can exert cellular effects by acti ng on membrane receptors or by acting on intracellular receptors. In the fi rst experiment we show that RU 486 can antagonize progesterone's inhibitory effect on maternal behavior. Since RU 486 acts as an antagonist to progest erone's action at its intracellular receptor, these results support the inv olvement of that receptor in maternal behavior control. The second experime nt employs immunocytochemical techniques to detect the number of cells in v arious forebrain regions which contain intracellular progesterone receptors during different reproductive states. The number of cells which contained progesterone receptors was higher toward the end of pregnancy (progesterone is presumably exerting its effects on maternal behavior at this time) when compared to either early pregnancy or lactation in the following forebrain regions: anteroventral periventricular nucleus of the preoptic area; media l preoptic area; ventral part of the bed nucleus of stria terminalis; ventr olateral division of the ventromedial nucleus; arcuate nucleus; anterior pa raventricular nucleus of the hypothalamus; and medial amygdala. The possibl e involvement of these regions as a site or sites where progesterone might exert its effects on maternal behavior is discussed. (C) 1999 Elsevier Scie nce B.V. All rights reserved.