Hypothalamic and serum factors influence on prolactin and luteinizing hormone release by the pituitary gland of the young turkey (Meleagris gallopavo)

Identification of avian vasotocin receptor subtype-specific antagonists involved in the stress response of the chicken, Gallus gallus

Vasotocin 1a and 1b receptors (V1aR and V1bR) have been shown to play important roles in the neuroendocrine regulation of stress responses via the anterior pituitary (AP) of birds. To identify effective subtype-specific antagonists for the chicken V1aR (cV1aR) and cV1bR, potential antagonists to the mammalian V1R were screened against the cV1aR and cV1bR 3D structural models by molecular docking analysis with determination of binding pocket/amino acid residues involved in the interaction. The antagonistic effects of the selected ligands were examined by measuring pro-opiomelanocortin (POMC) heteronuclear RNA (hnPOMC) levels following the in vitro stress administration to primary chicken AP cells. Results of in silico analysis showed that the Manning compound and several other antagonists were bound to cV1bR with higher affinity than the natural agonist, arginine vasotocin (AVT). Similarities and differences in the antagonist-receptor binding interface with receptors were characterized for each ligand. Non-peptide mammalian V1bR antagonists, SSR-149415 and L-368899, were shown to be effective and had an additive effect in blocking POMC hnRNA expression in pituitary cell culture studies. SR-49059 antagonized the effect(s) of AVT/CRH on the downregulation of the cV1aR and the upregulation of the cCRH-R2 expression but not the cV1bR and cCRH-R1. The Manning compound antagonized the downregulation of cV1aR, cV1bR and cCRH-R1 and the upregulation of cCRH-R2 expression. The specificity of antagonists apparently resulted from unique differences in the interacting residues and their binding affinities. Collectively, these results provide valuable leads for future development of novel compounds capable of blocking or attenuating the AP stress response of avian species and perhaps other non-mammalian vertebrates as well.

Regulation of gene expression of vasotocin and corticotropin-releasing hormone receptors in the avian anterior pituitary by corticosterone

The effect of chronic stress (CS) on gene expression of the chicken arginine vasotocin (AVT) and corticotropin-releasing hormone (CRH) receptors [VT2R, VT4R, CRH-R1, and CRH-R2] was examined by measuring receptor mRNA levels in the anterior pituitary gland of the chicken after chronic immobilization stress compared to acute stress (AS). Radioimmunoassay results showed that blood circulating corticosterone (CORT) levels in the CS group were significantly decreased compared to that of birds in the AS group (P<0.05). The VT2R and CRH-R2 mRNA in CS birds were significantly decreased to that of controls. The VT4R mRNA was significantly decreased compared to controls in AC birds and was further decreased in the CS group compared to controls (P<0.05). The CRH-R1 mRNA was significantly decreased in the AS birds compared to controls. However, there was no significant difference of CRH-R1 mRNA between acute stress and chronic stress birds. Using primary anterior pituitary cell cultures, the effect of exogenous CORT on VT/CRH receptor gene expression was examined. Receptor mRNA levels were measured after treatment of CORT followed by AVT/CRH administration. The CORT pretreatment resulted in a dose-dependent decrease of proopiomelanocortin heteronuclear RNA, a molecular marker of a stress-induced anterior pituitary. Without CORT pretreatment of anterior pituitary cell cultures, the VT2R, VT4R and CRH-R1mRNA levels were significantly increased within 15 min and then decreased at 1 h and 6 h by AVT/CRH administration (P<0.05). Pretreatment of CORT in anterior pituitary cells induced a dose-dependent increase of VT2R, VT4R and CRH-R2 mRNA levels, and a significant decrease of CRH-R1 mRNA levels at only the high dose (10 ng/ml) of CORT (P<0.05).Taken together, results suggest a modulatory role of CORT on the regulation of VT/CRH receptor gene expression in the avian anterior pituitary gland dependent upon CORT levels.

