Interaction between salsolinol (SAL) and thyrotropin-releasing hormone (TRH) or dopamine (DA) on the secretion of prolactin in ruminants

We have recently demonstrated that salsolinol (SAL), a dopamine (DA)-derived compound, is present in the posterior pituitary gland and is able to stimulate the release of prolactin (PRL) in ruminants. The aim of the present study was to clarify the effect that the interaction of SAL with thyrotropin-releasing hormone (TRH) or DA has on the secretion of PRL in ruminants. A single intravenous (i.v.) injection of SAL (5mg/kg body weight (b.w.)), TRH (1microg/kg b.w.), and SAL plus TRH significantly stimulated the release of PRL in goats (P<0.05). The cumulative response curve (area under the curve: AUC) during 120min was 1.53 and 1.47 times greater after the injection of SAL plus TRH than either SAL or TRH alone, respectively (P<0.05). A single i.v. injection of sulpiride (a DA receptor antagonist, 0.1mg/kg b.w.), sulpiride plus SAL (5mg/kg b.w.), and sulpiride plus TRH (1microg/kg b.w.) significantly stimulated the release of PRL in goats (P<0.05). The AUC of PRL during 120min was 2.12 and 1.78 times greater after the injection of sulpiride plus TRH than either sulpiride alone or sulpiride plus SAL, respectively (P<0.05). In cultured bovine anterior pituitary (AP) cells, SAL (10(-6)M), TRH (10(-8)M), and SAL plus TRH significantly increased the release of PRL (P<0.05), but the additive effect of SAL and TRH detected in vivo was not observed in vitro. In contrast, DA (10(-6)M) inhibited the TRH-, as well as SAL-induced PRL release in vitro. All together, these results clearly show that SAL can stimulate the release of PRL in ruminants. Furthermore, they also demonstrate that the additive effect of SAL and TRH on the release of PRL detected in vivo may not be mediated at the level of the AP, but that DA can overcome their releasing activity both in vivo and in vitro, confirming the dominant role of DA in the inhibitory regulation of PRL secretion in ruminants.     

Bovine Posterior Pituitary Extract Stimulates Prolactin Release from the Anterior Pituitary Gland In Vitro and In Vivo in Cattle

It has been reported that the posterior pituitary (PP) gland contains a potent, unknown prolactin (PRL)-releasing factor (PRF) in rats. PRFs are assumed to be produced in neurones located within the hypothalamus, and to be peptidergic in nature. However, little is known about PRFs in domestic animals. To characterize the PRF in the PP of domestic animals, the present study examined the PRL-releasing activity of an acidic extract from bovine PP (bPP) in vitro and in vivo in cattle. First, the PRL-releasing effect of bPP extract was compared with that of PRL-releasing peptide (PrRP), and thyrotropin-releasing hormone (TRH) from cultured bovine anterior pituitary cells. The extract significantly increased PRL concentrations in the culture medium, at doses of 0.002 and 0.02 eq./ml (one eq. is the PP extract from one animal), compared with the control (p < 0.05). PrRP failed to stimulate the release of PRL. TRH significantly increased PRL concentrations in the culture medium, at doses from 10(-9) to 10(-7) M, compared with the control (p < 0.05). The rate of increase in the PRL concentration, by 0.02 eq./ml bPP extract, was significantly greater than that in TRH (p < 0.05). Secondly, plasma PRL responses to the intravenous (i.v.) injection of bPP extract (0.5 eq./head), PrRP [3.59 mug/kg body weight (BW)], TRH (1 mug/kg BW), and a dopamine receptor antagonist (sulpiride, 0.1 mg/kg BW), were examined in calves. PrRP failed to stimulate PRL release; however, plasma PRL increased immediately following the injection of bPP extract, TRH and sulpiride. The PRL-releasing effect of i.v. injections of TRH and sulpiride was more potent than that of bPP extract. Finally, plasma PRL responses to the intra-hypothalamic injection of bPP extract were examined in calves. The intra-hypothalamic infusion (arcuate nucleus) of 0.0625 eq./head of bPP extract strongly stimulated PRL release in calves (p < 0.05). The present results show that PP contains a physiologically potent PRF in cattle.     

