Prolactin as a luteotrophin during late pregnancy in pigs

In vitro effect of leptin on anterior pituitary cells LH secretory activity during early pregnancy in pig

Leptin modulates reproductive activity but its potential influence on LH secretion from anterior pituitary (AP) cells during implantation period in pigs (days 14-16 of pregnancy) remained unexplored. This study focused on determination whether leptin affects basal and GnRH-induced LH secretion and intracellular accumulation and whether leptin receptor (OB-Rb) mRNA is expressed in the AP gland during implantation in pigs. Four individual AP glands were developed into separate primary cultures. 2×105 cells/ml were preincubated (72 h) and next, for 3.5 h, experimentally treated with GnRH (100 ng/ml), leptin (10-11, 10-9, 10-7, 10-6 M) alone, or given in respective combinations with GnRH. In the AP gland, OB-Rb mRNA expression was determined by real-time PCR method. Leptin activated LH secretion and its concentration-dependent effect was observed as stimulation shown in a full range tested (culture 1) and exhibited only at 10-6 M (culture 2). A pooled data analysis revealed that basal LH secretion increased at 10-9, 10-7 and 10-6 M, but GnRH-induced LH release decreased at 10-6 M. Leptin down-regulated GnRH-induced LH secretion in all cultures, but only culture 3 exhibited sensitivity for all concentrations tested. Basal LH accumulation was activated in culture 1 (at 10-11 M) and inhibited in culture 4 (at 10-9 M). In the presence of GnRH leptin up-regulated LH accumulation with individual culture leptin-sensitivity (culture 1-3), while down-regulated LH accumulation in culture 4. Obtained data indicate that OB-Rb mRNA is expressed in the AP gland and leptin alone and in combination with GnRH specifically modulates LH activity during early pregnancy in pigs.

Growth Repression in Diethylstilbestrol/Dimethylbenz[a]anthracene–Induced Rat Mammary Gland Tumor Using Hecate-CGβ Conjugate

Recently, we have shown that Hecate-CGβ conjugate, which is a fusion of the lytic peptide Hecate and a 15–amino acid fragment of the β-chain of chorionic gonadotropin (CGβ), selectively destroys mammary gland carcinoma cells that possess luteinizing hormone receptors (LHR) in vitro. We induced mammary gland tumors using combined prenatal exposure to synthetic diethylstilbestrol (DES) and additional postnatal exposure to dimethylbenz[a]anthracene (DMBA). Rats with tumors were equally randomized (10/group) and treated with either sham (control) or 12 mg/kg body wt of either Hecate or Hecate-CGβ once a week for 3 weeks by tail vein injections. One week after the last injection, rats were kilted. Reverse-transcription–nested polymerase chain reaction/Southern blotting revealed alternatively spliced mRNA for LHR in tumor tissues of 5 of 30 females, which was further confirmed by Western blot analysis. The percentage of tumor volume increase was lowest in the group treated with Hecate-CGβ (45.3 ± 27.6), compared with Hecate- and shamtreated, control group (324.8 ± 78.1 and 309.9 ± 51.2, respectively; P < 0.001). Hecate-CGβ induced a significant decrease in tumor burden compared with controls (9.5 ± 2.1 mg/g body wt vs. 21.6 ± 2.9; P < 0.01). A smaller reduction in tumor burden was also observed in Hecate-treated females (17.6 ± 1.6 mg/g body wt vs. 21.6 ± 2.9; P < 0.05). Our results prove the principle that Hecate-CGβ conjugate is able to repress mammary gland tumor growth, even in tumor tissues that lack or have very low levels of LHR. The mechanism of Hecate-CGβ conjugate action in repression of DES/DMBA-induced tumor growth needs to be further analyzed to clarify the molecular mechanisms of Hecate-CGβ conjugate action in vivo.

