Pertussis toxin blocks the inhibitory effects of somatostatin on cAMP-dependent vasoactive intestinal peptide and cAMP-independent thyrotropin releasing hormone-stimulated prolactin secretion of GH3 cells

It was shown that somatostatin (SRIF) inhibited cAMP-dependent vasoactive intestinal peptide (VIP)-stimulated prolactin (PRL) release by a GH3 clonal strain of rat pituitary tumor cells and decreased basal PRL secretion and inhibited PRL release in response to thyrotropin releasing hormone (TRH) whose action was independent of prior synthesis of cAMP. Pretreatment of these cells with pertussis toxin prevented SRIF's inhibitory effects on basal and TRH-stimulated hormone secretion as well as its VIP-stimulated responses. The blockade of SRIF's inhibitory effect on the actions of TRH or VIP was dependent on both the duration of preincubation and concentration of the toxin and was correlated with the ability of the toxin to catalyze the ADP-ribosylation of the 39,000-Da membrane protein. It is likely that this pertussis toxin substrate is involved in signal transduction of SRIF on cAMP-dependent actions of VIP and cAMP-independent action of TRH. However, the mechanism of SRIF's action on TRH is not clear, since SRIF did not affect the intracellular responses by TRH, neither intracellular Ca2+ mobilization nor the increase of 1,2-diacylglycerol formation following the breakdown of polyphosphoinositides.     

Modulatory role of substance P on gonadotropin and prolactin secretion in the rabbit.

Substance P (SP) is present in large quantities in the brainstem and hypophysiotropic areas of the brain, but its roles in gonadotropin and prolactin secretion are controversial. The aim of this study was to measure luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin (PRL) release from the pituitary after either intracerebroventricular (ICV) injection or infusion of SP or its C- and N-terminal fragments in intact (INT) and ovariectomized (OVX) conscious rabbits. A single injection of SP into the 3rd cerebral ventricle (3CVT) in INT and OVX rabbits augmented plasma LH concentrations, especially when SP was applied during the initial phase of an LH peak. Injection of SP during the declining phase of LH release was not effective. Injection of SP into the 3CVT was followed by increased plasma PRL concentrations in OVX but not in INT rabbits. Both SP 1-11 and SP 1-7 failed to alter LH, FSH, and PRL secretion when the peptides were slowly infused into the 3CVT, although ICV infusion of SP 6-11 did cause a delayed increase in LH release. The results support a stimulatory role of SP on LH and prolactin release. The results further indicate that although the stimulatory effect of SP on LH is ovarian steroid-independent, in the absence of ovarian steroids, SP is stimulatory only during the rising phase of an LH pulse. A dual role of SP-ergic transmission in modulating LH secretion is discussed.     

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

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

Ovariectomized Sprague-Dawley and Long-Evans rats release prolactin differently in response to estrogen

Mature female Sprague-Dawley (SD) and Long-Evans (LE) rats were ovariectomized (OVX), fitted with indwelling atrial catheters and given a single sc injection of either 25 or 100 μg polyestradiol phosphate (PEP); seven days later blood samples were withdrawn at two hour intervals from 1100 to 2100 hours to detect the presence of an afternoon surge of prolactin (PRL). Other groups of OVX rats of both strains also treated with PEP and catheterized as above were sampled before and at 2, 5, 10 and 30 min after iv administration of 1 μg synthetic thyrotropin releasing hormone (TRH). Pituitary (AP) and uterine weights were determined following sacrifice one day after TRH treatment. Separate groups of OVX rats of both strains treated with 100 μg PEP were decapitated 7 days later and each AP was removed and homogenized. The AP homogenates and plasma samples were assayed for PRL by radioimmunoassay. Rats of both strains had afternoon PRL surges and in both strains the magnitude and/or duration of the surges were enhanced by the higher dose of PEP. However, within each PEP dose LE rats released significantly more PRL during the surge than did SD rats. Rats of both strains also released PRL in response to TRH and this response was enhanced in both strains by the higher of the two doses of PEP. However, there were no differences between the strains at 25 μg PEP and at 100 μg PEP SD rats released significantly more PRL to TRH than did LE rats. Pituitary weight and PRL concentration were not different between the strains at either dose of PEP but LE rats had significantly heavier uteri at both doses of PEP compared to SD rats. These data not only show that strain differences exist in estrogen-induced or mediated PRL release in the rat but also indicate that the differences are not uniform. This latter observation suggests that the estrogen-induced mechanisms examined in this study are for the most part independent of each other.     

