Peptide T blocks GP120/CCR5 chemokine receptor-mediated chemotaxis

We previously reported that certain short gp120 V2 region peptides homologous to vasaoactive intestinal peptide (VIP), such as "peptide T," were potent inhibitors of gp120 binding, infectivity, and neurotoxicity. The present study shows that synthetic V2-region-derived peptides have potent intrinsic chemotaxis agonist activity for human monocytes and also act as antagonists of high-affinity (0.1 pM) gp120-mediated monocyte chemotaxis. Selectivity is shown in that peptide T is more potent at suppressing M-tropic than T-tropic gp120 chemotaxis. Peptide T was also able to suppress monocyte chemotaxis to MIP-1beta, a chemokine with selectivity for CCR5 chemokine receptors, while chemotaxis of the more promiscuous ligand RANTES was not inhibited, nor was chemotaxis mediated by SDF-1alpha. In order to determine if peptide T mediated its gp120 antagonistic effects via modulation of CCR5 receptors, RANTES chemotaxis was studied using a CCR5 receptor-transfected HOS cell line. In this case, RANTES chemotaxis was potently inhibited by V2-region-derived short peptides. Peptide T also partially suppressed (125)I-MIP1-beta binding to human monocytes, suggesting action at a subset of MIP1-beta receptors. The V2 region of gp120 thus contains a potent receptor binding domain and synthetic peptides derived from this region modulate CCR5 chemokine receptor chemotactic signaling caused by either gp120 or chemokine ligands. The results have therapeutic implications and may explain recent clinical improvements, in that HIV/gp120 actions at CCR5 receptors, such as occur in the brain or early infection, would be susceptible to peptide T inhibition.

HIV gp120 inhibits the somatotropic axis: a possible GH-releasing hormone receptor mechanism for the pathogenesis of AIDS wasting

AIDS is often associated with growth retardation in children and wasting in adults. The dissociated envelope protein of the HIV (HIV-1), gp120, can be found in significant concentrations in the parenchyma and cerebrospinal fluid of brains in infected individuals, even in the earliest stages of HIV-1 disease. On the basis of this and the fact that we observed pentapeptide sequence homology between GH-releasing hormone (GHRH) and the V2 receptor-binding region of gp120, we initiated experiments to determine whether gp120 could affect GH secretion and growth in vivo and/or interact with anterior pituitary GHRH receptors in vitro. Although acute IV administration of gp120 in conscious rats had no effect on plasma GH levels, acute administration of gp120 (400 ng) into the brain significantly suppressed pulsatile GH release over a 6-h period compared with saline-injected controls. Furthermore, the putative gp120 antagonist, Peptide T (DAPTA), prevented the suppression of GH by gp120. In support of these in vivo findings, gp120 also significantly (P < 0.05) suppressed GHRH-stimulated GH release in static cultures of dispersed pituitary cells and from cells undergoing perifusion with the peptides. DAPTA prevented the GH suppression by gp120 in both of the pituitary cell paradigms. Furthermore, chronic administration of gp120 into the third ventricle significantly reduced body weight in juvenile rats, compared with saline-injected controls. Thus, gp120 appears to act both at the hypothalamus and pituitary to suppress GH release, and its action at these two locations is associated with a significant loss in body weight in chronically treated young animals. These findings may suggest a specific mechanism for the pathogenesis of wasting in HIV-1 patients that involves blockade of endogenous GHRH receptors by gp120.

Endotoxin-induced suppression of the somatotropic axis is mediated by interleukin-1 beta and corticotropin-releasing factor in the juvenile rat

This study extends the neuroendocrine role of central interleukin-1 beta (IL-1 beta) during the stress of lipopolysaccharide (LPS) challenge to include inhibition of the somatotropic [GH-releasing hormone (GHRH)-somatostatin (SRIF)-GH] axis in juvenile male rats and clarifies the role of CRF in the mediation of LPS/IL-1-induced changes in GHRH and SRIF neurosecretion. The results of the in vivo component of this study demonstrated that LPS treatment (2.5 mg/kg twice daily for 5 days) caused a significant attenuation of body weight gain for 2 days (2.4 +/- 1.7% vs. 10.3 +/- 1.8% BW/day in saline controls; P < 0.05) and failure of catch-up growth thereafter even though a small transient suppression of food intake returned to normal by the second of 4 days of treatment. Associated with the first day of growth attenuation was an acute suppression of all plasma GH parameters, including GH mass (area under the curve, 1.972 +/- 0.1837 vs. 6.402 +/- 1.7 micrograms/ml.6 h for saline controls; P < 0.05), in animals receiving an acute bolus of LPS, which was blocked by prior microinjection of IL receptor antagonist protein (IRAP) into the third ventricle. In contrast, GH parameters associated with the second day of LPS-suppressed body weight gain were increased (GH mass, 9.4 +/- 2.2 vs. 3.5 +/- 0.5 micrograms/ml.4 h in saline controls; P < 0.05). These increases were reversed after another 2 days of LPS treatment. In a series of in vitro experiments using medial basal hypothalamic (MBH) explants incubated with LPS [100 ng/ml alone or with 10(-7) M IRAP or 10(-6) M CRF antagonist (CRF-ANT)], GHRH release from MBH incubated with LPS was significantly greater than that in controls (231 +/- 79% vs. 71 +/- 34% of baseline release; P < 0.05), and this stimulation was antagonized by both IRAP and CRF-ANT. SRIF release was significantly increased by incubation with LPS (163 +/- 28% vs. 97 +/- 20% of the baseline for controls; P < 0.05) and blocked (to 88 +/- 14% of the baseline) by IRAP, but not by CRF-ANT. Finally, when MBH explants were incubated with IL-1 beta (10(-9) M), there was a significant inhibition of in vitro GHRH release (37.9 +/- 6.7% vs. 74.9 +/- 16.6% for controls), which was reversed by IRAP and CRF-ANT, and a significant stimulation of SRIF release (168.7 +/- 37.5% vs. 98.0 +/- 11.6% for controls), which was reversed by IRAP, but not CRF-ANT.(ABSTRACT TRUNCATED AT 400 WORDS)

Early increase in pulsatile growth hormone release after unilateral nephrectomy in adult rats

Removal of one kidney results, within days, in accelerated growth of the remaining kidney. However, the mechanisms that underlie this compensatory renal hypertrophic response, particularly in the early time period following nephrectomy, are not understood. In this study we tested the hypothesis that removal of one kidney leads to a change in the pulsatile release of growth hormone (GH), which facilitates compensatory renal growth. Adult Wistar rats were implanted with Silastic cannulas in jugular veins and underwent either unilateral nephrectomy (UNX) or sham operation. Plasma levels of GH were determined 24 and 48 h after sham operation or UNX. Blood samples were taken every 20 min over a 6-h period from conscious, unrestrained animals. Pulsatile GH release was markedly elevated 24 h after UNX in both the amplitude of the surges as well as in the duration of release. Peak GH levels after 24 h were three- to fourfold higher in UNX rats compared with sham controls (417 +/- 75 vs. 119 +/- 23 ng/ml, P < 0.05). However, this enhanced release of GH appeared to be of short duration and began declining by 48 h post-UNX (peak level of 227 +/- 37 ng/ml, P < 0.05 vs. both 24 h UNX and sham controls). To examine whether this rise in GH release post-UNX contributed to the compensatory renal growth, rats underwent UNX and were immediately treated with an antagonist to GH-releasing factor (GRF-AN; i.e., [N-Ac-Tyr1,D-Arg2]GRF-(1-29) amide, 200 micrograms/kg twice daily), and the effects on GH release and renal growth were determined. Administration of GRF-AN significantly suppressed the increase in GH release post-UNX and was associated with a significant attenuation in renal growth 48 h post-UNX in GRF-AN-treated rats (8.7 +/- 2.6% vs. 22.7 +/- 3.0% in UNX controls, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Tandospirone stimulates prolactin secretion in the rat by an action at serotonin-1A receptors