Characteristics of prolactin-releasing response to salsolinol (SAL) and thyrotropin-releasing hormone (TRH) in ruminants

The secretion of prolactin (PRL) is stimulated by thyrotropin-releasing hormone (TRH), and inhibited by dopamine (DA). However, we have recently demonstrated that salsolinol (SAL), a DA-derived endogenous compound, is able to stimulate the release of PRL in ruminants. The aims of the present study were to compare the characteristics of the PRL-releasing response to SAL and TRH, and examine the relation between the effects that SAL and DA exert on the secretion of PRL in ruminants in vivo and in vitro. Three consecutive intravenous (i.v.) injections of SAL (5 mg/kg body weight (b.w.): 19.2 μmol/kg b.w.) or TRH (1 μg/kg b.w.: 2.8 nmol/kg b.w.) at 2-h intervals increased plasma PRL levels after each injection in goats (P < 0.05); however, the responses to SAL were different from those to TRH. There were no significant differences in each peak value between the groups. The rate of decrease in PRL levels following the peak was attenuated in SAL-treated compare to TRH-treated animals (P < 0.05). PRL-releasing responses to SAL were similar to those to sulpiride (a DA receptor antagonist, 0.1 mg/kg b.w.: 293.3 nmol/kg b.w.). In cultured bovine anterior pituitary (AP) cells, TRH (10⁻⁸ M) significantly increased the release of PRL following both 15- and 30-min incubation periods (P < 0.05), but SAL (10⁻⁶ M) did not increase the release during the same periods. DA (10⁻⁶ M) completely blocked the TRH-induced release of PRL for a 2-h incubation period in the AP cells (P < 0.05). Sulpiride (10⁻⁶ M) reversed this inhibitory effect but SAL (10⁻⁶ M) did not have any influence on the action of DA. These results show that the mechanism(s) by which SAL releases PRL is different from the mechanism of action of TRH. Furthermore, they also show that the secretion of PRL is under the inhibitory control of DA, and SAL does not antagonize the DA receptor's action.     

Characteristics of prolactin-releasing response to salsolinol in vivo in cattle

The aims of the present study were to clarify the effect of salsolinol (SAL), a dopamine (DA)-derived endogenous compound, on the secretion of prolactin (PRL) in cattle. The experiments were performed from April to June using calves and cows. A single intravenous (i.v.) injection of SAL (5 mg/kg body weight [BW]) or sulpiride (a DA receptor antagonist, 0.1 mg/kg BW) significantly stimulated the release of PRL in male and female calves (P < 0.05), though the response to SAL was smaller than that to sulpiride. The secretory pattern of PRL in response to SAL or sulpiride in female calves resembled that in male calves. A single i.v. injection of SAL or sulpiride significantly stimulated the release of PRL in cows (P < 0.05). There was no significant difference in the PRL-releasing response between the SAL- and sulpiride-injected groups in cows. A single intracerebroventricular injection of SAL (10 mg/head) also significantly stimulated the release of PRL in castrated calves (P < 0.05). These results show that SAL is involved in the regulatory process for the secretion of PRL, not only in male and female calves, but also in cows. The results also suggest that the potency of the PRL-releasing response to SAL differs with the physiological status of cattle.     

Bromocriptine inhibits salsolinol‐induced prolactin release in male goats

The secretion of prolactin (PRL) is under the dominant and tonic inhibitory control of dopamine (DA); however, we have recently found that salsolinol (SAL), an endogenous DA‐derived compound, strongly stimulated the release of PRL in ruminants. The aim of the present study was to clarify the inhibitory effect of DA on the SAL‐induced release of PRL in ruminants. The experiments were performed from late June to early July. Male goats were given a single intravenous (i.v.) injection of SAL (5 mg/kg body weight (BW)), a DA receptor antagonist (sulpiride, 0.1 mg/kg BW), or thyrotropin‐releasing hormone (TRH, 1 µg/kg BW) before and after treatment with a DA receptor agonist (bromocriptine), and the effect of DA on SAL‐induced PRL release was compared to that on sulpiride‐ or TRH‐induced release. Bromocriptine completely inhibited the SAL‐induced release of PRL (P < 0.05), and the area under the response curve (AUC) for a 120‐min period after the treatment with bromocriptine was 1/28 of that for before the treatment (P < 0.05). Bromocriptine also completely inhibited the sulpiride‐induced release (P < 0.05). The AUC post‐treatment was 1/17 that of pre‐treatment with bromocriptine (P < 0.05). Bromocriptine also inhibited the TRH‐induced release (P < 0.05), though not completely. The AUC post‐treatment was 1/3.8 that of pre‐treatment (P < 0.05). These results indicate that DA inhibits the SAL‐induced release of PRL in male goats, and suggest that SAL and DA are involved in regulating the secretion of PRL. They also suggest that in terms of the regulatory process for the secretion of PRL, SAL resembles sulpiride but differs from TRH.     

