Hypothalamic Releasing Hormone Research Articles

Reversal of Chronic Ethanol‐Induced Testosterone Suppression in Peripubertal Male Rats by Opiate Blockade

Teenage drinking continues to be a significant problem in the US., as well as abroad. We have previously demonstrated that opiate blockade with naltrexone, a drug currently used in patients to diminish alcohol craving, prevented the fall in serum testosterone seen after acute ethanol (EtOH) exposure in young, peripubertal male rats. To follow‐up on this reversal, a series of experiments was performed to determine if naltrexone would also prevent the testosterone suppression caused by chronic EtOH exposure. Peripubertal rats either 45 days old (mid‐pubertal) or 55 days old (late pubertal) were fed an EtOH‐containing liquid diet or pair‐fed control diet for 14 days. Each animal was implanted with either a naltrexone containing or placebo pellet before starting the liquid diet. In each age group, EtOH alone significantly suppressed testosterone, whereas naltrexone prevented this fall, although it had no effect alone. Serum luteinizing hormone was also suppressed by EtOH; however, naltrexone did not abrogate this fall. In the 45‐day‐old animals, β‐luteinizing hormone mRNA levels rose significantly in the EtOH group, but not when naltrexone was coadministered with EtOH. There was no change in hypothalamic luteinizing hormone releasing hormone (LHRH) mRNA, pro‐LHRH, or LHRH in any group at either age. Thus, naltrexone is able to partially prevent the EtOH‐induced suppression of gonadal testosterone of young, adolescent male rats. This effect appears to be mediated directly at gonadal level, because hypothalamic and pituitary hormone changes were minor and nonsignificant.

Inhibition of receptor-mediated calcium responses by corticotrophin-releasing hormone in the CATH.a cell line

A region of the brain believed to be important in the CNS response to stress is the locus coeruleus, the predominant site of noradrenergic cell bodies. Corticotrophin releasing hormone (CRH) is the primary hypothalamic releasing hormone responsible for the activation of the pituitary-adrenal axis in response to stress and, in this study, we employed a locus coeruleus-like cell line, CATH.a, to investigate the modulation of receptor signalling pathways by CRH. Pituitary adenylyl cyclase-activating polypeptide (PACAP) (10 nM), vasoactive intestinal peptide (VIP) (1 μM) and carbachol (1 mM) produced transient increases in intracellular [Ca 2+]. The inhibition of the carbachol (1 mM) response by CRH was concentration-dependent (EC 50=154±1.8 nM). Calcium responses to sub-maximally effective concentrations of PACAP (5 nM), VIP (400 nM) and carbachol (1 mM) were abolished by prior exposure to CRH (1 μM). At the concentrations employed, CRH and VIP both substantially increased intracellular [ 3H]-cyclic AMP accumulation. The adenylyl cyclase activator forskolin (10 μM) was also effective at eliminating the agonist-induced calcium responses. Incubation with the cell permeant cyclic AMP analogue dibutyryl cyclic AMP (dbcAMP) (1 mM), an activator of protein kinase A (PKA), for 12 min prior to agonist exposure similarly abolished the intracellular calcium response to carbachol. Carbachol increased [ 3H]-inositol phosphate ([ 3H]-IP) accumulation to a maximum of 2.4±0.11-fold basal (EC 50=6.75±0.26 μM). PACAP produced a much greater accumulation (19.9±2.1 fold basal; EC 50=24 nM). In the presence of forskolin (10 μM), neither carbachol- nor PACAP-induced [ 3H]-IP accumulation was significantly different from in its absence. These results demonstrate that CRH inhibits receptor-mediated intracellular calcium responses in a locus coeruleus-like cell line possibly via activation of PKA. This modulation could be important in controlling neuronal function in vivo in stressful situations in which the levels of CRH are increased in the locus coeruleus.

Secretory pattern of GH, TSH, thyroid hormones, ACTH, cortisol, FSH, and LH in patients with fibromyalgia syndrome following systemic injection of the relevant hypothalamic-releasing hormones.

