Antisense mRNA for NPY-Y1 receptor in the medial preoptic area increases prolactin secretion

Estrogen Induces Neuropeptide Y (NPY) Y1 receptor gene expression and responsiveness to NPY in gonadotrope-enriched pituitary cell cultures

We showed previously that neuropeptide Y1 receptor (Y1R) expression is increased in the hypothalamus on proestrus afternoon and that this up-regulation of Y1R mRNA may permit neuropeptide Y (NPY) to facilitate release of the preovulatory GnRH surge. Because NPY also modulates LH release directly, we examined steroid regulation of Y1R expression in the female rat anterior pituitary. Treatment of female rats with estrogen in vivo decreased the levels of Y1R mRNA in the whole pituitary gland. In lactotrope/somatotrope-enriched pituitary cells separated by unit gravity sedimentation, 17beta-estradiol (E(2)) treatment likewise suppressed Y1R expression. In contrast, E(2) elevated Y1R mRNA in gonadotrope-enriched cell populations, indicating that estrogen regulates Y1R mRNA expression differently in gonadotropes vs. other pituitary cell types. After exposure to E(2), NPY augmented GnRH-induced LH release from gonadotrope-enriched cells in a manner requiring Y1R activation. Without steroid exposure, this augmentation disappeared, and with progesterone alone, NPY reduced GnRH-induced LH release. In addition, NPY inhibited prolactin secretion from primary pituitary cells in a steroid-free environment, but not in the presence of estrogen. These findings demonstrate that E(2) can directly up-regulate gonadotrope responsiveness to NPY and suggest that this action is mediated at least in part by E(2)'s ability to stimulate Y1R gene expression in gonadotropes. Our observations are consistent with the idea that this regulatory mechanism represents a component of E(2)'s positive feedback actions in pituitary gonadotropes. The biological importance of E(2)'s opposite effects on Y1R expression in other pituitary cell types remains to be determined.

Y1 receptor activation is involved in the effect of exogenous neuropeptide Y on pup growth and the early termination of lactational diestrus in the postpartum rat

The effect of chronic administration of exogenous neuropeptide Y (NPY) and specific NPY receptor agonists and antagonists on reproductive function was examined in lactating rats. As previously demonstrated in our laboratory, chronic (7-day) intracerebroventricular (i.c.v.) NPY infusion (6 microg/day) from days 8-15 postpartum (pp) caused a significant decrease in milk production and an early termination of lactational diestrus. Similar application of the mixed Y1/Y4/Y5 receptor agonist (Leu31, Pro34) NPY (at 3, 6 and 9 microg/day) reproduced the effect of chronic NPY infusion on milk production in a dose-independent manner. Consistent with this effect, the potent Y1 antagonist/Y4 agonist, 1229U91, given concomitantly with NPY eliminated the decline in milk production. The Y2 receptor agonist, NPY13-36, had no effect on milk production at any of the doses used. Length of lactational diestrus was reduced following administration of the Y2 agonist at 18 microg/day but not at 9 microg or 27 microg/day whereas (Leu31, Pro34) NPY infusion had no effect on this parameter at any of the doses used. However, the group that was treated with NPY plus 1229U91 exhibited the usual length of lactational diestrus, indicating that there is at least some Y1 involvement in the effects of NPY on lactational infertility. To test the possibility that the effects of NPY infusion are mediated through changes in circulating prolactin and progesterone, plasma concentrations of these hormones were measured on day 15 pp in NPY-, (Leu31, Pro34) NPY- and vehicle-treated females. NPY-infused females had lower plasma prolactin concentrations than vehicle-infused dams but progesterone concentrations were similar across groups. Overall, these data indicate that chronic exogenous NPY-infusion in lactating females disrupts milk production and shortens lactational diestrus, most likely through reducing prolactin secretion, and that this effect is mediated via Y1 receptor activity.

Chronic neuropeptide Y infusion during lactation suppresses pup growth and reduces the length of lactational infertility in rats

In lactating rats, food restriction potentiates the already high levels of hypothalamic neuropeptide Y (NPY). To investigate the role that high levels of NPY might play in the prolongation of lactational infertility that typically accompanies a food restricted lactation we investigated the effects of chronic central infusions of NPY in ad libitum-fed lactating females. First, we compared the effects of intracerebroventricular (icv) infusion of NPY from Days 12-19 postpartum at a dose of 14.4 microg/day with a similar treatment in nonlactating females. In subsequent experiments we examined the effects of NPY infusions into the lateral ventricle at doses of 6 or 20 mug/day or unilaterally into the medial preoptic area at a dose of 1 microg/day from either Days 12-19 or 7-21 postpartum. Effects on food intake; female body weight; and, where appropriate, litter weight and length of lactational diestrus were compared between NPY and vehicle-treated females. As expected NPY infusion produced a robust increase in body weight and food intake in nonlactating females that was accompanied by a suppression of cyclicity. By contrast NPY treatment in lactating rats resulted in a marked decrease in litter growth and an earlier termination of lactational diestrus.

The central effects of orexin-A in the hypothalamic-pituitary-adrenal axis in vivo and in vitro in male rats

Orexin-A is synthesized in the posterolateral hypothalamus and immunoreactive fibres project to many central nervous system structures, including the paraventricular nucleus, which is rich in corticotropin releasing factor (CRF) neurones and neuropeptide Y (NPY) innervation. We investigated the central effects of orexin-A on the hypothalamic-pituitary-adrenal (HPA) axis by measuring plasma concentrations of corticosterone and adrenocorticotropic hormone (ACTH) in vivo. We explored the potential neuropeptide pathways involved by investigating the effects of orexin-A on CRF, NPY, arginine vasopressin (AVP) and noradrenaline release from hypothalamic explants in vitro. Intracerebroventricular (i.c.v.) injection of orexin-A (3 nmol) in male rats stimulated increases in plasma concentrations of corticosterone between 10 and 40 min after injection, and of plasma ACTH at 20 and 90 min after injection. Orexin-A significantly stimulated CRF and NPY release from hypothalamic explants in vitro. Orexin-A did not stimulate CRF release in the presence of the selective NPY Y1 receptor antagonist, BIBP3226. BIBP3226 alone did not alter CRF release from hypothalamic explants. Orexin-A had no effect in vitro on the release of other neuropeptides, AVP and noradrenaline, involved in the central regulation of the HPA axis. These results suggest that orexin-A is involved in activation of the HPA axis, and that these effects could be mediated via the release of NPY.