Effects of photoperiod on salsolinol‐induced prolactin secretion in goats

The aim of the present study was to clarify the relation between salsolinol (SAL)‐induced prolactin (PRL) release and photoperiod in goats. A single intravenous (i.v.) injection of SAL was given to adult female goats under short (8 h light, 16 h dark) or long (16 h light, 8 h dark) photoperiod conditions at two different ambient temperatures (20°C or 5°C), and the PRL‐releasing response to SAL was compared to that of thyrotropin‐releasing hormone (TRH) or a dopamine (DA) receptor antagonist, sulpiride. SAL, as well as TRH or sulpiride, stimulated the release of PRL promptly after each injection in both 8‐ and 16‐h daily photoperiods at 20°C (P < 0.05). The area under the response curve (AUC) of PRL for the 60‐min period after injections of saline (controls), SAL, TRH and sulpiride in the 16‐h daily photoperiod group was greater than each corresponding value in the 8‐h daily photoperiod group (P < 0.05). There were no significant differences in the AUC of PRL among the values produced after the injection of SAL, TRH and sulpiride in 16‐h daily photoperiod group; however, the values produced after the injection of TRH were smallest among the three in the 8‐h daily photoperiod group (P < 0.05). The PRL‐releasing responses to SAL, TRH and sulpiride under a short and long photoperiod condition at 5°C resembled those at 20°C. These results show that a long photoperiod highly enhances the PRL‐releasing response to SAL as well as TRH or sulpiride in either medium or low ambient temperature in goats.     

Effects of melatonin on salsolinol‐induced prolactin secretion in goats

The aim of the present study was to clarify the effect of melatonin (MEL) on the salsolinol (SAL)‐induced release of prolactin (PRL) in goats. Female goats were kept at 20°C with 16 h of light, 8 h of darkness, and orally administered saline or MEL for 5 weeks. A single intravenous (i.v.) injection of saline (controls), SAL, thyrotropin‐releasing hormone (TRH) or a dopamine receptor antagonist, sulpiride, was given to the goats 3 weeks after the first oral administrations of saline or MEL, and the responses were compared. The mean basal plasma PRL concentrations in the control group were higher for the saline treatments than MEL treatments (P < 0.05). SAL as well as TRH and sulpiride stimulated the release of PRL promptly after each injection in both the saline‐ and MEL‐treated groups (P < 0.05). The area under the response curve of PRL for the 60‐min period after the i.v. injection of SAL, TRH and sulpiride in the saline‐treated group was greater than each corresponding value in the MEL‐treated group (P < 0.05). These results show that daily exposure to MEL under a long day length reduces the PRL‐releasing response to SAL as well as TRH and sulpiride in goats.     

Effects of hypothalamic dopamine (DA) on salsolinol (SAL)‐induced prolactin (PRL) secretion in male goats

The aim of the present study was to clarify the effects of hypothalamic dopamine (DA) on salsolinol (SAL)‐induced prolactin (PRL) release in goats. The PRL‐releasing response to an intravenous (i.v.) injection of SAL was examined after treatment with augmentation of central DA using carbidopa (carbi) and L‐dopa in male goats under 8‐h (8 h light, 16 h dark) or 16‐h (16 h light, 8 h dark) photoperiod conditions. The carbi and L‐dopa treatments reduced basal PRL concentrations in the 16‐h photoperiod group (P < 0.05), while a reduction was not observed in the 8‐h photoperiod group. The mean basal plasma PRL concentration in the control group for the 8‐h photoperiod was lower than that for the 16‐h photoperiod (P < 0.05). SAL significantly stimulated the release of PRL promptly after the injection in both the 8‐ and 16‐h photoperiod groups (P < 0.05). PRL‐releasing responses for the 16‐h photoperiod were greater than those for the 8‐h photoperiod (P < 0.05). The carbi and L‐dopa treatments blunted SAL‐induced PRL release in both the 8‐ and 16‐h photoperiods (P < 0.05). These results indicate that hypothalamic DA blunts the SAL‐induced release of PRL in male goats, regardless of the photoperiod, which suggests that both SAL and DA are involved in regulating the secretion of PRL in goats.     