Short Communication: Effect of silymarin (Silybum marianum) treatment on prolactin concentrations in cyclic sows

Loisel, F., Quesnel, H. and Farmer, C. 2013. Short Communication: Effect of silymarin (Silybum marianum) treatment on prolactin concentrations in cyclic sows. Can. J. Anim. Sci. 93: 227–230. An extract (silymarin) from the plant Silybum marianum (milk thistle) was shown to increase circulating concentrations of prolactin in cycling rats. A dose-response study was undertaken to determine if silymarin does have hyperprolactinemic properties in cycling swine. Forty-four weaned sows were allotted to four groups receiving 0, 1, 2 or 4 g d−1 of silymarin over a period of 8 d. Blood samples were obtained on days 1 (first day of treatment starting 24 to 48 h after the onset of the standing estrus), 2, 3, and 9. Prolactin, progesterone, estradiol-17β and leptin concentrations were determined. Silymarin did not increase prolactin concentrations at any of the doses studied, nor did it affect concentrations of the other hormones (P>0.1).

Hormone interactions confer specific proliferative and histomorphogenic responses in the porcine mammary gland

Mammary gland growth and morphogenesis are regulated by interactions between hormones as much as by their individual actions. The effect of these interactions on the mammary gland phenotype in species other than rodents is relatively undefined. We investigated the individual and combined effects of estrogen (E), progestin (P), and prolactin (PRL) on mammary gland development in gilts. Pigs were shown to have a ductal-lobular parenchyma that underwent hormone-stimulated progression of terminal ductal lobular unit (TDLU) morphogenesis similar to that in the human breast. Ovariectomy plus hypoprolactinemia abolished mammary gland growth. Estrogen alone stimulated mammary epithelial cell proliferation, terminal bud formation, and the progression of TDLU1 structures to a TDLU2 morphotype. Maximal epithelial cell proliferation, DNA content, parenchymal area, and morphological development of the porcine mammary gland were realized following treatment with E+PRL or E+P+PRL. In contrast, P alone did not promote epithelial cell proliferation, TDLU type progression, mammary gland growth, or morphogenesis. These data indicate that interactions between E and PRL are the main determinants of growth and morphogenesis in the porcine mammary gland.

Pregnancy-associated glycoprotein family (PAG)--as chorionic signaling ligands for gonadotropin receptors of cyclic animals

The chorionic pregnancy-associated glycoprotein (PAG) family was identified in pigs, cattle and other eutherian mammals. The objective of this study was to examine whether secretory chorionic proteins (including PAGs), produced in vitro by explants of porcine and bovine placental membranes, may interact with other proteins, i.e. gonadal and extragonadal binding sites. Trophoblast (TRF) and trophectoderm (TRD) explants of pigs (n=38; 14-61 dpc-day post coitum) or cotyledons (CT) of cows (n=5; 40-110 dpc) were long-term cultured. Released chorionic proteins were ultra-fractionated from media (>10 kDa) or precipitated [20-75% of (NH(4))(2)SO(4)]. The PAGs were monitored by Western/PAGE (30-73 kDa). Secretory TRF/TRD/CT (+PAG) proteins (0.78-25 microg/ligand) were examined by radioreceptor assay (RRA) with iodinated hCG ((125)I-hCG) for binding-effectiveness by gonadotropin receptors of cyclic pigs and cows (cRc). Gonadal and extragonadal cRc isolated from luteal-phase corpora lutea and uteri (cCLRc, cMYORc and cENDRc) were tested with positive control ligands: porcine LH and hCG (0.39-50 ng/ml). Control proteins produced in vitro by endometrial (END) explants of cyclic (cEND), pseudopregnant (PsEND) and pregnant (pEND) gilts were utilised as negative ligands (0.78-25 microg/ligand). Positive control ligands competed with (125)I-hCG for binding by cCLRc, cMYORc and cENDRc (18-61%/B(0) for hCG and 27-57%/B(0) for LH). Negative ligands (cEND, PsEND and pEND) did not show cRc bindings. This is the first RRA report indicating that in vitro produced porcine TRF/TRD proteins (+PAG) competed (P< or =0.05) with (125)I-hCG for binding by cCLRc, cMYORc and cENDRc in a concentration- and pregnancy stage-dependent manner. The highest competition with (125)I-hCG (up to P< or =0.001) was found for ultra-fractionated TRF/TRD proteins (>10 kDa) during early pregnancy (<22 dpc). The greatest competition (P< or =0.05) of precipitated porcine TRD proteins (>30 dpc) was detected for fractions obtained by saturation with use of 20% of (NH(4))(2)SO(4). Bovine CT proteins revealed lower competition of (125)I-hCG for bovine cCLRc (during 45 dpc only) that was more efficient with CT (up to 71%) than with non-labelled hCG (82%). The PAG proteins may play a role as potential "signal molecules", because they were able to interact with gonadotropin receptors of luteal-phase animals. It seems that the pPAG proteins may be luteoprotective chorionic-origin signals during implantation and placentation, according to binding-effectiveness of the chorionic ligands that was comparable to LH/hCG ligands with gonadal and extragonadal receptors of cyclic animals.

