PROLACTIN AND GROWTH HORMONE SECRETION AND MAMMARY GLAND GROWTH DURING PSEUDOPREGNANCY IN THE MOUSE

SUMMARY Pseudopregnancy was induced in 61% of C57BL/St mice that were stimulated cervically with a glass rod for 30 s on the day of oestrus. Within 60 min of stimulation, the concentration of prolactin in the serum increased threefold and appeared to be released in a phasic manner. Subsequently, prolactin was secreted at an enhanced rate for approximately two-thirds of pseudopregnancy, as the level in the serum rose and the level in the pituitary gland dropped. The concentrations of growth hormone (GH) in the serum showed only a minor increase 8 h after cervical stimulation, with no significant rise during the course of pseudopregnancy. Pituitary GH concentrations increased between early and late pseudopregnancy. The DNA and RNA contents of the mammary glands, indices of mammary gland growth and metabolic activity, were both enhanced during pseudopregnancy and paralleled the increases in the secretion of prolactin, whereas mammary gland weight generally followed changes in the level of GH in the pituitary gland. Administration of the prolactin inhibitor 2-bromo-α-ergocryptine reduced the incidence of pseudopregnancy, but neither an inhibitor of GH release, somatostatin, nor an antiserum to mouse GH had any appreciable effects. Injections of ovine prolactin and mouse GH induced the dioestrous state in mice, but only ovine prolactin stimulated the growth of corpora lutea. These results suggest that prolactin is the main hormone involved in the induction and maintenance of pseudopregnancy in the mouse. Injections of ovine prolactin or mouse GH suppressed endogenous levels of the respective hormone; however, prolactin also reduced the level of GH and GH reduced the level of prolactin, suggesting that pituitary hormones can influence not only their own secretion but that of other pituitary hormones as well.