Tandospirone is an azapirone drug that has high affinity for serotonin-1A (5-HT-1A) receptors and preclinical effects predictive of antidepressant and/or anxiolytic efficacy. 5-HT-1A receptor agonists, such as 8-hydroxy-2-(di-n-propyl-amino)tetralin, increase the plasma prolactin concentration in rats, probably by an action in the brain that leads to an increase in prolactin release from the pituitary gland. The purpose of this study was to examine the effect of tandospirone on plasma prolactin concentration in awake, freely moving male rats. We found that i.v. administration of tandospirone results in a rapid and dose-related increase in the plasma prolactin concentration. The plasma prolactin level peaks about 10 min after injection and returns to base-line values within 30 min after injection. The ED50 of tandospirone to increase plasma prolactin levels is approximately 0.3 mg/kg. This effect of tandospirone is blocked by pretreatment with the 5-HT antagonists metergoline and NAN-190 and shows cross-desensitization with the 5-HT-1A agonist 8-hydroxy-2-(di-n-propyl-amino)tetralin. Thus the effect of tandospirone on plasma prolactin concentration appears to be mediated by 5-HT-1A receptors. In contrast to this effect of tandospirone, 1-(2-pyrimidyl)-piperazine (1-PP), the common metabolite of tandospirone and other azapirone drugs, has no effect on plasma prolactin levels. The effect of chronic administration of tandospirone was examined by measuring the prolactin response to an acute i.v. injection of tandospirone 24 hr after the last of chronic injections of tandospirone (10 mg/kg, s.c., twice a day for 2 weeks).(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of serotonin1A receptors increases release of prolactin in the rat

The effect of serotonin1A receptor agonists on release of prolactin was examined in awake, freely-moving male rats in which a catheter in the jugular vein allowed samples of blood to be collected periodically after intravenous injection of the agonist. The serotonin1A receptor agonist, 8-hydroxy-2(di-n-propylamino) tetralin (8-OHDPAT) increased concentrations of prolactin in plasma rapidly and in a dose-related manner. Concentrations of prolactin peaked within 9 min after intravenous injection of 8-OHDPAT and returned to baseline values within 30 min. Another serotonin1A receptor agonist, 5-methylurapidil (5-MeU), produced a similar response of prolactin. The effects of these agonists on release of prolactin were completely blocked by pretreatment with the serotonin receptor antagonists, methysergide and metergoline, administered 1 or 2 hr before the agonist. These results demonstrated that serotonin1A receptors can mediate the effects of serotonin on release of prolactin in the male rat.

Partial characterization of a neurotransmitter pathway regulating the in vivo release of prolactin

Nicotinic cholinergic, opiate and serotonergic agonists as well as dopaminergic antagonists induce the release of pituitary prolactin. The purposes of the present studies were to determine if nicotine, morphine and the serotonin1A (5-HT1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) utilize a common synaptic pathway to release prolactin and, if so, to establish the serial order of the receptors involved. We also sought to determine whether the pathway under investigation leads to the secretion of prolactin via a mechanism involving dopamine, the prolactin inhibitory factor. Male rats with indwelling jugular catheters were pretreated with saline, mecamylamine, naltrexone, methysergide or bromocriptine. In the saline-treated animals, administration of nicotine, morphine, 8-OH-DPAT and haloperidol resulted in significant increases in plasma prolactin levels. Mecamylamine pretreatment prevented the prolactin response to nicotine only. Naltrexone blocked the stimulation of prolactin release by morphine and by nicotine. Methysergide inhibited the effects of 8-OH-DPAT, morphine and nicotine but not haloperidol. Bromocriptine blocked the prolactin secretion induced by haloperidol as well as by each of the above agonists. Also, in dual-immunocytochemically stained sections, tyrosine hydroxylase-immunoreactive cells and serotonin-immunoreactive processes were detected in close anatomical proximity in the dorsomedial arcuate nucleus. These data indicate that nicotine, morphine and 8-OH-DPAT act to release prolactin via a common synaptic pathway expressing nicotinic cholinergic, opiate, and 5-HT1A receptors at synapses arranged serially in that functional order. Furthermore, the data indicate that the in vivo secretion of prolactin via this pathway may ultimately occur through the inhibition of dopamine release.

Effect of a growth hormone-releasing factor antagonist on compensatory renal growth, insulin-like growth factor-I (IGF-I), and IGF-I receptor gene expression after unilateral nephrectomy in immature rats

We have recently reported that pulsatile GH secretion is elevated 24 h after unilateral nephrectomy (UNX) in adult rats. In addition, suppression of the increase in GH with an antagonist to GH-releasing factor (GRF-AN) significantly attenuated compensatory renal growth (CRG) in adult rats. The present study examined the role of GH in CRG in immature animals. Pulsatile GH release was determined 24 h post-UNX in immature (26-28 days of age) sham-operated and UNX male Wistar rats. In contrast to the adult UNX rats, no increase in GH secretion was seen in the immature UNX rats compared with that in the controls. When pulsatile GH release was suppressed with GRF-AN, there was preferential growth of the remnant kidney despite the attenuated gain in whole body weight. In addition, insulin-like growth factor-I (IGF-I) and IGF-I receptor mRNA levels were elevated 3-fold in the remnant kidneys of GRF-AN-treated rats, despite the suppression of pulsatile GH release. These findings suggest that the initial phase of CRG is GH independent in the immature rat and, further, that CRG is associated with an increase in IGF-I and IGF-I receptor gene expression that is independent of episodic GH secretion.

Suppression of growth hormone release restores phosphaturic response to PTH in immature rats

Immature rats display a blunted rise in urinary phosphate but not adenosine 3',5'-cyclic monophosphate (cAMP) excretion in response to parathyroid hormone (PTH), perhaps as a consequence of the increased demand for phosphate during growth. Because a major driving force for growth is growth hormone (GH), and in view of the fact that GH has been shown to promote renal phosphate retention in the immature animal, it is possible that GH may attenuate the phosphaturic effect of PTH. The objective of this study was to determine whether suppression of pulsatile GH release, during administration of a synthetic peptide antagonist to GH-releasing factor, i.e., [N-acetyl-Tyr1-D-Arg2]-GRF-(1-29)-NH2 (GRF-AN), alters the renal response to increasing doses of PTH (1.5-15.0 micrograms.100 g-1.h-1) in the acutely thyroparathyroidectomized immature rat. Baseline fractional excretion of phosphate (FEPi), before administration of PTH, was negligible in all groups (less than 0.05%). Infusion of PTH resulted in an attenuated rise in FEPi in immature control rats compared with adult control rats (from 3.8 +/- 1.4% at lowest PTH dose to 16.7 +/- 3.1% at highest dose in immature rats vs. 21.1 +/- 3.5 to 31.9 +/- 4.4% in adult rats, P less than 0.05). In contrast, immature rats treated for 2 days with GRF-AN (100 micrograms/kg, twice daily) displayed an enhanced phosphaturic response (FEPi from 12.0 +/- 4.2 to 42.9 +/- 3.7%, P less than 0.05) compared with immature control rats, which was not different from that observed in control adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of renal phosphate reabsorption during development: implications from a new model of growth hormone deficiency