The effects of GnRH and adrenergic agents on PRL and beta-endorphin secretion by porcine pituitary cells in vitro

The direct effects of alpha- and beta-adrenergic agents on PRL and beta-endorphin (beta-END) secretion in vitro by porcine pituitary cells have been investigated. Pituitary glands were obtained from mature gilts, which were ovariectomised (OVX) one month before slaughter. Ovariectomised gilts, assigned to four groups, were primed with: (1) vehicle (OVX); (2) and (3) oestradiol benzoate (EB; 2.5 mg/100 kg b.w.) at 30-36 h (OVX+EB I) and 60-66 h (OVX+EB II) before slaughter, respectively; and (4) progesterone (P4; 120 mg/100 kg b.w.) for 5 consecutive days before slaughter (OVX+P4). Isolated anterior pituitary cells were submitted to 3.5 h incubation in the presence of GnRH, alpha- and beta-adrenergic agonists [phenylephrine (PHEN) and isoproterenol (ISOP), respectively], or alpha- and beta-adrenergic blockers [phentolamine (PHENT) and propranolol (PROP), respectively]. The culture media were assayed for PRL (exp. I) and beta-endorphin-like immunoreactivity (beta-END-LI) (experiment II). In experiment I, GnRH did not influence PRL release by pituitary cells in all experimental groups. Some of tested doses of adrenergic agonists, PHEN and ISOP, increased PRL release from pituitary cells of OVX gilts, but not from those of OVX+EB I animals. In the OVX+EB II group, PHEN alone, but ISOP with PROP, potentiated PRL secretion by the cells. In OVX+P4 animals, PHEN alone or in combination with PHENT and also ISOP alone or with PROP enhanced PRL output from the cells. In experiment II, addition of GnRH increased beta-END-LI release from pituitary cells only in the OVX+EB II group. PHEN and PHENT potentiated beta-END-LI secretion by pituitary cells in OVX+EB II and OVX+P4 groups, while ISOP and PROP increased beta-END-LI secretion by the cells of OVX and OVX+EB II animals. In turn, in the OVX+EB I group, effect of PHENT and PROP on PRL secretion by pituitary cells was inhibitory. In conclusion, our results suggest that adrenergic agents can modulate PRL and beta-END secretion by porcine pituitary cells in a manner dependent on the hormonal status of gilts.

Effect of a gonadotrophin-releasing hormone antagonist on luteinising hormone secretion and early pregnancy in gilts

The aims of the present study were: (1) to determine the duration of suppression of luteinising hormone (LH) following a single treatment with a gonadotrophin-releasing hormone (GnRH) antagonist (BIM-21009; Biomeasure) at a dose of 100 μg kg−1; (2) to block LH pulses only for certain days of pregnancy; and (3) to determine the period of early pregnancy most susceptible to suppression of LH. Three groups of gilts were injected with 100 μg kg−1 on Day 16 (n = 5), 14 (n = 6) or 19 (n = 4) of pregnancy. Blood for LH analysis was collected at 20-min intervals for 12 h on the day before treatment and during varying stages of early pregnancy. Blood for progesterone analysis was collected daily and development of pregnancy was followed using real-time ultrasound. Prior to treatment, gilts had 2.6 ± 0.7 LH pulses per 12 h. The GnRH antagonist abolished LH pulses for a period of 2.7 ± 1.8 days and, thereafter, suppressed the resumed LH pulses (P < 0.05). Pregnancy was disrupted in three pigs (20%) with a mean treatment-to-abortion period of 4.7 days concurrent with a mean treatment-to-progesterone decline interval of 4.3 days. In a proportion of pigs, short-term LH suppression may cause early disruption of pregnancy.

