The influence of sex and gonadectomy on the growth hormone and corticosterone response to hexarelin in the rat

The present study is designed to investigate the role of sex and gonadal status in the growth hormone (GH) and corticosterone response to hexarelin (HEXA), a GH-releasing peptide, which also causes a stimulatory action on the hypothalamic-pituitary-adrenal (HPA) axis. HEXA (80 microg/Kg) was administered intracarotid to anesthetized intact or gonadectomized male (ORC) and female (OVX) middle-aged rats. The GH stimulatory response to HEXA was gender-related since the GH increase was significantly (p < 0.001) higher in intact males (area under the curve, AUC= 12560 +/- 1784 ng/ml.45 min) than in females (AUC= 4628 +/- 257 ng/ml.45 min). This sex difference does not depend on circulating gonadal steroids since it persists in ORC (AUC = 11980 +/- 1136 ng/ml.45 min) and OVX (AUC = 5539 +/- 614 ng/ml.45 min) rats. The different effects of HEXA on corticosterone secretion detected in male and female rats are probably dependent on the prevailing activity of the HPA axis. In fact, in male rats that have low basal corticosterone levels, HEXA caused an increase in corticosterone secretion, which was significantly (p< 0.05) higher in ORC than in intact rats. The increase in corticosterone secretion by HEXA both in intact and OVX females was delayed, probably due to the elevated initial corticosterone levels, which could have activated the glucocorticoid negative feedback. We suggest that gender-specific patterns in the regulation of the hypothalamus-pituitary function could be responsible for the GH and corticosterone sexually differentiated responses to HEXA.     

Growth hormone and prolactin secretion after metoclopramide administration (DA2 receptor blockade) in fertile women.

In this report, we will describe the results of a cross-sectional study to assess PRL and GH secretion during the early follicular phase in 22 fertile patients after metoclopramide administration in order to achieve a dopaminergic DA2 receptor blockade. Blood samples were collected at - 15, 0, 15, 30, 45 and 60 minutes. PRL, GH, estradiol, IGF-I, TSH, glucose, and insulin were measured in the samples taken at - 15 and 0 minutes. The existence of a correlation between GH and PRL secretion was investigated. All patients presented normal serum levels of estradiol, prolactin, insulin, fasting glucose and IGF-I. Serum GH levels were not changed after metoclopramide infusion (p = 0.302), but there was a significant alteration in serum PRL (p = 0.0001) with the highest levels after 30 (mean: 237.20 ng/ml +/- 95.86) and 45 (mean: 211.80 ng/ml +/- 83.24) minutes. Serum GH levels did not correlate with serum PRL levels after the dopaminergic DA2 blockade. We conclude that GH secretion was not modulated by a direct effect of type 2 dopamine receptor.     

Fatty acid profiles in hepatic membranes of rats with different levels of circulating estrogen and prolactin

Plasma and liver microsomal fatty acid patterns of female rats (Rattus norvegicus) with either low or high serum levels of prolactin (PRL) were studied. Hyperprolactinemia was achieved by grafting anterior pituitary glands or by estradiol administration. One group treated with estradiol also received bromocriptine to inhibit PRL secretion. Ovariectomized (OVX) rats showed a decrease in PRL levels as compared with intact animals (controls). Rats possessing high levels of circulating PRL showed a significant decrease of linoleic acid in the fatty acid pattern of total and polar liver microsomal lipids. High PRL levels in the presence of normal estrogen levels significantly increased arachidonic acid in the same group of lipids. The group of rats treated with estrogen evidenced a decrease in arachidonic acid and in the unsaturation index. From these results it is possible to infer a decrease in the activity of the desaturases. The changes observed in the estradiol-treated group were not modified by bromocriptine administration. OVX rats showed no changes when compared with controls. It is concluded that, while PRL decreases the microsomal unsaturation index, estrogen administration causes a decrease in poly-unsaturated fatty acid biosynthesis and that this effect is independent of PRL levels.     