The effects of dopaminergic antagonism by sulpiride on TRH and VIP-induced prolactin release in nonsuckled lactating rats

Prolactin (PRL) release was studied in female rats during midlactation using pharmacologic manipulations designed to mimic the hypothalamic effects of suckling. In the first experiment pituitary dopamine (DA) receptors were blocked by sulpiride (10 micrograms/rat i.v.). One hour later, thyrotropin-releasing hormone (TRH, 1.0 micrograms/rat i.v.) was given to induce PRL release. TRH released significantly more PRL following DA antagonism than when no DA antagonism was produced, suggesting that DA receptor blockade increased the sensitivity of the AP to TRH. In a second experiment, VIP (25 micrograms/rat) increased plasma prolactin 3-4 fold but this effect was not enhanced significantly by prior dopamine antagonism with sulpiride. We conclude that dopamine antagonism enhances the PRL releasing effect of TRH but not VIP in lactating rats.     

Stimulatory effect of peptide histidine isoleucine amide 1-27 on prolactin release in the rat

Intravenous injection of pure peptide histidine isoleucine amide 1-27 (PHI) resulted in a prompt and significant increase of plasma prolactin (PRL) in conscious freely-moving male rats. Using a perifusion system of rat anterior pituitary tissues in vitro, effluent PRL levels were also increased by 10(-8)-10(-7) M PHI. A PRL releasing potency of PHI was almost similar with that of vasoactive intestinal polypeptide (VIP) or TRH both in vivo and in vitro. Coupled with the recent immunocytochemical studies showing the dense network of PHI immunoreactive fibers around the hypophysial portal vessels, PHI might be another candidate for PRL releasing factor.     

VIP enhances TRH-stimulated prolactin secretion of pituitary tumours. Studies with 31P NMR

Intravenous thyrotrophin releasing hormone (TRH) caused a 6.5-fold increase in plasma prolactin (PRL) in rats carrying implanted pituitary tumours. Vasoactive intestinal polypeptide (VIP) had no effect, but TRH given after VIP raised TRH stimulated secretion 13-fold above basal. 31P NMR spectroscopy showed that VIP caused a decrease in high energy metabolites (depleted phosphocreatine, elevated inorganic phosphate and lowered intracellular pH). TRH alone caused a similar but smaller effect; given after VIP, it caused no detectable depletion. We suggest that the changes in high energy metabolite concentrations reflect increased cellular energy consumption consistent with a priming process (stage 1) in PRL secretion, followed by hormone release (stage 2). VIP induces stage 1 whereas RTH induced both stages.     

The influence of vasoactive intestinal polypeptide and cholecystokinin of prolactin release in rat and human monolayer cultures

We have evaluated the effects of the gut-brain peptides, VIP and CCK, on pituitary PRL secretion in monolayer cultures of normal and tumor bearing rodent and human pituitary tissue. In cultures prepared with normal human pituitary tissue obtained from three patients with metastatic breast cancer, VIP at 10^?7M and 10^?9M (but not 10^?11M) significantly (p<.05) increased PRL secretion in the wells by 6 hrs. Similar concentrations of VIP also significantly (p<.05) promoted PRL release from pituitary tissue obtained by transphenoidal hypophysectomy from one of two prolactinoma patients. Dopamine (10^?5M) inhibition of PRL secretion was not affected by 10^?11 to 10^?7M VIP. In contrast to these findings VIP did not significantly influence 6 hr rat PRL release in monolayer cultures of normal or transformed cells (GH₃) with or without the addition of bacitracin (10^?5M).CCK₃₃ significantly (p<.01) increased rat PRL release in human pituitary monolayer cultures at 10^?5M. The more biologically potent CCK₈ significantly (p<.02) increased rat PRL release at a 10-fold lower concentration, 10^?6M. In contrast, CCK₈ 10^?8 to 10^?6M, did not significantly influence PRL release from normal human pituitary cultures or from tumor bearing human (prolactinoma) and rat (GH₃) cultures. We conclude that 1) the gut-brain peptides, VIP and CCK, can directly stimulate pituitary PRL release and 2) VIP may be a physiologic prolactin releasing factor in man.     

