Galanin (GAL) is widely distributed in the peripheral and the central nervous systems. In the brain, the highest GAL concentrations are observed within the hypothalamus and, particularly, in nerve terminals of the median eminence. This location, as well as GAL actions on prolactin, growth hormone, luteinizing hormone (LH), and LH-releasing hormone (LHRH) secretion, suggest the possibility that GAL may act as a putative hypothalamic-hypophysiotropic hormone. To establish this, GAL and LEIRH levels were measured in hypophyseal portal plasma samples using specific radioimmunoassays. Rat galanin (rGAL) concentrations in portal blood were -7-fold higher than those observed in peripheral plasma in male and female (estrus, diestrus) rats, indicating an active secretory process of rGAL into the portal vasculature. Frequent (10 min) sampling revealed that rGAL and LHRH were secreted into the portal circulation in a pulsatile manner with a pulse frequency of one pulse per hour. Interestingly, both hormone series depicted a high degree of coincident episodes. In fact, the probability of random coincidence, calculated by the algorithm HYPERGEO, was <0.01. Moreover, the retrograde tracer Fluoro-Gold, when given systemically, was taken up by GAL neurons in the hypothalamus, including a subset of neurons expressing rGAL and LHRH, strengthening the notion of the existence of a GAL neuronal system connected to the hypophyseal portal circulation. These observations reinforce the concept that GAL regulates pituitary hormone secretion. To analyze this in further detail, the effects of rGAL on LH secretion were evaluated under basal and stimulated conditions. rGAL induced a small but dosedependent increase in LH secretion from cultured, dispersed pituitary cells. Interestingly, rGAL enhanced the ability of LHRH to stimulate LH release. The tight link between GAL and LHRH neuronal systems is strengthened by the observaItion that during the estrous cycle of the rat, rGAL and LHRH contents in the median eminence show an identical profile (r = 1.00). These data indicate that GAL should be considered as a hypothalamic-hypophysiotropic hormone and as an important

The present studies were designed to determine whether an ultrashort loop feedback mechanism is involved in the regulation of LHRH secretion. Daily administration of a highly potent LHRH agonist (LHRH-AGO; [D-Ala6,Des-Gly10] LHRH ethylamide) immediately after orchidectomy (ORDX) significantly attenuated the rise of plasma LH from days 2 through 10 after ORDX. Concomitantly with the diminished LH rise after ORDX, a significant increase in LHRH content in the arcuate nucleus was observed in LHRH-AGO-treated rats. Measurement of LHRH levels in hypophyseal portal blood in rats 10 days after ORDX combined with daily agonist treatment revealed a significant decrease in LHRH values in portal plasma compared with those in orchidectomized controls. Arcuate nuclei-median eminence (ME) fragments obtained from ORDX rats treated in vivo with LHRH-AGO for 5 days showed a decreased basal secretion of LHRH and a diminished response to K+ stimulation compared with the release from fragments obtained from ORDX saline-treated controls. To evaluate whether a tonic LHRH inhibitory activity operates within the ME, additional experiments were performed in which ME fragments were incubated in vitro in the presence of a potent LHRH antagonist [( D-pGlu1,D-Phe2,D-Trp3,6]LHRH). The antagonist significantly enhanced the basal secretion of LHRH in a dose-dependent manner. The latter results suggest that LHRH antagonists may enhance LHRH release, perhaps by interacting with LHRH receptors playing an inhibitory role on the endogenous secretion of the decapeptide. These observations strongly suggest a tonic inhibitory or modulatory role of LHRH neurons in the regulation of their own function.

Virgin proestrous rats were acutely treated with 2 doses of morphine sulfate (MS) (5-8 and 40 mg/kg BW) or naloxone HCl (NH) (10 mg/kg BW) co-administered with the high dose of MS, and the pituitary portal plasma concentrations of gonadotropin releasing hormone (GnRH) were compared with those of untreated proestrous (PE) and diestrous (DE) control animals. LH and FSH were measured in systemic plasma obtained by venipuncture just prior to the collection of portal blood. Both doses of MS severely diminished the PE surges of LH and FSH, but NH reversed the effect of MS and restored the circulating gonadotropins to PE levels. However, only PE rats treated with the high dose of MS exhibited significantly reduced GnRH concentrations in portal plasma. This suggests that the reduction of gonadotropin concentrations is not due merely to reduced GnRH secretion by the hypothalamus but may involve other mechanism(s) as well. However, in rats given the high dose of MS the severe reduction in pituitary gonadotropin secretion is attributable in large part to the corresponding decrease in hypothalamic GnRH release, since NH restored the GnRH and LH/FSH plasma concentrations to PE levels.

An effective recovery procedure has been utilized for determining luteinizing hormone-releasing hormone (LHRH) concentrations in rat pituitary stalk plasma. With this new recovery protocol it was revealed that stalk plasma immunoreactive LHRH concentrations increased 5-fold to 206 pg/ml during proestrus (p < 0.001) and decreased to diestrous levels during estrus. In contrast, the LHRH concentration in systemic plasma extracts remained unchanged throughout the estrous cycle and did not exceed 4 pg/ml. The stalk plasma:systemic plasma ratio increased from 12:1 at diestrus to 76:1 at proestrus; it then decreased to 22:1 at estrus (p < 0.001). Correlative luteinizing hormone (LH) and follicle-stimulating hormone (FSH) surges of statistically significant magnitudes were observed in the systemic plasma of nonanesthetized cardiac-catheterized rats during proestrus. When administered before the onset of the critical period, Althesin and other anesthetic agents suppressed but did not completely inhibit the peripheral LH surge in cardiac-catheterized rats.The FSH surge was suppressed also but to a lesser degree. These results indicate that the brain triggers the preovulatory surge of LH and FSH via massive secretion of LHRHinto the pituitary portal circulation. They also reveal that, aside from any dosage considerations, the type of anesthetic used and the time of administration in relation to the critical period, can significantly affect the magnitude of pituitary LH and FSH secretion.