Lack of involvement of dopamine and serotonin during the orphanin FQ/Nociceptin (OFQ/N)-induced prolactin secretory response

The purpose of these studies was to examine possible mechanisms of Orphanin FQ/Nociceptin (OFQ/N)-induced prolactin release. We investigated the involvement of the dopaminergic neurons by quantifying DOPAC:DA levels in the median eminence and neurointermediate lobe following central administration of OFQ/N to female Sprague-Dawley rats. To specifically determine the involvement of the tuberoinfundibular dopaminergic neurons, immunocytochemical studies were conducted to visualize c-fos protein expression in the arcuate nucleus following central administration of OFQ/N. In addition, the role of serotonergic activation was examined in dose response studies using the selective serotonin antagonist ritansarin and the nonselective antagonist metergoline. Finally, the pharmacological specificity of the prolactin response was examined by pretreating animals with [Nphe1] NC (1-13)NH2, a drug reported to antagonize OFQ/N effects. The results of these studies indicate that the increase in prolactin release following central administration of OFQ/N does not inhibit tuberoinfundibular, tuberohypophyseal or periventricular hypophysial dopaminergic neuronal activity at 10 min after drug administration, a time when prolactin levels were significantly elevated. Furthermore, serotonergic activation is not involved since pharmacological blockade of serotonergic receptors did not alter the prolactin secretory response to OFQ/N. NC (1-13)NH2 did not antagonize the stimulatory effects of OFQ/N on prolactin secretion. The neural effects of OFQ/N on dopaminergic neuronal activity may occur following a different time course than that of the prolactin increase.     

Evidence that serotonin neurons stimulate secretion of prolaction releasing factor.

The purpose of the present study was to determine if serotonin was stimulatory to prolactin release by inhibition of the dopaminergic system or by stimulating release of a prolactin releasing factor (PRF). We measured the amount of prolactin secreted after administration of 30 mg/kg of 5-hydroxytryptophan (5-HTP) to male rats pretreated with fluoxetine (10 mg/kg) and compared it with the amount of prolactin released in male rats treated with αmethyl-p-tyrosine methyl ester (αMT) or various dopamine receptor blocking agents. In every experiment the serotonergic stimulus provided by 5-HTP in fluoxetine-pretreated rats released considerably more prolactin than did treatment with αMT or dopaminergic blockers. We conclude that serotonin releases prolactin not by inhibiting dopaminergic neurons but rather by stimulating the release of PRF.     

Effects of steroid administration on pituitary luteinizing hormone and follicle-stimulating hormone in ovariectomized pony mares in the early spring: pituitary responsiveness to gonadotropin-releasing hormone and pituitary gonadotropin content

These experiments tested the hypothesis that administration of steroid hormones to ovariectomized (OVX) mares during the vernal transition to the breeding season would influence LH and FSH secretion. Circulating gonadotropin concentrations, response to exogenous GnRH, and pituitary gonadotropin content were monitored. Experiments 1 and 2 were conducted, beginning 10 March, and 3 February, respectively, utilizing a total of 30 long-term OVX pony mares. In experiment 1, mares were administered vehicle (n = 5) or estradiol-17 beta (E2, n = 5, 5 mg/3 ml sesame oil), twice daily for 16 days. Blood samples were collected daily for assessment of circulating LH and FSH concentrations. On Day 10 of treatment, 400 micrograms GnRH were administered to all mares. LH increased significantly over days of treatment in the estradiol-treated group, but pituitary response to GnRH tended to be less than in control mares. Circulating FSH tended to decline over days of treatment in estradiol-treated mares, and the pituitary response to GnRH was significantly reduced. Pituitary LH, but not FSH, was increased on Day 16 of treatment with estradiol. In experiment 2, 20 OVX mares received, twice daily, vehicle (n = 5), E2, n = 5; 5 mg), progesterone (P4, n = 5; 100 mg), or progesterone plus estradiol (P4/E2, n = 5; 100 + 5 mg). Treatment continued for 14 days. GnRH (100 micrograms) challenges were administered on Days 6 and 13 of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of prolactin-releasing peptide on tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in estrogen-treated female rats

