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.     

牛垂体细胞体外培养条件下催乳素分泌的调节—TRH、 GnRH、LH和PRL的影响

利用牛垂体细胞体外无血清培养技术,研究了促甲状腺素释放素(TRH)、促性腺激素释放素(GnRH)、促黄体激素(LH)和催乳素(PRL)对牛垂体细胞PRL分泌的调节。细胞在培养箱内预培养24小时后开始各项处理,以后隔24小时采集培养液一次,并换新的相同处理的培养液。结果表明:GnRH和TRH在第一次处理时均能显著增加PRL的分泌。加入1、10、100ng/ml的GnRH可使PRL分泌量分别增加53％(P<0.05)、111％(P<0.01)和60％(P<0.01),而TRH只有在10、100ng/ml的剂量时明显引起PRL释放增加(P<0.01)。第二次处理时细胞对TRH的反应消失,但对GnRH的反应却发生了逆转,PRL分别降低了70％(P<0.05)、68％(P<0.05)和73％(P<0.05)。表明GnRH和TRH在培养初期具有促进垂体细胞释放PRL的作用。LH(1、10、100ng/ml)对牛垂体细胞PRL的分泌无影响。但LH和TRH合用时,100ng/ml LH可明显地降低由TRH引起的PRL分泌(P<0.05)。PRL本身对牛垂体细胞的分泌有自我调节作用,100、1000ng/ml的PRL处理细胞使其分泌的PRL显著下降,分别降低70％(P<0.05)和100％(P<0.001)。此外1000ng/ml的PRL可显著地抑制由GnRH在第一次处理时引起的PRL释放。本实验在垂体细胞水平说明多种激素对PRL的分泌调节具有调控作用。     

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.     

Thyrotropin-releasing hormone-induced growth hormone secretion in dogs with primary hypothyroidism

Primary hypothyroidism in dogs is associated with increased release of growth hormone (GH). In search for an explanation we investigated the effect of intravenous administration of thyrotropin-releasing hormone (TRH, 10 microg/kg body weight) on GH release in 10 dogs with primary hypothyroidism and 6 healthy control dogs. The hypothyroid dogs had a medical history and physical changes compatible with hypothyroidism and were included in the study on the basis of the following criteria: plasma thyroxine concentration < 2 nmol/l and plasma thyrotropin (TSH) concentration > 1 microg/l. In addition, (99m)TcO(4)(-) uptake during thyroid scintigraphy was low or absent. TRH administration caused plasma TSH concentrations to rise significantly in the control dogs, but not in the hypothyroid dogs. In the dogs with primary hypothyroidism, the mean basal plasma GH concentration was relatively high (2.3+/-0.5 microg/l) and increased significantly (P=0.001) 10 and 20 min after injection of TRH (to 11.9+/-3.5 and 9.8+/-2.7 microg/l, respectively). In the control dogs, the mean basal plasma GH concentration was 1.3+/-0.1 microg/l and did not increase significantly after TRH administration. We conclude that, in contrast to healthy control dogs, primary hypothyroid dogs respond to TRH administration with a significant increase in the plasma GH concentration, possibly as a result of transdifferentiation of somatotropic pituitary cells to thyrosomatotropes.     

冷应激对雏鸡下丘脑-垂体-甲状腺轴的影响

越来越多的证据表明,应激能够激活下丘脑-垂体-甲状腺轴,进而影响促甲状腺激素释放激素(TRH)、促甲状腺激素(TSH)及甲状腺激素的合成与分泌。为了探明冷应激对雏鸡下丘脑-垂体-甲状腺轴的影响,以公雏鸡为实验动物,进行急性(0.25、1、3、6、12h与24h)与慢性(5、10d与20d)冷应激处理(12±1)℃,检测了雏鸡下丘脑TRH mRNA的表达水平,血清TSH、FT(3T3的游离形式)及FT(4T4的游离形式)的含量。结果表明:急性应激时,TRH mRNA的表达水平在各应激时间点均显著升高,TSH变化不明显,FT3开始无变化,在6h时突然降低,而后又显著升高,FT4开始变化不大,6h后显著升高;慢性应激时,TRH mRNA的表达水平与相应的对照组相比显著降低,TSH变化仍不明显,FT3呈上升趋势,而FT4呈下降趋势。这说明冷暴露可以使雏鸡下丘脑-垂体-甲状腺轴的激素分泌发生改变,而且不同程度的冷应激对同一激素也会产生不同的影响。     

Rapeseed Products from Double-Low Cultivars as Feed for Dairy Cows: Effects of Long-Term Feeding on Thyroid Function,Fertility and Animal Health