It has been hypothesized that the high rate of renal phosphate (Pi) reabsorption in the immature animal is a consequence of the increased demand for Pi associated with the rapid rate of growth. Although growth hormone (GH) has been proposed to play a role in this process, investigations of the relationship between GH, growth and the renal Pi transport have been hampered by the lack of methods available to specifically alter circulating GH levels. This review summarizes the findings from recent studies using a newly developed peptidic antagonist to GH-releasing factor (GRF-AN) as a method of specifically inhibiting GH release. Systemic injection of GRF-AN was effective in suppressing the pulsatile release of GH, and in significantly attenuating the rate of growth, in both immature and adult rats. However, the inhibition of growth was associated with a reduction in net Pi retention only in immature rats, resulting in a doubling in the urinary excretion of Pi. GRF-AN treatment of immature rats lead to a decrease in the maximum tubular capacity to transport Pi-down to the level seen in adult rats. However, GRF-AN treatment did not alter renal Pi reabsorption in adult rats. We conclude that chronic administration of an antagonist to GRF in rats provides a new model of GH deficiency with which to study the interrelationships between growth, GH and other physiological systems. Furthermore, the findings suggest that the pulsatile release of GH, directly or indirectly, contributes to the high rate of renal Pi reabsorption in young, growing animals and may play a critical role in regulating Pi homeostasis during development.

Neuropeptide Y in the hypothalamus: effect on corticosterone and single-unit activity

The purpose of the present study was to determine whether neuropeptide Y (NPY) acts within the hypothalamic paraventricular nucleus (PVN) or the suprachiasmatic nucleus (SCN) to alter circulating levels of corticosterone and to evaluate the effects of NPY on the single-unit response of PVN and SCN neurons using the hypothalamic slice preparation. Blood levels of corticosterone were determined in groups of rats that received microinjections of NPY or saline (Sal) into the PVN or SCN. NPY injected into the PVN 4 h after light onset resulted in corticosterone levels of 13.15 +/- 2.18 (SE) micrograms/dl within 1 h, which were significantly higher than the corticosterone levels of 4.08 +/- 1.78 micrograms/dl seen in rats receiving Sal injections. In contrast, no significant differences were observed in circulating levels of corticosterone between groups of rats 1 or 4 h after NPY or Sal microinjection into the SCN. In the hypothalamic slice, NPY was found to produce primarily inhibitory responses in both SCN and PVN neurons. Forty-nine percent of the SCN units examined were inhibited. In addition, another 20% of the neurons tested in the SCN displayed excitation followed by more sustained inhibition. In the PVN, 45% of the units examined were inhibited by NPY, however, nearly 30% of the remaining neurons were significantly excited by NPY. In summary, NPY alters the electrical activity of both SCN and PVN neurons but appears to act only within the PVN to influence circulating levels of corticosterone. These and other data indicate that NPY acts as an important neurochemical messenger within several hypothalamic sites.

Effects of chronic administration of nicotine on prolactin release in the rat: inactivation of prolactin response by repeated injections of nicotine

The effects of chronic injections of nicotine on nicotine-induced prolactin release in the rat were measured and compared to the effects of this treatment on [3H]acetylcholine binding to nicotinic cholinergic sites in the hypothalamus. Treatment with nicotine for 10 days (s.c. injections twice daily) abolished prolactin release in response to an acute i.v. injection of nicotine given 2, 6 or 8 days after the last of the chronic injections of nicotine. At each of these time points, the binding of [3H]acetylcholine in the hypothalamus from rats treated chronically with nicotine was significantly higher than in the hypothalamus from control rats. By 14 days after the last chronic injection of nicotine, the prolactin response to an acute injection of nicotine was restored. Coinciding with the return of the nicotine-induced prolactin response, the binding of [3H]acetylcholine had returned to control values. These results are consistent with the hypothesis that nicotine inactivates nicotinic cholinergic receptors in brain by an allosteric mechanism, and that prolonged inactivation of nicotinic cholinergic receptors leads to their increased number.

Acute effects of nicotine on prolactin release in the rat: agonist and antagonist effects of a single injection of nicotine

The effects of nicotine on prolactin release were studied in conscious, unrestrained rats in which an indwelling jugular cannula allowed multiple samplings of blood after i.v. administration of nicotine. Intravenous administration of nicotine bitartrate dihydrate increases plasma prolactin concentrations in a dose-dependent manner with an ED50 of approximately 100 micrograms/kg (200 nmol/kg) and this effect is blocked completely by pretreatment with mecamylamine, indicating that it is mediated by a nicotinic cholinergic receptor. Intracerebral ventricular injection of 1 microgram of nicotine also increases plasma prolactin levels, but i.v. injection of this same amount of nicotine has no effect, indicating that nicotine acts within the brain to release prolactin. A single i.v. injection of nicotine resulted in desensitization of the prolactin response to a subsequent injection of nicotine given 1 to 2 hr later, thus confirming a previous report by Sharp and Beyer (J. Pharmacol. Exp. Ther. 238: 486-491, 1986). The prolactin response to nicotine was restored within 24 hr after a single injection. The acute desensitization after a single injection of nicotine appears to be specific to release of prolactin by nicotine because the prolactin response to morphine was unaffected 1 hr after injection of nicotine. A single injection of nicotine appears to desensitize the prolactin response to a subsequent injection of nicotine with an ED50 of approximately 20 micrograms/kg (40 nmol/kg), indicating that nicotine is even more potent in stimulating desensitization of nicotinic cholinergic receptors than in stimulating prolactin release. These results support the concept that nicotine acts as a time-averaged antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Destruction of the dorsal anterior hypothalamic region suppresses pulsatile release of follicle stimulating hormone but not luteinizing hormone

The region of the paraventricular nucleus-dorsal anterior hypothalamic area (PVN-DAHA) previously was implicated in the control of tonic FSH secretion. However, the role that this hypothalamic area plays in governing pulsatile FSH release is unknown. To examine this question, radiofrequency (RF) lesions were produced bilaterally in the PVN-DAHA of adult female rats which had been ovariectomized for 4 weeks. Control animals received sham lesions. After a recovery period of 1 week, all rats were fitted with jugular cannulae. The next day sequential blood samples were withdrawn from conscious, undisturbed rats at 10-min intervals for 3 h. Control animals displayed secretory peaks of FSH in plasma with a frequency of 4.0 +/- 0.44/3-hour period (or 1 peak/40-50 min). LH in plasma pulsed at a frequency of 5.8 +/- 0.49 peaks/3 h (or 1 peak/20-30 min). Both of these control values were in agreement with previous studies. RF lesion of the PVN-DAHA reduced FSH peak frequency to 1.2 +/- 0.37 peaks/3 h (p less than 0.001) and also significantly suppressed the mean peak height and trough values for FSH (p less than 0.001). In contrast, none of these parameters for pulsatile LH secretion was altered by the lesion. Immediately after the 3-hour sampling period, synthetic LHRH (50 ng/100 g BW) was infused intravenously into rats and blood samples withdrawn 10 and 40 min later to determine whether the responsiveness of the pituitary gland had changed as a result of the lesion.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide-Y suppresses pulsatile secretion of luteinizing hormone in ovariectomized rats: possible site of action

