Arginine vasotocin and mesotocin levels in theca and granulosa layers of the ovary during the oviposition cycle in hens (Gallus domesticus)

Arginine Vasotocin Directly Regulates Spermatogenesis in Adult Zebrafish (Danio rerio) Testes

The neuropeptide vasopressin is known for its regulation of osmotic balance in mammals. Arginine vasotocin (AVT) is a non-mammalian homolog of this neuropeptide that is present in fish. Limited information suggested that vasopressin and its homologs may also influence reproductive function. In the present study, we investigated the direct effect of AVT on spermatogenesis, using zebrafish as a model organism. Results demonstrate that AVT and its receptors (avpr1aa, avpr2aa, avpr1ab, avpr2ab, and avpr2l) are expressed in the zebrafish brain and testes. The direct action of AVT on spermatogenesis was investigated using an ex vivo culture of mature zebrafish testes for 7 days. Using histological, morphometric, and biochemical approaches, we observed direct actions of AVT on zebrafish testicular function. AVT treatment directly increased the number of spermatozoa in an androgen-dependent manner, while reducing mitotic cells and the proliferation activity of type B spermatogonia. The observed stimulatory action of AVT on spermiogenesis was blocked by flutamide, an androgen receptor antagonist. The present results support the novel hypothesis that AVT stimulates short-term androgen-dependent spermiogenesis. However, its prolonged presence may lead to diminished spermatogenesis by reducing the proliferation of spermatogonia B, resulting in a diminished turnover of spermatogonia, spermatids, and spermatozoa. The overall findings offer an insight into the physiological significance of vasopressin and its homologs in vertebrates as a contributing factor in the multifactorial regulation of male reproduction.

Effects of Dark Brooder Rearing and Age on Hypothalamic Vasotocin and Feather Corticosterone Levels in Laying Hens

Chickens cannot independently thermoregulate at hatch and lack opportunity to behaviorally thermoregulate with a hen in the egg layer industry, thus barns are heated to thermoneutral temperatures. Dark brooders are low-energy-consuming hot plates, which may be environmentally advantageous while providing welfare-enhancing aspects of maternal care (i.e., shelter and separation of active and inactive individuals). Dark brooder use has been demonstrated to decrease injurious pecking and mortality well into the production period of layers. To further understand hen development around lay onset and effects of dark brooders on the brain and HPA-axis, we examined effects of rearing with dark brooders on expression of vasotocin (AVT) in the hypothalamus and corticosterone (CORT) in the feathers of in total 48 layer Isa Warren hens at 16 w and 28 w of age (n = 12 per age and treatment). An age-dependent decreased number of AVT-positive neurons was seen in the medial preoptic area, medial preoptic nucleus, paraventricular nucleus, rostral part (prepeduncular hypothalamus), and lateral preoptic area. Trends to effects of brooder rearing were found in both anteromedial preoptic nucleus and supraoptic nucleus, with dark brooder reared animals showing higher mean counts of AVT-positive neurons in both areas. No interactions between brooder raising and age were observed in AVT-positive neuron count. CORT levels were higher in primary wing feathers from 28 week old hens than in those from 16 week hens. No main effects of rearing with dark brooders or interactions between age and treatment were found on CORT levels. The age-dependent effects seen in the hypothalamus and CORT aids in further understanding of the development of chickens around puberty. The use of brooders tended to increase AVT expression in the anteromedial preoptic nucleus and supraoptic nucleus, an indication that dark brooder rearing may affect physiological responses regulated by these areas. The lack of effect of dark brooders on CORT in feathers is at the least an indication that the use of dark brooders is not stressful; in combination with the benefits of dark brooders on injurious pecking, fearfulness and early mortality, this pleads for the use of dark brooders in on-farm situations.