Early disruption of pregnancy as a manifestation of seasonal infertility in pigs

All gilts and sows in production from which the detailed production information was available in a 160-sow unit were included to the study. In winter-spring, there were complete data available from 47 animals and in summer-autumn from 64 animals. The farm had a consistent history of the seasonally reduced farrowing rate in summer-autumn. Success of inseminations was monitored during a 4-month breeding period in winter-spring and in summer-autumn. Each animal was bled twice a week for 6 weeks starting a day before insemination and the blood samples were assayed to determine serum progesterone concentration. The blood samples were also assayed for cortisol to detect any acute infectious response. Starting on day 18, animals were pregnancy tested by transcutaneous real time ultrasound twice a week. In winter-spring, the farrowing rate was 72% (58 inseminations, 1.2 inseminations/sow) and in summer-autumn 63% (81 inseminations, 1.3 inseminations/sow). In winter-spring, there was only one detected case of early disruption of pregnancy (EDP), whereas nine such cases were recognised in summer-autumn. Five out of those nine animals returned to oestrus with a mean insemination to oestrus interval of 25.8+/-1.6 days. One sow returned to oestrus 35 days after insemination and three sows did not return to oestrus within 45 days. However, two of these sows had progesterone profiles that indicated an undetected oestrus around day 25. In those nine animals, no acute phase infectious response as indicated by a rise in serum cortisol was evident. Serum progesterone concentrations in the animals eventually loosing the pregnancy tended to be lower on day 13 (no significant difference) and were significantly lower on day 20 when compared with animals remaining pregnant. There was no difference in serum progesterone levels of pregnant animals between winter-spring and summer-autumn. Litter size was not affected by the season. The weaning to oestrus interval tended to be longer in summer-autumn. This study showed that the seasonally decreased farrowing rate is partly caused by EDP. The lowered progesterone concentrations in summer-autumn were demonstrable only in "problem animals".

The effect of stimulators and blockers of adrenergic receptors on LH secretion and cyclic nucleotide (cAMP and cGMP) production by porcine pituitary cells in vitro

The direct effects of alpha- and beta-adrenergic agents on luteinizing hormone (LH) secretion in vitro by porcine pituitary cells and the participation of secondary messengers, adenosine 3'5'-monophosphate (cAMP) and guanosine 3'5'-monophospate (cGMP), in transduction of signals induced by adrenergic agents and gonadotropin-releasing hormone (GnRH) in these cells have been investigated. Pituitary glands were obtained from mature gilts, which were ovariectomized (OVX) 1 month before slaughter. OVX gilts, assigned to four groups, were primed with: (1) vehicle (OVX); (2 and 3) estradiol benzoate (EB; 2.5mg/100kg b.w.) at 30-36h (OVX+EB I) or 60-66h (OVX+EB II) before slaughter, respectively; (4) progesterone (P(4); 120mg/100kg b.w.) for 5 consecutive days before slaughter (OVX+P(4)). Anterior pituitaries were dispersed with trypsin and then pituitary cells were cultured (10(6) per well) in McCoy's 5a medium containing horse serum (10%) and fetal calf serum (2.5%) for 3 days, at 37 degrees C and under the atmosphere of 95% air and 5% CO(2). On day 4 of the culture, the cells were submitted to 3.5h incubation in the presence of GnRH (a positive control), alpha- and beta-adrenergic agonists (phenylephrine (PHEN) and isoproterenol (ISOP), respectively), and alpha- and beta-adrenergic blockers (phentolamine (PHENT) and propranolol (PROP), respectively). The culture media were assayed for LH (experiment I) and cyclic nucleotides (experiment II). In experiment I, addition of GnRH (100ng/ml) increased LH secretion by pituitary cells taken from gilts of all experimental groups. The effects of alpha- and beta-adrenergic agents on LH secretion by the cells depended on hormonal status of gilts. The LH secretion by pituitary cells of OVX gilts was potentiated in the presence of PHEN (10, 100nM, and 1microM) and PHENT (1microM), alone or in combination with PHEN (100nM) and by the cells derived from OVX+EB I and OVX+P(4) animals in response to PHEN (100nM) and ISOP (1microM). ISOP (1microM) also stimulated LH secretion by the cells taken from OVX+EB II gilts. In experiment II, GnRH (100ng/ml) increased cGMP production by pituitary cells obtained from all groups of gilts and cAMP secretion by the cells taken from OVX and OVX+P(4) animals. PHEN (100nM) decreased and PROP (1microM) enhanced cAMP production by pituitary cells derived from OVX+EB I and OVX gilts, respectively. Moreover, PHEN (100nM) reduced, while PHENT (1microM) stimulated the release of cGMP by pituitary cells taken from OVX+EB II animals. In turn, ISOP (100nM) decreased and increased cGMP production by the cells derived from OVX+EB II and OVX+P(4) gilts, respectively. PROP (1microM) potentiated cGMP accumulation by pituitary cells taken from OVX+EB I and OVX+P(4) animals. In conclusion, our results suggest that adrenergic agents can modulate LH release by porcine pituitary cells acting through guanyl and adenylyl cyclase and in a manner dependent on hormonal status of gilts.

