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Abstracts from VI International Symposium on Avian EndocrinologyMarch 31 - April 5, 1996 Chateau Lake Louise, Alberta Continued Secretory Patterns of Plasma GH and IGf-1 Concentrations in Meat-Type and Laying-Type Chickens during Early Posthatch DevelopmentR.Q. Zhao1, E. Decuypere2, R. Grossmann Institute for Small Animal Research, Federal Research Centre
of Agriculture, Celle, Germany; 1Laboratory for Animal Physiology
and Biochemistry, Nanjing Agricultural University, nanjing, P.R.
China; 2Laboratory for Physiology of Domestic Animals, Catholic
University Leuven, Heverlee, Belgium The secretory patterns of plasma GH and IGF1 were studied in a meattype (Broiler) and a layingtype (White Leghorn) poultry strain during early posthatch development from 5 to 11 days (D) of age. Birds were catheterized via the jugular vein at D3 and serial blood samples were taken at 20min intervals for 100, 140 and 180 min at D5, D8 and D11, respectively. Plasma concentrations of GH and IGF1 were measured by radioimmunoassay. GH plasma concentrations followed a pulsatile pattern both in broilers and layers. However, the pattern of GH secretion showed considerable differences between both strains. The overall mean plasma GH concentration was higher (P<0.05) in the layer, mainly due to peak amplitudes which were almost 23 times higher (P<0.01) when compared to broilers at all age groups investigated. On the contrary, the nadir and the pulse frequency were significantly higher (P<0.05) in broilers compared with layers at all ages investigated. No significant developmental changes in GH concentrations were observed between D5 to D11 in both strains. The plasma IGF1 concentrations were significantly higher (P<0.05) in broilers at D8 and D11 when compared to layers. Furthermore, there was a continuous increase in IGF1 concentrations in broilers (P<0.01) from D5 to D11. These results show that the higher growth rate in broilers during the first two weeks after hatching corresponds to the higher GH baseline concentration and pulse frequency, as well as the higher IGF1 concentration in plasma. Investigations of the 5'-Region of the Growth Hormone-Encoding Gene in the ChickenT.S. Nichelmann, E. Mühlbauer, M. Tanaka1, K. Nakashima1, R. Grossmann Institute for Small Animal Research, Department of Physiology,
Dörnbergstrasse 25-27, 29223 Celle, Germany; 1Department of
Biochemistry, Mie University School of Medicine, Edobashi Tsu Mie
514, Japan In order to investigate regulatory mechanisms of growth hormone (GH) gene expression in the chicken a PstIfragment (Tanaka M. et al., 1992, Gene, 122) of the promoter region was cloned into the luciferase reporter gene vector GL2Basic (Promega). The constructs were transfected into both a rat pituitary cell line (GC cells) and primary hypophyseal cells obtained from 28 day old broiler chickens. As a negative control the GL2Basic vector without inserted DNA was also used. The luciferase activity of the reporter gene was 4-10 fold higher than the background activity, in the case of the PstIfragment (which included 500 bp upstream of the transcription start point) but the same as background activity when a deletion construct without the proximal TATA box and part of a putative Pit1 binding site was used. The results show that the chicken growth hormone promoter is active in these expression systems. However, since GH gene expression is usually upregulated during posthatch growth in chickens, the reporter activity is much lower than expected. It is, therefore, concluded that (a) regulation of the GH promoter is achieved by differing factors in avian species compared to mammals and (b) one or more relevant regulatory elements for GH gene expression may be located further upstream of the PstI fragment tested. (Supported by the Deutsche Forschungsgemeinschaft [DFG grants Gr 838/31, 32]) Development of a RNase Protection Assay for Quantitative Measurement of Chicken Growth Hormone Receptor TranscriptsN-C. Mao, J. Burnside, L.a. Cogburn Department of Animal and Food Sciences, University of