The hypothesis that neuropeptide-Y (NPY) suppresses pulsatile LH secretion in ovariectomized (OVX) rats was examined. Rats were bilaterally OVX and 6 weeks (Exp 1) or 2 weeks (Exp 2) later a stainless steel cannula was implanted in the third cerebral ventricle (3V). Seven to 10 days later, an intraatrial cannula was inserted. The next day, a blood sample was withdrawn, and each conscious unrestrained animal received a 3V injection of synthetic porcine NPY (5 or 0.5 micrograms/2 microliters saline) or vehicle in Exp 1. Blood samples were taken every 10 min for 2 h and centrifuged, and the plasma was analyzed for LH by RIA. In Exp 2, OVX rats received a 3V injection of NPY (5 micrograms/2 microliters) or vehicle. Blood samples were taken before and 60 min after injection. At 60 min, LHRH (10 ng/100 g BW) was injected iv, and blood was withdrawn 10, 20, 60, and 120 min later. NPY caused a dramatic dose-related reduction in the pulsatile release of LH compared to that in vehicle-treated rats. The 5.0-micrograms dose of NPY significantly reduced LH pulse frequency (P less than 0.05), pulse amplitude (P less than 0.01), and trough levels (P less than 0.01) compared to those in saline-injected controls. The lower dose of NPY (0.5 micrograms) significantly decreased the mean LH levels throughout the 2-h sampling period and slightly, though not significantly, the pulse frequency. Administration of LHRH increased plasma LH levels by 124% in control animals and by 1239% in NPY-injected rats. The results of these studies indicate that the suppressive effects of NPY on pulsatile LH release appear to be exerted through inhibition of pulsatile LHRH secretion from the hypothalamus.

Antagonist to GH-releasing factor inhibits growth and renal Pi reabsorption in immature rats

Compared with adult rats, the immature rat has an enhanced tubular capacity for phosphate reabsorption, which presumably facilitates the growth process. Since the main driving force for growth is thought to be the pulsatile release of growth hormone, we examined the possibility that the adaptation in phosphate handling by the immature kidney is promoted by growth hormone (GH). To address this issue, we used a synthetic peptide antagonist to GH-releasing factor (GRF-AN) that we have shown blocks episodic GH secretion, and attenuates somatic growth. Immature male Wistar rats (4-5 wk of age) were catheterized with Silastic jugular cannulas and placed in metabolic cages. The rats were injected intravenously with either saline or GRF-AN (100 micrograms/kg) twice daily for 4 days. On the 4th day, they were prepared for renal clearance experiments to assess the maximum capacity for phosphate transport (TmPi). In animals treated with GRF-AN, there was an attenuated gain in body weight over 4 days of treatment (5 +/- 2 vs. 23 +/- 2% in saline controls, P less than 0.01). The suppressed growth was associated with a doubling of daily urinary phosphate excretion, and a reduction in the TmPi (3.3 +/- 0.1 vs. 4.6 +/- 0.3 mumol/ml in controls, P less than 0.01). A single injection of the antagonist to a separate group of immature rats did not alter TmPi. Thus injections of this new antagonist to GH-releasing factor over a 4-day period inhibit the pulsatile release of GH and significantly attenuate growth. The decline in growth of the immature rat was associated with a decrease in the renal capacity for phosphate reabsorption, down to levels seen in normal adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of pulsatile growth hormone (GH) secretion and somatic growth in immature rats with a synthetic GH-releasing factor antagonist

We previously reported that systemic administration of the recently described GRF peptide antagonist (N-Ac-Tyr1,D-Arg2)GRF-(1-29)-NH2 to adult male rats would suppress the pulsatile release of GH. In the present study, we have sought to determine whether this same antagonist would be efficacious in immature male rats to block spontaneous GH secretion and, as a result, retard several parameters of somatic growth. Indwelling Silastic catheters were placed into the jugular veins of immature male rats (120-140 g) at 29 days of age. After a recovery period of 48 h, beginning at 1000 h, 100-400 micrograms/kg GRF antagonist or its vehicle (controls) were injected iv immediately after withdrawing an initial blood sample from conscious undisturbed animals. Subsequent samples were obtained every 20 min until 1520 h. Red blood cells were resuspended in a restorative volume of saline and reinjected after each blood sample. Results showed that both doses of antagonist prevented the two major periods of episodic GH release observed in controls. For example, mean plasma GH (+/- SEM; nanograms per ml) at 1120 h was 9.0 +/- 2.7 in antagonist-treated rats and 37.1 +/- 5.1 in controls (P less than 0.05). Mean plasma GH (+/- SEM) at 1340 h was 10.8 +/- 3.7 in antagonist-treated rats and 38.8 +/- 9.6 in controls (P less than 0.05). Injection of 400 micrograms/kg of the structurally related VIP antagonist (N-Ac-Tyr1,D-Phe2)GRF-(1-29)-NH2, iv failed to suppress spontaneous GH release. GRF antagonist (100 micrograms/kg) was next administered twice daily iv for 4 days to 31-day-old rats in metabolic cages. This treatment essentially arrested the normal rapid body weight gain, significantly suppressed increases in body and tail lengths, and reduced increases in heart and kidney weights (P less than 0.01). Food intake and fecal output were unchanged by antagonist treatment and, therefore, did not contribute to the observed effects. These results support the idea that a number of tissues and organs are stimulated by the pulsatile secretion of GH and that a peptidic GRF receptor antagonist is useful in blocking episodic GH release in immature animals. As a consequence, this specific antagonist is effective in suppressing numerous aspects of somatic growth.

Blockade of growth hormone-releasing factor (GRF) activity in the pituitary and hypothalamus of the conscious rat with a peptidic GRF antagonist

Microinjection of synthetic GRF into the cerebroventricles or hypothalamus of the rat produces a number of neural effects, including the suppression of GH secretion, possibly representing a negative ultrashort loop autoregulation of GRF and/or stimulation of somatostatin neurosecretion. To demonstrate that such neuromodulation acts physiologically through endogenous GRF activity, the peptidic GRF antagonist (N-Ac-Tyr1,D-Arg2)GRF-(1-29)-NH2 was used to block the action of GRF on its presumed receptors in the hypothalamus. First, to establish the efficacy of the antagonist to block GRF receptors in the anterior pituitary, we injected the antagonist iv at doses of 2, 20, and 50 micrograms or saline (controls) into conscious male rats fitted with jugular cannulae. Sequential blood sampling every 15 min for 6 h between 1000-1600 h showed that 50 micrograms antagonist, iv, significantly suppressed the two periods of spontaneous release of radioimmunoassayable GH in controls in the morning and afternoon. A dose of 20 micrograms, iv, lowered mean plasma GH between 1400-1500 h (P less than 0.025), while the 2-microgram dose was without effect. The GRF antagonist was then microinjected into the third ventricle (3V) of conscious male rats at doses of 0.5 and 8.0 ng in 2 microliter sterile saline. The 8.0-ng dose of 3V antagonist elicited a 3-fold increase in the morning peak of GH (nanograms per ml): 3V antagonist, 159.0 +/- 62.0; 3V control, 51.0 +/- 21.9 (P less than 0.05). The 0.5-ng dose was without effect. Finally, we observed that pretreatment with the GRF antagonist 3V (10 ng), followed 15 min later by 10 ng rat GRF administered 3V, completely blocked the GRF-induced suppression of pulsatile GH release observed earlier. Both the systemic and central effects of the antagonist were specific to the control of GH, since PRL concentrations were unaltered. These results 1) have demonstrated the ability of a peptidic GRF antagonist to specifically suppress pulsatile GH release after its systemic administration, presumably by acting on pituitary GRF receptors, and 2) support the notion that GRF receptors are also present in the hypothalamus and are available for the physiological mediation of GRF-induced inhibition of GH release by a central mechanism.