Effects of serotonin injected into the third ventricle on prolactin and growth hormone secretion in Holstein steers

In order to clarify the role of serotonin (5-HT) in the regulation of pituitary hormones, the effects of 5-HT injected into the third ventricle (3V) on prolactin (PRL) and growth hormone (GH) release were investigated in Holstein steers. A chronic cannula was implanted in 3V by stereotaxic surgery under general anesthesia. After sufficient recovery from surgery, 5-HT (0, 0.1, 1.0, 2.0 mg) was injected into via the cannula and blood samples were collected over 4 h. Plasma PRL and GH concentrations were determined by radioimmunoassay. PRL release was significantly stimulated by the injection of 5-HT. The increase in PRL was observed at 20 min after the injection at three doses and the highest dose (2.0 mg) was the most effective in stimulating PRL release. The injection of 5-HT into 3V, at all doses tested, did not alter GH release significantly. Our results suggest that 5-HT is involved in the regulation of PRL release partly through the hypothalamus in cattle.     

Luteinizing hormone in the bovine pars tuberalis: secretion in response to luteinizing hormone releasing hormone and intracellular isoforms

The objective of this study was to examine the physiological characteristics of gonadotropes in the bovine (b) pars tuberalis as assessed by their ability to release Luteinizing Hormone (LH) in response to LH-Releasing Hormone (LHRH) and the intracellular distribution of LH isoforms. At slaughter, the stalk median eminence and associated pars tuberalis as well as the anterior pituitary gland were collected from each of 7 castrate males. Each stalk median eminence and pituitary gland was mid-sagitally sectioned and weighed. One half of each tissue was immediately frozen and subsequently homogenized to determine the intracellular distribution of bLH isoforms. Tissue extracts were desalted by flow dialysis against water and chromatofocused on pH 10.5-7.0 gradients. The remaining half of the pituitary was sliced with a Staddie-Riggs slicer. The pituitary slices and the remaining half of the stalk median eminence were perifused (0.1 ml/min) for a total of 360 min with effluent samples (1.0 ml) collected every 10 min. At 130 min tissues were stimulated with 5 x 10(-8) M LHRH. Concentrations of LH in the effluent samples and the fractions collected from chromatofocusing were determined by radioimmunoassay. The release of LH in response to LHRH was 43.9% and 47.0% above basal secretion for the pars tuberalis and pituitary, respectively, suggesting similar degrees of responsiveness. Pars tuberalis and pituitary extracts resolved into nine LH isoforms during chromatofocusing and were coded with letters beginning with the most basic form. No differences (P greater than .05) were observed in distribution of LH isoforms between the pars tuberalis and the pituitary gland.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of intracerebroventricular injections of prolactin‐releasing peptide on prolactin release and stress‐related responses in steers

Some evidence suggests that there might be a species difference in the effect of intracerebroventricularly administered (ICV) prolactin‐releasing peptide (PrRP) between rodents and sheep. We compared the levels of cortisol (CORT) and prolactin (PRL), rectal temperature (RT) and behavioral responses to ICV bovine PrRP (bPrRP) in steers. ICV bPrRP (0.2, 2 and 20 nmol/200 µL) tended to evoke a dose‐related increase in CORT concentrations and 0.2 nmol of bPrRP induced transient increase in PRL concentrations. A significant time–treatment interaction was observed for the percent change of CORT (P < 0.05) and PRL (P < 0.05) from pre‐injection value. The time–treatment interaction for changes in RT was not significant (P = 0.50). There tended to be a difference among the four treatments in terms of maximum change in RT from the pre‐injection value between 0 and 90 min (P < 0.1). Stress‐related behavioral signs were not observed in the present experiment. These findings indicate that ICV bPrRP increased CORT and PRL levels, suggesting that central PrRP might participate in controlling the hypothalamo‐pituitary‐adrenal axis and PRL release in cattle, unlike sheep. In contrast, central PrRP is unlikely to be involved in controlling the behavior of this species because ICV bPrRP did not induce marked changes in their behavior.     