Delaware, Newark, DE 19717, USA Expression of the chicken growth hormone receptor (cGHR) gene is developmentally regulated and could be a rate limiting factor in GH-dependent growth. Three major cGHR transcripts (0.8, 3.2 and 4.3 kb), derived from three different polyadenylation signals in the primary transcript, are detected in liver by Northern blot analysis. We have developed a more sensitive, quantitative RNase protection assay (RPA) to distinguish between the full-length (3.2 and 4.3 kb) and truncated (0.8 kb) cGHR transcripts. A riboprobe complementary to nt. 267 to 443 of the cGHR cDNA was constructed to span a portion of the extracellular domain. This riboprobe is fully protected by the 3.2 and 4.3 kb transcripts (a 176 bp protected fragment) from the cGHR gene. Another riboprobe, complementary to nt. 399 to 625 of cGAPDH cDNA (a 226 bp protected fragment), was constructed and included for normalization of RNA. The RPA is linear for quantitation of the two protected fragments of cGHR and cGAPDH when 10 to 40 mg of total RNA are assayed. For further validation, we compared the RPA with Northern blot analysis for quantitation of the developmental expression of cGHR transcripts in normal (DwDw) and cGHR-deficient sex-linked dwarf (dwdw) broiler chickens. Northern blot analysis shows a 3-fold increase in cGHR gene expression in both DwDw and dwdw chickens from 1 to 7 weeks of age. The RPA shows a similar developmental increase (2 to 3-fold) in abundance of cGHR transcripts in DwDw chickens and a 5-fold increase in expression of the truncated transcript in dwdw chickens. The RPA shows that expression of the truncated cGHR transcript in dwdw chickens is 7-times greater than the abundance of all three cGHR transcripts in DwDw chickens at 7 weeks of age, while Northern blot analysis shows only a 4-fold difference at 5 weeks. These differences in cGHR gene expression reflect the increased sensitivity of the RPA when compared to Northern blot analysis. Generation of a Turkey Pit-1 cDNA Variant by Alternative Transcription InitiationE.a. Wong, L. Sharova, K. Kurima, K. Weatherly Department of Animal and Poultry Sciences, Virginia
Polytechnic Institute and State University, Blacksburg, VA
24061-0306, USA The transcription factor Pit1/GHF1 belongs to the POU class of homeodomain proteins and is involved in the transcriptional regulation of the growth hormone, prolactin, and thyroid stimulating hormonebeta genes and its own gene. To study the regulation of Pit1 gene expression in the turkey, multiple Pit1 cDNAS and the gene have been cloned and characterized. The turkey Pit1 gene contains seven exons and spans greater than 12 kilobases. Two Pit1 transcripts, encoding different amino termini, were identified by 5' RACE (rapid amplification of cDNA ends) analysis. One transcript encoded a Pit1 amino terminus that was comparable to the rat Pit1a (1 b, GHF2) isoform, which contains an extra 26 amino acids relative to the major Pit1 transcript. This transcript arises by the splicing of exon I to the alternative acceptor splice site in exon II. The other turkey Pit1 transcript was generated following transcription initiation upstream of exon II and entirely lacks exon I. This transcript is unique to turkey Pit1 mRNAs. Both turkey Pit1 transcripts contained an extra 38 amino acid coding exon that has not been found in mammalian Pit1 mRNAs. This exon is located between exons II and III and is designated exon IIa. In summary, alternative transcription initiation of the turkey Pit1 gene generates Pit1 isoforms with different amino termini. The ability of these Pit1 isoforms to activate transcription is currently being investigated. Differential Autoregulation of the Growth Hormone Receptor in Central and Peripheral TissuesK. Hull, C. Ball, R.W. Lea, S. Harvey Department of Physiology, University of Alberta, Edmonton, AB