Both systemic and central effects of a newly discovered prolactin (PRL)-releasing factor (PRF), prolactin-releasing peptide (PrRP), were determined in this study. Systemic injection of PrRP (1 and 10 microg/rat, i.v.) stimulated PRL secretion in ovariectomized, estrogen-treated rats similar to the effect of another PRF, thyrotropin-releasing hormone (TRH). Pretreatment with a dopamine D2 receptor antagonist, sulpiride (1 microg/rat, i.v.), potentiated the stimulatory effect of both PrRP and TRH on PRL secretion. Using the double-labeling immunohistochemical method, PrRP-immunoreactive terminals were found in close contact with tyrosine-hydroxylase-immunoreactive neurons in the hypothalamic arcuate nucleus. Central administration of PrRP (0.1-1,000 ng/rat, i.c.v.) stimulated tuberoinfundibular but not nigrostriatal dopaminergic neuronal activity in 15 min. Levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence and striatum were used as indices for tuberoinfundibular dopaminergic (TIDA) and nigrostriatal dopaminergic neuronal activities, respectively. The serum PRL level, however, was not significantly changed. Similar treatment with TRH (10 ng/rat, i.c.v.) stimulated and inhibited TIDA neuronal activity and serum PRL, respectively, at 30 min. In summary, PrRP may play a role in both the central and peripheral control of PRL secretion.     

Effect of prolactin and estradiol on rat striatal dopamine receptors

Chronic estrogen treatment has been found to increase the level of rat striatal dopamine receptors. Since it is well known that estrogen treatment increases circulating prolactin levels, we have investigated the possibility that the stimulatory effect of estrogens on dopamine receptors is exerted via prolactin. Ovariectomized female or intact male rats were implanted with three adenohypophyses under the kidney capsule or treated with 17 β-estradiol (10 μg, twice daily) for 2 weeks. In animals of both sexes, the pituitary-implanted and estradiol-treated rats showed higher levels of [³H]spiperone binding to striatal dopamine receptors. This effect of estradiol or pituitary implants on dopamine receptors was further investigated in ovariectomized rats. The pituitary-implanted and estradiol-treated rats had elevated plasma prolactin levels and an increased density of striatal dopamine receptors without alteration of their affinity. The role of the pituitary in the effect of estradiol was next investigated using hypophysectomized female rats treated with 17 β-estradiol (10 μg, twice daily), o-prolactin (500 μg, twice daily) or bearing three anterior pituitary implants. The implants as well as the treatment with estradiol or prolactin increased the level of striatal dopamine receptors in hypophysectomized rats while, as expected, the estradiol-treated animals did not have elevated plasma prolactin levels. The present data indicate that high prolactin levels lead, as observed with chronic estradiol treatment, to an increased density of striatal dopamine receptors. However, the effect of estradiol may not be explained exclusively by increased prolactin levels since a similar stimulatory effect is observed in hypophysectomized animals.     

Reinitiation of ovulatory cycles in incubating female turkeys by an inhibitor of serotonin synthesis, P-chlorophenylalanine

Nest deprivation of incubating turkeys caused a decrease in serum prolactin (Prl) levels from 1184.5 +/- 116.4 ng/ml to 896.8 +/- 83.0 ng/ml 1 day after initiation of deprivation, with a further decline to 156.5 +/- 111.7 ng/ml at the end of the 22-day experimental period. Serum luteinizing hormone (LH), progesterone and estradiol levels following nest deprivation were similar to those in birds allowed to incubate (controls). Oral administration of p-chlorophenylalanine (PCPA, 50 mg/kg) to incubating turkeys for 3 consecutive days reduced nesting frequency (P less than 0.05) on the 4th day after initiation of treatment and the nesting virtually ceased by the 9th day. Pretreatment Prl was 1655 +/- 210 ng/ml and declined (P less than 0.05) after PCPA administration to a low of 28.6 +/- 2.8 ng/ml. In addition, PCPA caused a sustained rise in serum LH peaking (5.59 +/- 1.09 ng/ml) 3 days after treatment initiation. Contrary to nest deprivation, serum levels of progesterone and estradiol increased (P less than 0.05) as a consequence of PCPA treatment. Seven of 8 PCPA-treated birds later came into lay when their Prl levels and nesting frequency increased again. The results suggest a role for serotonin (5-HT) in incubation behavior, and Prl and LH secretion in turkeys.     