Arginine vasopressin as a thyrotropin-releasing hormone

Although hypothyroidism (with concomitant increased levels of thyroid-stimulating hormone) has been associated with elevated plasma vasopressin, the role that vasopressin plays in controlling thyroid-stimulating hormone secretion from the adenohypophysis is not understood. In two in vitro pituitary cell systems, vasopressin caused a specific and dose-related release of thyroid-stimulating hormone from cells that was equal in potency to that elicited by thyrotropin-releasing hormone, the primary acknowledged regulator of thyroid-stimulating hormone release. When injected into the hypothalamus, however, vasopressin specifically inhibited the release of thyroid-stimulating hormone. Thus, vasopressin may exert differential regulatory effects on thyroid-stimulating hormone secretion in the hypothalamus and pituitary gland.

Purification of FSH-releasing factor: its dissimilarity from LHRH of mammalian, avian, and piscian origin

Sheep stalk median eminence fragments were lyophylized, extracted and filtered through a column of Sephadex G-25. The fractions were then assayed for the presence of LHRH by radioimmunoassay (RIA) and bioassayed for FSH and LH-releasing activity following their IV injection into ovariectomized, estrogen progesterone-blocked rats. The radioimmunoassayable LHRH emerged from the column at the same position from which it emerged many years before when LH was measured by bioassay. This same region also contained the LH-releasing activity as measured by bioassay. FSH-releasing activity was present in two tubes just preceding the emergence of the bio- and immunoassayable LHRH. The activity was highly significant and there was no LH-releasing activity in the fractions. They contained much less LHRH as determined by RIA than is sufficient to evoke LH release in this assay. The FSH-releasing activity was recovered in the same fractions in which it was found many years ago with this same assay but with measurement of plasma FSH by bio-rather than immunoassay as employed here. A dose-related release of LH was obtained by injection of LHRH in this assay but there was no significant FSH release even with a dose of 27 ng of LHRH per rat. To determine if one of the LHRHs of lower forms might be FSH-RF, Chicken I and II LHRH and Salmon LHRH were also assayed for FSH- and LH-releasing activity. Each of these peptide possessed LH-releasing activity, albeit much less than that of the mammalian peptide but had no FSH-releasing activity whatsoever.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a physiological role for oxytocin in the control of prolactin secretion

The presence of oxytocin (OT) in neuronal elements of the external layer of the median eminence and in hypophysial portal plasma suggests a role for the peptide in the control of anterior pituitary function. We have reported previously that OT stimulates PRL release in vitro; therefore, we attempted to establish evidence for a physiological PRL-releasing role for OT. Plasma OT levels rose significantly just before the PRL surges occurring during a suckling stimulus in lactating rats (10 min after pup reinstatement vs. 15 min for PRL) and 48 h after estrogen injection in ovariectomized (OVX) rats (at 1200 h vs. 1300 h). Dispersed anterior pituitary cells harvested from lactating female rats and OVX estrogen-primed rats released PRL in a specific, significant, and dose-related fashion when perifused in vitro with incubation medium containing 10(-7)-10(-9) M OT, doses similar to levels found previously in hypophysial portal plasma. Infusion of antiserum specific for OT into lactating females before pup reinstatement and into estrogen-primed OVX rats 2 h before the expected release of endogenous OT delayed and significantly reduced subsequent PRL surges compared to levels in saline-or normal rabbit serum-infused rats; however, PRL release was not completely abolished. These data indicate that OT plays a physiological role in the hypothalamic control of PRL secretion and further suggest the importance of multiple factors in coordinated regulation of PRL release.

Effects of intravenous and intraventricular injection of antisera directed against corticotropin-releasing factor on the secretion of anterior pituitary hormones

To determine the physiological significance of corticotropin-releasing factor (CRF) in the control of pituitary hormone secretion, highly specific antibodies directed against the peptide were injected either intravenously or intraventricularly (third ventricle) and the effect on plasma levels of pituitary hormones was determined before and after application of ether stress for 1 min. The intravenous injection of CRF antiserum (0.5 ml) did not significantly alter basal corticotropin (ACTH) levels in freely moving ovariectomized rats but largely blocked the increase in plasma ACTH resulting from ether stress. These antibodies had no effect on the ether-induced decline in plasma growth hormone (GH), and they failed to modify plasma luteinizing hormone levels. In a second experiment, CRF antiserum (3 microliter) or normal rabbit serum was injected into the third ventricle. A blood sample was drawn 24 hr later and immediately thereafter another injection of CRF antiserum or normal rabbit serum was made. There was no modification in the level of any of the hormones 24 hr after the first injections, and they were similar in CRF antiserum and normal rabbit serum-injected animals. After imposition of ether stress, the response of plasma ACTH was nearly completely blocked by the intraventricular CRF antiserum, but the degree of blockade was slightly less than that obtained by intravenous injection. The decline in plasma GH after ether stress was blocked by the intraventricular CRF antiserum. There was no effect of the intraventricular injection of the antiserum on the levels of the other pituitary hormones. The results with intravenous injection of the antisera indicate that CRF plays an extremely important but probably not completely indispensable role in the release of ACTH after ether stress. The results of the intraventricular injection of the antiserum suggest strongly that endogenous CRF may also modify its own release in response to stress, augmenting it by a positive ultrashort loop feedback, and that the antisera against the peptide blocked this action; however, an action at the pituitary of these intraventricularly injected antibodies cannot be completely ruled out. The blockade of the stress-induced suppression of GH release by the CRF antibodies suggests that CRF released intrahypothalamically during ether stress brings about an alteration in the hypothalamic control of GH secretion such that the stress-induced inhibition of GH release is blocked.

Effects of intraventricular growth hormone-releasing factor on growth hormone release: further evidence for ultrashort loop feedback

We examined the effects of cerebroventricular injection of synthetic human GH-releasing factor [hGRF-(1-44)] on regulation of GH release in conscious male rats. These results were compared with the direct effects of hGRF on hormone released from dispersed anterior pituitary cells. Administration of two higher doses of hGRF (200 and 2000 ng) into the third ventricle (3V) produced a dose-related increase in plasma GH levels (P less than 0.001). Injection of hGRF into the 3V at two lower doses actually reduced GH release. Infusion of 20 ng (5 pmol) hGRF reduced plasma GH from 5-60 min (P less than 0.005), with a maximum suppression of 66%. The 2-ng (0.5-pmol) dose decreased GH secretion by 45% (P less than 0.05). hGRF stimulated a significant and dose-dependent release of GH from dispersed pituitary cells at concentrations of 10(-10) and 10(-9) M (P less than 0.025). The specificity of GRF for GH control, whether stimulatory or inhibitory, was seen by the failure of GRF to modify PRL, TSH, or LH release. Our results indicate that injection of larger doses of GRF into the 3V produce GH release, but at lower doses, 3V GRF may exert an action centrally to inhibit GH release. We propose that hypothalamic GRF may decrease its own neurosecretion by negative ultrashort loop feedback.

Naloxone-induced dissociation of oxytocin and prolactin releases

Immobilization of adult male rats resulted in concomitant, significant releases of prolactin (PRL) and oxytocin (OT). Naloxone (0.2 mg/kg, i.p.) administration 30 min prior to initiation of restraint resulted in a significant diminution of the PRL response to stress. On the other hand, a significant augmentation of OT release was observed. These results demonstrate that concomitant releases of OT and PRL in response to a given physiologic stimulus (stress) can be dissociated and suggest that the hypothalamic events which are responsible for the release of these hormones during stress are different.