In situ localization of vasotocin receptor gene transcripts in the brain-pituitary-gonadal axis of the catfish Heteropneustes fossilis: a morpho-functional study

In the catfish Heteropneustes fossilis, three vasotocin (VT) receptor subtype genes, v1a1, v1a2, and v2a, were cloned and characterized previously. In the present study, using RNA probes, we localized the distribution of the gene transcripts in the brain-pituitary-gonadal (BPG) axis. The V1a-type receptor, v1a1 and v1a2, genes showed similar and overlapping distribution in the brain. The gene paralogs are distributed in the radial glial cells (RGCs) of the telencephalic ventricle and around the third ventricle in the hypothalamus and thalamus, olfactory tract, nucleus preopticus, nucleus lateralis tuberis, nucleus recessus lateralis and posterioris, nucleus saccus vasculosi, thalamic nuclei, habenular nucleus, habenular commissure, basal part of pineal stalk, accessory pretectal nucleus, optic tectum, corpus and valvula of the cerebellum, and facial and vagal lobes. The V2a receptor gene (v2a) has restricted distribution and is largely confined to the anterior subependymal region of the telencephalon. The localization pattern shows that the V1a-type receptors are distributed in major sensorimotor processing centers and the neuroendocrine/reproductive centers of the brain. In the pituitary, the receptor genes were localized differentially in the three divisions with the V1a-type receptor genes strongly expressed in the rostral pars distalis compared to the v2a paralog. In the ovary, the V1a-type receptor genes were localized in the follicular layer while v2a was localized in the oocyte membrane. In the testis, v1a2 and v2a are densely distributed in the interstitial tissue and seminiferous epithelium but the v1a1 is lowly expressed. The results suggest that the VT receptor genes have an extensive but differential distribution in the BPG axis. Future experimental studies are required to correlate the cellular localizations with specific functions of VT in the BPG axis.

Structural and functional diversity of nonapeptide hormones from an evolutionary perspective: A review

The article presents an overview of the comparative distribution, structure and functions of the nonapeptide hormones in chordates and non chordates. The review begins with a historical preview of the advent of the concept of neurosecretion and birth of neuroendocrine science, pioneered by the works of E. Scharrer and W. Bargmann. The sections which follow discuss different vertebrate nonapeptides, their distribution, comparison, precursor gene structures and processing, highlighting the major differences in these aspects amidst the conserved features across vertebrates. The vast literature on the anatomical characteristics of the nonapeptide secreting nuclei in the brain and their projections was briefly reviewed in a comparative framework. Recent knowledge on the nonapeptide hormone receptors and their intracellular signaling pathways is discussed and few grey areas which require deeper studies are identified. The sections on the functions and regulation of nonapeptides summarize the huge and ever increasing literature that is available in these areas. The nonapeptides emerge as key homeostatic molecules with complex regulation and several synergistic partners. Lastly, an update of the nonapeptides in non chordates with respect to distribution, site of synthesis, functions and receptors, dealt separately for each phylum, is presented. The non chordate nonapeptides share many similarities with their counterparts in vertebrates, pointing the system to have an ancient origin and to be an important substrate for changes during adaptive evolution. The article concludes projecting the nonapeptides as one of the very first common molecules of the primitive nervous and endocrine systems, which have been retained to maintain homeostatic functions in metazoans; some of which are conserved across the animal kingdom and some are specialized in a group/lineage-specific manner.

Vasotocin--A new player in the control of oocyte maturation and ovulation in fish

In this article, the physiological role of ovarian vasotocin (VT) on fish final oocyte maturation (FOM) and ovulation is reviewed based on the studies mainly available in the catfish Heteropneustes fossilis. The VT system is characterized in the follicular layer of the oocytes by both immunocytochemical and in situ hybridization techniques. The distribution was confirmed in isolated follicular layer preparations by HPLC characterization and quantification. Three VT receptor subtype genes are identified: V1a1 and V1a2 subtypes are distributed in the follicular layer and V2 subtype is present along the granulosa-oocyte membrane junction. The expression of peptide, VT precursor gene and VT receptor genes shows seasonal and periovulatory changes in the ovary. VT secretion is modulated by E2 differentially in a season-specific manner, and by progestin steroids positively. VT modulates E2 in a biphasic manner in early recrudescent phase and induces a steroidogenic shift inhibiting E2 and stimulating progestin steroid (P4, 17P4 and 17,20β-DP) pathways in the late recrudescent phase. VT stimulates prostaglandin secretion, germinal vesicle breakdown (GVBD), oocyte hydration and ovulation. VT acts through different receptors to stimulate these processes. It uses the V1 type receptor to stimulate GVBD and ovulation, and the V2 type to stimulate oocyte hydration. VT acts as an important link in the cascade of gonadotropin control of FOM and ovulation. More research is required in other species.