Kisspeptin‐10 stimulates the release of luteinizing hormone and testosterone in pre‐ and post‐pubertal male goats

The aims of the present study were to clarify the effect of kisspeptin‐10 (Kp10) on the secretion of luteinizing hormone (LH) and testosterone (T) in pre‐pubertal and post‐pubertal male ruminants. Four male goats (Shiba goats) were given an intravenous (i.v.) injection of Kp10 (5 µg/kg body weight (b.w.)), gonadotoropin‐releasing hormone (GnRH, 1 µg/kg b.w.), or 2 mL of saline as a control at the ages of 3 (pre‐pubertal) and 6 (post‐pubertal) months. A single i.v. injection of Kp10 significantly stimulated the release of LH and T in both groups. The area under the response curve (AUC) of LH for a 60‐min period after the i.v. injection of Kp10 was significantly greater in the pre‐pubertal goats (P < 0.05). The AUC of T for a 120 min period post‐injection did not differ between the two age groups. A single i.v. injection of GnRH also significantly stimulated the release of LH and T in both groups (P < 0.05). The secretory pattern of LH and T in response to GnRH resembled that in response to Kp10. These results show that the LH‐releasing response to Kp10 is greater in pre‐pubertal than post‐pubertal male goats. They also show that Kp10, as well as GnRH, is able to stimulate the release of T in male goats.     

Effects of leptin on the release of luteinizing hormone, growth hormone and prolactin from cultured bovine anterior pituitary cells

The effects of leptin on the release of luteinizing hormone (LH), growth hormone (GH) and prolactin (PRL) were studied in cultured bovine anterior pituitary (AP) cells in vitro. The AP cells were obtained from fully‐fed Japanese Black steers and were incubated for 3 h with 10^?13 to 10^?7 mol/L of leptin after incubating in Dulbecco's modified Eagle's Medium for 3 days. Leptin significantly increased the concentration of LH in the culture medium by 45 and 44% at doses of 10^?8 and 10^?7 mol/L, respectively, compared with the controls (P < 0.05). Leptin significantly increased the concentration of GH in the culture medium by 14 and 12% at doses of 10^?8 and 10^?7 mol/L, respectively (P < 0.05). Leptin also significantly increased the concentration of PRL in the culture medium by 26% compared with the controls at a dose of 10^?7 mol/L (P < 0.05). These results show that leptin stimulates the release of LH, GH and PRL by acting directly on bovine AP cells from fully‐fed steers.     

Secretion of dopamine and norepinephrine in hypophyseal portal blood and prolactin in peripheral blood of Holstein cattle

The objective was to test the hypothesis that dopamine regulates prolactin (PRL) secretion by determining acute changes in catecholamine concentrations in hypophyseal portal blood of cattle, and their relation to peripheral blood concentration of PRL in hypophyseal stalk-transected (HST) and sham-operated controls (SOC). Holstein heifers (606 +/- 21 kg BW; mean +/- SE) were subjected to neurosurgery for 8 h to collect hypophyseal portal blood with a stainless steel cannula designed with a cuff placed under the pituitary stalk and peripheral blood via a jugular vein catheter. PRL plasma concentration was measured by radioimmunoassay, and dopamine and norepinephrine in portal plasma by radioenzymatic assay. During anesthesia before HST or SOC, PRL plasma concentration ranged from 20-40 ng/ml throughout 255 min. PRL abruptly increased and remained above 90 ng/ml after HST compared with a steady decrease to <20 ng/ml in SOC heifers throughout 440 min. Within 5 min after severing the hypophyseal stalk, dopamine in portal blood (>8 ng/ml) was significantly increased (P < 0.05) compared with peripheral blood (<2 ng/ml). Norepinephrine concentration in portal blood was significantly greater (P < 0.05) than in peripheral blood during the first 60 min. The sustained high PRL level in peripheral plasma after severing the hypophyseal stalk stimulated hypothalamic dopamine secretion from hypophyseal portal vessels during the prolonged period of blood collection. Norepinephrine concentration in these cattle was greater in hypophyseal portal than in peripheral blood, implicating both an important hypothalamic source of the catecholamine as well as an adrenal gland contribution during anesthesia.     