Neuropeptide Y affects secretion of luteinizing hormone and growth hormone in ovariectomized rats

Neuropeptide Y (NPY) has recently been localized in the rat hypothalamus. We have evaluated the effects of NPY on hypothalamic and pituitary function by injecting NPY into the third ventricle in vivo and by examining its action on perifused pituitary cells in vitro. Injections of NPY into the third ventricle of conscious ovariectomized rats led to a dramatic and highly significant reduction in plasma luteinizing hormone (LH) relative to pretreatment levels in these animals or to those of controls injected with physiological saline. Significant inhibition was obtained with doses ranging from 0.02 to 5.0 micrograms (4.7-1175 pmol) of NPY. These inhibitory effects on LH release were dose dependent and lasted for at least 120 min after injection of 5.0 micrograms of NPY. Intraventricular injection of NPY also significantly decreased plasma growth hormone; however, the threshold dose was 2.0 micrograms (470 pmol), a dose 100-fold greater than the lowest dose that inhibited LH release. Plasma follicle-stimulating hormone was unaffected by injection of NPY. NPY (10(-6) and 10(-7) M) stimulated secretion of LH, growth hormone, and follicle-stimulating hormone from perifused anterior pituitary cells loaded in a Bio-Gel P-2 column. These results indicate that NPY acts on structures adjacent to the third ventricle to inhibit the secretion of LH and growth hormone but not follicle-stimulating hormone, whereas it can directly stimulate the secretion of all three hormones from the cells of the anterior pituitary in vitro. Since NPY has been found in the hypothalamus and median eminence, it is quite likely that it plays a physiologically significant role at both hypothalamic and pituitary sites: influencing secretion of pituitary hormones.

Pancreatic polypeptides affect luteinizing and growth hormone secretion in rats

We have examined the effects of third cerebroventricular (3V) injections of avian and bovine pancreatic polypeptide (APP and BPP) and the C-terminal hexapeptide amide of human PP (CHPP) on the secretion of anterior pituitary hormones in conscious ovariectomized rats. Injection of APP (2.0 micrograms; 472 pmoles) or BPP (5.0 micrograms; 1191 pmoles) decreased plasma levels of luteinizing hormone (LH) when compared to pre-injection levels in these animals or to saline-injected controls. The lower dose of BPP (0.5 micrograms; 119 pmoles) decreased plasma LH versus pre-injection levels and control animals, however, these effects diminished at later times. Plasma growth hormone (GH) also decreased following 3V injections of APP (2.0 micrograms) or BPP (5.0 micrograms). The lower dose of BPP (0.5 microgram) initially inhibited GH release, however, this effect was rapidly reversed and GH levels were significantly greater than those in controls at 60 and 120 min. Injections of BPP or APP did not alter prolactin (PRL) or thyroid stimulating hormone (TSH) secretion. Administration of 2.0 micrograms and 0.2 microgram of CHPP (2488 and 249 pmoles) produced no significant effects on plasma LH, GH, PRL or TSH. APP and BPP had no consistent effects on hormone secretion from dispersed anterior pituitary cells. The results indicate that APP and BPP exert potent central effects which inhibit LH and GH release from the pituitary gland.

Effect of destruction of the dorsal anterior hypothalamus on follicle-stimulating hormone secretion in the rat

The role of the paraventricular nucleus-dorsal anterior hypothalamus (PVN-DAHA) in the control of anterior pituitary gland secretion of FSH and LH in castrated male and female rats was examined. Bilateral radiofrequency lesions of the PVN-DAHA in chronically ovariectomized (OVX) rats lowered plasma FSH levels by 33% (P less than 0.005) compared to values in unoperated and sham-operated control rats; plasma LH concentrations were unaltered. RIA of median eminence (ME) LHRH concentrations in these animals revealed no differences among the three experimental groups. Other categories of diencephalic destruction did not result in this pattern of selectively reduced FSH release. Bilateral radiofrequency destruction of the PVN-DAHA also attenuated by 50% (P less than 0.025 to P less than 0.005) the progesterone-induced surge of FSH in estrogen-primed OVX rats. Progesterone-induced LH release was unaffected by PVN-DAHA lesions. Other lesion categories failed to show the same result. Bilateral ablation of the PVN-DAHA in male rats resulted in a selective diminution of the postcastration rise of plasma FSH beginning 48 h postcastration (P less than 0.05 to P less than 0.005) and persisting for 14 days (P less than 0.005) after orchidectomy, thus revealing the time course and permanence of this procedure on plasma FSH levels. The postcastration rise of plasma LH levels was not affected by PVN-DAHA lesions. The concentration of ME LHRH was the same among orchidectomized male rats whether they bore PVN-DAHA lesions, sham lesions, or no lesions. In summary, destruction of the PVN-DAHA was found to reduce significantly the elevation of plasma FSH, but not LH, in the OVX rat and the estrogen-progesterone-stimulated OVX rat. PVN-DAHA lesions also attenuated the postcastration rise of FSH, but not that of LH, in the male. The failure of lesions of the PVN-DAHA to alter ME LHRH concentrations in the face of decreased FSH release does not prove that LHRH release is totally unaffected by this procedure. This finding is, however, consistent with the concept that diminished FSH secretion could be the result of a deficiency of a hypothalamic releasing factor (FSH-releasing factor?) other than that of the LHRH decapeptide.

Intrahypothalamic action of corticotrophin-releasing factor (CRF) to inhibit growth hormone and LH release in the rat

The effects of intravenous or intraventricular injection of synthetic ovine corticotrophin-releasing factor (oCRF) on plasma levels of anterior pituitary hormones were studied in conscious, ovariectomized (OVX) female rats and compared with the actions of the peptide on dispersed anterior pituitary cells from OVX female rats incubated in the presence of CRF. Third ventricular injection of oCRF in freely moving rats caused a significant increase in plasma levels of ACTH in a dose-related manner with a minimal effective dose of less than 0.5 micrograms (0.1 nmol). The effect was observable at 5 min after injection and persisted for the 60 min duration of the experiment. In contrast, growth hormone levels were significantly depressed within 15 min with a minimal effective intraventricular dose of 0.5 micrograms. The suppression persisted for the duration of the experiment but there was no additional effect of the higher dose of 5 micrograms. Plasma LH levels were also lowered by the highest dose of 5 micrograms (1.0 nmol) of oCRF, with the first significant lowering at 30 min. Lower doses had no effect on plasma LH. Plasma TSH levels were not significantly altered. Control injections of the 0.9% NaCl diluent were without effect on the levels of any of the hormones. Intravenous injection of similar doses of oCRF had no effect on plasma levels of GH or LH. The ACTH-releasing action of the oCRF preparation was confirmed by in vitro incubation of the peptide with dispersed anterior pituitary cells for 2 h. A dose-related release of ACTH occurred in doses ranging from 0.1-10 nM, but there were no effects on the release of the other anterior pituitary hormones. The results suggest that oCRF may act within the hypothalamus to suppress the release of GH and to a lesser extent LH. The stimulation of ACTH release following intraventricular CRF is presumably related to its uptake by portal blood vessels with delivery to the pituitary and stimulation of the corticotrophs.

Presence and possible site of action of secretin in the rat pituitary and hypothalamus

Secretin-like immunoreactivity was detected in extracts of several rat brain structures by radioimmunoassay, most notably in the pituitary, hypothalamus, pineal and septum. Its localization to these structures suggested that it might play a role in neuroendocrine events similar to its structural homolog vasoactive intestinal peptide. Dose-related stimulations (MED, 10(-7) M) of prolactin (PRL) release were observed after incubation of synthetic secretin with dispersed, cultured pituitary cells from male and ovariectomized (OVX) female rats. In OVX females, i.v. infusion of a high dose of secretin (10 micrograms) resulted in a significant elevation of PRL levels. Doses of secretin as low as 0.1 micrograms when administered into the third cerebroventricle were capable of significantly inhibiting PRL release in both males and OVX females, suggesting an ultrashort-loop, negative feedback of secretin. Secretin can now be added to the growing list of putative PRL-releasing agents.

