Role of prolactin the the expression of casein genes in the virgin rabbit.

Prolactin was injected into mature virgin rabbits. Casein mRNA, already present prior to the treatment was enhanced by prolactin. This accumulation of casein mRNA was not accompanied by a parallel significant synthesis of casein. Th effect of prolactin took place without a previous cellular multiplication. The mammary cell is therefore sensitive to prolactin before the first pregnancy but prolactin then displays only an abortive effect on the expression of casein genes.