Biomedical subjects

M E El Halawani

Publications and source records attributed to M E El Halawani.

At least 19 recordsLinked to original sources

PubMed2006

Identification of dopamine, gonadotrophin-releasing hormone-I, and vasoactive intestinal peptide neurones activated by electrical stimulation to the medial preoptic area of the turkey hypothalamus: a potential reproductive neuroendocrine circuit.

The neural and neurochemical substrates regulating reproduction in birds remain vaguely defined. The findings that electrical stimulation in the medial preoptic area (ES/MPOA) or intracerebroventricular infusion of dopamine (DA) stimulated luteinising hormone (LH) and prolactin (PRL) release in female turkeys, led to the suggestion that ES/MPOA might help to clarify the DA circuitry regulating LH and PRL. We used c-fos mRNA and tyrosine hydroxylase immunoreactivity as measured by double in situ hybridisation/immunocytochemistry (ISH/ICC) to determine which group/subgroup of DA neurones was activated following unilateral ES/MPOA. To establish that the reproductive neuroendocrine system was activated, double ISH/ICC was also conducted on c-fos/gonadotrophin-releasing hormone-I (GnRH-I) and c-fos/vasoactive intestinal peptide (VIP). Changes in circulating LH and PRL were determined by radioimmunoassay. Unilateral ES/MPOA (100 microA, right side) of anaesthetised laying turkeys for 30 min increased circulating LH and PRL levels. It also induced c-fos mRNA expression on the ipsilateral side by all GnRH-I neurones within the septopreoptic region, implying that GnRH-I neurones in this region share similar circuitry. VIP neurones within the nucleus infundibularis were the only VIP group to show c-fos mRNA expression, suggesting their involvement in ES/MPOA induced PRL release. c-fos mRNA expression was also observed in a subgroup of DA neurones in the nucleus mamillaris lateralis (ML). To our knowledge, the present study is the first to show that activation of DAergic cells in the ML is associated with the activation of GnRH-I and VIP neurones and the release of LH and PRL. It is likely that ES/MPOA activated VIP/GnRH-I neurones via activation of DA neurones in the ML, as this was the only DA subgroup that showed c-fos mRNA expression.

Analysis of Variance↗

PubMed2005

Regulation of prolactin gene expression by vasoactive intestinal peptide and dopamine in the turkey: role of Ca signalling.

Our recent work has demonstrated that dopamine, acting through D2 dopamine receptors on pituitary cells, inhibits the stimulatory effects of vasoactive intestinal peptide (VIP) on prolactin release and prolactin gene transcription. It is hypothesised that the stimulatory and inhibitory roles of VIP and dopamine, respectively, on prolactin synthesis and release are mediated by their opposite effects on intracellular Ca2+ concentration ([Ca2+]i) in lactotrophs. The present study aimed: (i) to investigate the effect of VIP and dopamine on [Ca2+]i of cultured turkey anterior pituitary cells and (ii) to examine the role of Ca2+ signalling in mediating the regulatory effects of VIP and dopamine on prolactin mRNA levels and prolactin release. Changes in [Ca2+]i were measured spectrofluorometrically using Fura-2/AM as a fluorescent Ca2+ indicator. Semi-quantitative reverse transcription-polymerase chain reaction and radioimmunoassay were used to determine prolactin mRNA levels and prolactin release, respectively. VIP or the L-type Ca2+ channel activator, Bay K8644 (Bay) increased [Ca2+]i in a concentration- and time-dependent fashion, an effect abolished by preincubating the cells with R(-)-propylnorapomorphine HCl, a D2 dopamine receptor agonist (D2AG) or Verapamil (VR), a specific L-type Ca2+ channel blocker. Similarly, either VR or D2Ag diminished the VIP/Bay stimulatory effect on prolactin expression and release. On the other hand, pretreatment of pituitary cells with thapsigargin (TG) or neomycin (NEO), to deplete the intracellular Ca2+ stores, showed no effect on basal or VIP-stimulated prolactin mRNA levels; although VIP-induced prolactin release was partially inhibited by NEO but not TG. These results suggest that intracellular Ca2+ represents a common signal transduction pathway through which VIP and dopamine can exert antagonistic control on prolactin synthesis and release in avian lactotrophs.

Animals↗

PubMed2004

The role of prolactin in reproductive failure associated with heat stress in the domestic turkey.