Evidence that physiological levels of circulating leptin exert a stimulatory effect on luteinizing hormone and prolactin surges in rats.

Increasing evidence suggests that leptin, an adipocyte-derived hormone, may positively regulate the reproductive axis, and serve as a critical metabolic signal linking nutrition and the reproductive function. However, along this line there remains an as-of-yet unresolved important issue whether physiological levels of circulating leptin exert a stimulatory effect on the reproductive axis. It is also unknown whether hyperleptinemia affects the reproductive function. In this study, we attempted to examine these unexplored issues, employing as an indicator the estradiol/progesterone-induced luteinizing hormone (LH) and prolactin (PRL) surges in ovariectomized female rats. Experiments were performed on normally fed, 3-day starved, 3-day starved + murine leptin (100 microg/kg/day), and normally fed + murine leptin (300 microg/kg/day) groups. Leptin was administered utilizing osmotic minipumps during 3 days immediately before experimentation. From 11:00 to 18:00 h, blood was collected every 30 min to measure LH and PRL. The 3-day starvation completely abolished both LH and PRL surges, but 3-day starved + leptin (100 microg/kg/day) group, whose plasma leptin levels (3.7 +/- 0.4 ng/ml) were similar to those in normally fed group (3.4 +/- 0.5 ng/ml), showed a significant recovery of the hormonal surges. On the other hand, the magnitudes of LH and PRL surges in normally fed + leptin (300 microg/kg/day) group, whose leptin levels were 10.8 +/- 1.5 ng/ml, were statistically the same as those in normally fed group. These results indicate for the first time that physiological concentrations of circulating leptin exert a stimulatory effect on the steroid-induced LH and PRL surges in the rat. It was also suggested that mild hyperleptinemia of 3 days' duration may not significantly affect the hormonal surges.     

Prolactin modulates steroidogenesis by rat granulosa cells: I. Effects on progesterone

Either testosterone or follicle-stimulating hormone (FSH) stimulates progesterone secretion by granulosa cells from rats but the combination of the two hormones increases progesterone production in a synergistic manner. We have investigated the effects of graded doses of prolactin (0, 0.02, 0.2, 2, or 10 micrograms/ml) alone or in combination with testosterone (0.5 microM), FSH (300 ng/ml), or FSH + testosterone on progesterone secretion by granulosa cells at two stages of differentiation. Relatively undifferentiated granulosa cells from immature, diethylstilbestrol-treated, hypophysectomized (HPX) rats were cultured in defined (serum-free) medium for 3 days. More highly differentiated granulosa cells were obtained on the morning of proestrus from the preovulatory follicles of 30-day-old rats induced to undergo an estrous cycle by injection with 4 IU pregnant mare's serum gonadotropin; these cells were cultured in medium containing 10% fetal bovine serum. Prolactin alone did not enhance the negligible secretion of progesterone by cells from HPX rats, but increased progesterone secretion by cells from proestrous rats. Prolactin significantly enhanced the stimulatory effects of testosterone or FSH alone on cells from both HPX and proestrous rats. When cultures containing both FSH + testosterone were treated with prolactin, progesterone secretion by cells from proestrous rats was significantly enhanced, whereas secretion by cells from HPX rats was significantly depressed. Therefore when cells from HPX rats were cultured with both FSH and testosterone, the direction of the effect of prolactin was reversed from that observed with prolactin + FSH or testosterone alone, and from that observed when cells from proestrous rats were cultured with prolactin + FSH + testosterone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Different neuroendocrine systems modulate pulsatile luteinizing hormone secretion in photosuppressed and photorefractory ewes.