Identification of antagonists to the vasotocin receptor sub-type 4 (VT4R) involved in stress by molecular modelling and verification using anterior pituitary cells

The vasotocin receptor family is homologous to the mammalian vasopressin G-protein coupled receptor (GPCR) family. The vasotocin receptor 2 (VT2R) and 4 (VT4R) have recently been shown to play important role(s) in the neuroendocrine regulation of stress in birds. A homology-based structural model of VT4R of the domestic chicken, Gallus gallus, was built using the sophisticated SYBYL-X suite. The structure of VT4R built with and without extra- and intracellular unstructured loops showed a seven-helix transmembrane domain, which is a characteristic feature of GPCRs. Several agonists and antagonists were screened by molecular docking to map their potential binding sites on the structure of VT4R. Interestingly, the presence of the N-terminal, intracellular and extracellular loops and C-terminal amino acid sequences emerging from the transmembrane domains during molecular docking appeared to influence the binding interface of the peptide agonists and peptide/non-peptide antagonists on the VT4R. The presence of unstructured loops, however, did not affect the relative binding affinity ranking of the peptide antagonists to VT4R. In general, the natural ligand, arginine vasotocin and the peptide/non-peptide antagonists were observed to be more deeply buried in the receptor. Results of in vitro inhibition experiments, using cultured anterior pituitary cells, showed excellent agreement with the binding affinity of the antagonists predicted by molecular docking. The results of this study provide valuable clues for the rational design of novel pharmaceutical compounds capable of blocking or attenuating the stress response.

The relative importance of vasoactive intestinal peptide and peptide histidine isoleucine as physiological regulators of prolactin in the domestic turkey

In mammals, prolactin (PRL) secretion is regulated by vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI). In birds, however, VIP is considered a PRL-releasing factor (PRF), while the role of PHI is unknown. The purpose of this study was to compare the effects of turkey PHI (tPHI) and turkey VIP (tVIP) on PRL secretion in vitro, and to study their physiological significance in vivo through active immunization against tPHI and tVIP. In vitro studies were conducted using pituitary cell cultures from female turkeys. In the in vivo study, female turkeys were immunized with keyhole limpet hemocyanin (KLH; control), synthetic tPHI conjugate (KLH-tPHI), or synthetic tVIP conjugate (KLH-tVIP). Both tVIP and tPHI stimulated PRL secretion from anterior pituitary cells in a dose response manner. However, tPHI was 100-fold less potent than tVIP in stimulating maximum PRL secretion in vitro. In addition, the highest dose (10(-4) M) of tPHI inhibited its own PRL-releasing activity as well as that of VIP-stimulated PRL release. Whereas, circulating PRL levels and nesting activity remained low and unchanged during the photo-induced reproductive cycle (i.e., experimental period) in tVIP-immunized birds, control and tPHI-immunized turkeys showed a significant increase in plasma PRL levels and in the incidence of incubation behavior over time following photostimulation. These findings, taken together with earlier results, indicate that VIP is the sole physiological PRF in the turkey (avian species).

Influence of VIP on prolactinemia in turkey anterior pituitary cells: role of cAMP second messenger in VIP-induced prolactin gene expression

Vasoactive intestinal peptide (VIP) is the avian prolactin (PRL)-releasing factor. In the turkey, hypothalamic VIP immunoreactivity and mRNA content, as well as VIP levels in hypophyseal portal blood, are closely related to the state of prolactinemia and the reproductive stage. The present study investigated the role of VIP on prolactinemia in turkey anterior pituitary (AP) cells through PRL gene expression and the role of a cAMP second messenger system on VIP-induced PRL expression. In primary AP cells harvested from hens in different prolactinemic states, steady state promoter activities were positively correlated with secreted PRL levels. VIP increased PRL promoter activities in AP cells from hens with intermediate PRL levels (laying), but not in AP cells from hypoprolactinemic hens (nonphotostimulated reproductively quiescent). However, in AP cells from hyperprolactinemic hens (incubating), PRL promoter activity was down-regulated by VIP. PRL mRNA steady state levels were significantly decreased by the cAMP analogue, 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), and PRL secretion was down-regulated by the phosphodiesterase blocker, 3-isobutyl-1-methylxanthine (IBMX) in a dose-dependent manner, suggesting that the cAMP second messenger system might be involved in the inhibitory action of dopamine upon VIP-stimulated PRL secretion and gene expression at the pituitary level. In a study of VIP immediate and long-term effects on c-fos expression in relation to PRL expression, VIP dramatically induced c-fos mRNA expression within 5 min, suggesting that VIP-induced c-fos expression might be involved in VIP-stimulated PRL secretion and gene expression. These results provide additional evidence of the functional significance of VIP in PRL gene expression and suggest that changes in PRL promoter activity by VIP may be one of the important inductive mechanisms leading to prolactinemia.