T6G 2H7 Canada Growth hormone differs from other pituitary hormones in that it can affect a wide spectrum of cellular activities in many different tissues. These disparate actions are, however, mediated by a common receptor, suggesting tissue-specific differences in the post-receptor mechanisms and/or tissue sensitivity to GH stimulation. Tissue responsiveness also reflects receptor abundance and GH affinity and tissue-specific autoregulation of GH receptors (GHRs) could therefore contribute to differential tissue responsiveness of GH action. The autoregulation of GHR gene transcription in novel central (whole brain and/or hypothalami) and peripheral (thyroid, bursa, spleen and thymus) tissues was therefore examined in domestic fowl. For comparative purposes, GHR gene expression was also examined in the liver, which has traditionally been considered the major GH- target site. In all tissues, the abundance of the 4.4 kb and 2.8 kb transcripts was inversely related to peripheral GH status, as it was reduced by a bolus systemic injection of recombinant chicken GH, designed to mimic an episodic burst of endogenous GH release. This autoregulatory response was observed within 2 h of GH administration and was of greater magnitude in the brain than in peripheral tissues. Intracerebroventricular (icv) injections of GH also downregulated GHR gene expression in the brain, although peripheral GHR transcripts were unaffected by central GH administration. The autoregulation of GH receptors appears to be tissue specific and greater in non-traditional GH-target sites, particularly immune and neural tissues, than in liver. It may therefore be pertinent that the brain and immune system, but not the liver, are extrapituitary sites of GH synthesis, and downregulation may provide a mechanism to modulate GH autoregulation. The specific upregulation of hypothalamic and extrahypothalamic GHR transcripts following passive GH immunoneutralization supports this view, since hepatic GHR mRNA was unaffected by GH immunoneutralization. The ability of peripheral and central GH to autoregulate brain GHRs also suggests that the blood brain barrier (BBB) does not impede humeral GH entry, although the failure of icv GH to downregulate peripheral GHRs may indicate that movement through the BBB is not bidirectional. These results suggest that GHR mRNA transcription and/or transcript stability are autoregulated in most tissues, particularly the brain. The marked downregulation of tissue GHRs may partly account for the inability of exogenous GH to promote growth in growing domestic fowl. (Supported by NSERC and NATO)6.01-Oral Principles in Protein Hormone Evolution: the Neurohypophysial Peptides As Avian Evolutionary TracersR. Acher, J. Chauvet, M.T. Chauvet, G. Michel Laboratory of Biological Chemistry, University of Paris VI, 96
Bd Raspail, 75006 - Paris, France Any protein displays a polydomainal structure, each functional domain modelled by a separate evolutionary history. In a prohormone protein, the hormone domain has been conformed by coevolution with a specific receptor and therefore represents a reliable evolutionary tracer. The other nonhormone domains have coevolved with distinct protein partners. Vertebrate neurohypophysial hormones form a remarkably homogeneous superfamily of 13 peptides : all are alpha-amidated nonapeptides with a disulfide bridge linking positions 1 to 6. Two main evolutionary lineages can be distinguished: 1) isotocin (bony fish) - mesotocin (nonmammalian tetrapods) - oxytocin (placentals) ; 2) vasotocin (nonmammalian vertebrates) - vasopressins (mammals). All flying or flightless birds examined to date have mesotocin ([I1e8]-oxytocin) and vasotocin ([I1e3]-vasopressin). The neurophysin domains of promestotocin and provasotocin can be used for measuring phylogenetic distances. Whereas the function of mesotocin is obscure, vasotocin is clearly the antidiuretic hormone. In the kidney, vasotocin is active on the glomerulus and nephron tubule. Vasotocin receptors have been partially identified in the chicken and the toad. The striking evolutionary stability of vasotocin in nonmammalian vertebrates suggests a corresponding stability of the hormone-binding site of the receptor. However several receptors could exist in distinct target cells of the same species and fulfil different functions through their G protein-binding domains. Indeed, the three types of mammalian vasopressin receptors V1a, V1b and V2 likely have avian vasotocin receptor counterparts. Arginine Vasotocin in the Domestic FowlT.I. Koike, L.E. Cornett Department of Physiology and Biophysics, University of