The mechanisms by which vasoactive intestinal peptide (VIP) and thyrotropin releasing hormone (TRH) stimulate prolactin release from pituitary cells

The effect of vasoactive intestinal peptide (VIP) on prolactin (PRL) secretion from pituitary cells is reviewed and compared to the effect of thyrotropin releasing hormone (TRH). These two peptides induced different secretion profiles from parafused lactotrophs in culture. TRH was found to increase PRL secretion within 4 s and induced a biphasic secretion pattern, while VIP induced a monophasic secretion pattern after a lag time of 45–60 s.The secretion profiles are compared to changes in adenylate cyclase activity, production of inositol polyphosphates, changes in intracellular calcium concentrations and changes in electrophysiological properties of the cell membrane.Abbreviations AC adenylate cyclase - DG diacyglycerol - GH growth hormone - GTP guanosine trisphosphate - G_i GTP binding proteins that mediate inhibition of adenylate cyclase and that are pertussis toxin sensitive - G_s GTP binding protein that mediates stimulation of adenylate cyclase - GH cells clonal rat pituitary tumor cells producing PRL and/or growth hormone - GH₃ GH₄C₁ and GH₄B6 subclones of GH cells - PKA protein kinase A - PKC protein kinase C - PLC phospholipase C - PRL prolactin - TPA 12-O-tetradecanoyl phorbol 13-acetate - TRH thyrotropin releasing hormone - VIP vasoactive intestinal peptide     

Prostaglandin D2 stimulates vasoactive intestinal polypeptide release into rat hypophysial portal blood

The effect of prostaglandin D2 (PGD2) on vasoactive intestinal polypeptide (VIP) release from the hypothalamus was examined by determining plasma VIP levels in rat hypophysial portal blood. Intraventricular injection of PGD2 (5 micrograms/rat) caused a 3-fold increase in the concentration of plasma VIP in hypophysial portal blood in anesthetized rats. A PGD2 metabolite, 13,14-dihydro-15-keto PGD2, did not affect VIP levels in portal blood. The flow rate of hypophysial portal blood was not changed after the injection of PGD2. The intraventricular injection of PGD2, but not PGD2 metabolite, resulted in an increase in peripheral plasma prolactin (PRL) levels in the rat. These findings suggest that PGD2 plays a stimulatory role in regulating VIP release from the hypothalamus into hypophysial portal blood and causes PRL secretion from the pituitary in rats.     

Influence of VIP on prolactinemia in turkey anterior pituitary cells: role of cAMP second messenger in VIP-induced prolactin gene expression

Vasoactive intestinal peptide (VIP) is the avian prolactin (PRL)-releasing factor. In the turkey, hypothalamic VIP immunoreactivity and mRNA content, as well as VIP levels in hypophyseal portal blood, are closely related to the state of prolactinemia and the reproductive stage. The present study investigated the role of VIP on prolactinemia in turkey anterior pituitary (AP) cells through PRL gene expression and the role of a cAMP second messenger system on VIP-induced PRL expression. In primary AP cells harvested from hens in different prolactinemic states, steady state promoter activities were positively correlated with secreted PRL levels. VIP increased PRL promoter activities in AP cells from hens with intermediate PRL levels (laying), but not in AP cells from hypoprolactinemic hens (nonphotostimulated reproductively quiescent). However, in AP cells from hyperprolactinemic hens (incubating), PRL promoter activity was down-regulated by VIP. PRL mRNA steady state levels were significantly decreased by the cAMP analogue, 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), and PRL secretion was down-regulated by the phosphodiesterase blocker, 3-isobutyl-1-methylxanthine (IBMX) in a dose-dependent manner, suggesting that the cAMP second messenger system might be involved in the inhibitory action of dopamine upon VIP-stimulated PRL secretion and gene expression at the pituitary level. In a study of VIP immediate and long-term effects on c-fos expression in relation to PRL expression, VIP dramatically induced c-fos mRNA expression within 5 min, suggesting that VIP-induced c-fos expression might be involved in VIP-stimulated PRL secretion and gene expression. These results provide additional evidence of the functional significance of VIP in PRL gene expression and suggest that changes in PRL promoter activity by VIP may be one of the important inductive mechanisms leading to prolactinemia.     

