J.L. Silsby

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

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

Isolation of replication-competent molecular clones of visna virus

Visna virus is the prototypic member of a subfamily of retroviruses responsible for slow infections of animals and humans. As a part of our investigation of the functions of viral gene products in virus replication, we have isolated three infectious molecular clones and determined the complete nucleotide sequences of two of the clones. We have also characterized the progeny of the biologically cloned viral stocks and of the infectious clones and document considerable heterogeneity in plaque size and antigenic phenotype of the former that is reduced to near homogeneity in the progeny of the infectious clones. It thus should now be possible to trace the emergence of antigenic variants of visna virus as well as ascribe defined functions to structural and regulatory genes of the virus in determining neurovirulence and the slow tempo of infection.

Effects of estrogen and progesterone on serum prolactin and luteinizing hormone levels in ovariectomized turkeys (Meleagris gallopavo)

Serum prolactin (PRL) and luteinizing hormone (LH) levels from mature female turkeys were determined following ovariectomy and daily subcutaneous injections for 4 or 9 days of 0.002-2.0 mg/kg estradiol benzoate (EB), 0.05-2.0 mg/kg progesterone (P), or their combinations. Serum PRL levels were increased (P less than 0.05) at the onset of sexual maturity in intact controls, but not in ovariectomized turkeys. Injection of 0.02 mg/kg EB into ovariectomized turkeys resulted in elevated (P less than 0.05) serum PRL levels after 8 treatment days. Higher doses failed to elicit a response of greater magnitude. EB at doses of 0.02-2.0 mg/kg reduced LH levels while 0.002 mg/kg EB increased LH levels above (P less than 0.05) those of ovariectomized controls. The 1.0-mg/kg P dose increased (P less than 0.05) serum PRL within 24-48 hr of administration while the 0.05-, 0.5-, or 2.0-mg/kg P doses failed to elicit a response. When P was injected in doses of 0.5-2.0 mg/kg, LH levels were decreased in a dose-response manner. The injection of EB combined with P had a variable effect on serum PRL levels. The 0.5- or 1.0-mg/kg dose of P facilitated (advanced in time) the rise in serum PRL level induced by 0.02 mg/kg of EB. In contrast, the positive feedback effect of 0.2 mg/kg of EB was blocked when administered with 0.5 mg/kg of P. Serum LH levels were dramatically decreased by all steroid combinations used. These results indicate that daily injections of EB and/or P in ovariectomized turkeys have a variable effect on serum PRL and LH levels depending upon the dose, the duration of treatment, or the ratio between EB and P.

SERUM PROLACTIN LEVELS OF TURKEY HENS IN RELATION TO REPRODUCTIVE FUNCTION

Publisher Summary This chapter discusses the serum prolactin levels of turkey hens in relation to reproductive function. Serum prolactin levels of somatically mature, sexually immature photosensitive female turkeys, housed in a 6L: 18D photoperiod are low. In different experiments with birds of varying history mean levels of prolactin have varied from less than 5 ng/ml to as high as 20 ng/ml. The data were obtained from 8 large white strain turkeys. The hens had been held on a 6L:18D photoperiod and were changed to 15L: 9D on the first day of the experiment. Blood was drawn from the birds at 1300–1400 h each day. It appears from the data that the drop in feed consumption follows the behavioral manifestation of broodiness. The time course of changes in feed consumption and serum prolactin suggest that the drop in feed consumption is not a casual factor in increasing the serum prolactin. The composite data suggest that the prolactin levels increase before the increase in nesting. Most birds sampled follow this general pattern, however, one did not. This individual bird showed the behavioral manifestations of broodiness, the concomitant drop in feed consumption, and cessation of egg production but her serum prolactin level did not increase until after the onset of broodiness.