Arkansas for Medical Sciences, Little Rock, AR 72205, USA In chickens arginine vasotocin (AVT), the vasopressinlike peptide, acts on both the kidneys and uterus and is selectively released by osmotic stress or oviposition. Hemorrhage, on the other hand, has not been considered an effective stimulus for hormone release in this species. AVT gene transcripts, detected by reverse transcriptase polymerase chain reaction, are present in several extrahypothalamic tissues, suggesting that AVT may have a autocrine or paracrine action in nonhypothalamic tissues. In birds, osmotic stress, induced by water deprivation, results in an increase in AVT gene expression not only in a given hypothalamic neuron, but also by activating neurons that are not normally observed to contain AVT transcripts in euhydrated animals. An increase in size of the AVT mRNA, due to lengthening of the poly[A+] tail, is also observed in waterdeprived animals. The effects of hemorrhage (30% hemorrhage) in urethaneanaesthetized chickens were compared to effects of acute osmotic stress (5 3 M NaCl/kg i.P.) in conscious animals. Hemorrhage caused an approximate 3fold increase in plasma AVT, a 2fold increase in the amount of hypothalamic AVT mRNA, and an increase in the number of cells expressing AVT. These observations suggest that urethane removes inhibitory inputs to the AVT system that may be present in conscious or barbiturateanaesthetized animals. AVT mRNA levels also increase following estradiol administration, indicating that gene expression is enhanced by an estrogen response element . Detection of cfos mRNA was used to determine the neural structures that are activated by osmotic and cardiovascular inputs to the AVT system. (Supported by the National Science Foundation) An Avian Gene in the GHRH-PACAP FamilyN.M. Sherwood, J.E. McRory Department of Biology, University of Victoria, Victoria, BC,
Canada Growth hormonereleasing hormone (GHRH) and pituitary adenylate cyclase activating polypeptide (PACAP) are two neuropeptides known to be encoded within the same gene in teleosts, but on different genes in mammals. To investigate avian GHRHPACAP structure and expression, we sequenced the chicken GHRHPACAP gene. Exons 4 and 5 encoded GHRH and PACAP, respectively. In addition, the cDNAs from the embryo, juvenile and adult were sequenced. In the developing chick embryo on days 13, foreshortened mRNA encoding exons 4 and 5, but not 13 is expressed. A longer mRNA with exons 35 is expressed on day 4. Beginning on embryonic day 5, the full length mRNA with 5 exons is detected and is processed into three different transcripts. The first mRNA encodes full length GHRH (46aa); the second mRNA is the result of exon sliding at the intron4:exon 5 boundry and encodes a shorter GHRH (43aa); and the third mRNA skips exon 4 leaving only GHRH3346. We also determined that juvenile chicken GHRH PACAP mRNA tissue expression is in the brain and gonads, but not in the pituitary, heart, liver, kidney, crop, small intestine, large intestine, eye and muscle. TRH-Related Peptides and GH SecretionS. Harvey, K.L. Hull, K.L. Geris, L.a. Cogburn1, M. Bulant2, a. Ladram2 1Department of Animal and Food Science, University of
Delaware, Delaware 1917-1303, Department of Physiology,
University of Alberta, Edmonton, AB T6G 2H7 Canada; USA;
2Laboratoire de Bioactivation des Peptides, Institute Jacques