To study the hormonal perturbations in FMS patients we injected sixteen FMS patients and seventeen controls a cocktail of the hypothalamic releasing hormones: Corticotropin-releasing hormone (CRH), Thyrotropin-releasing hormone (TRH), Growth hormone-releasing hormone (GHRH), and Luteinizing hormone-releasing hormone (LHRH) and observed the hormonal secretion pattern of the pituitary together with the hormones of the peripheral endocrine glands. We found in FMS patients elevated basal values of ACTH and cortisol, lowered basal values of insulin-like growth factor I (IGF-I) and of triiodothyronine (T3), elevated basal values of follicle-stimulating hormone (FSH) and lowered basal values of estrogen. Following injection of the four releasing-hormones, we found in FMS patients an augmented response of ACTH, a blunted response of TSH, while the prolactin response was exaggerated. The effects of LHRH stimulation were investigated in six FMS patients and six controls and disclosed a significantly blunted response of LH in FMS. We explain the deviations of hormonal secretion in FMS patients as being caused by chronic stress, which, after being perceived and processed by the central nervous system (CNS), activates hypothalamic CRH neurons. CRH, on the one hand, activates the pituitary-adrenal axis, but also stimulates at the hypothalamic level somatostatin secretion which, in turn, causes inhibition of GH and TSH at the pituitary level. The suppression of gonadal function may also be attributed to elevated CRH by its ability to inhibit hypothalamic LHRH release, although it could act also directly on the ovary by inhibiting FSH-stimulated estrogen production. We conclude that the observed pattern of hormonal deviations in FMS patients is a CNS adjustment to chronic pain and stress, constitutes a specific entity of FMS, and is primarily evoked by activated CRH neurons.

The anti-gonadotropic effects of cytokines: the role of neuropeptides

The inhibitory effect of inflammation and endotoxins on the secretion of reproductive hormones from the hypothalamo-pituitary axis is well documented. A comparison of the luteinizing hormone (LH) suppressing effects of several pro-inflammatory cytokines revealed that centrally administered IL-1β was the most potent inhibitor of pituitary LH secretion; interleukin (IL)-1α and tumor necrosis factor (TNF)α were relatively less effective, whereas IL-6 was ineffective. This order of potency suggested that the anti-gonadotropic effects of an immune challenge are most likely attributable to the action of centrally released IL-1β, and this was supported by the demonstration that IL-1β suppressed hypothalamic luteinizing hormone releasing hormone (LHRH) release. We used a multifaceted approach to identify the afferent signals in the brain that convey immune messages to hypothalamic LHRH neurons. Pharmacological studies with specific antagonists of opioid receptor subtypes demonstrated that activation of the μ1 receptor subtype was required to transmit the cytokine signal. Furthermore, icv IL-1β upregulated hypothalamic POMC mRNA and increased the concentration and release of β-endorphin, the primary ligand of μ1 receptors. We have obtained evidence that IL-1β also enhanced the gene expression and concentration of tachykinins, a family of nociceptive neuropeptides in the hypothalamus. Blockade of tachykinergic NK2 receptors attenuated IL-1β induced inhibition of LH secretion. Collectively, these results demonstrate that IL-1β, generated centrally in response to inflammation, upregulates the opioid and tachykinin peptides in the hypothalamus. These two groups of neuropeptides are critically involved in relaying the cytokine signal to neuroendocrine neurons and causing the suppression of hypothalamic LHRH and pituitary LH release.

Reversal of Ethanol‐Induced Testosterone Suppression in Peripubertal Male Rats by Opiate Blockade

Teenage drinking continues to be a significant problem in the U.S., as well as abroad. We have previously demonstrated that opiate blockade with naltrexone, a drug currently used in patients to diminish alcohol craving, prevented the fall in serum testosterone seen after acute ethanol (EtOH) exposure in young, peripubertal male rats. To follow-up on this reversal, a series of experiments was performed to determine if naltrexone would also prevent the testosterone suppression caused by chronic EtOH exposure. Peripubertal rats either 45 days old (mid-pubertal) or 55 days old (late pubertal) were fed an EtOH-containing liquid diet or pair-fed control diet for 14 days. Each animal was implanted with either a naltrexone containing or placebo pellet before starting the liquid diet. In each age group, EtOH alone significantly suppressed testosterone, whereas naltrexone prevented this fall, although it had no effect alone. Serum luteinizing hormone was also suppressed by EtOH; however, naltrexone did not abrogate this fall. In the 45-day-old animals, beta-luteinizing hormone mRNA levels rose significantly in the EtOH group, but not when naltrexone was coadministered with EtOH. There was no change in hypothalamic luteinizing hormone releasing hormone (LHRH) mRNA, pro-LHRH, or LHRH in any group at either age. Thus, naltrexone is able to partially prevent the EtOH-induced suppression of gonadal testosterone of young, adolescent male rats. This effect appears to be mediated directly at gonadal level, because hypothalamic and pituitary hormone changes were minor and nonsignificant.

