C. E. Grosvenor

Sensory stimuli from pups involved in inhibition of milk secretion in rats during late lactation

Abstract Lactating rats of two strains, Holtzman and Sprague-Dawley-Rolfsmeyer, were removed from the animal room on postpartum Day 20 and were placed in a testing room where there were no other rats. Milk secretion was stimulated in mothers of each strain 24 hr later in response to 30 min of exposure to a rack of lactators and their pups, placed 3 ft in front of the test mothers; this stimulation to milk secretion was prevented, provided their own pups were placed under, but not when they were placed alongside, the mothers 3–4 min beforehand. The milk secretion responses on the whole were quantitatively less in the Holtzman rats. The sensory signals emanating from the pups which were responsible for the inhibitory effect upon milk secretion were analyzed in Holtzman rats. Sound and odor each proved to be inhibitory; sight, however, was not. The function of the exteroceptive inhibition of milk secretion in the control of milk secretion in late lactation is discussed.

Failure of self-licking of nipples to alter pituitary prolactin concentration in lactating rats☆

Abstract Primiparous lactating rats were separated from their litters of 6 pups each for 8 hr on postpartum Day 14. The pups then were replaced for 30 min under their mother so they could be seen, smelled, and heard but not touched. In this situation, the mother licked her nonnipple areas more often, but was not stimulated to lick her nipples more often or to lick a greater number of nipples. Prolactin concentration in the pituitary glands of the mothers was significantly reduced by 30 min exposure to their pups in respect to that in pituitaries of mothers not exposed to their pups but killed at the end of the 8-hr isolation period. The painting of the nipples of another group of rats with a nonalcoholic orange-flavored syrup at the end of the 8-hr isolation period markedly increased self-licking of the nipples and the number of nipples licked. The intensified licking of the nipples, however, did not provoke any alteration in the pituitary concentration of prolactin. We conclude that the fall in pituitary prolactin concentration which occurs in response to the presence of the pups and without suckling is not the result of self-licking of the nipples, but is due to exteroceptive cues emanating from the pups.

Effect of Nonsuckling Interval in the Ether-Induced Release of Immunoprecipitable Prolactin into the Plasma of the Lactating Rat

Summary The concentration of prolactin in the plasma of lactating rats increased from 24 to 74 ng/ml following acute ether stress as the length of the preceding period of nonsuckling diminished from 12 to 8 hr. In a second experiment the prolactin concentration increased from 24 to 79 to 104 ng/ml as the frequency of 10-min sucklings during a 12-hr period was altered from once at the end of the twelfth hr to once every 6 hr to once every 4 hr and the mothers then exposed to ether 12 hr after the last suckling. These results indicate that more frequent activation of the prolactin-releasing mechanism facilitates the subsequent release of prolactin in response to ether stress. We are grateful to the NIAMDD Rat Pituitary Hormone Distribution Program for the rat prolactin radioimmunoassay kit, and to Mrs. Tanya McGee for technical assistance.

The release of prolactin in the lactating rat: effect of chloroquine

The storage form of prolactin (PRL) was converted into the releasable form in the lactating rat pituitary gland within 10 min of suckling by 6 pups following 4-5 h of nonsuckling on postpartum day 13-14. The characteristics of the PRL discharge from the releasable pool into the circulation was then studied using a stimulus which is known to effectively release PRL (exposure of the mother to the exteroceptive stimuli emanating from 2 pups) but which is not of sufficient strength to influence the conversion of storage PRL. We found that the concentration of PRL could be repeatedly elevated to the same extent in the plasma with repetitive 10-min applications of this stimulus. With continuous 75 min of exposure to 2 pups, the plasma PRL concentration of the mother rose to maximum within 15 min which then was sustained for the remaining 60 min, suggesting a steady release of PRL into the circulation had occurred. These data indicate that, unlike the storage form of PRL, the discharge of the releasable form occurs in relation to the length of time the stimulus is applied and exhibits neither summation nor refractoriness. Subsequently it was noted that PRL could be released up to 8 h after the releasable pool had been formed and that the plasma concentration curves were not altered by injecting 5 mg of the lysosome inhibitor, chloroquine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of Prolactin Upon Milk Secretion in the Spinal Cord-Sectioned Rat.

Summary Milk secretion in the six abdominal mammary glands of lactating rats was assessed after spinal cord transection at T-11 and after prolactin replacement, to further define the role of somatic sensory pathways in the suckling-induced release of prolactin. The six pectoral mammary glands were rendered nonfunctional by ligation of the main milk ducts on postpartum day 2. Somatic sensory denervation of the 6 abdominal mammary glands was accomplished on day 10–11. Each mother nursed 6 young. The growth of litters of cord-transected lactating rats was greatly reduced in comparison to that of sham-operated rats. Prolactin (17 IU) plus oxytocin (1 IU) 3 × daily resulted in significant improvement in litter growth and in food and water intake of the mothers in comparison with oxytocin-injected control rats, but was without detectable effect upon the loss in maternal body weight which follows spinal cord transection. Litter growth following prolactin treatment was less than that of sham-operated rats. Prolactin did not increase litter growth when the mothers were pair-fed to oxytocin-injected mothers. These data indicate that the spinal cord serves as a pathway for sensory impulses from the mammary gland to the CNS which are fundamental in the reflex release of prolactin from the adenohypophysis. The data also suggest that prolactin stimulates food intake in some manner with the additional food converted more readily into milk than into maternal tissue.

Metabolic Clearance Rate of Rat Prolactin in Preweanling Rat Pups

Abstract The metabolic clearance rate (MCR) of immunoreactive rat prolactin (rPRL) was determined in urethane-anaesthetized 14 to 16-day-old rat pups using the constant infusion to equilibrium method. Two different doses of rPRL were used, 329 and 472 ng/min each at a volume of 0.0025 ml/min for 30 min. The MCR was 0.216 ml/min following infusion of 329 ng PRL/min dose and 0.337 ml/min following infusion of the 472-ng/min dose. When calculated on the basis of metabolic body weight the MCR of rPRL in rat pups was comparable to that of adult rats.

Dopamine and epinephrine infusion reduce clearance rate of rat prolactin in the female rat.

Summary The iv infusion of dopamine (250 or 500 ng/min) or epinephrine (500 or 1000 ng/min) into adult female cycling rats in each instance caused a threefold elevation in the equilibrium concentration of prolactin (PRL) in the plasma resulting from the continuous infusion of 200 ng rat PRL/min. A single iv injection of 1 μg dopamine had no effect upon equilibrium PRL levels. The metabolic clearance rate of PRL was significantly reduced from 1.35 to 0.44 ml/min as a result of infusing either 250 or 500 ng dopamine/min, and from 1.14 to 0.37 and from 1.56 to 0.65 ml/min from infusing 500 and 1000 ng/min, respectively, of epinephrine. I am grateful to the National Institute of Arthritis, Metabolism and Digestive Diseases (NIAMDD) Rat Pituitary Hormone Distribution Program for the gift of rat PRL, and to Tanya McGee for expert technical assistance.