Reproductive failure associated with heat stress is a well-known phenomenon in avian species. Increased prolactin (PRL) levels in response to heat stress have been suggested as a mechanism involved in this reproductive malfunction. To test this hypothesis, laying female turkeys were subjected to 40 degrees C for 12 h during the photo-phase daily or maintained at 24-26 degrees C. Birds in each group received oral treatment with parachlorophenyalanine (PCPA; 50 mg/kg BW/day for 3 days), an inhibitor of serotonin (5-HT) biosynthesis, or immunized against vasoactive intestinal peptide (VIP). Both treatments are known to reduce circulating PRL levels. Nontreated birds were included as controls. In the control group, high ambient temperature terminated egg laying, induced ovarian regression, reduced plasma luteinizing hormone (LH) and ovarian steroids (progesterone, testosterone, estradiol) levels, and increased plasma PRL levels and the incidence of incubation behavior. Pretreatment with PCPA reduced (P < 0.05) heat stress-induced decline in egg production, increase in PRL levels, and expression of incubation behavior. Plasma LH and ovarian steroid levels of heat stressed birds were restored to that of controls by PCPA treatment. As in PCPA-treated birds, VIP immunoneutralization of heat-stressed turkeys reduced (P < 0.05) circulating PRL levels and prevented the expression of incubation behavior. But it did not restore the decline in LH, ovarian steroids, and egg production (P > 0.05). The present findings indicate that the detrimental effect of high temperature on reproductive performance may not be related to the elevated PRL levels in heat-stressed birds but to mechanism(s) that involve 5-HT neurotransmission and the induction of hyperthermia.

Animals↗

PubMed2003

Method for collecting semen from the ostrich (Struthio camelus) and some of its quantitative and qualitative characteristics.

1. Four methods of semen collecting that involved interruption of mating in two breeding ostrich pairs were tested: an artificial vagina was tested without promising results; the funnel method, in which a funnel was placed under the phallus of the tested male immediately after mating allowing semen drips to be collected; the vacuum method, using a turkey semen collector, inserted into the seminal canal; and the tube method, conducted by placing a test tube inside the seminal canal, allowing semen to enter by gravity. 2. For the funnel, vacuum and tube methods, respectively, average semen volume was 0.1 +/- 0.02, 1.12 +/- 0.22, and 0.58 +/- 0.13 ml, sperm concentration was 0.66 +/- 0.14, 2.35 +/- 0.26, and 2.13 +/- 0.27 x 10(9) cells/ml, and percentage of abnormal cells was 5.82 +/- 1.79%, 4.68 +/- 1.19%, and 7.09 +/- 1.72%. 3. Semen characteristics varied throughout the reproductive season reaching peak concentration in June-July. 4. The vacuum method proved to be the most efficient and was a low stress, restraint-free method for collecting ostrich semen.

Analysis of Variance↗

PubMed2003

Effect of embryonic photostimulation on the posthatch growth of turkey poults.

Artificial illumination, including light quality, is important in modern meat-type poultry management. In the present study, the effect of in ovo monochromatic green light photostimulation on posthatch growth of turkey poults was investigated. In experiment 1, 182 turkey eggs were divided into two light treatment groups (n = 91). The first group was intermittently photostimulated (3 min on and 3 min off) with green light provided by five light-emitting diodes (LED) per egg at 0.1 W/m2 at the upper eggshell surface. The second group was incubated in the dark and served as the control. Posthatch BW were recorded at 0, 2, 6, 13, 20, 28, 35, and 59 d of age. A heavier BW, occurring at 28 d of age and persisting until the end of the experiment (59 d of age), was observed in the in ovo green light stimulated females as compared to their corresponding controls. In experiment 2, 273 turkey eggs were divided into three light treatment groups (n = 91). The first group was intermittently photostimulated (15 min on and 15 min off) with green light provided by seven LED per egg at 0.14 W/m2. The second group was photostimulated with white light provided by one mini-incandescent lamp per egg at light intensity and schedule similar to the first group. Eggs of the third group were incubated in the dark and served as controls. Posthatch BW were recorded at 0, 7, 14, 28, 42, 56, and 79 d of age. No differences were found among the BW of males incubated under different light conditions. As in experiment 1, female turkeys with stimulated green light in ovo had greater BW compared to their corresponding control and white light groups from 28 d of age until termination of the experiment at 79 d of age. Breast muscle weight was greater in female turkeys incubated under green light when compared to white and dark incubation treatment groups. We suggest that in ovo green light photostimulation enhances the posthatch BW of female turkey poults.