Tanycytes and pituicytes: Morphological and functional aspects of neuroglial interaction

The hypothalamo-hypophyseal system is supplied with two types of specialized glial cells that interact in neuroendocrine functional dynamics: the tanycytes and the pituicytes. Tanycytes are the dominating glial cells within the median eminence. Similar to radial glia, they extend from the floor of the third ventricle to the neurohemal surface of the median eminence. Pituicytes, as specialized astrocytes, are the main glial cells of the neural lobe. They are in intimate contact with the perivascular space of the sinusoidal vessels. Morphological similarities between the two cell types focus on their interaction with terminal branches of hypothalamic neurons in both regions of the neurohypophysis, the median eminence and the neural lobe. Release of hypothalamic hormones is apparently influenced by pituicytes and tanycytes. For instance, both types of cells are capable of closing or opening the access to the vessels. Thereby, in contrast to the "blood-brain-barrier" function of astrocytes, pituicytes and tanycytes display "brain-blood-barrier" functions. Pituicytes are characterized by the expression of specific membrane-bound receptors for opioids, vasopressin, and beta-adrenoceptors, indicating that they receive input by numerous neuroactive substances. Integration of these incoming signals may result in a regulation of neurosecretion, especially by morphological changes and by modulation of extracellular ion concentrations. Comparable modulatory mechanisms of tanycytes have not yet been elucidated in a convincing manner. Besides possible regulatory functions, tanycytes are considered to possess guiding functions for hypothalamic axons and to be involved in transport mechanisms between ventricle and blood vessels of the portal system.

Multi-responsiveness of single anterior pituitary cells to hypothalamic-releasing hormones: a cellular basis for paradoxical secretion.

The classic view for hypothalamic regulation of anterior pituitary (AP) hormone secretion holds that release of each AP hormone is controlled specifically by a corresponding hypothalamic-releasing hormone (HRH). In this scenario, binding of a given HRH (thyrotropin-, growth hormone-, corticotropin-, and luteinizing hormone-releasing hormones) to specific receptors in its target cell increases the concentration of cytosolic Ca2+ ([Ca2+]i), thereby selectively stimulating the release of the appropriate hormone. However, "paradoxical" responses of AP cells to the four well-established HRHs have been observed repeatedly with both in vivo and in vitro systems, raising the possibility of functional overlap between the different AP cell types. To explore this possibility, we evaluated the effects of HRHs on [Ca2+]i in single AP cells identified immunocytochemically by the hormone they stored. We found that each of the five major AP cell types contained discrete subpopulations that were able to respond to several HRHs. The relative abundance of these multi-responsive cells was 59% for lactotropes, 33% for thyrotropes, and in the range of 47-55% for gonadotropes, corticotropes, and somatotropes. Analysis of prolactin release from single living cells revealed that each of the four HRHs tested were able to induce hormone release from a discrete lactotrope subpopulation, the size of which corresponded closely to that in which [Ca2+]i changes were induced by the same secretagogues. When viewed as a whole, our diverse functional measurements of multi-responsiveness suggest that hypothalamic control of pituitary function is more complicated than previously envisioned. Moreover, they provide a cellular basis for the so-called "paradoxical" behavior of pituitary cells to hypothalamic hypophysiotropic agents.