Changes in brain gonadotropin-releasing hormone- and vasoactive intestinal polypeptide-like immunoreactivity accompanying reestablishment of photosensitivity in male dark-eyed juncos (Junco hyemalis)

In seasonally breeding, photoperiodic birds, the development of photorefractoriness is associated with decreased brain expression of gonadotropin-releasing hormone-like immunoreactivity (GnRH-li ir) and increased expression of vasoactive intestinal polypeptide-like immunoreactivity (VIP-li ir). Dissipation of photorefractoriness and reestablishment of photosensitivity are associated with increased GnRH-li ir brain production, but concurrent changes in VIP-li ir expression have not been investigated. To address this question, we compared the expression of VIP-li ir in the infundibulum (INF) of adult male dark-eyed juncos (Junco hyemalis) that were made photorefractory (PR) by prolonged exposure to long days with that of birds that were not photostimulated (PS), but had regained photosensitivity by exposure to short days for 5 (short-term-PS, ST-PS) or 13 (long-term-PS, LT-PS) consecutive months. Photosensitive males had smaller INF VIP-li ir cell bodies than PR males, but the numbers of INF VIP-li ir cells were independent of photoperiodic condition. Changes in infundibular VIP-li ir were correlated with changes in preoptic area (POA) GnRH-li expression. Specifically, photosensitive males had more and larger POA GnRH-li ir cells and more GnRH-li ir fibers in this region than PR males. Further, LT-PS males had more GnRH-li ir POA fibers and larger testes than ST-PS juncos. Thus, induction of photorefractoriness is associated with increased VIP and decreased GnRH brain expression whereas dissipation of photorefractoriness concurs with decreased VIP and increased GnRH brain expression. These results suggest a physiological role for VIP in the control of changes in GnRH expression as a function of the photosensitive condition.

Evidence of a role for the turkey posterior pituitary in prolactin release

The aims of this study were: (1) to examine whether the posterior pituitary contains prolactin releasing factor (PRF) activity, (2) to determine to what extent known neurohypophyseal peptides contribute to this activity, and (3) to compare posterior pituitary PRF activities of hens in different reproductive stages. Anterior pituitary cells derived from juvenile female turkeys were incubated with posterior pituitary extracts or test substances for 3 hr. Posterior pituitary extracts (0.1-0.8 equivalent) contained a potent substance(s) which stimulated PRL release in a concentration-dependent manner (2.4 +/- 0.08 to 6.5 +/- 0.23 micrograms/500 k cells). Arginine vasotocin (AVT) and vasoactive intestinal peptide (VIP) antisera (1:500) completely abolished the PRL-releasing activities of their respective peptides but partially reduced (P less than 0.05) the PRF activity of the posterior pituitary (AVT, 19.9%; VIP, 55.1%). Mesotocin antiserum did not alter (P greater than 0.05) PRL release induced by posterior pituitary extract. Posterior pituitary extract (0.01-0.5 equivalent) from hens in each of the various stages of the reproductive cycle induced a concentration dependent PRL release. The 0.5 posterior pituitary equivalent dose from reproductively quiescent (nonphotostimulated), laying, photorefractory, and incubating hens increased PRL release 2.4-, 2.9-, 3.8-, and 11.1-fold, respectively. The turkey posterior pituitary contains a potent PRF activity, partially accounted for by VIP and AVT, at the assayed concentrations, which varies with the reproductive cycle.

Enhanced vasoactive intestinal peptide-induced prolactin secretion from anterior pituitary cells of incubating turkeys (Meleagris gallopavo)

During incubation, female turkeys exhibit elevated circulating prolactin (PRL) which may be the result of enhanced pituitary responsiveness to vasoactive intestinal peptide (VIP). This hypothesis was tested by comparison of spontaneous and porcine VIP-induced PRL secretion from anterior pituitary cells of hens in various reproductive conditions. The effect of VIP and luteinizing hormone releasing hormone (LHRH), alone and in combination, on luteinizing hormone (LH) secretion was also examined. Incubation with pVIP (10(-10) to 10(-6) M) significantly stimulated PRL secretion at all incubation times tested (1-5 hr). This increase was greatest in cells from incubating hens, with those from laying, photorefractory, and quiescent (nonphotostimulated) hens secreting successively less PRL. These responses were obtained when spontaneous PRL secretions were compared. VIP induced approximately a similar 1.5-fold increase in LH secretion, in all reproductive groups. Also, VIP enhanced LHRH-induced LH secretion (1.2- to 1.6-fold; P less than 0.0001). It is concluded that PRL secretion in vitro by pituitary cells from turkey hens in various reproductive stages reflects the circulating levels of PRL at these stages.