The objective of this study was to determine whether two photoperiod regimens that induce anestrus in the ewe-short-day photorefractoriness (SDPR) and long-day photosuppression (LDPS)--act by different neuronal mechanisms. In separate experiments, ovary-intact (INTACT), ovariectomized (OVX), and ovariectomized estradiol-treated (OVX + E) ewes were subjected to three different photoperiodic regimens that resulted in reproductive quiescence: (1) exposure to long days (16L:8D), which caused photosuppression (INTACT, n = 9; OVX, n = 6; OVX + E, n = 5; (2) prolonged exposure to short days (10L:14D)), which caused photorefractoriness (INTACT, n = 10; OVX, n = 6; OVX + E, n = 5); (3) exposure to natural photoperiod, which induced seasonal anestrus (INTACT, n = 11; OVX, n = 6; OVX + E, n = 5). Effect of photoregimen was monitored by measuring progesterone or LH. Drug challenges were made after two sequential estrous cycles were missed in INTACT ewes, after mean LH concentrations dropped below 1 ng/ml in OVX + E ewes, and after LH interpulse intervals increased in OVX ewes. Effects of drug on LH pulse pattern were determined by taking blood samples at 12-min intervals for 8 h after i.v. diluent injection; then for 8 h after i.v. injection of cyproheptadine, a serotonin antagonist (3 mg/kg); and again 7 days later after i.v. injection of diluent or pimozide, a dopamine antagonist (0.25 mg/kg). Cyproheptadine had little effect except to decrease (p = 0.05) mean LH in INTACT anestrous ewes and decrease (p less than 0.01) pulse amplitude in OVX + E SDPR ewes. Pimozide did not affect LH pulse frequency in LDPS ewes. However, pimozide increased LH pulse frequency (p less than 0.005) and mean concentrations (p less than 0.005) in SDPR OVX + E ewes, whereas it suppressed LH pulse frequency (p less than 0.05) and amplitude (p less than 0.03) in SDPR INTACT and SDPR OVX ewes. The results suggest that (1) the role of the dopaminergic system differs in SDPR and LDPS ewes, and that different neuronal systems may effect SDPR and LDPS, (2) the effect of pimozide in SDPR ewes is altered by ovarian steroids, and (3) the serotonergic system has relatively little role in regulating pulsatile LH secretion in any of the three different states of anestrus.     

Effects of the dopamine agonist cabergoline on the pulsatile and TRH-induced secretion of prolactin, LH, and testosterone in male beagle dogs

In the present study, the pulsatile serum profiles of prolactin, LH and testosterone were investigated in eight clinically healthy fertile male beagles of one to six years of age. Serum hormone concentrations were determined in blood samples collected at 15 min intervals over a period of 6 h before (control) and six days before the end of a four weeks treatment with the dopamine agonist cabergoline (5 microg kg(-1) bodyweight/day). In addition, the effect of cabergoline administration was investigated on thyrotropin-releasing hormone (TRH)-induced changes in the serum concentrations of these hormones. In all eight dogs, the serum prolactin concentrations (mean 3.0 +/- 0.3 ng ml(-1)) were on a relatively constant level not showing any pulsatility, while the secretion patterns of LH and testosterone were characterised by several hormone pulses. Cabergoline administration caused a minor but significant reduction of the mean prolactin concentration (2.9 +/- 0.2 ng ml(-1), p < 0.05) and did not affect the secretion of LH (mean 4.6 +/- 1.3 ng ml(-1) versus 4.4 +/- 1.7 ng ml(-1)) or testosterone (2.5 +/- 0.9 ng ml(-1) versus 2.4 +/- 1.2 ng ml(-1)). Under control conditions, a significant prolactin release was induced by intravenous TRH administration (before TRH: 3.8 +/- 0.9 ng ml(-1), 20 min after TRH: 9.1 +/- 5.9 ng ml(-1)) demonstrating the role of TRH as potent prolactin releasing factor. This prolactin increase was almost completely suppressed under cabergoline medication (before TRH: 3.0 +/- 0.2 ng ml(-1), 20 min after TRH: 3.3 +/- 0.5 ng ml(-1)). The concentrations of LH and testosterone were not affected by TRH administration. The results of these studies suggest that dopamine agonists mainly affect suprabasal secretion of prolactin in the dog.     