The Impact of Acute Ethanol on Reproductive Hormone Synthesis, Processing, and Secretion in Female Rats at Proestrous

It is the purpose of this study to investigate the effects of acute ethanol (EtOH) on the female rat hypothalamic-pituitary-gonadal (HPG) axis. The molecular and cellular mechanistic details of such effects have been studied intensively in the male rat. However, there has been relatively little in-depth study of EtOH's effects on the adult, postpubertal female rat. Adult female rats with confirmed 4- or 5-day estrous cycles were given a single injection of EtOH or saline between noon and 1:00 PM on proestrous and were killed at 4:00 PM. EtOH caused a sharp 97% reduction in luteinizing hormone (LH) serum levels (p < 0.001), compared with controls with no concomitant change in LH mRNA. EtOH also significantly reduced hypothalamic LH releasing hormone (LHRH) by 49% (p < 0.01), with no change in content of the precursor pro-LHRH compared with saline-injected controls. The ratio of LHRH to pro-LHRH was also significantly reduced by EtOH (p < 0.05), compared with control. There was no EtOH-induced change in LHRH mRNA. Compared with saline, EtOH reduced both serum estradiol by 37% (p < 0.02) and progesterone by 47% (p < 0.001). These results show that EtOH has profound disruptive effects on the female HPG axis. Our data suggests that EtOH decreases the releasable LHRH pool either by decreasing conversion of pro-LHRH to LHRH and/or by increasing local LHRH degradation. This acutely restricts the release of LH and subsequent estradiol and progesterone secretion.

The Impact of Acute Ethanol on Reproductive Hormone Synthesis, Processing, and Secretion in Female Rats at Proestrous

It is the purpose of this study to investigate the effects of acute ethanol (EtOH) on the female rat hypothalamic-pituitary-gonadal (HPG) axis. The molecular and cellular mechanistic details of such effects have been studied intensively in the male rat. However, there has been relatively little in-depth study of EtOH's effects on the adult, postpubertal female rat. Adult female rats with confirmed 4- or 5-day estrous cycles were given a single injection of EtOH or saline between noon and 1:00 PM on proestrous and were killed at 4:00 PM. EtOH caused a sharp 97% reduction in luteinizing hormone (LH) serum levels (p < 0.001), compared with controls with no concomitant change in LH mRNA. EtOH also significantly reduced hypothalamic LH releasing hormone (LHRH) by 49% (p < 0.01), with no change in content of the precursor pro-LHRH compared with saline-injected controls. The ratio of LHRH to pro-LHRH was also significantly reduced by EtOH (p < 0.05), compared with control. There was no EtOH-induced change in LHRH mRNA. Compared with saline, EtOH reduced both serum estradiol by 37% (p < 0.02) and progesterone by 47% (p < 0.001). These results show that EtOH has profound disruptive effects on the female HPG axis. Our data suggests that EtOH decreases the releasable LHRH pool either by decreasing conversion of pro-LHRH to LHRH and/or by increasing local LHRH degradation. This acutely restricts the release of LH and subsequent estradiol and progesterone secretion.

Hypothalamic-pituitary axis remains intact after interferon-alpha treatment in hematologic diseases.

Many endocrinologic disturbances have been reported during and after interferon-alpha (IFN-alpha) treatment. These disturbances have often been caused by autoantibodies. The aim of this prospective study was to evaluate whether IFN-alpha causes hormonal changes and if it is necessary to search for such disturbances routinely. Ten patients with hematologic malignancies were examined before and after 4 months of IFN-alpha treatment. Pituitary function was tested by hypothalamic releasing hormones (thyrotropin-releasing hormone, TRH, growth hormone-releasing hormone, GHRH, gonadotropin-releasing hormone, GnRH). The adrenal glands were tested with the adrenocorticotropin (ACTH) test. The human chorionic gonadotropin (hCG) test was performed on the men (n = 4). The IFN treatment was well tolerated, and no long-term hormonal side effects were found. The testosterone/sex hormone binding globulin (SHBG) index tended to improve. There were no significant differences between the hormone responses before and after IFN-alpha treatment. We conclude that the hypothalamic-pituitary axis remains intact after IFN-alpha treatment. There is no need to follow patients endocrinologically if the patients are not predisposed by autoantibodies.

Adrenalectomy Lowers the Body Weight Set-Point in Rats

Gosselin, C. and M. Cabanac. Adrenalectomy lowers the body weight set-point in rats. Physiol Behav 62(3) 519–523, 1997.—In order to evaluate the impact of a complete depletion of glucocorticoids on rats’ body weight set-point, rats were adrenalectomized, and their set-points were estimated before, and after surgery. Body weight set-points were obtained from a quantitative behavioral method based on the rats’ food-hoarding response to weight deficit. In addition, body fat contents were measured in vivo using a total body electrical conductivity analyzer (TOBEC). The hoarding of food showed that adrenalectomized rats had significantly lower body weight set-points than the sham-operated controls (337 ± 11 g vs. 385 ± 8 g) and were also significantly lighter. TOBEC measurements showed that the sham-operated control rats had a higher body fat content than the adrenalectomized rats (16.7 ± 1.1% vs. 10.2 ± 1.2%). The present study demonstrates that adrenalectomy lowers the body weight set-point in rats, suggesting that the concentrations of glucocorticoid hormones, and in turn, the hypothalamic corticotrophin releasing hormone (CRH) participates in the adjustment of the body weight set-point.