Recent studies on the role of brain peptides in control of anterior pituitary hormone secretion

Recent work in our laboratory on the role of peptides to influence release of pituitary hormones by direct action on the gland and also some of the interactions of these peptides at the hypothalamic level to alter release of pituitary hormones will be reviewed. Considerable evidence from hypothalamic stimulation and lesion studies suggests the existence of a separate FSH-releasing factor (FSHRF). We have been able to purify a bioactive FSHRF which appears to be distinct from LHRH. Consequently, we believe that a distinct FSHRF will ultimately be isolated. With regard to prolactin, it is now clear that it is under dual control by both prolactin-inhibiting (PI) and prolactin-releasing factors (PRF). Although dopamine acts as a PIF, our recent fractionation studies indicate the existence of a peptidic PIF in hypothalamic extracts which can be separated from dopamine and GABA. The peptidic PIF is eluted from Sephadex in the same position originally described by us a number of years ago. Thus, inhibitory control is probably mediated by a combination of factors which would include dopamine, possibly GABA and a peptidic PIF. A number of peptides have been shown to have PRF activity which include TRF and also VIP. In recent studies, we have shown a prolactin-releasing action of oxytocin on male hemipituitaries or dispersed pituitary cells. Furthermore, high doses of oxytocin given intravenously released prolactin in male rats. There is a correlation between estrogen-induced prolactin release and an increase in plasma oxytocin and a correlation between suckling-induced oxytocin and prolactin release. These results suggest that oxytocin may be an important PRF.(ABSTRACT TRUNCATED AT 250 WORDS)

Pulsatile release of follicle-stimulating hormone in ovariectomized rats is inhibited by porcine follicular fluid (inhibin)

While the pulsatile release of LH in ovariectomized animals has been well established, little information exists concerning the pulsatile release of FSH and the factors involved in its regulation. The present studies analyzed the characteristics of episodic FSH secretion and examined the effects of ovarian feedback signals on that pattern. From conscious unrestrained ovariectomized rats, blood was collected at 10-min intervals for a period of 3 h. A pulsatile pattern of FSH was observed in the plasma of control rats, with an average frequency of 4 peaks/3, h, while LH oscillations occurred at an average of almost 6 peaks per 3 h. The injection of porcine follicular fluid (PFF; 0.5 ml), a source of inhibin, reduced FSH peak frequency to 2.5 peaks/3 h (P less than 0.005). In the third hour of blood sampling, PFF also significantly lowered the average FSH peak amplitude (P less than 0.05), the trough value, and the average mean value for that hour. In contrast, none of these parameters for LH was altered by PFF. Estradiol benzoate (EB; 20 micrograms, sc, 24 h before bleeding) significantly reduced LH peak frequency, mean trough value, and average mean value, whereas it only reduced the mean FSH average value and mean trough value. Combined treatment with EB and PFF synergized to decrease the mean value, trough value, peak frequency, and peak amplitude of episodic LH release. Pituitary responsiveness to LHRH was evaluated after the treatments described above. PFF inhibited the response of FSH to LHRH, but not that of LH. EB by itself potentiated the release of both hormones. The combined EB/PFF treatment resulted in a decreased pituitary response in terms of FSH and LH compared to that of the EB-treated control group. We conclude that the release of FSH occurs in a pulsatile fashion and, further, that this pattern of release is regulated by PFF (inhibin). It was also shown that an intriguing synergism between estradiol and inhibin exists to suppress LH release.

Motilin: a novel growth hormone releasing agent

Motilin, a gastrointestinal peptide recently detected in the rat brain, was capable of stimulating growth hormone (GH) release from dispersed anterior pituitary cells in a dose-related fashion. In initial experiments, the minimum effective concentration was 10(-7) M and the effect was specific for just GH. Subsequent experiments demonstrated that concentrations of synthetic motilin as low as 10(-9) M could significantly stimulate GH release. Only large IV doses (100 micrograms) of motilin significantly elevated circulating GH levels in vivo. However, administration of antiserum to porcine motilin (100 microliters, IV) significantly depressed plasma GH levels, suggesting a physiologic role for median eminence and hypothalamic motilin in the control of GH secretion. Furthermore, infusion of motilin into the third ventricle of conscious rats resulted in a significant depression of GH levels, suggesting an ultrashort loop feedback action of motilin on the release of motilin itself or somatostatin. In light of motilin's only minor structural similarity to human pancreatic tumor GH-releasing factor (GRF) and the ability of passive immunoneutralization of motilin to lower GH, this 22-amino acid peptide must now be considered a physiologic GRF.

Evidence for an FSH-releasing factor in the posterior portion of the rat median eminence

To test the hypothesis that an FSH-releasing factor might be contained within the posterior portion of the median eminence (ME), the anterior half of the ME (aME) and the posterior half of the ME (pME) were removed separately from the brains of adult male rats and extracted in 0.2 N acetic acid. LH and FSH-releasing activities of the extracts were measured in vitro by incubating 8 hemipituitaries from adult male rats for 6 h at a dose of 5 tissue equivalents and determining the radioimmunoassayable LH and FSH released into the medium. LH release induced by the aME extracts was significantly greater than that induced by the pME in both experiments, whereas there were no differences in FSH release between aME and pME extracts. A significant dose-related increase in FSH release was noted in this system when 1 and 2 ng of synthetic LHRH were tested which indicates that the assay was sensitive to different amounts of LHRH with regard to FSH-releasing action. The content of immunoreactive LHRH in the extracts was almost twice as high in the aME as in the pME. Therefore, the results indicate that the pME has greater FSH-releasing activity than can be accounted for by its content of LHRH. The additional FSH-releasing activity is presumably due to an FSH-releasing factor distinct from LHRH.

Prolactin-releasing activity of porcine intestinal peptide (PHI-27)

Porcine intestinal peptide (PHI), a twenty-seven amino acid peptide isolated from porcine gut extracts, is a close structural homolog of the secretin family hormones. The structural and biological similarities of PHI to vasoactive intestinal peptide (VIP) together with its presence in the rat hypothalamus suggested a possible role for the peptide in the control of prolactin (PRL) secretion. PHI induced significant, dose-related stimulations of PRL release from cultured, dispersed rat pituitary cells in vitro. The minimum effective dose is 10(-7) molar, compared to 10(-9) molar for VIP. No interactive effect with thyrotropin-releasing hormone was observed; however, PHI partially overcame the dopamine inhibition of PRL release.

Purification of a bioactive FSH-releasing factor (FSHRF)

Before the advent of radioimmunoassay (RIA), FSH-releasing factor (FSHRF) appeared to be separable from LH-releasing hormone (LHRH) by chromatography followed by bioassay for FSH. In this study, we re-examined hypothalamic extracts for the existence of an FSHRF distinct from LHRH, utilizing the Steelman-Pohley bioassay as well as RIA for identification of FSH. Acid extracts of rat hypothalamic fragments were chromatographed on Sephadex G-25. LH- and FSH-releasing activities of each fraction were assessed by bio- and immunoassay of FSH and immunoassay of LH released after incubation with hemipituitaries from adult male rats. The immunoreactive LHRH(IR-LHRH) concentration of each fraction was also measured by RIA. In order to evaluate the FSH-releasing activity of LHRH, three doses of synthetic LHRH were tested and FSH-releasing activity determined by bio- and immunoassay. By RIA, the FSH-releasing activity of each column fraction could be accounted for by IR-LHRH contamination. However, greater FSH-releasing activity than could be predicted by IR-LRH contamination was detected by Steelman-Pohley assay in fractions eluted prior to the LHRH peak in 2 separate fractionations. These fractions from the second fractionation were pooled and eluted from a CMC column with ammonium acetate buffers. Again greater FSH-releasing activity than could be accounted for by IR-LHRH was detected prior to the IR-LHRH peak by Steelman-Pohley assay. These results agree with early work from our laboratory and suggest the presence of a bioactive FSHRF in hypothalamic extracts.