Prolactin modulates steroidogenesis by rat granulosa cells: II. Effects on estradiol

The effects of prolactin on secretion of estradiol by rat granulosa cells obtained at two stages of differentiation and cultured under various conditions were determined. Relatively undifferentiated granulosa cells were obtained from immature, diethylstilbestrol-treated, hypophysectomized (HPX) rats and cultured in serum-free medium or with 10% serum. More highly differentiated granulosa cells were obtained on the morning of proestrous from the preovulatory follicles of immature rats in which an estrous cycle had been induced with 4 IU pregnant mare's serum gonadotropin; these cells were cultured in medium containing 10% serum. Cells were cultured for 3 days with graded doses of prolactin (0, 0.02, 0.2, 2, or 10 micrograms/ml) alone or in combination with follicle-stimulating hormone (FSH; 300 ng/ml), testosterone (0.5 microM), or FSH + testosterone. In control cultures (no prolactin) the relatively undifferentiated granulosa cells from HPX rats secreted negligible quantities of estradiol except when both FSH and testosterone were supplied. Prolactin alone or in combination with FSH or testosterone had no effect on estradiol secretion, but prolactin in combination with FSH + testosterone significantly decreased secretion in a dose-dependent fashion. This set of prolactin treatments was applied in both serum-free medium and medium containing 10% serum, with similar results under both culture conditions. The inhibitory effects of prolactin appeared to be reversible if cells were cultured with prolactin for only 1 day, but were not reversed if cells were cultured with prolactin for 2 or 3 days.(ABSTRACT TRUNCATED AT 250 WORDS)     

Estrogen modulates neuromedin B effects on thyrotropin and prolactin release in vitro

Neuromedin B(NB), a bombesin-like peptide, has been shown to inhibit thyrotropin (TSH) release in pituitary explants of male rats and to stimulate Prolactin (PRL) release in male pituitary cell cultures. We investigated the effect of estrogen status of female rats on the response of thyrotrophs and lactotrophs to neuromedin B (NB) in vitro. Ovariectomized rats were treated with near-physiological or high doses of 17beta estradiol benzoate (0.7 or 14 EB microg/100 gBW/daily, 10 days) or with vehicle (OVX). EB treatment induced a dose-dependent increase in serum prolactin and an increase in pituitary NB content, measured by specific RIA, that was similar in both EB groups (P < 0.05). TSH release from isolated hemipituitaries of OVX rats was significantly reduced (P < 0.05) in the presence of 10(-7) M NB. OVX + EB0.7 glands responded to NB with a not statistically significant dose-dependent decrease in TSH release. However, glands from hyperestrogenized rats (OVX + EB14) required a higher dose (10(-5) M) of NB to inhibit TSH release (P < 0.05). PRL release was highly increased (p < 0.001) by the presence of 10(-5) M NB only in glands of hyperestrogenized rats, while no effect of NB was observed in the other groups. In conclusion, estrogen status of female rats modulates the inhibitory effect of NB on TSH release in vitro and hyperestrogenism is required for stimulatory effect of NB on PRL release in vitro. It is suggested that the induction of PRL release by neuromedin B is a pharmacological rather than a physiological effect, but neuromedin B may contribute to the increased release of PRL associated with hyperestrogenism.     

