Inhibitory effects of bromocriptine on mammary development and function in lactating mice

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Abstract
Five experiments were conducted. In the first, the plasma prolactin concentration of lactating mice was significantly reduced 3, 6 and 12 h after a single injection of 0·2 mg bromocriptine at 09.00 h, but recovered to normal levels at 24 h. A second injection at 17.00 h (i.e. total dose of 0·4 mg) completely prevented this recovery. Prolactin concentration was also reduced after the s.c. implantation of a solid pellet of bromocriptine. Once daily injections of bromocriptine for 7 days starting 1-day prepartum (early lactation group) or on day 5 of lactation (established lactation group) significantly reduced milk yield, assessed from the weight gain of the litter or estimated by a tritiated water dilution procedure. In early lactation the degree of inhibition was positively related to the size of the suckling litter (three pups, no inhibition; nine pups, 16·4% inhibition; 14 pups, 40·8% inhibition), but in established lactation an inhibition of 25–30% was observed, regardless of litter size. Twice-daily injections and s.c. implants of bromocriptine both reduced milk yield by a greater amount (approximately 45%), but in no case was secretion completely suppressed. The inhibitory effect of bromocriptine was prevented by the simultaneous administration of ovine prolactin (0·4 mg). Lactogenesis was apparently not affected by bromocriptine treatment. At the end of the early lactation treatment period, the mammary glands of bromocriptine-treated animals were significantly smaller and contained fewer cells than those of controls, but this was not the case in animals treated during established lactation. It is suggested that suppression of prolactin secretion by bromocriptine treatment may reduce milk yield both indirectly, by inhibiting development of the gland and thereby reducing the size of the secretory cell population, and directly, by reducing the activity of individual secretory cells. The observation that lactation was not completely inhibited suggests that prolactin may not be the only galactopoietic hormone in the mouse. J. Endocr. (1986) 110, 263–270