Effects of ovariectomy or force feeding on the plasma concentrations of prolactin and luteinizing hormone in incubating turkey hens

The effect of ovariectomy (OVX) on plasma concentrations of prolactin (PRL) and luteinizing hormone (LH) in incubating turkey hens was studied. Neither the sham-operated nor the OVX hens exhibited any change in the pattern of incubation behavior as a result of the surgery. Plasma concentrations of estradiol decreased to less than approximately 3 pg/ml by 2 days after surgery in the OVX hens. There were no significant differences in plasma levels of PRL between the sham-operated and OVX hens throughout the study. The concentration of PRL did not change in either the sham-operated or OVX hens and was maintained at high levels after surgery and during incubation of the eggs. By 2 days after hens were placed into cages, plasma levels of PRL significantly decreased and were maintained at low levels in both groups. The concentration of LH did not change in either group during the two wk after surgery when the hens were incubating eggs. After the hens were placed into cages, the concentration of LH increased in the OVX hens and was maintained at significantly higher levels than in the sham-operated hens. By contrast, the concentration of LH increased within 4 days after OVX of out-of-lay but nonincubating hens. The delay in the postcastration increase in plasma level of LH in the OVX hens was not associated with anorexia of incubating hens, since plasma levels of LH were not affected by force-feeding unless plasma levels of PRI were suppressed by nest deprivation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Systemic oxytocin induces a prolactin secretory rhythm via the pelvic nerve in ovariectomized rats

We have shown previously that an intravenous injection of oxytocin (OT) in ovariectomized (OVX) rats initiates a circadian rhythm of prolactin (PRL) secretion similar to that observed after cervical stimulation (CS). In this study, we investigated the pathway through which OT triggers the PRL rhythm. We first tested whether an intracerebroventricular injection of OT could trigger the PRL secretory rhythm. As it did not, we injected OT intravenously while an OT receptor antagonist was infused intravenously. This antagonist completely abolished the PRL surges, suggesting that a peripheral target of OT is necessary for triggering the PRL rhythm. We hypothesized that OT may induce PRL release, which would be transported into the brain and trigger the rhythm. In agreement with this, OT injection increased circulating PRL by 5 min. To test whether this acute increase in PRL release would induce the PRL rhythm, we compared the effect of intravenously administered thyrotropin-releasing hormone (TRH) and OT. Although TRH injection also increased PRL to a comparable level after 5 min, only OT-injected animals expressed the PRL secretory rhythm. Motivated by prior findings that bilateral resection of the pelvic nerve blocks CS-induced pseudopregnancy and OT-induced facilitation of lordosis, we then hypothesized that the OT signal may be transmitted through the pelvic nerve. In fact, OT injection failed to induce a PRL secretory rhythm in pelvic-neurectomized animals, suggesting that the integrity of the pelvic nerve is necessary for the systemic OT induction of the PRL secretory rhythm in OVX rats.     

Integrin expression and integrin-mediated adhesion in vitro of human multipotent stromal cells (MSCs) to endothelial cells from various blood vessels

Estradiol (E2) stimulates not only secretion of prolactin (PRL) and proliferation of PRL-producing cells (PRL cells) in the anterior pituitary, but also the expression of growth factors. In insulin-like growth factor-I (IGF-I) knockout (KO) mice, the number of PRL cells is decreased and administration of IGF-I does not increase either the number of PRL cells or plasma PRL levels, indicating that IGF-I plays a pivotal role in PRL cells. The effect of E2 on PRL cells in KO mice was investigated by immunohistochemistry and real-time RT-PCR. The number of PRL cells in KO mice was significantly lower than in the wild-type (WT) control mice. E2 increased the PRL mRNA in WT and KO mice; however, an increase of PRL mRNA in KO was less than that in WT. In addition, no vasoactive intestinal peptide (VIP)-immunoreactive cells were found in KO mice, therefore IGF-I is essential for VIP expression. To investigate the roles of IGF-I on PRL cells in the postnatal development, double-immunostaining with PRL and BrdU was performed in WT and KO mice from days 5–20. The percentages of PRL cells and BrdU-labeled cells in the anterior pituitary of KO mice were lower than in WT mice. Thus, IGF-I may be responsible for proliferation and differentiation of PRL cells in this postnatal period. Differentiation and the proliferation of PRL cells are controlled by IGF-I during the postnatal development, and IGF may be a mediator of E2 action through VIP induction in PRL cells of adults.     