Effects of pimozide and domperidone administration on prolactin levels in neonatally estrogenized female rats

The effects of two dopaminergic blockers, pimozide and domperidone, on the prolactin secretion were investigated in adult female rats treated neonatally with estrogens (100 micrograms of estradiol benzoate s.c. on day 1). These rats showed hyperprolactinemia (556 micrograms/l vs 57.7 in oil-injected) and treatment with pimozide or domperidone failed to increase prolactin levels in the adult age. These results suggest that the hyperprolactinemia in neonatally estrogenized female rats is produced by loss of the dopaminergic inhibition on prolactin secretion, so that the pharmacological blockade of dopaminergic receptors is uneffective. The dopamine levels in hypothalamus were similar in control and estrogenized females suggesting that failure in dopaminergic inhibition is due to a decrease in dopamine secretion to portal vessels.     

Indomethacin fails to alter basal or phenothiazine-induced prolactin concentrations in man.

In order to evaluate the possible role of prostaglandins in pituitary prolactin (PRL) secretion, PRL was serially measured following perphenazine (Trilafon) ingestion in 8 men before and after 5 days of indomethacin administration. Since estrogens have been shown to modulate prolactin secretion in man, serum steroids including estrone (E1), estradiol (E2), progesterone (P) and testosterone (T) were measured before and after indomethacin ingestion. Serum E1, P and T levels were similar during the pre- and post-indomethacin study periods: 56 +/- 4 (1 SEM) vs 48 +/- 5 pg/ml, 298 +/- 28 vs 315 +/- 32 pg/ml, and 8.1 +/- 0.7 vs 8.6 +/- 0.7 ng/ml, respectively. Serum E2 levels were slightly, but significantly, lower following indomethacin treatment at 30 +/- 3 vs 37 +/- 3 pg/ml (p less than .01). Basal serum PRL concentrations were unaffected by indomethacin administration (9 +/- 3 pre- vs 8 +/- 2 ng/ml post-drug treatment). Integrated perphenazine-induced PRL responses were likewise similar during the 2 study periods: 101 +/- 16 ng . hr/ml during the control period and 104 +/- 14 ng . hr/ml following indomethacin. Thus, short-term indomethacin treatment had no effect on basal or perphenazine-stimulated PRL secretion in men.     

Involvement of peptide histidine isoleucine (PHI) in prolactin secretion induced by serotonin in rats

The possible role of hypothalamic peptide histidine isoleucine (PHI) in prolactin (PRL) secretion induced by serotoninergic mechanisms was investigated in male rats using a passive immunization technique. Intracerebroventricular injection of serotonin (5HT, 10 micrograms/rat) raised plasma PRL levels both in urethane-anesthetized rats and in conscious rats pretreated with normal rabbit serum (0.5 ml/rat, iv, 30 min before). Plasma PRL responses to 5HT were blunted in these animals when they were pretreated with rabbit antiserum specific for PHI (0.5 ml/rat, iv, 30 min before) (mean +/- SE peak plasma PRL: anesthetized rats 271.3 +/- 38.3 ng/ml vs 150.0 +/- 12.6 ng/ml, p less than 0.01, conscious rats 54.3 +/- 6.8 ng/ml vs 30.7 +/- 4.1 ng/ml, p less than 0.025). These results suggest that hypothalamic PHI is involved, at least in part, in PRL secretion induced by central serotoninergic stimulation in the rat.     

