Dynamic Changes in Neuropeptide Y Concentrations in the Median Eminence in Association with Preovulatory Luteinizing Hormone Release in the Rat

Abstract

Neuropeptide Y (NPY) stimulates luteinizing hormone (LH) release in the rat by a dual action, one in the hypothalamus to excite LHreleasing hormone (LHRH) release and the other at the level of pituitary gonadotrophs to activate and/or potentiate LH release induced by LHRH. Because NPY produces effects similar to norepinephrine with which it may comprise an excitatory hypothalamic circuit, it was hypothesized that NPY concentrations in the hypothalamus would change in a time-and site-specific manner in association with the preovulatory LH surge on proestrus. Concentrations of NPY in individual nuclei of the preoptic-tuberal pathway and serum LH levels were estimated by radioimmunoassays in rats during diestrus 2 and proestrus. On proestrus, serum LH levels were basal between 1000 and 1400 h, rose significantly at 1500 h and plateaued between 1600 and 1800 h. Of the five neural sites examined, only NPY in the median eminence displayed marked fluctuations in close association with the LH surge. NPY concentrations were low between 1000 and 1300 h, and rose abruptly at 1400 h (P< 0.05) preceding the onset of LH rise at 1500 h. These elevated levels were maintained until 1600 h, during which time serum LH rose to a plateau and then fell at 1800 h to the low range seen between 1000 and 1300 h. In contrast, the pattern of changes in NPY levels in the arcuate nucleus, suprachiasmatic nucleus and medial preoptic area, three additional sites in the preoptic-tuberal pathway known to participate in the preovulatory LH surge, were markedly different from that seen in the median eminence. In each of these three sites, NPY levels rose significantly at 1800 h from the values at 1000 to 1200 h with a slightly different time-course of increment. None of these areas exhibited changes in NPY concentrations on diestrus 2; NPY concentrations also were unaltered on diestrus 2 or proestrus in the ventromedial nucleus. The present observations of site-specific, dynamic changes in NPY levels on proestrus, in a manner previously documented for LHRH, support the hypothesis that a subset Of NPY neurons terminating in the median eminence may be a component of excitatory neural circuitry that either independently or in co-action with the adrenergic system is responsible for the induction of preovulatory LH release.The preovulatory release of luteinizing hormone (LH) during the critical period on the afternoon of proestrus occurs as a result of increased episodic discharge of LH-releasing hormone (LHRH) from the hypothalamus (1-7). The timing and duration of LHRH-induced pituitary activation on proestrus has been fully characterized (2,3,8). In addition, it is generally believed that neurogenic stimuli entrained to the daily photoperiod clock abruptly activate LHRH secretion into the hypophysial portal veins during the critical period (2, 9). The origin and route of transmission of these neural signals to LHRH neurons localized in the preoptic-tuberal pathway (2, 3), have not yet been delineated.A number of hypothalamic neural systems h...

Neuropeptide Y stimulates in vivo gonadotropin secretion in teleost fish

“…In vitro NPY stimulation of GtH secretion from pituitary dispersed cells in the trout (Breton et al, 1990) suggests similarities with in vivo actions, as has also been postulated in mammals (Chabot et al, 1988). The most plausible hypothesis in mammals is that NPY is a component of an excitatory neural circuit responsible for the induction of preovulatory LH release (Sahu et al, 1989) mainly interacting with the LHRH system, on which it could have a dual action. An initial stimulation of LHRH release from the median eminence (Khorram et al, 1987;Allen et al, 1987;Pau et al, 1989) is one about which no information is available in fish.…”

“…An initial stimulation of LHRH release from the median eminence (Khorram et al, 1987;Allen et al, 1987;Pau et al, 1989) is one about which no information is available in fish. A second action postulates a direct effect on pituitary gonadotrophs to activate and/or potentiate LH release induced by LHRH Sutton et al, 1987;Sahu et al, 1989). Previously Breton et al (1990) dem- onstrated that the combined effects of GnRH and NPY on the stimulation of gonadotropin secretion by trout pituitary fragments in vitro were greater than the cumulative effects of each secretagogue individually.…”

Multimodal Influence of Estrogen upon Gonadotropin-Releasing Hormone Neurons

“…Nerve terminals containing NPY have been found to synapse on GnRH cell bodies in the rat (284) and sheep (285), but not the monkey (286), and NPY concentrations within the vicinity of the GnRH cell bodies increase on proestrus (287) and during the estradiol-induced LH surge in the rat (288). Recent observations using retrograde tracing techniques indicate that NPY inputs to the immediate vicinity of the GnRH cell bodies in the rostral preoptic area come from neurons located in both the brainstem and arcuate nucleus and that a small number of the arcuate NPY neurons contain ER␣ (A. E. Herbison and S. X. Simonian, unpublished data).…”

Molecular Endocrinology and Physiology of the Aging Central Nervous System

“…In particular, hypothalamic NPY neurosecretion appears crucial for the preovulatory LH surge in young rats (363)(364)(365)(366)(367). To investigate the possibility of age-related changes, NPY mRNA was quantitated by ribonuclease protection assays in microdissected hypothalamic nuclei from young (2.5-3 months old) and middle aged (7-9 months old) regularly cycling rats (368).…”