Prolactin involvement in the regulation of the hypothalamic-pituitary-ovarian axis during the early luteal phase of the porcine estrous cycle

Our previous in vivo and in vitro studies revealed that prolactin (PRL) affected luteal function during the first days of the porcine estrous cycle. Since the mechanism by which the luteotrophic action of PRL might be mediated was not elucidated, the goal of the present study is to investigate the effects of short term, in vivo administration of PRL on in vitro functions of hypothalamic explants, adenohypophyseal cells and luteal cells of sows. Injections of PRL or saline (performed every 2h) started shortly after the preovulatory LH surge and lasted for 2 or 3 days. Peripheral blood plasma for determination of LH, PRL and progesterone (P(4)) was sampled at 4h intervals. Ovaries, pituitaries and the stalk median eminence (SME) dissected after slaughter were used for in vitro studies. Luteal and adenohypophysial cells as well as hypothalamic tissue were incubated/cultured with different treatments. Medium and plasma levels of GnRH, LH and P(4) were quantified by radioimmunoassays (RIAs). Corpora lutea (CL) were used for LH/human chorionic gonadotrophin (hCG) receptor analysis. In vivo and in vitro treatment with PRL increased the in vitro GnRH release by hypothalamic explants (P<0.05). GnRH-stimulated LH production was enhanced in PRL-treated sows compared to that of control sows (P<0.05). PRL injections had no effect on plasma P(4) concentrations during the treatment period. However, luteal secretion of P(4) (P=0.06) and LH/hCG receptor concentration (P=0.079) tended to be higher in PRL-treated sows in comparison to those of controls. The results indicate that PRL may be involved in the regulation of the hypothalamic-pituitary-ovarian axis at the beginning of the luteal phase of the porcine estrous cycle.

The influence of GnRH, oxytocin and vasoactive intestinal peptide on the secretion of beta-endorphin and production of cAMP and cGMP by porcine pituitary cells in vitro