Functional interactions between vasotocin and prostaglandins during final oocyte maturation and ovulation in the catfish Heteropneustes fossilis

Functional interactions between vasotocin (VT) and prostaglandins (PGs) in the regulation of final oocyte maturation (FOM) and ovulation were investigated in the catfish Heteropneustes fossilis. Incubation of post-vitellogenic follicles with VT resulted in significant increases of both PGF2α and PGE2 at 8 and 16h intervals. The rise was higher at 16h except in the 1000nM VT group, in which the PG levels decreased compared to the 100nM group (biphasic effect). VT was more effective to increase the PG levels in comparison to hCG or IT. The co-incubation of the follicles with both hCG (20IU/ml) and VT (100nM) increased significantly PGF2α level at 8h, higher than that elicited by each when incubated alone. Pre-incubation of the follicles with V1 receptor antagonist, alone or in co-incubation with VT, significantly inhibited the VT-stimulated PGF2α and PGE2 levels. Under similar conditions, V2 receptor antagonist did not affect the PGE2 levels. Both VT (100nM) and PGs stimulated FOM (germinal vesicle breakdown) and ovulation in a dose- and duration dependent manner, PGF2α was more effective. Incubation of postvitellogenic follicles with indomethacin (a non selective cyclooxygenase inhibitor) per se did not affect FOM and ovulation but significantly decreased VT and PG effects upon pre-incubation. The results suggest that the VT stimulation of PGs may be mediated mainly through the V1 receptor though the involvement of V2 receptor cannot be excluded. The article also discussed the positive interplay of gonadotropin, maturation-inducing steroid, VT and PG during FOM and ovulation.

Age, photoperiod and estrogen dependent variations in the shell gland and the expression of AVT in the ovary of Japanese quail

Present work was undertaken to describe (i) age dependent (prepuberal-3, 4, 5 and 6 weeks old, puberal and actively laying 8 and 12 weeks old and aged 78 weeks old) (ii) photoperiodic response dependent (photosensitive and photorefractory) and sex steroid dependent (estradiol benzoate and its antagonist tamoxifen treated) variation in the ovary and shell gland activity of Japanese quail (Coturnix coturnix japonica). Further, in view of the role of neurohypophysial peptide arginine vasotocin (AVT) in many physiological processes including age/reproduction related oviposition, expression of ir-AVT was also monitored in the ovary of quail. All the parameters associated with histodifferentiation increased rapidly during the developing stages followed by a decrease in old age, which also increased in reproductively quiescent photorefractory birds following estradiol treatment and decreased in reproductively active photosensitive quail following tamoxifen treatment. Using AVT-specific antibody, expression of immunoreactive AVT (ir-AVT) observed in the ovary of photosensitive quail was not detected in the photorefractory quail. However, administration of estrogen in the photorefractory quail stimulated the growth and activity of ovary and shell gland also resulted in the expression of ovarian ir-AVT. On the other hand, tamoxifen eliminated the localization of ir-AVT in the ovary of photosensitive quail in addition to a decrease in the shell gland protein and alkaline phosphatase activity. It is concluded that estrogen not only affects the growth and differentiation of ovary and oviduct including shell gland but also regulates the expression of ovarian AVT. It is also suggested that in addition to reported paracrine effect of AVT in the shell gland of Japanese quail for oviposition, ovarian AVT may also affect ovarian function (ovulation), and in part, this regulation is estrogen dependent.

Vasotocin induces final oocyte maturation and ovulation through the production of a maturation-inducing steroid in the catfish Heteropneustes fossilis