Growth hormone treatment in aging: state of the art and perspectives.

The regulation of GH secretion in the aged human is still not clearly understood. Results from a large number of studies suggest that aging decreases both spontaneous pulsatile GH secretory activity, and baseline serum GH concentrations. In addition, the GH response to physiological and pharmacological stimuli, including the administration of exogenous GH-releasing hormone (GHRH) have also been reported to be decreased in aged subjects. Recent studies have suggested that the inhibitory effect of aging on GH secretion may be due to an enhancement of hypothalamic somatostatin release both in the rat and in man. Shortly after the documentation of decreases in GH secretion in humans, other studies showed that the amplitude of spontaneous GH pulses decreased with age in rodents and that these changes were associated with a decline in plasma IGF-I. These studies immediately progressed to an investigation of the mechanisms responsible for the decline in GH secretion. Research efforts were eventually directed to the level of hypothalamic releasing (GHRH) and inhibiting (somatostatin) hormones. Results of these studies provided evidence that increased somatostatin tone and decreased GHRH tone at the hypothalamic level may both be a contributing factor in the decline in GH secretion with age. These conclusions were supported by research in humans, where administration of cho

An antisense oligodeoxynucleotide to lipocortin 1 reverses the inhibitory actions of dexamethasone on the release of adrenocorticotropin from rat pituitary tissue in vitro.

Our previous studies have demonstrated that lipocortin 1 (LC1, also called annexin 1) is an important mediator of glucocorticoid action in the neuroendocrine system, particularly with regard to the powerful inhibitory actions of the steroids on the secretion of ACTH and its hypothalamic releasing hormones. In the present study, we have used an antisense oligodeoxynucleotide (ODN) unique to LC1 to investigate further the role of this protein in the regulatory effects of dexamethasone on ACTH release in vitro from rat anterior pituitary cells. Pituitary cells dispersed with collagenase retained their functional and morphological integrity in vitro and sequestered ODNs in a time-dependent manner from the incubation medium. LC1 was readily detected in the cells by Western blot analysis or by immunoprecipitation/autoradiography after preloading with 35S-methionine/cysteine; the bulk of the protein was contained within an intracellular pool but a small amount was attached to the outer cell surface (pericellular). Dexamethasone (100 nm, 2.5 h) initiated de novo synthesis of LC1; it also increased the amount of LC1 in the pericellular pool detected by either method and caused a concomitant decrease in intracellular LC1. The responses to the steroid were prevented by the inclusion in the medium of an LC1 antisense ODN (50 nM, 3.5 h) but the corresponding sense and scrambled ODN sequences were inert. None of the ODN sequences tested influence the expression of annexin 5 in the pituitary tissue. CRH-41 (100 pM-1 mM), forskolin (1 nM-1 mM) and an L-Ca2+-channel opener BAY K8644 (100 pM-1 microM) initiated concentration dependent increases in immunoreactive- (ir-) ACTH release from the pituitary cells that were reduced (P < 0.01) by preincubation with dexamethasone (100 nM, 2.5 h). The inhibitory effects of the steroid were reversed by the LC1 antisense ODN (50 nM, P < 0.01), whereas the LC1 sense and scrambled control sequences (50 nM) were both ineffective in this respect (P > 0.05). The results add further support to the view that the acute inhibitory effects of glucocorticoids on the secretion of ACTH by the pituitary gland are dependent on the generation of lipocortin 1.

Differential regulation of hypothalamic pituitary corticotropin releasing hormone receptors during development of adjuvant-induced arthritis in the rat.