Effects of hypothalamic dopamine on growth hormone‐releasing hormone‐induced growth hormone secretion and thyrotropin‐releasing hormone‐induced prolactin secretion in goats

The aim of the present study was to clarify the effects of hypothalamic dopamine (DA) on the secretion of growth hormone (GH) in goats. The GH‐releasing response to an intravenous (i.v.) injection of GH‐releasing hormone (GHRH, 0.25 μg/kg body weight (BW)) was examined after treatments to augment central DA using carbidopa (carbi, 1 mg/kg BW) and L‐dopa (1 mg/kg BW) in male and female goats under a 16‐h photoperiod (16 h light, 8 h dark) condition. GHRH significantly and rapidly stimulated the release of GH after its i.v. administration to goats (P < 0.05). The carbi and L‐dopa treatments completely suppressed GH‐releasing responses to GHRH in both male and female goats (P < 0.05). The prolactin (PRL)‐releasing response to an i.v. injection of thyrotropin‐releasing hormone (TRH, 1 μg/kg BW) was additionally examined in male goats in this study to confirm modifications to central DA concentrations. The treatments with carbi and L‐dopa significantly reduced TRH‐induced PRL release in goats (P < 0.05). These results demonstrated that hypothalamic DA was involved in the regulatory mechanisms of GH, as well as PRL secretion in goats.     

Effects of intracerebroventricular injections of leptin on the release of luteinizing hormone and growth hormone in castrated calves

The purpose of the present study was to clarify the hypothalamic action of leptin on the secretion of luteinizing hormone (LH) and growth hormone (GH) in cattle. Intracerebroventricular (the third ventricle) injections of leptin were given to fully fed castrated Holstein calves. Blood samples were collected at 10‐min intervals for 60 min after injection and 20‐min intervals for 60 min before injection and for 60–180 min after injection through an indwelling catheter in the external jugular vein. Plasma LH and GH levels were examined by homologous radioimmunoassay. The administration of 10 µg of leptin stimulated a significant (P < 0.05) release of GH but not LH. Average GH levels began to rise after 30 min and were significantly increased at 40, 50 and 60 min after the injection, compared with the respective control values (P < 0.05). The present result suggests that leptin may act partly on the hypothalamus to stimulate the release of GH in castrated calves.     

Characterization and expression of the bovine growth hormone-releasing hormone (GHRH) receptor

The hypothalamic hormone, growth hormone-releasing hormone (GHRH) and its pituitary receptor are principal regulators of pituitary growth hormone (GH) synthesis and release. In the present study, we cloned and sequenced a complete bovine pituitary GHRH receptor cDNA in order to study its expression in cattle. The lengths of the exons in the bovine GHRH receptor gene were determined by comparison of the cloned cDNA with genomic sequences obtained from a bovine genomic library clone. As in other species, the bovine cDNA sequence encodes a 423-amino acid protein containing seven hydrophobic domains characteristic of a G protein-coupled receptor. The predicted bovine amino acid sequence shares 93, 90, 89, 87, and 85% identity with the ovine, porcine, human, rat and mouse sequences, respectively. Expression of the receptor in bovine ileum, ovary, anterior pituitary, testis, hypothalamus, pancreas and liver was examined by RT-PCR. Of those tissues examined, GHRH receptor expression was detected in the anterior pituitary gland and hypothalamus. To gain a better understanding of GHRH receptor gene regulation in ruminants, we examined the effect of bovine somatotropin (bST) treatment on pituitary GHRH receptor expression in dairy heifers using relative and real-time RT-PCR. In the present study, bST treatment of dairy heifers resulted in no significant decline in pituitary GHRH receptor expression.     

Serum prolactin response to thyrotropin releasing hormone in estrogen treated hypophysial stalk-transected gilts

