Norepinephrine release in the immature ovary is regulated by autoreceptors and neuropeptide-Y

Experiments were undertaken to study the role that neuropeptide-Y (NPY) and adrenergic autoreceptors may play in the regulation of norepinephrine (NE) release from the rat ovary. Ovaries from 28- to 32-day-old rats were preincubated with [3H]NE, and the release of the recently taken up catecholamine in response to electric field stimulation was assessed. The release was strictly dependent on the presence of extracellular calcium and decreased when the frequency of stimulation was increased. This drop in [3H]NE release was significantly reversed by exposure of the ovaries during stimulation to yohimbine, a selective alpha 2-adrenoreceptor blocker. The existence of prejunctional alpha 2-adrenergic autoreceptors in ovarian nerves was further suggested by the ability of exogenous NE to mimic the inhibitory effect of high frequency stimulation. NPY inhibited by 40% the release of [3H]NE induced by electrical stimulation. The specificity of this effect and its prejunctional nature were demonstrated by the finding that avian pancreatic polypeptide, a structural homolog of NPY that is not recognized by prejunctional NPY receptors of the Y2 subtype, failed to alter the induced release of [3H]NE. Neither NPY, avian pancreatic polypeptide, nor peptide-YY, another member of the pancreatic polypeptide fold family, altered progesterone or estradiol secretion from whole ovaries or granulosa cells in culture, suggesting that NPY (and its structural homologs) does not directly affect ovarian steroidogenesis. The results suggest that 1) the release of NE from ovarian sympathetic nerves is subjected to a dual modulatory influence provided by NE itself and NPY; and 2) this regulatory effect is exerted via specific prejunctional receptors. Such NE/NPY actions are likely to regulate the availability of NE to its postsynaptic receptors during ovarian development and adult function. The fact that NPY is mostly released during high frequency stimulation raises the possibility of NPY involvement in ovarian dysfunctions associated with situations of enhanced sympathetic discharge, such as strenuous exercise and psychogenic amenorrhea.