The expression of corticotropin releasing hormone (CRH) in the hypothalamic paraventricular nucleus (PVN) and CRH receptor mRNA in the PVN and anterior pituitary was studied during development of adjuvant-induced arthritis in Piebald-Viral-Glaxo rats, using in situ hybridization techniques. As previously shown with i.p. hypertonic saline injection, basal and immobilization stress-stimulated CRH mRNA levels in the PVN were significantly lower than in controls 14 days after adjuvant injection. However, 7 days after injection, preceding the onset of inflammation, the increase of CRH mRNA following immobilization was significantly higher than in control rats. In contrast to other chronic stress paradigms, inflammation stress failed to induce type-1 CRH receptor (CRH-R1) mRNA in the PVN, either at 7 days, or at 14 days after adjuvant injection, when inflammation is present. The ability of acute immobilization to induce CRH-R1 mRNA in the PVN was not affected 14 days after adjuvant injection but parallel to the CRH peptide mRNA response it was markedly potentiated at 7 days. Pro-opiomelanocorpin (POMC) mRNA levels in the anterior pituitary increased significantly 14 days after adjuvant injection, and they were unaffected by 1 h immobilization. While CRH binding in the pituitary decreased significantly 14 days after adjuvant injection, CRH-R1 mRNA was unchanged. This study shows biphasic hypothalamic responses to acute stress during development of adjuvant-induced arthritis, with enhanced CRH peptide and CRH-R1 mRNAs responses at 7 days, preceding the onset of inflammation, and blunted CRH mRNA responses at 14 days at the height of the inflammatory response. The lack of CRH receptor expression in the PVN in this model of chronic inflammation stress associated to low hypothalamic CRH peptide levels supports the view that positive feedback regulation by CRH is necessary to maintain enhanced CRH expression during chronic stress.

Assessment of pituitary function after transsphenoidal hypophysectomy in beagle dogs

Pituitary function was assessed in healthy adult beagle dogs before and after hypophysectomy. Anterior pituitary function was tested by use of the combined anterior pituitary (CAP) function test, which consisted of sequential 30-sec intravenous injections of four hypothalamic releasing hormones, in the following order and doses: 1 μg of cortrticotropin-releasing hormone (CRH)/kg, 1 μg of growth hormone-releasing hormone (GHRH)/kg, 10 μg of gonadotropin-releasing hormone (GnRH)/kg, and 10 μg of thyrotropin-releasing hormone (TRH)/kg. Plasma samples were assayed for adrenocorticotropin (ACTH), cortisol, GH, luteinizing hormone (LH), and prolactin (PRL) at multiple times for 120 min after injection. Pars intermedia function was assessed by the α-melanotropin (α-MSH) response to the intravenous injection of the dopamine antagonist haloperidol in a dosage of 0.2 mg/kg. Posterior pituitary function was assessed by the plasma vasopressin (AVP) response to the intravenous infusion of 20% saline. Basal plasma ACTH, cortisol, thyroxine, LH, PRL, and AVP concentrations were significantly lower at 10 wk after hypophysectomy than before hypophysectomy. In the CAP test and the haloperidol test, the peaks for the plasma concentrations of ACTH, cortisol, GH, LH, PRL, and α-MSH occurred within 45 min after injection. At 2 and 10 wk after hypophysectomy, there were no responses of plasma GH, LH, PRL, and α-MSH to stimulation. In four of eight hypophysectomized dogs, there were also no plasma ACTH and cortisol responses, whereas in the other four dogs, plasma ACTH and cortisol responses were significantly attenuated. The basal plasma ACTH and cortisol concentrations were significantly lower in the corticotropic nonresponders than in the responders. Plasma AVP responses were completely abolished by hypophysectomy, although water intake by the dogs was normal. Histopathological examinations at 10 wk after hypophysectomy revealed that adrenocortical atrophy was much more pronounced in the corticotropic nonresponders than in the responders. No residual pituitary tissue was found along the ventral hypothalamic diencephalon. However, in all hypophysectomized dogs that were investigated, islets of pituitary cells were found embedded in fibrous tissue in the sella turcica. A significant positive correlation was found between the number of ACTH-immunopositive cells and the ACTH increment in the CAP test at 10 wk after hypophysectomy. It is concluded that 1) stimulation of the anterior pituitary with multiple hypophysiotropic hormones, stimulation of the pars intermedia with a dopamine antagonist, and stimulation of the neurohypophysis with hypertonic saline do not cause side effects that would prohibit routine use, 2) in the routine stimulation of the anterior pituitary and the pars intermedia, blood sampling can be confined to the first 45 min, 3) the ACTH and cortisol responses to hypophysiotropic stimulation are the most sensitive indicators for residual pituitary function after hypophysectomy, 4) small islets of pituitary cells in the sella turcica, containing corticotropic cells, are the most likely source of the attenuated corticotropic response that may occur after hypophysectomy, and 5) residual AVP release from the hypothalamus after hypophysectomy is sufficient to prevent diabetes insipidus, despite the fact that the AVP response to hypertonic saline infusion is completely abolished.