Twelve crossbred gilts, 169 ± 3 days of age and 72.8 ± 3.4 kg body weight, were hypophysial stalk-transected (HST)1 or sham hypophysial stalk-transected (S-HST). Gilts were ovariectomized 6 days later and assigned to four treatments of 3 gilts each in a 2 × 2 factorial arrangement. One-half of the HST and S-HST gilts received 5 mg estradiolbenzoate (EB) or corn oil vehicle im at 0800 hr daily for 5 days beginning 64 ± 3 days after HST or S-HST. Blood was collected by jugular vein cannula at 0830 and 0900 hr the day after the last injection of EB or oil. Immediately after the 0900 hr sample, 200 μg thyrotropin releasing hormone (TRH) were injected (iv). Mean basal serum prolactin (PRL) concentration was similar for HST (10.3 ± 1.0 ng/ml) and S-HST (12.3 ± 1.7 ng/ml) gilts, however mean basal serum PRL concentration was greater (P<.05) for EB-treated gilts (13.7 ± 1.3 ng/ml) than for oil-treated gilts (8.8 ± .5 ng/ml). Mean serum PRL concentration of all gilts increased within 10 min and returned to approximately 20 ng/ml by 150 min after TRH. Maximum serum PRL concentrations at 10 min after TRH were greater (P<.01) for S-HST (255.9 ± 29.6 ng/ml) than HST gilts (83.4 ± 18.8 ng/ml), but were not different for EB (198.0 ± 50.6 ng/ml) and oil-treated gilts (141.4 ± 36.3 ng/ml). Area under the serum PRL response curve after TRH was greater (P<.005) for S-HST than HST gilts and for EB than oil-treated gilts (P<.05). These results do not eliminate the possible influence of estrogen on PRL secretion at the hypothalamus, but do indicate that estrogen directly stimulated the anterior pituitary gland to secrete PRL.     

Suppression of somatotroph function induced by growth hormone treatment in neonatal pigs

The effect of recombinant porcine growth hormone (pGH) treatment on pituitary function was evaluated in young pigs. Piglets received intraperitoneal recombinant pGH implants (0.5 mg/d sustained release) or vehicle implants beginning at 3 d of age. Ten piglets were sacrificed at 4 and 6 wk of age (five piglets/treatment group) for the collection of pituitary glands, blood, and liver tissue. Blood samples also were drawn at 3 and 12 d of age. Serum concentrations of GH, prolactin (PRL), thyroid-stimulating hormone (TSH), insulin-like growth factor-1 (IGF-1) and IGF-2 were evaluated. Levels of IGF-1 and IGF-2 mRNA were determined in liver samples. Treatment with GH increased circulating levels of GH and IGF-1 (P < 0.01), but not PRL, TSH, or IGF-2. Hepatic IGF-1, but not IGF-2, mRNA levels were increased by pGH (P < 0.001). Cultured pituitary cells from each animal were challenged with 0.1, 1, and 10 nM GH-releasing hormone (GHRH); 2 mM 8-Br-cAMP; or 100 nM phorbol myristate acetate. The release of GH from cultured pituitary cells was stimulated by all secretagogues (P < 0.001). The secretion of GH, but not PRL or TSH, in culture was inhibited by previous in vivo GH treatment (P < 0.001). Similarly, cellular GH, but not PRL or TSH, content was lower in the GH-implant group (P = 0.005). Cell cultures from 6-wk-old piglets secreted more GH, but not PRL or TSH, than cultures from 4-wk-old piglets (P < 0.05). Likewise, cellular GH, but not PRL or TSH, content was greatest in cultures from 6-wk-old animals (P = 0.002). Piglet growth was not affected by exogenous GH treatment (P = 0.67). These results demonstrate that exogenous pGH treatment selectively down-regulates somatotroph function in young pigs.     

Effects of extracerebral dopamine on salsolinol‐ or thyrotropin‐releasing hormone‐induced prolactin (PRL) secretion in goats

The aim of the present study was to clarify the effect of extracerebral dopamine (DA) on salsolinol (SAL)‐induced prolactin (PRL) secretion in goats. An intravenous injection of SAL or thyrotropin‐releasing hormone (TRH) was given to female goats before and after treatment with an extracerebral DA receptor antagonist, domperidone (DOM), and the PRL‐releasing response to SAL was compared with that to TRH. DOM alone increased plasma PRL concentrations and the PRL‐releasing response to DOM alone was greater than that to either SAL alone or TRH alone. The PRL‐releasing response to DOM plus SAL was similar to that to DOM alone, and no additive effect of DOM and SAL on the secretion of PRL was observed. In contrast, the PRL‐releasing response to DOM plus TRH was greater than that to either TRH alone or DOM alone and DOM synergistically increased TRH‐induced PRL secretion. The present results demonstrate that the mechanism involved in PRL secretion by SAL differs from that by TRH, and suggest that the extracerebral DA might be associated in part with the modulation of SAL‐induced PRL secretion in goats.