The objective of this study was to determine whether gonadotrophin-releasing hormone (GnRH), oxytocin (OT) and vasoactive intestinal polypeptide (VIP) modulate beta-endorphin-like immunoreactivity (beta-END-LI) secretion by dispersed anterior pituitary cells of pigs and in vivo priming with steroid hormones, estradiol benzoate (EB) and progesterone (P(4)), influences the cell reactivity to peptide hormones tested. Additionally, the aim of this research was to examine the involvement of cyclic nucleotides (cAMP and cGMP) in transduction of signals induced by GnRH, OT and VIP in porcine pituitary cells. Pituitaries were collected from ovariectomized (OVX) gilts that were divided into four experimental groups. Animals of group 1 (OVX) received 1ml corn oil (placebo)/100 kg body weight (b.w.), group 2 (OVX+EB I) and group 3 (OVX+EB II) were treated with EB at the dose 2.5mg/100 kg b.w., 30-36 and 60-66 h before slaughter, respectively. Animals of group 4 (OVX+P(4)) were injected with P(4) at the dose 120 mg/100 kg b.w. for 5 subsequent days before slaughter. Anterior pituitaries were dispersed with trypsin and then pituitary cells were cultured (10(6) per well) in McCoy's 5A medium containing horse serum (10%) and fetal calf serum (2.5%) for 3 days at 37 degrees C under an atmosphere of 95% air and 5% CO(2). Subsequently, plates were rinsed with fresh McCoy's 5A medium and pituitary cells were treated with one of the following agents: GnRH (100 ng/ml), OT (10(-6)M) or VIP (10(-7)M) and incubated for 3.5h at 37 degrees C.GnRH did not affect beta-END-LI secretion by pituitary cells of OVX (group 1) and OVX+P(4) (group 4) gilts. When the pituitary cells were incubated in the presence of OT and VIP, significant increases were observed. After priming of OVX gilts with EB, 30-36 h before slaughter (group 2), we noted a significant increase in beta-END-LI release from pituitary cells only in the presence of VIP. Pituitary cells from gilts treated with EB, 60-66 h before slaughter (group 3), produced markedly elevated amounts of beta-END-LI after GnRH, OT or VIP addition.GnRH markedly stimulated cGMP release from cultured pituitary cells in all experimental groups and significantly increased cAMP production by the cells from OVX, OVX+EB II and OVX+P(4) animals. The addition of OT enhanced both cAMP and cGMP output in all experimental groups of pigs. VIP stimulated cAMP release from pituitary cells derived from OVX, OVX+EB I and OVX+EB II animals. cGMP output was markedly elevated under the influence of VIP from pituitary cells of OVX, OVX+EB II and OVX+P(4) gilts. In conclusion, our results suggest that GnRH, OT and VIP can modulate beta-endorphin release from porcine pituitary cells and imply the involvement of cAMP and cGMP in transduction of signals induced by studied peptides in the cells.

Role of prolactin in regulating the onset of winter fur growth in mink (Mustela vison): A reconsideration

The objectives of this study were to determine: (1) if the onset of winter hair growth (anagen) in mink could be delayed or inhibited by elevating endogenous PRL concentrations; (2) if bilaterally adrenalectomy (ADX)-induced winter anagen occurs concomitantly with a reduction in serum PRL concentrations, and (3) if exogenous dehydroepiandrosterone (DHEA), an adrenal steroid or Delta(5)-DIOL (a peripherally produced metabolite of DHEA), would delay or inhibit the onset of winter anagen. During early July, while in the resting (telogen) stage of the hair growth cycle, mink were treated with slow release implants containing haloperidol (HAL, a dopaminergic antagonist), melatonin (MEL), deoxycorticosterone (DOC), DHEA and Delta(5)-DIOL. In addition, mink were ADX'd and supplemented with DOC and DHEA. MEL reduced PRL levels to basal levels and induced winter anagen 7 weeks earlier than controls. Surprisingly, HAL initiated winter anagen 7 weeks earlier than controls (P < 0.05), although serum PRL levels were not different between the two groups. Mink that were ADX'd or ADX + DHEA-treated exhibited winter anagen 6 weeks earlier than controls (P < 0.05), but serum PRL concentrations were not different between the three groups. The administration of DHEA or Delta(5)-DIOL to mink with intact adrenals had no effect on the time of onset of winter anagen or serum PRL levels. Our findings suggest that a reduction in circulating PRL levels is not essential for onset of winter anagen in the mink and that the apparent inhibitory effects of the adrenal glands on initiation of winter anagen is not mediated through DHEA or its metabolite Delta(5)-DIOL. J. Exp. Zool. 284:437-444, 1999.

