Neuropeptide y suppresses prolactin secretion from rat anterior pituitary cells: Evidence for interactions with dopamine through inhibitory coupling to calcium entry

Expression elf neuropeptide Y (NPY) in the medial basal hypothalamus is increased during lactation, and at least part of this increase is due to the appearance of the peptide in hypothalamic tuberoinfundibular dopamine neurons, a cell population that does not exhibit NPY expression in other physiological conditions. The present studies tested the hypothesis that NPY affects PRL secretion by modulating the action of dopamine (DA) at the lactotroph. In static incubations of cultured anterior pituitary (AP) cells, the addition of either NPY or DA in concentrations of 0.1-500 nM resulted in dose-dependent inhibition of PRL secretion, and the combination of DA and NPY in submaximal concentrations produced an additive inhibition of PRL release. NPY also inhibited PRL secretion induced by TRH in perifused AP cells, and the effects were again additive with DA. The interactions of NPY and DA on TRH-induced elevations in cytosolic Ca2+ ([Ca2+]i) were examined by loading cultured AP cells of lactating rats with the fluorescent calcium probe fura-2. TRH produced a dose-dependent stimulation of [Ca2+]i, which was characterized by a rapid transient spike and a more prolonged plateau. Both phases were attenuated by either DA or NPY at 100 nM and were nearly abolished by the combination of DA and NPY, whereas neither DA nor NPY altered resting [Ca2+]i. DA and NPY also inhibited the increases in PRL secretion and [Ca2+]i induced by elevated extracellular K+ in an additive manner. Stimulation of AP cells with TRH in the absence of extracellular Ca2+ resulted in an attenuated spike of PRL release and [Ca2+]i and no plateau phase. Under these conditions, DA still inhibited the residual [Ca2+]i and PRL responses, but the inhibitory effects of NPY on PRL secretion and [Ca2+]i, and the potentiation by NPY of DA inhibition, were abolished. These results suggest that one physiological function of the NPY expressed in tuberoinfundibular dopamine neurons in lactation is to amplify the inhibitory action of DA on PRL secretion through negative coupling to the Ca2+ messenger system, particularly the entry of extracellular Ca2+.