Lunch induces an increase in the plasma prolactin concentration, but not vasoactive intestinal peptides or cholecystokinin

Ingestion of lunch is known to be associated with acute release of prolactin (PRL). The neuroendocrine mechanism of this release was examined by measuring changes in serum vasoactive intestinal peptides (VIP) and cholecystokinin (CCK) after the noon meal in six normal men. The serum PRL concentration was significantly increased (40% above the level before lunch) from 30 min to 1 h 45 min after beginning to eat. However, the ingestion of lunch had no remarkable effects on the plasma immunoreactive concentration of VIP and CCK. Thus the changes in the plasma concentration of these two gut-brain peptides did not coincided with acute PRL release after ingestion of lunch, suggesting either that these two gut-brain peptides are probably not involved in PRL release after lunch, or that the level of these two gut-brain peptides in the general circulation may not represent that in the hypophyseal portal plasma.     

Effect of opioid peptides and potassium on the release of vasoactive intestinal polypeptide and thyrotropin releasing hormone from perifused rat hypothalami

Opioid peptides have been demonstrated to stimulate prolactin secretion, and it has been postulated that this is mediated, at least in part, by an effect on hypothalamic prolactin releasing and release-inhibiting factors and neurotransmitters. The aim of this study was to investigate the effect of opioid peptides and depolarizing concentrations of K+ on the release of both vasoactive intestinal polypeptide (VIP) and thyrotropin releasing hormone (TRH) from perifused rat hypothalami. Both met-enkephalin and beta-endorphin stimulated the release of VIP significantly whilst not affecting the release of TRH. In addition, leu-enkephalin was found to have no effect on the release of either VIP or TRH. In contrast, depolarizing concentrations of K+ (50 mM) were found to cause the immediate release of TRH, but not VIP, from the same perifusion. The results suggest a role for VIP, but not TRH, in opioid peptide stimulated release of prolactin. In addition, the data indicates that a substance may be released in response to K+ depolarization which is inhibitory to the release of VIP.     

Effect of serotonin depletion by p-chlorophenylalanine on serum prolactin levels in estrogen-treated ovariectomized rats: insights concerning the serotoninergic, dopaminergic and opioid systems.

The stimulatory effect of serotonin on prolactin secretion is well documented, and the administration of an inhibitor of serotonin synthesis (p-chlorophenylalanine - pCPA) has the expected inhibitory action on prolactin release in most experimental situations. However, there is evidence that in certain physiological or experimental conditions, activation of the serotoninergic system can also determine inhibition of prolactin secretion. The aim of the present study was to investigate the ability of estrogen to modify the effect of pCPA on prolactin secretion and to evaluate the participation of opioid and/or dopaminergic systems in regulating pCPA-induced prolactin secretion in estradiol-treated rats. We observed that pCPA administration (200 mg/kg/day, s.c., 2 days) to ovariectomized (OVX) female rats treated with estradiol benzoate (300 microg/week for 2 weeks, or 50 microg/week for 4 weeks, s.c.) causes a significant increase in serum prolactin, whereas no effect is observed in intact rats or in OVX rats without treatment. Bromocriptine administration completely reversed prolactin values previously increased by estradiol and by pCPA [OVX rats + estradiol = 86.50 ng/ml (68.90-175.02), OVX + estradiol + pCPA = 211.30 ng/ml (142.03-311.00), OVX + estradiol + pCPA + bromocriptine = 29.35 ng/ml (23.01 - 48.74), p<0.05. Naloxone administration partially reduced estrogen-induced high prolactin concentrations, but did not affect prolactin secretion stimulation determined by pCPA. Overall, the data from this report confirm the involvement of the dopaminergic system and, to a lesser degree, of endogenous opioids in prolactin secretion stimulation determined by estradiol. Furthermore, our results suggest that the stimulatory action of pCPA on prolactin secretion in estradiol-treated OVX rats is mediated by serotonin, which may also act indirectly on dopamine neurons.     