Effects of lipopolysaccharide on neurokinin A content and release in the hypothalamic-pituitary axis

The administration of bacterial lipopolysaccharide (LPS) markedly affects pituitary secretion, and its effects are probably mediated by cytokines produced by immune cells or by the hypothalamo-pituitary axis itself. Since neurokinin A (NKA) plays a role in inflammatory responses and is involved in the control of prolactin secretion, we examined the in vivo effect of LPS on the concentration of NKA in hypothalamus and pituitary (assessed by RIA) and serum prolactin levels in male rats. One hour after the intraperitoneal administration of LPS (250 microg/rat), NKA content was decreased in the posterior pituitary but not in the hypothalamus or anterior pituitary. Three hours after injection, LPS decreased NKA concentration in the hypothalamus and anterior and posterior pituitary. In all the conditions tested, LPS significantly decreased serum prolactin. We also examined the in vitro effects of LPS (10 microg/ml), interleukin-6 (IL-6, 10 ng/ml) and tumor necrosis factor alpha (TNF-alpha, 50 ng/ml) on hypothalamic NKA release. Interleukin-6 increased NKA release without modifying hypothalamic NKA concentration, whereas neither LPS nor TNF-alpha affected them. Our results suggest that IL-6 may be involved in the increase of hypothalamic NKA release induced by LPS. NKA could participate in neuroendocrine responses to endotoxin challenge.     

Restoration of LH output and 17beta-oestradiol responsiveness in acutely ovariectomised holstein dairy cows pre-treated with a GnRH agonist (deslorelin) for 10 days

The objectives of the study were firstly to identify the role of the ovary in maintaining plasma luteinising hormone (LH) concentrations in cows treated with an implant of a potent GnRH agonist (deslorelin), and secondly to characterise the changes in LH following ovariectomy (OVX) in the same animals. Oestrus was synchronised in mature Holstein dairy cows and deslorelin implants were inserted 17 days later into two-third of the cows. A further 10 days later (day 0) all cows had bilateral OVX performed. A control group (CON; n=4) received no treatment and had blood samples collected at 15-min intervals for 8h on the day prior to OVX (day -1) and similarly on days 4 and 10. One group (DES_IN; n=4) had implants in place for the duration of the study while another group had implants removed (DES_OUT; n=4) at the time of OVX. DES_IN cows were sampled hourly at each sampling session (days -1, +4 and +10), whereas DES_OUT cows were sampled similarly to CON except on day -1 when hourly samples were collected.Predictable post-operative increases in mean LH (0.61 ng/ml versus 1.79 ng/ml; P<0.01) and LH pulse amplitude (0.66 ng/ml versus 1.56 ng/ml; day -1 versus day +10; P<0.01) occurred after CON cows were ovariectomised. Smoothed LH means showed a delayed effect of time compared to arithmetic means. Pulse frequency was unchanged following OVX in CON cows. A comparison of all cows that had been treated with deslorelin from day -1 showed a significant elevation of smoothed mean LH compared to untreated cows (0.80 ng/ml versus 0.34 ng/ml; DES_IN and DES_OUT versus CON; P<0.05). DES_IN cows had a 54% reduction in mean LH from day -1 to +4 following OVX (1.05 ng/ml versus 0.48 ng/ml; P<0.01) indicating the probable involvement of the ovary in the maintenance of elevated basal LH. No further reduction was detected by day +10. The LH response to an intramuscular (IM) injection of 500 microg 17beta-oestradiol (E2) on day +11 varied significantly between treatment groups (P<0.01). CON cows showed a typical LH surge, reaching maximum concentrations (10.3 ng/ml) at 17.3h post-injection. Even though low amplitude LH pulsatility had been restored in DES_OUT cows by day +4, there was an inconsistent response to E2 on day +12; one cow had an apparently normal surge yet, others showed only attenuated responses. Pulse amplitude in DES_OUT cows was lower at days +4 and +10 compared to CON (P<0.05). DES_IN cows did not produce any surge after E2. Mean LH prior to OVX (day -1) remained unchanged following the 500 microg oestradiol injection (0.38 ng/ml versus 0.45 ng/ml pre-E2 versus post-E2 compared to 1.05 ng/ml pre-OVX).The results of this experiment implicated ovarian involvement in maintaining elevated basal LH output in cows that were chronically treated with a GnRH agonist. Individual cows varied in their LH surge response to exogenous E2 given 12 days after implant removal, even though LH pulse amplitude and frequency had been restored.