Monod, 75251 Paris, Cedex, France It is well established that thyrotropin-releasing hormone (TRH) (pGlu-His-ProNH2) is a growth hormone (GH) releasing factor in birds, since it directly stimulates pituitary GH release and increases stimulatory hypothalamic tone. The GH-releasing activity of TRH is, however, modulated at CNS and pituitary sites by numerous neural and humeral factors, including TRH itself and TRH-related peptides: TRH Autoregulation Although TRH stimulates GH release, it rapidly downregulates pituitary TRH receptors and induces refractoriness. It may also increase inhibitory hypothalamic tone, since it depletes the content of hypothalamic somatostatin. The induction of thyroid function by TRH also provides a feedback mechanism to inhibit the hypothalamo-pituitary axis, particularly somatotroph responsiveness to TRH stimulation. TRH Biotransformation The metabolism of TRH results in the formation of diketopiperazine (DKP) and cyclo (His-Pro), both of which enhance basal and TRH-induced GH release in vivo, although DKP is a TRH antagonist in vitro. TRH Gene-related Peptides In addition to TRH, the processing of the TRH precursor may, as in rats, generate a number of `cryptic' peptides that flank TRH progenitor sequences. Indeed, immunoreactivity (ir) for one of these peptides (P4) is present in extra-hypothalamic (principally the bulbus olfactorius) tissues of the chicken brain and is concentrated in the hypothalamus, particularly within preoptic and paraventricular regions and within the median eminence. P4-ir is also present in the chicken adeno- and neuro-hypophysis. However, while P4 is a TRH-potentiating peptide in rats, (augmenting TRH-induced thyrotropin and prolactin secretion) it suppresses basal and TRH-induced GH secretion in chickens in vivo. Another cryptic peptide, P5, similarly impairs in vivo GH secretion, although neither P4 nor P5 modulate basal or TRH-induced GH release from pituitary glands in vitro, suggesting CNS sites of action. TRH-like Peptides TRH-like peptides, for instance pGlu-Glu-ProNH2, are also produced in the hypothalamo-pituitary axis. This peptide interacts (albeit at pharmacological doses) with TRH-binding sites and inhibits TRH-induced GH release from pituitary glands incubated in vitro. It does, however, potentiate TRH-induced GH secretion in vivo, suggesting differential effects at hypothalamic and pituitary sites. In summary, although TRH is a potent GH secretagogue in birds, the GH releasing activity of TRH is subtly modulated at hypothalamic and pituitary sites by TRH and several TRH-related peptides. (Supported by NSERC of Canada) Vasoactive Intestinal Peptide as the Avian Prolactin-Releasing FactorM. El Halawani, O. Youngren1, G. Pitts Departments of Animal Science and 1Ecology, Evolution and
Behavior, University of Minnesota, St. Paul, MN 55108, USA It has long been established that prolactin (PRL) secretion in birds is tonically stimulated. The factor responsible for this stimulation appears to be vasoactive intestinal peptide (VIP), which meets many of the criteria required for releasing factors. Abundant VIP immunoreactive neurons are found within the medio-basal hypothalamus, giving rise to axons that extend into the external layer of the median eminence. The number of VIP neurons in the infundibulum, VIP immunoreactivity in the median eminence, hypothalamic VIP concentration and VIP mRNA content increase under conditions of hyperprolactinemia. Recent in vitro studies show that VIP is secreted from the hypothalamus in a pulsatile manner. VIP binds to specific receptors in the anterior pituitary gland where it stimulates PRL secretion and pituitary PRL mRNA at nanomolar concentrations, both in vivo and in vitro. Pre-incubation of turkey hypothalamic extract with VIP antiserum abolishes hypothalamic extract-induced PRL secretion. VIP immunoneutralization in vivo reduces circulating PRL, pituitary PRL mRNA and PRL gene transcription rate. Active immunization with VIP prevents the secretion of PRL induced by electrical stimulation of the hypothalamus, photostimulation, or monoamine infusion into the third ventricle. All of which suggests that VIP is the mediator of PRL secretion. VIP concentrations in hypophysial portal blood are 2.5- to 13.5-fold greater than in general circulation and mirrors that of circulating PRL. Taken together, these findings support the hypothesis that VIP is the avian PRL-releasing factor. INFLUENCE OF GONADAL HORMONES ON PEPTIDERGIC PATHWAYS OF THE QUAIL BRAIN. N. Aste, C. Viglietti-Panzica, J. Balthazart1, G.C. Panzica. Department of Human Anatomy, University of Torino, Torino,
Italy; 1Laboratory of Biochemistry, University of Liège, Liège,