Involvement of serotonin in prolactin release induced by electrical stimulation of the hypothalamus of the turkey (Meleagris gallopavo)

In anesthetized female turkeys electrical stimulation of the ventromedial nucleus (VMN) for 30 min caused increases in plasma prolactin (Prl); with maximum increase above the prestimulation level being 33.6 +/- 5.7 ng/ml for laying hens and 768.1 +/- 187.6 ng/ml for incubating hens. The possibility that serotonin (5-HT) plays a role in electrical stimulation-induced Prl release was investigated after administration of methysergide, a 5-HT receptor blocker (20 mg/kg), and stimulation in either the VMN or the infundibular nuclear complex-median eminence (INF-ME) region. Electrical stimulation in both the VMN and INF-ME region caused increases (P less than 0.05) in plasma Prl. Pretreatment with methysergide prevented the increase in plasma Prl that follows electrical stimulation in the VMN but had no effect on electrical stimulation-induced Prl release in the INF-ME region. We conclude that Prl release in the female turkey requires the functional integrity of serotonergic neurons within the VMN.

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

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

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

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

Involvement of dopamine in prolactin release induced by electrical stimulation of the hypothalamus of the female turkey (Meleagris gallopavo)

Controversy exists regarding the role of dopamine (DA) in the regulation of avian prolactin (PRL) secretion. Consequently, we injected apomorphine, a DA agonist, and pimozide, a DA receptor blocker, into laying and nest-deprived incubating turkeys and studied their effect on PRL secretion before (-20, -10, 0 min), during (5, 10, 20, 30 min), and after (5, 15, 30 min) electrical stimulation in the ventromedial nucleus of the hypothalamus. Apomorphine (10 mg/kg, ip) completely abolished the electrical stimulation-induced PRL increase in both laying and nest-deprived incubating hens. Pimozide (2 mg/kg, ip) potentiated electrical stimulation-induced PRL secretion in laying hens. In the two pimozide experiments, peak responses were 10.9-fold for the pimozide-treated group vs 2.9-fold for the control group, and 5.4-fold for the pimozide-treated group vs 2.6-fold for the control group. In nest-deprived incubating hens, PRL response to electrical stimulation was unaffected by pimozide treatment. These data support the concept that DA is inhibitory to the neuroendocrine system which stimulates PRL secretion in laying hens. In incubating hens, the dopaminergic inhibition is diminished, allowing for the increased PRL level observed during incubation.

The influence of acute or repeated immobilization on plasma prolactin levels in the turkey (Meleagris gallopavo)

Three experiments were conducted to evaluate the effect of immobilization on plasma prolactin (PRL) levels in the immature turkey. Acute immobilization for 120 min resulted in elevated PRL levels (P less than 0.05) starting 15 min after the onset of immobilization. Release from immobilization caused PRL levels to return to those of the nonimmobilized controls by 120-180 min following replacement of the turkeys in cages. Repeated immobilization for 3 or 5 consecutive days diminished PRL response (P less than 0.05) to subsequent immobilization. It is suggested that the PRL controlling mechanism(s) of the young turkey is both susceptible and able to habituate to changes induced by immobilization stress.

Characterization of dissimilar steroid productions by granulosa, theca interna and theca externa cells during follicular maturation in the turkey (Meleagris gallopavo).

Unlike the established models for steroidogenesis in the rat and human, we have previously demonstrated that in the turkey progesterone (P), androgen (A), and estrodiol (E) are primarily produced by the granulosa, theca interna, and theca externa cells, respectively. In the present study, experiments were conducted to further characterize steroid productions by these cell types during follicular maturation. In Experiments 1 and 2, granulosa cells and theca internal cells, respectively, from the larger (F1) and fifth largest (F5) preovulatory follicles were incubated (1 x 10(5) cells/ml) with ovine luteinizing hormone (oLH) or porcine follicle stimulating hormone (pFSH) for 5 hr. Granulosa production of P from both follicles was stimulated in response to oLH, with both basal and LH-stimulated P production greater in the larger F1 than in the smaller F5 follicle. No A or E production was detected in granulosa cells from either follicle size tested. Theca interna cell productions of P and A were stimulated by oLH in the smaller F5, but not in the larger F1, with both basal and stimulated levels of A greater in F5 than in F1. Medium content of E was non-detectable in cultures of theca interna cells from all follicles tested. In Experiment 3, theca externa cells were incubated (1 x 10(6) cells/ml) from F1 and F5 follicles. The theca interna cells from F5 and the seventh largest follicle (F7) were pooled (1 x 10(5) cells/ml) to provide substrate(s) for theca externa steroidogenesis. Theca externa cells were incubated alone and in combination with the pooled theca interna cells and with or without oLH or pFSH.(ABSTRACT TRUNCATED AT 250 WORDS)