Plasma Prolactin Levels and Hypothalamic Catecholamine Synthesis During Suckling

The effects of suckling on the release of prolactin and the synthesis rates of hypothalamic catecholamines were determined in lactating rats. 3H-tyrosine was administered to rats at the onset of suckling and the rate of its incorporation into 3H-dopamine and 3H-norepinephrine was measured at various intervals. Suckling caused an increase in the plasma prolactin concentration within 4 min. Although an increase in the rate of dopamine synthesis did occur during suckling, it was not significant until after 15–25 min of suckling. There were no significant changes in the rate of norepinephrine synthesis. The results suggest that the increase in prolactin release was probably not caused by the increase in dopaminergic neuron activity.