The study reports for the first time vasotocin (VT) induction of final oocyte maturation and ovulation through the production of the maturation-inducing steroid 17, 20β-dihydroxy-4-pregnen-3-one (MIS, 17, 20β-DP). Post-vitellogenic follicles of the catfish Heteropneustes fossilis were incubated with different concentrations of VT (1, 10, 100 and 1000 nM) for different time periods. Germinal vesicle breakdown [GVBD, as a marker of final oocyte maturation (FOM)] and ovulation were scored. In another series of experiments, the follicles were incubated with VT alone or in combination with VT receptor (V(1) and V(2)) antagonists, and GVBD and ovulation were increased with progesterone, 17-hydroxy-4-pregnene-3, 20-dione (17-P) and 17, 20β-DP levels. VT stimulated both GVBD and ovulation in a concentration and time-dependent manner, and the responses were inhibited to varying degrees in groups incubated with the VT receptor antagonists. The V(1) antagonist inhibited the responses by 2- to 3-fold and more than the V(2) antagonist, and the combination was more potent than the separate incubation. Progestins increased time-dependently in the VT groups and the fold increase was greater for the MIS. The VT-induced steroid stimulation was significantly inhibited to near the control levels in co-incubations with both V(1) and V(2) receptor antagonists, in the order 17, 20β-DP > 17-P > P(4). The inhibition by the V(1) receptor antagonist was greater than that with the V(2) blocker, and followed the same order of inhibition described above. The results suggest that VT induces FOM and ovulation mainly through the V(1) receptors.

The roles of pH in regulation of uterine contraction in the laying hens

In the laying hens, the uterus (shell gland) plays essential roles in calcium transfer for calcification of the eggshell and expulsion of the egg through the vagina for oviposition. Much is known about the effects of pH changes on eggshell production of the uterus. However, very little is understood about the effects of pH changes on uterine contractility. We investigated the effects of pH changes on uterine contraction in the laying hens. The laying hens were humanely killed, and strips of uterine smooth muscles were isolated. Isometric force was measured and the effects of intracellular and extracellular pH changes studied. The results show that alterations of pH clearly have marked effects on force in the hen uterus. Both intracellular and extracellular acidifications significantly decreased uterine activity, whether it arises spontaneously or in the presence of agonists such as prostaglandin F(2alpha) and arachidonic acid. Alkalinization produced the opposite effects. Thus, changes in pH can regulate uterine contraction. This insight into pH regulation of the uterine activity provides a focus for egg production management directed at physiological and pathological oviposition in the laying hens.

Relative in vitro seasonal effects of vasotocin and isotocin on ovarian steroid hormone levels in the catfish Heteropneustes fossilis

In the present investigation, catfish (Heteropneustes fossilis) ovarian tissues were incubated in vitro with vasotocin (VT) or isotocin (IT) to demonstrate their effects on estradiol-17beta (E(2)), progesterone (P(4)), 17alpha-hydroxy-4-pregnene-3, 20-dione (17-P) and 17alpha, 20beta-hydroxy-4-pregnen-3-one (17, 20beta-DP). Parallel incubations with human chorionic gonadotropin (hCG) alone or in combination with VT were used for a comparison. In pre-vitellogenic phase (preparatory phase, GSI-0.48+/-0.03%), both VT and hCG stimulated E(2) significantly, VT in a biphasic manner and hCG in a dose-dependent manner. In pre-spawning (post-vitellogenic, GSI-9.05+/-0.11%) and spawning (post-vitellogenic, GSI-8.01+/-0.12%) phases, both hormones decreased E(2) levels in a dose- and duration-dependent manner; the VT effect being biphasic in the spawning phase. The co-incubation with VT+hCG stimulated E(2) in the preparatory phase but inhibited it in the pre-spawning and spawning phases. The incubations with VT or hCG increased P(4) levels in a dose- and duration-dependent manner, the magnitude of the effect was higher in the pre-spawning and spawning phases. The co-incubation with VT+hCG stimulated P(4) without any additive effect. The P(4) derivatives (17-P and 17, 20beta-DP) showed similar changes except 17-P in the spawning phase which decreased at 16h of the incubation. The incubations with IT produced similar but low responses. In conclusion, like hCG, VT has differential effects on ovarian steroidogenesis and may be involved directly or indirectly in ovarian functions, as a paracrine/autocrine factor or a neurohormone.