Aging↗

PubMed2001

Molecular cloning and expression analysis of the turkey vasoactive intestinal peptide receptor.

Vasoactive intestinal peptide (VIP) is a prolactin (PRL)-releasing factor whose activity in avian species is believed to be mediated by a specific VIP receptor (VIP-R). Circulating PRL levels are closely related to hypothalamic VIP immunoreactivity, hypothalamic VIP mRNA content, and hypophysial-portal blood VIP concentrations in turkeys. In the present study, a turkey VIP-R (tVIP-R) cDNA was cloned and its mRNA abundance was quantified in various tissues during different reproductive stages. The 2347-bp tVIP-R cDNA encoded a 457 amino acid protein, with a predicted Mr of 52 kDa. The full-length cDNA shares approximately 55% similarity with the mammalian VIP receptor-1. Northern blot analysis revealed that a major 2.7-kb transcript was expressed in laying hen pituitaries. Furthermore, two minor tVIP-R transcripts of 3.7 and 3.4 kb were observed. Semiquantitative reverse transcription polymerase chain reaction (RT-PCR) was performed using RNA from various turkey brain and peripheral tissues throughout the reproductive cycle. The steady-state levels of pituitary tVIP-R mRNA changed during the reproductive cycle, whereas mRNA expression in other tissues was not affected. The steady-state levels of tVIP-R mRNA were only affected in the pituitary, whereas mRNA expression in any of the other tissues examined following the immunization of turkeys against VIP were not affected.

Amino Acid Sequence↗

PubMed2001

Photostimulated prolactin release in the Turkey hen: effect of ovariectomy and environmental temperature.

Ambient temperature modulates prolactin (PRL) secretion in birds. It is not known whether this modulation directly affects the PRL controlling mechanism(s) or whether it indirectly influences them through the onset of sexual maturity and/or the onset of incubation behavior. These experiments were designed to investigate the effect of elevated (32 degrees ) and reduced (10 degrees ) ambient temperatures on PRL secretion. Somatically mature, ovariectomized female turkeys were used to avoid the confounding effects of reproductive stage, nesting, and egg stimuli on PRL secretion. Hens were ovariectomized 5 weeks before, on the day of, or 10 days after the inception of photostimulation. Temperature treatments included chronic exposure (5 weeks) to 32 or 10 degrees and acute exposure (i.e., temperature was reversed from 32 to 10 degrees or from 10 to 32 degrees on or after the day of photostimulation). Chronic exposure to either 32 or 10 degrees had no effect on the rise in serum PRL that followed photostimulation in both sham-operated controls and ovariectomized hens. Acute exposure to 10 or 32 degrees altered the photoperiodically stimulated rise in plasma PRL. Birds switched from 10 to 32 degrees showed a significantly greater PRL increase than birds shifted from 32 to 10 degrees. Ovariectomy enhanced the PRL response to the gonadal stimulating photoperiod. The effect was most pronounced in hens photostimulated prior to ovariectomy. These findings suggest that ambient temperature and/or ovariectomy have a modulating effect on the PRL response to long days.

Animals↗

PubMed2001

Molecular cloning and expression of turkey inhibin-alpha and -betaA subunits.

We isolated cDNA encoding turkey inhibin-alpha (tINH-alpha) and -betaA (tINH-betaA) subunits from the turkey ovary using reverse transcription-polymerase chain reaction (RT-PCR). The isolated alpha subunit and betaA subunit included the entire open reading frames encoding 329 and 424 amino acids, respectively. The amino acid sequences of mature tINH-alpha subunit and tINH-betaA subunit (12.6 and 12.9 kDa proteins, respectively), established via DNA sequence analysis, were highly conserved between the chicken and various mammals. Northern blot analysis revealed that the transcripts of tINH-alpha and tINH-betaA subunits were approximately 1.7 and 8.4 kb, respectively. In various stages of follicular development, tINH-alpha mRNA was highly expressed in small white follicles as compared to postovulatory and regressed follicles, whereas tINH-betaA mRNA was predominately expressed in preovulatory F5 follicles.

