UNMASKING OF NEUROPEPTIDE-Y INHIBITORY EFFECTS ON IN-VITRO GONADOTROPIN-SECRETION FROM PITUITARIES OF METESTROUS, BUT NOT PROESTROUS, RATS

We have recently demonstrated that GnRH-stimulated gonadotropin secret ion is augmented by coadministration with neuropeptide-Y (NPY) in ante rior pituitaries removed in the afternoon from proestrous, but not met estrous, rats. To test the hypothesis that these effects of NPY are du e to an interaction with progesterone (P-4), we conducted another cycl e stage experiment using NPY, adding an in vitro treatment with P-4. P ituitaries were taken from rats at 0900 h (before the rise of P-4 on p roestrus) on proestrus or metestrus and were perifused for 8 h, with a nd without GnRH pulses. P-4 was given continuously in half of each of the basal or GnRH-stimulated perifusions. Pulsatile NPY was administer ed in half of the basal or GnRH-stimulated runs in the presence or abs ence of P-4. P-4 alone had a stimulatory effect on GnRH-induced LH sec retion only on the day of metestrus. P-4 did not confer responsiveness to NPY stimulation on either day. Unexpectedly, NPY reversed the P-4 augmentation of GnRH-stimulated LH release on metestrus. With respect to FSH secretion, NPY or P-4 alone had a striking suppressive effect o n GnRH-stimulated FSH secretion rates from pituitaries of metestrous, but not proestrous, donors. These data suggest that the previous endog enous endocrine environment may be crucial in determining the divergen t and interrelated effects of P-4 and NPY on gonadotropin secretion. I n particular, the metestrous environment appears to promote a suppress ive role for NPY.