Vasoactive intestinal peptide is a hypothalamic prolactin-releasing neuropeptide in the turkey (Meleagris gallopavo)

The hypothesis that vasoactive intestinal peptide (VIP) functions as a hypothalamic prolactin (PRL)-releasing peptide in the turkey was tested by determining the effects of hypothalamic VIP immunoneutralization and pituitary VIP receptor blockade on hypothalamic extract (HE)-induced PRL secretion from dispersed anterior pituitaries. Incubation of cells with porcine VIP (pVIP; 0.5 or 10 nM) significantly stimulated PRL secretion. This effect was inhibited in a dose-related manner by 1-hr preincubation of pVIP with a VIP antisera (A/S; 1:500-1:50,000). Likewise, HE (0.3 equivalent)-stimulated PRL secretion was inhibited by preincubation with VIP A/S (P less than 0.0001). A 96-98% reduction in PRL secretion was obtained from cells cultured with HE, that was previously incubated with 1/500 dilution of antiserum. Pretreatment of pituitary cells for 15 min with [4Cl-D-Phe6,Leu17] VIP, a VIP receptor antagonist (10(-5) M), significantly depressed the PRL response to 0.5 nM VIP (9.9 +/- 0.5 micrograms/500,000 cells vs 4.9 +/- 0.1 micrograms/500,000 cells; 22.4 +/- 0.9 micrograms/500,000 cells vs 14.7 +/- 0.4 micrograms/500,000 cells) or 0.3 eq HE (8.8 +/- 0.6 micrograms/500,000 cells vs 5.2 +/- 0.2 micrograms/500,000 cells; 15.3 +/- 0.3 micrograms/500,000 cells vs 8.2 +/- 0.2 micrograms/500,000 cells). These results suggest that hypothalamic stimulation of PRL secretion appears to be mediated by receptors specific for VIP and that VIP is an endogenous hypothalamic PRL-releasing peptide in the turkey.

Gonadal steroid modulation of basal and vasoactive intestinal polypeptide-stimulated prolactin release by turkey anterior pituitary cells

Porcine vasoactive intestinal polypeptide (VIP; 10(-9) to 10(-7) M) was a potent stimulator of prolactin (PRL) release by anterior pituitary cells from immature and laying turkey hens. Basal and VIP-induced PRL release of cells from laying hens were diminished (P less than 0.05) when the cells were cultured for 48 hr in the presence of charcoal-stripped laying hen serum, but not when the cells were cultured in the presence of whole laying hen serum. This change in VIP-induced PRL release was not evident when cells were derived from immature hens. Basal PRL release by cells from laying hens was not altered by the presence of estradiol (E2; 10(-12) to 10(-5) M), although such release was generally enhanced in cultures of cells from immature hens containing E2. The presence of E2 enhanced (P less than 0.05) the magnitude of the VIP-induced PRL release by cultures of cells from laying hens and diminished (P less than 0.05) the magnitude of this release in cultures of cells from immature hens. Cells from immature and laying hens exposed to progesterone (P4; 10(-5) M) for 96 hr exhibited enhanced basal PRL release, though lower P4 concentrations had no effect. Utilizing cells from laying hens, P4 exposure for 24 hr resulted in diminished (P less than 0.05) VIP-induced PRL release, while P4 exposure for 96 hr resulted in markedly enhanced (P less than 0.05) VIP-induced PRL release. Basal PRL release was generally not altered by the presence of testosterone (T). The VIP-induced PRL release by cells derived from immature and laying hens was diminished (P less than 0.05) by the presence of T. Prolactin release in the turkey is likely modulated by gonadal steroids acting directly on the cells of the anterior pituitary.

Basal and hypothalamic extract-induced luteinizing hormone and prolactin secretion by cultured anterior pituitary cells from female turkeys in various stages of the reproductive cycle

The wet weight of the anterior pituitary gland of the domestic turkey increases as the hen progresses through the reproductive cycle. The greatest wet weight was observed with anterior pituitary glands from incubating hens, whose weight was twice that of anterior pituitary glands from nonphotostimulated hens. Anterior pituitary glands from hens in each of the various stages of the reproductive cycle were dissociated and cultured, and subsequently basal and hypothalamic extract (HE)-induced releases of prolactin (PRL) and luteinizing hormone (LH) were compared between cultures containing a defined number of anterior pituitary cells. Basal and HE-induced releases of PRL were greatest (P less than 0.05) in the cultures of anterior pituitary cells derived from incubating hens, with those of laying, photorefractory, and nonphotostimulated hens being successively less (P less than 0.05), respectively. HE-induced release of LH was greatest (P less than 0.05) in cell cultures derived from nonphotostimulated hens, with cultures of cells derived from laying, photorefractory, and incubating hens exhibiting successively smaller releases of LH (P less than 0.05), respectively. The concentration of HE that induced the first significant increase in the release of PRL or LH above that of basal levels also varied with the reproductive stage of the donor hens. The changes in circulating PRL levels during the various stages of the reproductive cycle reflect changes in anterior pituitary sensitivity to hypothalamic releasing activity and/or changes in the PRL releasing capacity of the anterior pituitary. In contrast, there does not appear to be a strong correlation between anterior pituitary LH releasing capacity in vitro and circulating LH levels in the domestic turkey hen.