Amino Acid Sequence↗

PubMed2000

Three different turkey luteinizing hormone receptor (tLH-R) isoforms I: characterization of alternatively spliced tLH-R isoforms and their regulated expression in diverse tissues.

Using combinations of reverse transcription-polymerase chain reaction (RT-PCR) and 5'- and 3'-rapid amplification of cDNA ends, three different, alternatively spliced, partial turkey LH receptor (tLH-R) cDNA isoforms were characterized from ovarian mRNA. The first cDNA (tLH-R(intact)) showed 98% and 72-75% similarity with chicken and mammalian LH-R sequences, respectively. The second cloned cDNA isoform (tLH-R(insert)) contained an in-frame TGA stop codon within an 86-base pair insertion that was located in the extracellular domain of the seven-transmembrane region. The tLH-R(insert) isoform could encode a truncated soluble protein isoform that lacked the transmembrane region. The third cDNA isoform truncated the transmembrane region (tLH-R(trunc)) and was derived by the deletion of the last exon by incomplete splicing. Generation of multiple transcripts by alternative splicing was elucidated by partial characterization of tLH-R genomic sequences. The differentially regulated expression of the tLH-R mRNA isoforms in nongonadal tissues and ovarian stromal tissues during various reproductive stages was quantified and analyzed by Northern blot and/or RT-PCR. Alternatively spliced tLH-R isoforms were differentially expressed in a tissue-specific manner in most of the tissues examined. The steady-state levels of tLH-R mRNA isoforms were relatively high in the hypothalamus and optic nerve and relatively low in the cortex, pituitary, and cerebellum when compared to levels in ovarian follicles. In nongonadal reproductive tissues, the steady-state levels of tLH-R mRNA isoforms were relatively high in the uterus and infundibulum and relatively low in the isthmus, oviduct, and magnum. In addition, in the nongonadal peripheral tissues, the steady-state levels of tLH-R isoforms were relatively high in the thyroid gland and relatively low in the spleen, adrenal gland, kidney, skin, bursa, and muscle. The present study suggests that the alternative splicing of LH-R transcripts occurs in a tissue-specific manner and has been evolutionarily conserved (similar results were obtained in chicken and swine). These results raise fundamental questions as to the function of LH-R isoforms in nongonadal tissues.

Alternative Splicing↗

PubMed2000

Three different turkey luteinizing hormone receptor (tLH-R) isoforms II: characterization of differentially regulated tLH-R messenger ribonucleic acid isoforms in the ovary.

We have recently characterized three different, alternatively spliced, partial turkey LH receptor (tLH-R) cDNA isoforms by the combination of reverse transcription-polymerase chain reaction (RT-PCR) and 5'- and 3'-rapid amplification of cDNA ends. The first cDNA (intact form: tLH-R(intact)) showed 98% and 72-75% similarity with chicken and mammalian LH receptor sequences, respectively. The other two cloned cDNA isoforms (insertion and truncated forms: tLH-R(insert) and tLH-R(trunc)) could encode truncated soluble protein isoforms that lack the transmembrane region. Northern blot analysis detected two transcripts of 3.0 kilobases (kb) (tLH-R(intact)) and 1.5 kb (tLH-R(trunc)) in the turkey ovary but could not discriminate a third alternatively spliced transcript (tLH-R(insert)) due to the small 86-base pair difference in the size range of approximately 3.0-kb mRNAs. But with the combination of RNase protection assay, RT-PCR, and Northern blot analysis, three different alternatively spliced tLH-R mRNA isoforms were quantified. Differential expression of the tLH-R mRNA isoforms was demonstrated in ovarian stromal tissue during various reproductive stages and in the theca and granulosa layer through follicular development. To gain a better understanding of the physiological significance of the three different tLH-R isoforms, total RNA from the theca layer through follicular development after prolactin (PRL) treatment was analyzed by RT-PCR. PRL treatment for 8-14 days significantly increased the steady-state levels of total tLH-R mRNAs, including tLH-R(insert) and tLH-R(trunc) mRNAs, compared to those in nontreated controls. In contrast, the steady-state levels of tLH-R(intact) mRNA during the same period was not significantly changed when compared to that in nontreated controls. The present study shows that tLH-R transcripts are alternatively spliced in a tissue-specific manner in the turkey and that the mechanism may, in part, be controlled hormonally.

Alternative Splicing↗

PubMed1999

Molecular cloning and tissue distribution of an avian D2 dopamine receptor mRNA from the domestic turkey (Maleagris gallopavo).