Hypothalamic and pituitary sites of action of oxytocin to alter prolactin secretion in the rat

To determine whether oxytocin (OT) could alter the release of PRL and other hormones from the anterior pituitary gland, the effects of OT were examined in two in vitro and two in vivo test systems. Cells dispersed from anterior pituitary glands of intact adult male rats were incubated in medium containing OT at doses of 10(-8), 10(-7), 10(-6), and 10(-5) M in two trials. OT stimulated PRL release 1.5-fold (P less than 0.01) and 2- to 3-fold (P less than 0.001) above control levels at 10(-8) and 10(-7) M doses, respectively, thus indicating a dose-dependent relationship. Higher doses did not produce a further elevation above that obtained with 10(-7) M OT. Arginine vasopressin (AVP) caused a slight decrease in PRL release from dispersed cells while TRH produced a small (25%), significant, but nondose-related increase in PRL release. Hemipituitary glands from adult male rats, incubated with 10(-6) and 10(-5) M OT, released twice as much PRL (P less than 0.01) into the medium as paired controls, but 10(-7) M OT was ineffective. The iv injection of 1 or 10 micrograms OT into conscious male rats elevated plasma PRL by 50% (P less than 0.05) or 500% (P less than 0.001), respectively, above basal values at 5 min only. Vehicle or 0.1 microgram OT were without effect. When 0.1 microgram OT was microinjected into the third ventricle (3V) of conscious male rats, it paradoxically reduced plasma PRL by 40% at 30 min (P less than 0.05), whereas 1 microgram OT significantly lowered PRL at 5-60 min, with the maximum suppression (60%, P less than 0.001) occurring at 30 min. These latter findings may indicate that an ultrashort loop feedback mechanism exists whereby exogenous OT decreases hypothalamic OT secretion, thereby reducing the OT stimulus for PRL release. The specificity of the OT effect on PRL was attested to by the failure of OT to alter significantly FSH, LH, and TSH in each system. GH was unchanged except that 3V-injected OT (1 microgram only) elevated (P less than 0.001) plasma GH at 15-30 min. These results support the view that OT acts directly on the cells of the anterior pituitary gland at low to high doses to release PRL specifically and in a dose-related fashion. In contrast, 3V injection of OT reduces PRL secretion, thereby suggesting that OT may decrease its own neurosecretion by ultrashort loop feedback and thus reduce an OT stimulus for PRL release.

Differential hypothalamic control of FSH secretion: a review

There are many circumstances in which the release of FSH and LH is dissociated; however, many of these are now thought to be brought about by interactions of LH-releasing hormone (LHRH), which stimulates not only LH but also FSH release, and the gonadal peptide, inhibin, which acts at the pituitary to suppress FSH release selectively. There are also many examples which can only be explained by postulating separate hypothalamic control of FSH and LH release. For example, electrochemical stimulation of the medial preoptic area elicited only LH release, whereas stimulation further caudally elicited equivalent LH release but FSH release as well. Points of stimulation particularly in the dorsal anterior hypothalamic area (DAHA) evoked only FSH release. Furthermore, implantation of prostaglandin E2 in various hypothalamic loci in a region extending from the DAHA caudally and ventrally to the caudal median eminence (ME) selectively elicited FSH release. Lesions of the DAHA resulted in a decrease of plasma FSH but not LH in castrated male and female rats and also suppressed the post-castration rise in FSH in males. In ovariectomized estrogen-primed rats with DAHA lesions, injection of progesterone provoked a normal LH surge but a significantly depressed FSH surge. Anterior ME lesions in castrates lowered LH levels more than FSH levels. Extracts of the DAHA evoked greater FSH and LH release in vitro than could be accounted for by the content of LHRH in the extracts, but there was no preferential release of FSH. On the other hand, extracts of the organum vasculosum lamina terminalis (OVLT) evoked dramatically increased FSH release above that which could be accounted for by the content of LHRH. Lastly, posterior ME extracts had more FSH-releasing activity than could be accounted for by their content of LHRH. All these results suggest the existence of an FSH-releasing factor (FSHRF) and lead to the speculation that the cell bodies of FSHRF neurons are located in the DAHA, with axons projecting to the OVLT and to the posterior ME. In other experiments, attempts were made to purify rat and sheep hypothalamic extracts by gel filtration on Sephadex G-25 and to assay the FSH-releasing activity by both bio- and immunoassay. Using this approach, we obtained evidence for the early emergence of a bioactive FSHRF prior to the emergence of LHRH from the column. Although much more work remains to be done, the accumulated evidence strongly supports the concept of a distinct FSHRF.

Gastrointestinal hormones: central nervous system localization and sites of neuroendocrine actions

It was established that VIP (vasoactive intestinal peptide), secretin, CCK (cholecystokinin), gastrin and motilin can be localized to the CNS by immunologic means. Whether or not these immuno-crossreactivities represent peptides identical to those in the g.i. tracts remains to be established. The neuronal localization of these five peptides in the gut predicted, however, their presence in neurons of the CNS. Furthermore, their presence within the hypothalamus and pituitary suggested physiological roles for these hormones in anterior pituitary function. We have now demonstrated the direct actions of VIP, secretin, gastrin and motilin on pituitary hormone release in vitro. Perhaps more importantly, we have described a hypothalamic site of action of VIP, secretin, CCK and gastrin to alter hormone release in vitro. Our data, taken in concert with those of other groups, suggest a modulatory role for the g.i. hormones and indicate the possible symphonic control by many hormones and transmitter candidates of distinct secretory events in the pituitary. Indeed, these data indicate the complexity underlying the finally tuned hypothalamus-pituitary-target tissue axis.

Motilin stimulates growth hormone release in vitro

Motilin, a twenty two amino acid polypeptide originally isolated from duodenal extracts, has been detected recently in the mammalian hypothalamus and pituitary. We have investigated the possibility that motilin might play a role in neuroendocrine events and report here the ability of synthetic porcine motilin (10(-6)M) to stimulate growth hormone release from rat hemipituitaries and dispersed anterior pituitary cells in vitro. No significant effects on luteinizing hormone, thyroid stimulating hormone or prolactin release were observed. Motilin may be added therefore to the growing list of gastrointestinal hormones which can act directly at the level of the anterior pituitary to alter hormone release.

Paradoxical elevation of growth hormone by intraventricular somatostatin: possible ultrashort-loop feedback

Somatostatin, the growth hormone-inhibiting factor, when microinjected into the third ventricle of the rat brain, paradoxically induced the release of growth hormone. A pituitary site of action having been ruled out, this result supports the concept that exogenous somatostatin within the hypothalamus acts either to suppress the release of somatostatin from somatostatin-containing neurons, possibly via an ultrashort-loop feedback mechanism, or to augment release of hypothalamic growth hormone-releasing factor, thereby inducing a release of growth hormone. Injection of somatostatin into the third ventricle also decreased plasma concentrations of luteinizing hormone, follicle-stimulating hormone, and thyroid-stimulating hormone, probably by inhibiting the release of luteinizing hormone-releasing factor and thyrotropin-releasing factor.