Influence of adrenergic antagonists and apomorphine on prolactin release induced by serotonergic antagonists in the monkey.

The influence of adrenergic receptor blockers on the prolactin releasing effect of methysergide and cyproheptadine was examined in sexually mature female monkeys under ketamine anesthesia. Propranolol, a β-adrenergic blocker, at a dose of 1 mg/kg did not alter the prolactin releasing action of 0.1 mg/kg of methysergide but significantly potentiated (P < 0.025) the prolactin releasing action of 0.5 mg/kg of cyproheptadine. Phentolamine and phenoxybenzamine, both α-adrenergic blockers, at 1 mg/kg blunted the prolactin releasing effect of methysergide and cyproheptadine, but the pattern of prolactin blockade was different between the two putative antiserotonergic drugs. The prior administration of apomorphine, 4 mg/kg, a dopamine receptor stimulator, blocked the prolactin releasing effect of methysergide and cyproheptadine. Evidence presented here and from the literature indicate that the prolactin releasing action of methysergide and cyproheptadine is mediated by an antidopaminergic action directly on the pituitary.     

The influences of GnRH, oxytocin and vasoactive intestinal peptide on LH and PRL secretion by porcine pituitary cells in vitro.

The aim of the present study was to evaluate the possible direct effects of GnRH, oxytocin (OT) and vasoactive intestinal peptide (VIP) on the release of LH and PRL by dispersed porcine anterior pituitary cells. Pituitary glands were obtained from mature gilts, which were ovariectomized (OVX) one month before slaughter. Gilts randomly assigned to one of the four groups were treated: in Group 1 (n = 8) with 1 ml/100 kg b.w. corn oil (placebo); in Group 2 (n = 8) and Group 3 (n = 8) with estradiol benzoate (EB) at the dose 2.5 mg/100 kg b.w., respectively, 30-36 h and 60-66 h before slaughter; and in Group 4 (n = 9) with progesterone (P4) at the dose 120 mg/ 100 kg b.w. for five consecutive days before slaughter. In gilts of Group 2 and Group 3 treatments with EB have induced the negative and positive feedback in LH secretion, respectively. Isolated anterior pituitary cells (10(6)/well) were cultured in McCoy's 5a medium with horse serum and fetal calf serum for 3 days at 37 degrees C under the atmosphere of 95% air and 5% CO2. Subsequently, the culture plates were rinsed with fresh McCoy's 5A medium and the cells were incubated for 3.5 h at 37 degrees C in the same medium containing one of the following agents: GnRH (100 ng/ml), OT (10-1000 nM) or VIP (1-100 nM). The addition of GnRH to cultured pituitary cells resulted in marked increases in LH release (p < 0.001) in all experimental groups. In the presence of OT and VIP we noted significant increases (p < 0.001) in LH secretion by pituitary cells derived from gilts representing the positive feedback phase (Group 3). In contrast, OT and VIP were without any effect on LH release in Group 1 (placebo) and Group 2 (the negative feedback). Pituitary cells obtained from OVX gilts primed with P4 produced significantly higher amounts (p < 0.001) of LH only after an addition of 100 nM OT. Neuropeptide GnRH did not affect PRL secretion by pituitary cells obtained from gilts of all experimental groups. Oxytocin also failed to alter PRL secretion in Group 1 and Group 2. However, pituitary cells from animals primed with EB 60-66 h before slaughter and P4 produced markedly increased amounts of PRL in the presence of OT. Neuropeptide VIP stimulated PRL release from pituitary cells of OVX gilts primed with EB (Groups 2 and 3) or P4. In contrast, in OVX gilts primed with placebo, VIP was without any effect on PRL secretion. In conclusion, the results of our in vitro studies confirmed the stimulatory effect of GnRH on LH secretion by porcine pituitary cells and also suggest a participation of OT and VIP in modulation of LH and PRL secretion at the pituitary level in a way dependent on hormonal status of animals.