Effects of hCG and ovarian steroid hormones on vasotocin levels in the female catfish Heteropneustes fossilis

Effects of hCG, ovariectomy and estradiol replacement on brain, plasma and/or ovarian vasotocin in vivo, and estradiol, progesterone, 17alpha, 20beta-hydroxy-4-pregnen-3-one and hCG on ovarian vasotocin in vitro were investigated in the catfish. A 100IU/fish of hCG induced ovulation and elicited both periovulatory and post-ovulatory changes in vasotocin concentrations with a significant increase up to 8h in the brain and up to 16h in both plasma and ovary. After stripping the fish at 16h, the peptide concentration decreased significantly with time, up to 4 days. Ovariectomy in early pre-spawning phase resulted in a duration-dependent significant reduction of both brain and plasma vasotocin. Estradiol replacement in 3-week ovariectomized fish produced dosage-dependent biphasic effects: the lower dosage (0.1microg/g) restored the vasotocin level while the higher dosage (0.5microg/g) decreased it significantly below the control level. In vitro incubation of ovarian tissues with estradiol produced season-dependent effects on vasotocin. The incubation of pre-vitellogenic ovarian pieces with estradiol (1, 10, and 100ng/ml) elevated vasotocin level in a dose- and duration-dependent manner while that of post-vitellogenic follicles resulted in a significant decrease. The incubation of intact post-vitellogenic follicles or follicular envelope with progesterone and 17alpha, 20beta-hydroxy-4-pregnen-3-one (1microg/ml) or hCG (20IU/ml) for 8 and 16h significantly increased vasotocin in a duration-dependent manner. The results show that both gonadotropin and ovarian steroids modulate vasotocin titer, which may influence follicular growth, ovulation and spawning in the catfish.

Immunocytochemical localization, HPLC characterization, and seasonal dynamics of vasotocin in the brain, blood plasma and gonads of the catfish Heteropneustes fossilis

Immunocytochemical distribution and dynamics of vasotocin (VT) were studied in the air-breathing catfish Heteropneustes fossilis in relation to the reproductive cycle. Vasotocin was localized in the brain and ovary by streptavidin-biotin immunocytochemistry. The immunoreactivity was found throughout the hypothalamo-hypophysial neurosecretory system consisting of the magnocellular and parvocellular neurons of the nucleus preopticus, neurosecretory axonal tract and neurohypophysis (NH). The VT neurons showed seasonal changes; they were numerically less in resting phase but increased during the recrudescent phase. The neurons were hypertrophied and degranulated in pre-spawning phase and heavily degranulated and vacuolated in spawning phase. In the NH, the density of VT fibers increased up to the pre-spawning phase and decreased thereafter. In the ovary, VT immunoreactivity was noticed in the follicular layer and varied with the growth of the follicles. Vasotocin was characterized and quantified by a high performance liquid chromatography with UV detection method in the brain, plasma and ovary. Brain and plasma VT concentrations were also assayed with an EIA method, which was more sensitive than the HPLC method with values about 2-fold higher. Vasotocin levels showed significant seasonal and sexual differences with higher concentrations in females in the recrudescent (preparative, pre-spawning and spawning) phase. Brain VT recorded the highest concentration in the preparative phase (both sexes) while plasma (both sexes) and ovarian VT in the spawning phase. The ovarian concentration of VT was 15- and 25-fold higher in the pre-spawning and spawning phases (when expressed per mg protein), respectively, than plasma but lower than brain levels. In testis, VT concentration was relatively low and apparently did not show any significant seasonal variation. The seasonal activity patterns and gonadal distribution of VT indicate a reproductive function of the peptide.

The anatomy and physiology of the avian endocrine system

The endocrine system of birds is comparable to that of mammals, although there are many unique aspects to consider when studying the anatomy, physiology, and biochemistry. Avian endocrinology is a field of veterinary medicine that is unfamiliar to many practitioners; however, it is important to have a comprehensive understanding when evaluating companion birds in clinical practice. This article covers the anatomy and physiology of the normal avian, and readers are referred to other articles for a more detailed explanation of altered physiology and pathology.

The role of protein kinase A and cyclin-dependent (CDC2) kinase in the control of basal and IGF-II-induced proliferation and secretory activity of chicken ovarian cells