The reverse transcriptase-polymerase chain reaction (RT-PCR), in combination with 5' and 3' rapid amplification of cDNA ends (RACE), was used to clone a G protein-coupled receptor from turkey brain mRNA. This cDNA clone has an open reading frame of 1,311 base pairs encoding a 436-residue protein with seven transmembrane-spanning domains and exhibits high homology with previously cloned mammalian D2 dopamine receptors. Northern blot analysis of turkey brain mRNA detected an approximate 2.4-kb transcript. RT-PCR and subsequent nucleotide sequence analysis of turkey brain and peripheral tissue mRNA also demonstrated the presence of an alternatively spliced mRNA corresponding to the predicted D2 short isoform. RT-PCR experiments demonstrated a widespread distribution of alternatively spliced D2 dopamine receptor transcripts throughout the turkey brain and in select peripheral tissues as well. In situ hybridization experiments detected strong autoradiographic signals over much of the turkey telencephalon, diencephalon, mesencephalon, cerebellum, pituitary, and pineal gland. Dopamine has several important functions as a neurotransmitter and hormone in mammals and may have similar actions in avian species. The cloning and tissue distribution of the D2 receptor subtype should enable the investigation of any functional role dopamine and dopamine receptors exert on the physiology and behavior of birds.

Amino Acid Sequence↗

PubMed1999

D1 and D2 dopamine receptor messenger ribonucleic acid in brain and pituitary during the reproductive cycle of the turkey hen.

The regulation of prolactin secretion during the reproductive cycle of seasonal breeding birds appears to be largely under the stimulatory influence of hypothalamic vasoactive intestinal peptide (VIP). However, the factors influencing VIP secretion, and hence prolactin release, in birds remain largely unexplored. Recent evidence has demonstrated that dopamine and dopamine receptors may affect VIP and prolactin release in birds. The differential expression of dopamine receptors on hypothalamic VIP-releasing neurons may affect the degree of prolactinemia observed during the reproductive cycle of birds. In order to examine this hypothesis, we used reverse transcription-polymerase chain reaction to quantitate the levels of D1 and D2 dopamine receptor subtype mRNAs in the brain of the domestic turkey hen during the reproductive cycle. No significant difference in hypothalamic expression of D1 or D2 dopamine receptor subtypes during the reproductive cycle was observed. However, pronounced differences in D1D and D2 mRNAs were detected in cortex and cerebellum. Interestingly, there was a dramatic increase in pituitary D1D receptor mRNA during the reproductive stages of laying and incubation of eggs, which paralleled the hyperprolactinemic state of the turkey reproductive cycle. In addition, pituitary D2 receptor mRNA steadily increased throughout the reproductive cycle. In light of these observations, a modified hypothesis regarding the effects of dopamine on prolactin secretion is discussed.

Animals↗

PubMed1999

An opioid pathway in the hypothalamus of the turkey that stimulates prolactin secretion.

Circulating prolactin (PRL) levels increase when dynorphin is infused into the turkey brain. This study tested the hypothesis that centrally infused dynorphin requires an intact vasoactive intestinal peptide (VIP) system in order to stimulate turkey PRL secretion. It also investigated the roles of the dopaminergic and serotonergic systems in dynorphin-induced PRL release. Drugs were infused into the third ventricle of anesthetized laying turkeys via stereotaxically guided cannulae and circulating blood was assayed for changes in PRL levels. When a highly selective kappa opioid receptor antagonist was given prior to dynorphin injection, the PRL response to dynorphin was almost totally blocked. The coinfusion of either a serotonin (5-HT) or a D(1) dopamine (DA) receptor antagonist with dynorphin prevented the increase in PRL observed in birds when dynorphin was infused alone. On the other hand, the kappa opioid receptor antagonist failed to prevent the 5-HT-induced release of PRL. In hens actively immunized against VIP, infused dynorphin was unable to increase plasma PRL levels and infused VIP gave a muted PRL rise, while large increases in PRL were seen in nonimmunized birds receiving the same infusions. These data show that: (1) dynorphin stimulates PRL secretion by activating kappa opioid receptors in the avian hypothalamus, and (2) dynorphin, 5-HT, DA, and VIP stimulate avian PRL secretion via a common pathway expressing kappa opioid, serotonergic, dopaminergic, and VIPergic receptors at synapses arranged serially in that functional order, with the VIPergic system as the final mediator (releasing factor).

