Fig. 38. Schematic drawing of the major pathways involved in lactation. Suckling leads to neurogenic responses mediated through the medulla to inhibit dopamine secretion in arcuate nucleus (ARC) neurons and stimulate oxytocin (OXY) secretion in paraventricular (PVN) neurons. Prolactin and oxytocin exert effects on the breast, but prolactin also gains entry into the CNS to affect maternal behavior and inhibit reproductive function by acting on medial preoptic neurons (mPOA), and further inhibit the secretion of dopamine from ARC neurons. Milk production leads to a fall in circulating levels of leptin causing an increase in NPY and AGRP and inhibition of POMC in ARC neurons. NPY is also increased by neurogenic signals from the brainstem. Inhibitory effects of NPY on GnRH-producing neurons in the mPOA contributes to inhibition of reproductive function. NPY also exerts direct effects on the PVN to induce increase feeding and promote energy conservation.