Does prolactin mediate induced nest-building behaviour in pseudopregnant gilts treated with PGF2alpha?

Nest-building behaviour occurs 6-24 h before parturition in pigs (gestation=116 days). Pseudopregnancy in pigs (induced with oestradiol valerate injections) lasts 50-80 days. We have shown that prostaglandin F2alpha (PG) administration on day 47 of pseudopregnancy induces nest-building and changes to plasma prolactin, oxytocin, cortisol and progesterone similar to those seen before normal parturition. Peripheral prolactin has been proposed as a modulator of nest-building. This study assessed nest-building behaviour in prolactin-deprived gilts. Jugular vein catheters were inserted on day 39 of pseudopregnancy and blood samples collected daily from days 40-48. Animals were injected im with either 40 mg bromocriptine in 2 ml 70% ethanol (n=8) or vehicle (n=7) at 17.00 h on day 46 and 09.00 h on day 47 of pseudopregnancy. PG (15 mg Lutalyse: Upjohn) was injected im at 11.00 h on day 47. Blood and behavioural samples were taken from 90 min before PG to 6 h post-PG. Plasma prolactin increased in control but not bromocriptine treated animals following PG (P<0.05). Elevations in oxytocin, cortisol and progesterone (P<0.05) above pre-PG concentrations were also seen, but of these only progesterone showed between group differences [greater (P<0.05) in control gilts on both days 47 and 48]. PG significantly (P<0.05) increased both the rate and proportion of total time spent performing straw/floor-directed behaviours not including foraging (an index of nesting behaviour) in both treatment groups with no significant differences between groups. There were also no significant differences between groups in time spent performing pen fixture directed activities before or after PG. Bromocriptine suppressed the rise in prolactin concentrations after PG without suppressing nest-building behaviour. We conclude that peripheral prolactin is not an essential component of the nest-building complex in pigs.

In vivo investigation of luteal function in dogs: effects of cabergoline, a dopamine agonist, and prolactin on progesterone secretion during mid-pregnancy and -diestrus

The role of prolactin on luteal function in dogs was investigated in vivo. The function of prolactin in mid-luteal phase was compared in pregnant and nonpregnant dogs. A dopamine agonist, cabergoline, known for its prolactin secretion inhibitory effects, was injected subcutaneously at a dose of 5 micrograms/kg body weight in five pregnant and five nonpregnant Beagle bitches. Mean plasma prolactin and progesterone were dramatically suppressed for 4 to 5 days after injection in both groups when compared with control pregnant and non-pregnant animals, whereas no effect on luteinizing hormone (LH) secretion was observed. The decline in plasma progesterone occurred after that in prolactin, suggesting plasma progesterone was impaired by inhibition of prolactin secretion. These results confirm the luteotropic importance of prolactin in pregnant bitches, and also demonstrate its importance in luteal phase of the nonpregnant dog. Second, to demonstrate that the effects of cabergoline were mediated by prolactin inhibition and not by a direct action on the corpus luteum, concomitant administration on Day 30 of cabergoline and prolactin (375 micrograms i.v. twice daily on Days 30 and 31) or cabergoline and LH (750 micrograms i.v. twice daily on Days 30 and 31) was affected in two groups of five pregnant animals each. Results showed that only prolactin was able to reverse the negative effects of cabergoline on circulating progesterone. This confirms the indirect mode of action of the dopamine agonist, cabergoline on corpus luteum function. Third, further investigation on the precise luteotropic role of prolactin was made by IV injection of 375 micrograms pure canine prolactin twice daily in five pregnant bitches on Days 30 and 31, and in five pregnant bitches on Days 40 and 41. No direct stimulatory effect of prolactin on plasma progesterone secretion occurred. Nor was there a noticeable effect on plasma LH secretion. These results suggest that prolactin is unable to directly stimulate progesterone secretion by the corpus luteum of pregnancy. The results of this study suggest that prolactin is an essential luteotropin in the dog from mid-luteal phase in both pregnant and nonpregnant animals. However, it appears to act by sustaining corpus luteum lifespan and function rather than by direct stimulatory effects on progesterone secretion.