Effects of furazolidone or nitrofurazone on the concentrations of hypothalamic amines and plasma luteinizing hormone (LH) and prolactin (PRL), levels in young turkeys

1. Furazolidone (30 mg/kg) given orally for 14 days produced significant increases in the hypothalamic concentrations of noradrenaline, adrenaline and dopa. The amine concentrations returned to normal one week after withdrawal of the drug. 2. Nitrofurazone (30 mg/kg) given orally for 14 days did not affect significantly the hypothalamic amine concentrations. 3. Nitrofurazone (30 mg/kg, 14 days) increased significantly the plasma concentration of prolactin (PRL) and decreased luteinizing hormone (LH). The concentrations of the two hormones returned to normal one week after the drug withdrawal. Furazolidone (7.5, 15 or 30 mg/kg) did not influence the concentrations of either hormone. 4. Furazolidone (30 mg/kg, 14 days) did not influence the release of LH or PRL after treatment of LHRH or TRH, respectively. Nitrofurazone at the same dose, however, reduced the increases in LH and PRL concentrations induced by LHRH and TRH, respectively.

Gonadal steroid-mediated alteration of luteinizing hormone secretion by anterior pituitary cells of young turkeys

The magnitude of luteinizing hormone (LH) release during a 3-hr test incubation was diminished (P less than 0.05) when anterior pituitary cells from young turkeys were cultured for 24 to 120 hr. This trend was evident with basal LH release and with LH release induced by luteinizing hormone-releasing hormone (LH-RH) or hypothalamic extract. Anterior pituitary cells were cultured with various concentrations (10(-14) to 10(-6) M) of estradiol (E2), progesterone (P4), or testosterone (T) for 24 hr and then exposed to LH-RH or control medium for 3 hr, still in the presence of steroids. Basal LH release was potentiated (P less than 0.05) when cells were cultured with 10(-8) or 10(-6) M T, but not with E2 or P4. When cells were cultured with E2, LH release in the presence of 10(-8) M LH-RH was enhanced (P less than 0.05) in a dose-dependent fashion. LH-RH mediated LH release was also enhanced (P less than 0.05) when cells were cultured with 10(-8) M P4 or 10(-6) M T. Gonadal steroids can act directly on the anterior pituitary of the young domestic turkey to modulate LH release, with T enhancing basal LH release and E2 potentiating LH-RH-mediated LH release.

The influence of thyrotropin releasing hormone on in vivo prolactin release and in vitro prolactin, luteinizing hormone, and growth hormone release from dispersed pituitary cells of the young turkey (Meleagris gallopavo)

Intravenous administration of 0.025, 0.25, or 2.5 micrograms/kg thyrotropin releasing hormone (TRH) to 4-week-old female turkeys induced a dose-dependent increase (P = 0.004) in serum prolactin (PRL) 15 min post-treatment. Dispersed anterior pituitary cell cultures were utilized to determine the effect of TRH on cellular release of PRL, luteinizing hormone (LH), and growth hormone (GH). In the first experiment, cells from 13-week-old male turkeys were initially incubated for 24 hr in Medium 199 (M-199) plus 10% turkey serum and then placed in M-199 plus 10(-10) to 10(-4) M TRH for 5 hr. Incubation with TRH produced no change in PRL release from that of spontaneous release (P = 0.854). However, 10(-5) and 10(-4) M TRH induced LH release (P less than 0.0001). The TRH-induced GH response was parabolic (P less than 0.0001), with the maximal release at 10(-8) M. The second experiment, utilizing pituitary cells from 7-week-old females, studied these responses on 3, 5, and 7 days of monolayer incubation. TRH failed to induce a PRL release in all tests (P greater than 0.162), although hypothalamic extract induced a large release (P less than 0.0001) of PRL each time. Both 10(-6) and 10(-4) M TRH induced a LH release on Day 3 while only 10(-4) M did so on Day 5, and none of the doses elicited a release on Day 7. The parabolic GH response generally persisted in all tests.(ABSTRACT TRUNCATED AT 250 WORDS)