Substance P immunoreactivity in the rat mammary nipple and the effects of capsaicin treatment on lactation.

Tissue concentrations of substance P immunoreactivity (SP-I) were measured in rat mammary nipples and were significantly greater than in ventral abdominal skin in nonpregnant and pregnant rats. In contrast, the concentration of nipple SP-I was lower than that of skin in twelve day lactating animals. The mean total SP-I content of the pooled twelve nipples from each rat was not significantly different in nonpregnant, pregnant or lactating rats. However, the mean weight of the pooled twelve nipples from each rat was significantly higher in the lactating rats than in pregnant rats. Immunohistochemistry revealed SP-I nerve trunks and single fibers throughout the nipples of lactating rats. Nerve fibers were observed among smooth muscle and along blood vessels throughout the dermis and in association with epidermal structures. Some SP-I fibers were also observed in association with the main lactiferous duct and mammary gland secretory parenchyma. Radioimmunoassay and immunohistochemistry of nipples from lactating rats treated with capsaicin as neonates revealed a marked depletion of SP-I. Rats treated with capsaicin as neonates had a normal gestation period and produced litters of normal size and birth weight. However, the litters of these lactating rats grew at a significantly slower rate than litters from controls. The quantity of milk obtained from capsaicin-treated lactating dams, following a one hour suckling period on the twelfth day of lactation, was significantly less than that obtained by litters of control dams. It is concluded that capsaicin-sensitive primary sensory nerves of the mammary nipple play a role in the afferent limb of the suckling reflex. One transmitter candidate for these nerves is substance P.