Belgium. Previous studies from our laboratory showed that multiple peptidergic innervation occurs in prosencephalic regions of quail that are characterized by a wide amount of sex steroid receptors or LHRH elements (lateral septum, nucleus striac terminalis, preoptic region). These data suggested that neuropeptides could influence neuronal circuitries involved in the control of reproductive activities in birds. In the present study we investigated the existence of a testosterone (T) dependent modulation of peptidergic pathways in the septo-preoptic region of quail. We quantitatively analyzed the immunocytochemical distribution of vasotocin (VT), substance P, neuropeptide Y, and vasoactive intestinal polypeptide of intact, castrated, or castrated plus T male quail. Our results show that the reduction of T circulating levels after castration decreases the VT innervation of the medial preoptic nucleus (POM) and of the lateral septum. The innervation is completely restored by T treatment. No changes, or only slight variations of the other peptides were observed in the same birds. Our results confirm previous data on T-dependent VT innervation of the septal area of canary and of several mammalian species. Additionally, in quail the VT-ergic innervation of preoptic region is T modulated. Subsequent studies should clarify whether the T-sensitive VT innervation of the POM comes from intrinsic or extrinsic pathways. (Partly supported by EU and ESF grants) Brain and Peripheral Estrogen Synthesis in the Brown-Headed Cowbird (Molothrus ater).B.a. Schlinger Department of Physiological Science and Laboratory of
Neuroendocrinology, BRI, UCLA, Los Angeles, CA 90095, USA Estrogens exert considerable influence on the brain and behavior of birds. In the brain of adult non-songbirds, aromatase (estrogen synthetase) is most active in the hypothalamus, POA (HPOA), and in limbic regions of the telencephalon (TEL). In addition, in adult zebra finches aromatase is abundant in non-limbic parts of the TEL. In males, aromatase is undetectable in peripheral tissues, but it is sufficiently abundant in brain to enrich circulating estrogen (E) levels. Curiously, aromatase in the adult zebra finch brain is expressed in regions with and without E-receptors, and is expressed poorly, if at all, in song control nuclei. Thus, a linkage between aromatase and brain E action is not established in this species. Songbirds exhibit considerable species diversity in neural and behavioral properties of song. Identifying anatomical differences in aromatase expression across species may help us understand the function of this enzyme in the songbird brain. Aromatase activity in brain and peripheral tissues of wild and in captive brown-headed cowbirds has therefore been measured. Unlike zebra finches, aromatase was detected in the testes of breeding males, but activity was much lower than that present in the ovary of breeding females. Like zebra finches, aromatase was present in the TEL and HPOA of both males and females. Also, after a peripheral injection of 3H-androgen in males, 3H-E levels were 4-fold greater in jugular than in carotid plasma, and levels of 3H-E in the TEL, but not testes or adrenals, were increased. Together with in situ hybridization studies with a zebra finch aromatase cDNA clone, these studies show high aromatase expression in the cowbird TEL. However, a balance between brain and peripheral aromatization may be required to achieve significant concentrations of E in E-dependent neural circuits of different songbird species. (Support: NSF: IBN-9120776) Monoclonal Immunoaffinity Purification and Partial Characterization of a Family of N-Terminal Pro-Opiomelanocortin(Pomc)-Derived Polypeptides from the Chicken PituitaryL.R. Berghman, B. Devreese, J. Van Beeumen, L. Grauwels, F. Vandesande Laboratory for Neuroendocrinology and Immunological
Biotechnology, Zoological Institute, Katholieke Universiteit