Effects of a GnRH analogue on human smooth muscle cells cultured from normal myometrial and from uterine leiomyomal tissues.

Leiomyomas are tumours of uterine smooth muscle tissue that are oestrogen and progesterone dependent. When explants of these tumours were grown in culture, the proliferating tissue formed characteristic ball-like aggregates (BLA), rather than the usual hill and valley (HV) pattern of growth of normal myometrial tissue in culture. Immunocytochemical staining with fluorescein isothiocyanate (FITC)-labelled gonadotrophin-releasing hormone (GnRH) revealed that both myometrial and leiomyomal cells have membrane receptors for this hypothalamic releasing hormone. Furthermore, polymerase chain reactions (PCR) with primer sets that were specific for GnRH receptor mRNA, as well as GnRH mRNA, showed that transcripts for both of these nucleic acids are present in myometrial and leiomyomal tissues. The treatment of cultured explants of leiomyomal tissue with a GnRH analogue (buserelin, HOE766) diminished the formation of BLA, but this synthetic hormone had only a moderate effect on the HV topography of normal myometrial tissue. A colorimetric assay indicated that GnRHa inhibited cell proliferation in leiomyomal tissue in a dose-dependent manner. Western blotting, to detect the expression of G1 phase cell cycle-related gene products, showed that cyclin E and p33cdk2 formation in leiomyomas were inhibited by high concentrations of GnRHa. In conclusion, GnRHa might suppress leiomyomal growth by interfering with the expression of cell cycle factors.

Effects of azido-3'-deoxy-thymidine on luteinizing hormone, follicle-stimulating hormone and prolactin release by the pituitary-hypothalamus complex.

Azido-3'-deoxy-thymidine (AZT) is a drug extensively used in the treatment of AIDS. AZT was incubated in vitro either with the pituitary-hypothalamus complex (PHc) or the intact pituitary (PI) of male rats. The PHc is comprised of the hypothalamus and the attached pituitary gland. After a preincubation period, the PHc or PI was incubated for 1 or 2 h with Krebs-Ringer bicarbonate buffer or either of two different concentrations of AZT (1 and 10 microM). In the control incubations, the PHc released less prolactin (PRL) and more follicle-stimulating hormone (FSH) and luteinizing hormone (LH) than the PI, indicating that hypothalamic control of the pituitary was exerted in vitro, presumably by diffusion of releasing and inhibiting hormones from the neurohypophysis to the anterior lobe of the hypophysis. Both concentrations of AZT evoked a significant increase in release of PRL and a decreased release of LH and FSH from the PHc. In the case of LH, the higher concentration of AZT partially suppressed LH release within 1 h. The other effects were not dose-related and were observed after incubating the tissue with AZT for 2 h. However, incubation of the PI with AZT failed to alter anterior pituitary hormone release, illustrating that the site of action of AZT is in the hypothalamus. We hypothesize that AZT blocks DNA synthesis resulting in suppression of synthesis and consequent release of hypothalamic hormones that control release of pituitary hormones in vitro. The results raise the possibility that AZT may alter hypothalamic-pituitary function in vivo.