Animals↗

PubMed1999

Expression of vasoactive intestinal peptide/peptide histidine isoleucine in several hypothalamic areas during the turkey reproductive cycle: relationship to prolactin secretion.

The present study examined the changes in vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) gene expression in the hypothalamus and compared their level of expression to prolactin (PRL) secretion throughout the turkey reproductive cycle. Both VIP and PHI expression were determined using in situ hybridization histochemistry and reverse transcription-polymerase chain reaction/Southern blot analysis. Plasma PRL levels were determined by radioimmunoassay. The changes in total hypothalamic VIP and PHI mRNA steady-state levels mirrored the changes seen in plasma PRL across the reproductive stages. In situ hybridization revealed that VIP mRNA was highly expressed throughout the hypothalamus and predominated within the ventromedial nucleus (VMN), inferior hypothalamus, and infundibular nuclear complex (INF). PHI mRNA was expressed in very low abundance within the same areas expressing VIP mRNA. VIP mRNA abundance within the VMN and INF was lowest in nonphotostimulated hens (VMN, 13.8 +/- 1.7; INF, 17.0 +/- 1.8 arbitrary densitometric units (ADU)), intermediate in laying hens (VMN, 29.6 +/- 3.3; INF, 35.4 +/- 4.3 ADU), and highest in incubating hens (VMN, 76.4 +/- 10.2; INF, 119.2 +/- 3.4 ADU). Levels decreased when birds shifted from incubation to photorefractoriness (VMN, 75%; INF, 82%). This relationship was not observed within other areas of the hypothalamus. The expression of PHI mRNA was also highest in the VMN and INF of incubating hens but no correlation between PHI mRNA and the other reproductive states was observed. This study provides additional evidence that VIP is the avian PRL-releasing factor, and suggests that the central site for avian PRL regulation lies within the INF of the hypothalamus.

Animals↗

PubMed1998

Serotonergic stimulation of avian prolactin secretion requires an intact dopaminergic system.

In order to ascertain if prolactin (PRL) secretion stimulated by serotonin (5-HT) was mediated by the dopaminergic system, both dopamine (DA) and 5-HT were infused into the third ventricle of anesthetized laying turkeys in conjunction with the D1 DA receptor antagonist R(+)-SCH-23390 HCl or the 5-HT receptor antagonist Mianserin HCl. When infused alone at the rate of 10 nmol/min, both DA and 5-HT increased circulating PRL levels significantly (P < 0. 05) after 20 min (111.0 +/- 19.0 to 334.0 +/- 115.1 ng/ml and 93.3 +/- 13.2 to 173.8 +/- 33.3 ng/ml, respectively). When infused in conjunction with the D1 DA antagonist, the stimulatory effect of both DA and 5-HT on PRL release was significantly suppressed. The 5-HT antagonist completely blocked 5-HT-stimulated PRL release when infused at the rate of 10 nmol/min. When the 5-HT antagonist was co-infused with DA, PRL levels significantly increased (33.5 +/- 3.3 to 112.2 +/- 19.7 ng/ml) after 30 min, an increase that did not differ significantly from that caused by DA alone (63.2 +/- 16.4 to 144.5 +/- 44.1 ng/ml). These data suggest that 5-HT stimulation of PRL secretion requires an intact dopaminergic system and that 5-HT synapses are located further upstream than DA synapses along the PRL-releasing pathway within the avian hypothalamus.

Animals↗

PubMed1998

Vasoactive intestinal peptide secretion by turkey hypothalamic explants.

The objective of this study was to culture turkey hypothalami and examine vasoactive intestinal peptide (VIP) release during the turkey reproductive cycle. The release of VIP was studied employing a computer-guided perifusion system. Hypothalami were perifused with Krebs-Ringer bicarbonate medium for 10 or 15 h at a flow rate of 40 microliter/min, and perifusate was collected at 5-min intervals. Basal VIP secretion increased (p < 0.05) over time, and no differences in release rate were noted between reproductive stages. Basal VIP release during perifusion was episodic throughout each experimental period. Perifusion with dopamine (DA; 10 and 100 nmol/min) in incubating hens stimulated VIP release in a dose-dependent manner. There were no significant differences (p > 0.05) in VIP release in response to DA stimulation between hypothalamic fragments obtained from nonphotostimulated and incubating birds. The data suggest that 1) a VIP pulse generator appears to be located within the turkey hypothalamus, on the basis of the observed pulsatile release of VIP; 2) hypothalamic secretion of VIP is augmented by removal of inhibitory factors residing outside of the hypothalamus, or by the loss of negative feedback mechanisms that inhibit VIP release; and 3) mechanisms responsible for altering VIP release during different reproductive conditions may lie external to the hypothalamus.