Leuven, Naamsestraat 59, 3000 Leuven, Belgium A monoclonal antibody obtained by immunization with a ConApurified mixture of chicken pituitary glycoproteins was found to label specifically the corticotropic cell population in the cephalic lobe of the chicken pituitary. The antibody was purified and immobilized on Sepharose 4B for use in immunoaffinity chromatography. A crude pituitary extract was applied to the immunoaffinity column and the adsorbed components were eluted by acidification. Reversedphase HPLC analysis of the eluate revealed a mixture of several closely related polypeptides. MALDI mass spectrometry of the chromatogram confirmed the existence of a number of molecular species with molecular masses ranging from approximately 9600 to 3000 Da. The major peak was analyzed by automatic Edman degradation and revealed an Nterminal sequence which is homologous to the Nterminus of ostrich, mammalian and amphibian POMC. Another fraction displayed an identical Nterminus, suggesting differential carboxyterminal processing sites in the Nterminal portion of the chicken POMC molecule, resulting in a family of partially overlapping peptides with a common Nterminus. Immunohistochemical Localization of Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) in the Chicken ForebrainK. Peeters, L.R. Berghman, F. Vandesande Laboratory for Neuroendocrinology and Immunological
Biotechnology, Zoological Institute, Katholieke Univresiteit,
Leuven, Naamsestraat 59, B-3000 Leuven, Belgium Pituitary adenylate cyclase activating polypeptide (PACAP) is a member of the secretin/glucagon/vasoactive intestinal polypeptide (VIP) family. The primary structure of PACAP is highly conserved during evolution. It exists in two bioactive amidated forms: PACAP38 and PACAP27. PACAP immunoreactivity is found in high concentration in the central nervous system of mammals (sheep, rat, monkey and human) and amphibians (frog). Using polyclonal rabbit and sheep antisera to synthetic cysPACAP1027 and cysPACAP2438, we have investigated the distribution of PACAPcontaining neurons in the forebrain of juvenile, colchicineinjected chickens. PACAPimmunostained perikarya and fibers were observed in the preoptic and supraoptic region and throughout the hypothalamus (nucleus suprachiasmaticus (SCN), nucleus anterior medialis hypothalami (AM), nucleus periventricularis hypothalami (PHN), nucleus paraventricularis magnocellularis (PVN), nucleus ventromedialis hypothalami (VMN), regio lateralis hypothalami (LHy) and nucleus geniculatus lateralis (GLV)).Occasionally, immunoreactive fibers were found in the external zone of the median eminence. In the pituitary gland, numerous immunoreactive nerve fibers were observed in the posterior lobe. Circadian Variation in Neuropeptides of Chickens in Relation to Osmotic Stimulation and Egg LayingC.M. Chaturvedi, P. Marks1, T.I. Koike1 Department of Zoology, Banaras Hindu University, Varanasi-221005, India; 1Department of Physiology and Biophysics, University of
Arkansas Medical Sciences, Little Rock, AR 72205, USA Arginine vasotocin (AVT) and mesotocin (MT) are the two neuropeptides synthesized by the magnocellular neurons of the avian hypothalamus. While AVT is involved in both osmoregulation and oviposition, the role of MT is not yet known. The present study was undertaken to observe the circadian variation in the plasma AVT and MT levels of chickens in relation to age and egg laying and to osmotic stimulation (4 days water deprivation). Cyclic release of AVT was observed in actively laying hens (adult) with a peak value after the onset of dark phase (12L: 12D). On the other hand, while no cyclicity was noted in nonlaying (old) hens, a cycle of low amplitude was expressed in sexually immature (chick) hens. Further, water deprived adult hens did not show a circadian rhythm in AVT concentration, but the AVT level was significantly higher than in controls. No cyclic pattern was observed in the MT concentrations in any of these groups. These findings indicate that the osmotic stimulus eliminates the circadian pattern of AVT release. It is also suggested that cyclicity in the release of posterior pituitary hormones is selective for AVT and that the hormone variation may be related to egg laying. (Supported by the National Science Foundation) |