The aim of these experiments was to study the role of protein kinase A (PKA), cyclin-dependent kinase 2 (CDC2) and insulin-like growth factor II (IGF-II) in the control of ovarian function in domestic fowl, as well as the role of PKA and CDC2 in mediating the effects of IGF-II on the ovary. For this purpose, we studied the influence of an inhibitor of PKA (KT5720; 50 ng/ml), a CDC2 blocker (olomoucine; 1 microg/ml), IGF-II (0, 1, 10 or 100 ng/ml) and their combinations on cultured fragments of chicken ovarian follicular wall. Accumulation of PKA and CDC2 and secretion of progesterone (P4), testosterone (T), estradiol (E2) and arginine-vasotocin (AVT) were evaluated by using SDS-PAGE-Western blotting and RIA/EIA. IGF-II addition to culture medium stimulated T, E2 and AVT secretion and inhibited P4 secretion. These changes were associated with an increase in PKA and a decrease in CDC2 accumulation. The PKA blocker KT5720, when given alone, increased accumulation of PKA and secretion of T and E2, but not AVT and inhibited P4 secretion. The PKA blocker also prevented and even reversed the effects of IGF-II on PKA and steroid hormones secretion, but enhanced the action of IGF-II on AVT. The inhibitor of CDC2, olomoucine, when given alone, suppressed the expression of CDC2 and the secretion of P4 and AVT (but not T and E2). When given together with IGF-II, it augmented IGF-II-induced suppression of CDC2 and reversed the effects of IGF-II on P4 (but not on T, E2 or AVT). These observations demonstrate the involvement of PKA, CDC2 and IGF-II in regulating the secretory activity of avian ovarian cells. Our data also suggest the involvement of PKA in the mediation of IGF-II effects on P4, T, E2 and AVT secretion. CDC2 can mediate the effects of IGF-II on ovarian P4 secretion but not on other hormones.

Changes of AVT levels in plasma, neurohypophysis and hypothalamus in relation to oviposition in the laying hen

Arginine vasotocin (AVT) is a neurohypophysial hormone involved in the reproductive function in avian species. We measured AVT concentrations in the neurohypophysis and plasma in relation to indomethacin blocked oviposition and to prostaglandin (PG) E2-induced premature oviposition in the hen. In addition, AVT concentration in the hypothalamus was measured in relation to spontaneous oviposition. In the control group the concentration of AVT decreased in the neurohypophysis and increased in plasma at oviposition. In hens that were administrated indomethacin, oviposition was delayed for several hours and no changes in the levels of AVT in neurohypophysis and plasma were observed at the predicted time of oviposition. Premature oviposition was induced within 5 min after the administration of PGE2. The levels of AVT decreased in the neurohypophysis and increased in plasma at the PGE2-injected premature oviposition. The increases of AVT levels in paraventricular and supraoptic nuclei were observed 2 and 5 h after spontaneous oviposition. The results indicate that oviposition causes an increase in the release of AVT from neurohypophysis and suggest that the synthesis of hypothalamic AVT may be stimulated by oviposition.

Arginine vasotocin gene expression in neuroendocrine, reproductive and gastrointestinal tissues of the domestic fowl: detection by reverse transcriptase polymerase chain reaction

Arginine vasotocin gene transcripts in various tissues of the domestic fowl were detected by reverse transcriptase polymerase chain reaction followed by Southern blot analysis using a 209 bp fragment from the 3'-region of a cDNA encoding chicken arginine vasotocin as the probe. Relatively strong signals were observed with hypothalamic, adenohypophysial and proventricular RNA as the starting material. Lesser signals were obtained from RNA isolated from shell gland, adrenal gland, post-ovulatory follicles and ovarian thecal cells. Arginine vasotocin gene transcripts were undetectable in the posterior pituitary gland, small intestine and large intestine. These results suggest that in addition to its well-known antidiuretic and oxytocic actions, arginine vasotocin may act as a local neuromodulator or mediator and have other important autocrine or paracrine actions in non-hypothalamic tissues.

Arginine vasotocin gene expression and hormone synthesis during ontogeny of the chicken embryo and the newborn chick

Chicken embryos at different developmental stages (embryonal day (E) 6 to 21) and chicks at posthatch day 1 (D1) were monitored for the development of their hypothalamo-neurohypophysial system as indicated by the kinetics of arginine vasotocin (AVT) gene expression via mRNA concentration and brain AVT content. Our data concerning the onset of gene expression support previous results from our laboratory and others about an early activation of the AVT gene transcriptional and translational activity around E6. We could detect measurable amounts of AVT in chicken embryo brains at E6 and an exponential increase during further development until D1. Dot blots of hypothalamic RNA extracts indicated that AVT gene transcript concentrations rose between E12 and E17 and slightly dropped thereafter. Northern hybridization showed that this drop was caused by a decrease of full length message and an increase of smaller transcripts during late embryonal and D1 stages, probably an AVT mRNA specific degradation phenomenon. The dissociation between the increase of AVT concentration and AVT mRNA concentration visible at the D1 stage might be due to accumulation and storage of AVT in the magnocellular neurons, preferentially in their axon terminals in the neurohypophysis. Blood samples taken from E14 onwards revealed a constant increase in plasma osmolality and plasma AVT concentration. Our data suggest that, in the chicken, AVT seems to be required early during embryonal development, either for osmoregulatory or further unknown functions.

