Effect of urinary gonadotropin-inhibiting substance on rat prolactin

Administration of the partially purified urinary gonadotropin inhibiting substance (GIS) to the rat induced the following effects on prolactin: a) under basal conditions, 4 hrs after a single dose, or after 3 days of treatment, serum prolactin decreased by 36%, 35% respectively (p less than 0.05) and was associated with a nonsignificant decrease in pituitary content and concentration of this substance; b) under endogenous stimulation (castration) the 49% (p less than 0.05) decrease in serum prolactin was associated with a decrease to the limit of statistical significance of its pituitary content and concentration; c) under exogenous stimulation with TRH, the significant decrease in serum prolactin was associated with a nonsignificant decrease in its pituitary content and concentration. The action of lowering prolactin serum in relation to the similar effect of the pineal extract is discussed with reference to the possible pineal origin of GIS.