Eighty-five dairy cows of the Swedish Red and White Breed (SRB) were included in a long-term experiment during 3 consecutive lactations. The cows were divided into 3 different dietary groups that received no rapeseed (NR), up to 1.2 kg dry matter (DM) 00-rapeseed meal plus 0.2 kg DM full-fat 00-rapeseed (MR), and up to 2.5 kg DM 00-rapeseed meal plus 0.9 kg DM full-fat 00-rapeseed (HR) per day. No significant differences in culling rates or disease rates were found between the feeding groups at any time during the experiment. The interval from calving to conception among the primiparous cows was longer for the HR-group (125 days) than for the NR-group (100 days). The response to a thyrotroph releasing hormone around 90 days postpartum during the first lactation was significantly higher for the HR-group (86.7 µ/L/h) than for the NR-group (55.2 µg/L/h). This indicates that at the highest level of rapeseed feeding, glucosinolates had a very mild, suppressive influence on thyroid hormone release, apparently compensated for by an increased activity along the hypothalamic-pituitary-thyroid axis. No significant differences in fertility or thyroid function were found among the pluriparous cows. During 2nd lactation the concentration of serum urea was higher in the NR-group (7.31 mmol/L) than in the HR-group (6.83 mol/L). The effects of independent environmental factors influenced fertility and thyroid function to a much greater extent than the rapeseed feeding. It was concluded that the feeding of rapeseed products from certified double low varieties of B. napus to adult dairy cows in amounts up to 3 kg rapeseed meal per cow and day would not have any negative effects on animal health or fertility.     

Stimulated growth hormone release in juvenile cattle genetically selected for high and low milk yield

The purpose of the present study was to test if plasma growth hormone (GH) concentrations in juvenile male and female cattle before or after intravenous stimulation with secretagogues was affected by selection for high (H) vs. low (L) milk yield in lines of Norwegian cattle. In the first of two experiments (A), 32 yearling heifers (16 H and 16 L, at 307–424 days of age) were tested by use of four doses of growth hormone releasing factor (GRF); 0.02, 0.10, 0.50 and 2.50 μg/kg live weight, on 4 consecutive days. The animals were fed ad libitum on a silage-based ration before and during the experiment. Growth hormone was assayed in plasma from blood samples taken at ?15, ?5, 0, 5, 10, 15, 20, 30, 45 and 60 min from stimulation. Plasma GH concentrations were log transformed before statistical analyses. Response variables were; PRIOR (mean of ?15, ?5 and 0 min samples) and PEAK (mean of 10, 15 and 20-min samples). In experiment B, 37 calves (19 H+18 L, 22 males and 15 females, age 114–259 days) were subjected daily to one of three intravenous stimulation tests (GRF, 0.10 and 0.50 μg/kg or thyrotrophin releasing hormone (TRH) at 0.20 μg/kg live weight) on each of 3 consecutive days. Feeding was restricted to cover estimated maintenance requirements only. Rations were given once daily during the test days and 3 days prior to first test. Blood sampling and variables followed those of experiment A. Selection line did not significantly affect GH variables in experiments A or B at any dose of GRF or TRH. GH response increased with increasing dose of GRF up to 0.50 μg/kg. At the highest GRF dose, the response was delayed and persisted longer. Doses giving intermediate to large response increased repeatability of GH measurements. It is concluded that GH secretion in juvenile cattle can be accurately assessed using GRF based stimulation tests combined with restricted and controlled feeding, but it is not affected by selection for milk yield in Norwegian cattle.     

Effects of cold stress on hypothalamic corticotrophin-releasing and thyrotropin-releasing hormone messenger RNA levels in chickens

There is increasing evidence that stress can activate the hypothalamic-pituitary-adrenal-axis and hypothalamic-pituitary- thyroid-axis,and further affect the synthesis and secretion of corticotrophin-releasing hormone(CRH)and thyrotropin-releasing hormone(TRH).To evaluate the effect of cold stress on the hypothalamic CRH and TRH messenger RNA(mRNA)levels in Yisha chickens,male Yisha chickens were subjected to acute(1,6,12 h)and chronic(5,10,20 d)cold stress(12±1)℃.Hypothalami were collected for assessment of mRNA levels by semi-quantitative RT-PCR.Acute stress resulted in a significant decrease of CRH mRNA levels at 6 and 12 h,and a significant increase of TRH mRNA levels at every stress time point.Chronic cold stress resulted in a significant increase of CRH mRNA levels and a significant decrease of TRH mRNA levels compared with the control group at every stress time point.The results suggest that the two genes differently respond to cold stress at the mRNA levels.And the different degrees of cold stress will produce different effects on the identical gene.     

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.