The chicken vasotocin gene

cDNA clones corresponding to the vasotocin precursor polypeptide were isolated from a chicken hypothalamic library and sequenced. The derived amino-acid sequence indicates a precursor of comparable structural organization to that described for members of the vasotocin/vasopressin gene family from other species. Unlike in mammals the C-terminal glycopeptide moiety appears not be cleaved off from the neurophysin. Subsequent screening of a chicken genomic library permitted an analysis also of the vasotocin gene structure and exonic composition. The 5'region upstream of the first exon was sequenced and revealed an unusual pattern of 49 repetitive -YYCYCYAAAYY- motifs, together with a polyadenyl region supporting a bend in the DNA, and a long pyrimidine-rich sequence. Three AP2-like elements, identified in the mammalian vasopressin gene, were also observed in the immediate upstream region. There was no obvious homology to the promoter regions of the known oxytocin genes, nor to any other sequence deposited in available databases, nor to other known cis-elements.

Arginine vasotocin gene expression during osmotic challenge in the chicken

The avian hypothalamic nonapeptide arginine vasotocin (AVT) is released from axon terminals in the neural lobe upon the application of osmotic stimuli. We have investigated whether, and to what extent, hormone secretion from the neurohypophysis is related to gene expression in the hypothalamus. Results from hybridization experiments with an AVT-specific cDNA probe indicate that in adult chickens stimulated by water deprivation or by hypertonic saline (2% w/v) drinking water, an upregulation of the AVT mRNA pool takes place, since consistently higher AVT mRNA levels compared to controls were monitored in osmotically challenged birds. This stimulatory effect was even visible at the transcriptional level after 19 h of water deprivation when osmolality was still near the basal value. In hens osmotically challenged by hypertonic saline drinking water for 5 days, a dissociation between osmolality and AVT plasma concentration was visible: extremely high plasma osmolality was accompanied by only moderately increased plasma AVT concentration. This might be caused either by exhaustion of stored hormone, or by downregulation of the system after chronic challenge. The latter suggestion is supported by the fact that the AVT mRNA concentration after 5 days of hypertonic saline challenge was well below the AVT mRNA levels of the groups with the more short-term stimuli of water deprivation for 19 or 48 h. In 30-day-old chicks the hypothalamic AVT mRNA concentration hardly reached 70% of the adult value, although AVT plasma concentrations were similar to those in the mature bird. We conclude that osmotic challenge of the hypothalamo-neurohypophysial system not only causes secretion of AVT from stores in the neural lobe but is accompanied by upregulation of AVT gene expression. Upregulation already occurs after marginal increase in plasma osmolality, as seen after 19 h of water deprivation in hens. In 30-day-old chicks gene expression is only slightly upregulated after short-term water deprivation while increase in plasma AVT is even greater compared to hens.

In vitro uterine contractions in the viviparous lizard Tiliqua rugosa: effects of gestation and steroid pretreatment in vivo

Uterine contractility was investigated in the viviparous lizard Tiliqua rugosa. Arginine vasotocin (AVT) induces rhythmic contractions in vitro in strips of uterine tissue from ovariectomized female T. rugosa. The strength of these contractions was related to the dosage of AVT and reduced by pretreatment in vivo with both progesterone and estradiol-17 beta. The frequency of spontaneous and AVT-induced contractions was enhanced by estradiol-17 beta pretreatment. The strength of AVT-induced contractions in pregnant females was not significantly different from that recorded in nonpregnant females. Spontaneous rhythmic contractions were present only in pregnant females. Ovariectomy did not affect either spontaneous or AVT-induced contractions in pregnant females. The data indicate that ovarian steroids modulate uterine contractility in T. rugosa. It is suggested that, following a decline in plasma progesterone levels, estrogen may be involved in the onset of parturition.