Animals↗

PubMed1998

Regulation of prolactin secretion by dopamine and vasoactive intestinal peptide at the level of the pituitary in the turkey.

This study investigated the capability of dopamine (DA) to prevent avian prolactin (PRL) secretion by antagonizing the PRL-releasing factor, vasoactive intestinal peptide (VIP), at the level of the pituitary. To test this hypothesis, combined intracranial and intrapituitary infusions of DA, DA agonists, and VIP, plus electrical stimulation of the medial preoptic area (ES/POM), were used to characterize the actions of DA on PRL secretion in anesthetized laying turkey hens. Infused into the third ventricle at the rate of 10 nmol/min, DA induced a 2.8-fold increase in circulating PRL levels (63.8 +/- 15.1 to 181.3 +/- 44.3 ng/ml, p < 0. 05), similar to the 3.1-fold PRL increase induced by ES/POM (65 +/- 10.3 to 199.1 +/- 57.3 ng/ml, p < 0.05). Infused into the anterior pituitary at the rate of 15 ng/min, VIP induced a 2.2-fold increase in PRL (78.6 +/- 22.9 to 173.6 +/- 39.5 ng/ml, p < 0.05). When DA (10 nmol/min) was infused into the anterior pituitary it completely blocked both ES/POM- and VIP-induced PRL secretion. The D2 DA receptor agonist R- (-) -Propylnorapomorphine HCl inhibited VIP-induced PRL secretion at the level of the anterior pituitary, allowing only an insignificant rise in PRL (54.8 +/- 14.3 to 73.9 +/- 21.6 ng/ml, p > 0.05), while the D1 DA receptor agonist (+/-)-SKF-38393 HCl failed to prevent VIP-induced PRL release, allowing PRL to rise 2.5-fold (49.1 +/- 10.8 to 121.0 +/- 34.8 ng/ml, p < 0.05). Pituitary infusion of DA, DA agonists or vehicle alone caused no change in PRL levels. The data showed that DA prevented avian PRL secretion by blocking the action of VIP at the level of the anterior pituitary. DA effected this blockade of PRL via D2 DA receptors residing within the anterior pituitary. The data also suggested that there were no stimulatory D1 DA receptors related to PRL secretion in the avian anterior pituitary.

Animals↗

PubMed1998

Vasoactive intestinal peptide stimulates turkey prolactin gene expression by increasing transcription rate and enhancing mRNA stability.

This study evaluates the transcriptional and post-transcriptional regulation of prolactin (PRL) by vasoactive intestinal peptide (VIP). Pituitary nuclei from laying (control), incubating (with enhanced VIP secretion), and VIP-immunized laying turkey hens, and from pituitary cells cultured with or without VIP were used in nuclear run-on transcription assays. Cytoplasmic PRL mRNA was analyzed by slot blot hybridization. PRL transcription was greater in hyperprolactinemic incubating birds (PRL/beta-actin=3.33) than in laying birds (PRL/beta-actin=1.83). VIP-immunoneutralized birds had 47% and 51% decreases in PRL transcription and cytoplasmic PRL mRNA, respectively when compared with laying birds. In primary pituitary cell cultures, VIP significantly increased the transcription rate of PRL (3.8-fold) and cytoplasmic PRL mRNA (3.2-fold) compared with that of non-VIP-treated pituitary cells. The stability of pre-existing PRL mRNA was measured by Northern blot analysis after addition of actinomycin D. PRL mRNA half-lives were calculated using a two-component model, with a first-long component of 18.0+/-1.0 h and a second-short component of 3.7+/-0.7 h in non-VIP-treated pituitary cells. Both half-lives were significantly increased (53. 2+/-6.9 and 26.3+/-4.3 h) in VIP-treated cells. The present data show that VIP acts to stimulate PRL expression by up-regulating the transcription rate of PRL and by enhancing PRL mRNA stability.

Animals↗