Angiotensinergic Neurons Physiologically Inhibit Prolactin, Growth Hormone, and Thyroid-Stimulating Hormone, But Not Adrenocorticoptropic Hormone, Release in Ovariectomized Rats

FRANCI, C. R., J. A. ANSELMO–FRANCI AND S. M. McCANN.Angiotensinergic neurons physiologically inhibit prolactin, growth hormone and thyroid-stimulating hormone, but not adrenocorticotropic hormone, release in ovariectomized rats.PEPTIDES 18(7) 971–976, 1997.—Angiotensin II (AII)-containing neurons with cell bodies in the rostral medial hypothalamus and axons project to the external layer of the median eminence, so that AII maybe released into the hypophyseal portal vessels for actions on the pituitary gland. Indeed, intrahypothalamic actions of the peptide on the release of hypothalamic hormones and direct actions on the pituitary have been reported. To determine the role of endogenously released AII in hypothalamic–pituitary hormone release, we have determined the effects of central immunoneutralization of AII upon the plasma concentrations of prolactin (PRL), growth hormone (GH), thyroid-stimulating hormone (TSH), and adrenocorticotropic hormone (ACTH). Specific antiserum directed against AII (AB-AII) or normal rabbit serum (NRS), as a control, was microinjected into third ventricular (3V) cannulae of conscious, ovariectomized (OVX) rats. Immediately before and at various intervals after this procedure, blood samples were withdrawn through previously implanted external jugular catheters. Three hours after injection of the AB-AII, plasma PRL levels diverged from those of the NRS-injected animals and progressively increased from 4 to 24 h after administration of the antiserum. Results were similar with respect to plasma GH, except that the increase in the AB-AII animals above that in the NRS-injected controls from 4 to 6 h was not significant, but was highly significant on measurement 24 h after injection, at which time plasma GH was three times higher than in control rats. Similarly, following injection of AB-AII, plasma TSH values did not diverge significantly from those of the NRS-injected controls until 3 h after injection. From 3 to 5 h they remained constant and significantly elevated above values in the NRS-injected controls with a further nonsignificant increase at 6 h. At 24 h, there was no longer a difference between the values in both groups. In contrast to the significant elevations in plasma hormone levels observed with respect to PRL, GH, and TSH following injection of the antiserum, there was no change in plasma ACTH between the AB-AII-injected and NRS-injected animals throughout the same period of observation. Previous results by others have shown that intraventricular injection of AII has a suppressive action on the release of PRL, GH, and TSH. Consequently, we believe that the antiserum is acting intrahypothalamically to block the action of AII within the hypothalamus, resulting in the elevation of the three hormones mentioned. Therefore, the AII neurons appear to have a physiologically significant suppressive action on the release of hypothalamic neurohormones controlling the release of PRL, GH, and TSH. In contrast, there apparently is no effect of intrahypothalamically released AII on the secretion of corticotropin-releasing factors under these nonstress conditions. We cannot rule out an action of the antiserum at the pituitary level; however, in view of the fact that the actions of AII directly on the gland are to stimulate PRL, GH, TSH, and ACTH release, it appears that the antiserum was acting at the hypothalamic level.

Effects of sex steroid hormones on the neuroendocrine system

Estrogen and progesterone are the most important ovarian steroid hormones regulating female fertility. They have a profound effect on the central nervous system. Target functions of sex steroids in the brain are: pituitary and hypothalamic hormone release, thermoregulatory and cardiocirculatory activities and behavior and mood changes. Furthermore, several studies have shown a correlation between brain neurotransmitters, neuropeptides and sex steroid hormones: they influence synthesis and release of norepinephrine, dopamine, serotonin, gonadotropin releasing hormone, β-endorphin, corticotropin releasing factor and prolactin. Thus, oral hormone contraceptives inhibit the ovulatory process by blocking the activity of the hypothalamus-pituitary—gonadal axis. This inhibitory effect seems to be due to the action of both estrogens and progestins.

Lactation and fertility.

In almost all mammals lactation, or more correctly the suckling stimulus, induces a period of infertility designed to provide the optimal birth spacing for survival of the offspring. The duration of lactational infertility depends on the sucking activity of the young with little evidence to support a role for nutritional status. Suckling disrupts the normal pulsatile pattern of hypothalamic gonadotrophin releasing hormone (GnRH)2 secretion resulting in reduced LH secretion from the pituitary. Secretion of FSH returns to its normal cyclic pattern early in lactation and ovarian follicles may develop under its influence. However, until suckling declines, the follicles fail to secrete amounts of estradiol adequate to stimulate an LH surge and ovulation. The suckling stimulus may affect GnRH secretion by affecting prolactin, opiate and dopaminergic tone in the hypothalamus but no unifying mechanism has yet been proposed convincingly. The birth spacing effects of breastfeeding in women have a profound effect on infant well-being, and breastfeeding still prevents more pregnancies than all forms of artificial contraception.