Enterotoxin activity of a Salmonella typhimurium of equine origin in vivo in rabbits and the effect of Salmonella culture lysates and cholera toxin on equine colonic mucosa in vitro

To determine whether a strain of Salmonella typhimurium (UCD 1755) of equine origin had enterotoxin activity, 2 ml of a cell-free culture lysate of strain UCD 1755 and approximately 10(9) viable strain UCD 1755 organisms were inoculated into ligated small intestinal segments of rabbits. Intestinal segments inoculated with viable strain UCD 1755 organisms and those inoculated with a cell-free culture lysate of strain UCD 1755 had significant (P less than 0.05) accumulations of fluid 10 hours after inoculation when compared with ligated intestinal segments either inoculated with sterile brain-heart infusion broth or left empty. There was not a statistically significant difference between fluid accumulation of intestinal segments inoculated with viable strain UCD 1755 and that of segments inoculated with a cell-free culture lysate of strain UCD 1755. The responses of equine colonic mucosa to culture filtrates of 2 strains of salmonella typhimurium (UCD 1755 and SL 1027) and purified cholera toxin were studied in vitro. Isolated smaples of colonic mucosa were incubated for 4 hours at 37 C in Krebs-Henseleit bicarbonate buffer (KHB) alone, KHB plus culture lysate of strain UCD 1755, KHB plus culture lysate of strain SL 1027, and KHB plus 1 microgram of cholera toxin/ml. Cyclic adenosine monophosphate (cAMP) content of each sample was determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of Escherichia coli endotoxin and thyrotropin-releasing hormone on prolactin in lactating sows

Primiparous gilts were given subcutaneous injections of saline solution or 8 mg of Escherichia coli endotoxin (055:B5 strain) in saline solution on postpartum days (PPD) 2 and/or 6 and saline solution at the same site on PPD 1, 3, 5, and 7 at 1000 hours. On PPD 1 to 3 and on PPD 5 to 7, pigs were given 100 micrograms of thyrotropin-releasing hormone (TRH) IV at 1300 hours to evaluate TRH-induced prolactin (PRL) release. Blood samples were analyzed for PRL, cortisol, triiodothyronine (T3), and tetraiodothyronine (T4) concentrations. Rectal temperatures were monitored at hourly intervals between 0800 and 1500 hours on PPD 2 and 6. The PRL declined after endotoxin administration on PPD 2, but a similar decline was not seen after saline solution administration on PPD 1, 2, or 3. The PRL concentrations remained unchanged on PPD 5, 6, and 7 in gilts exposed to endotoxin for the 1st or 2nd time on PPD 6 and to saline solution on PPD 5 and 7. The TRH injection caused increases in PRL in all animals, but the PRL increase after TRH injection was significantly lower (P less than 0.05) in gilts treated with endotoxin on PPD 2. Cortisol concentrations increased after endotoxin exposure on PPD 2 and 6. Rectal temperatures increased after endotoxin exposure on PPD 2 and 6 with peak temperatures of 41.8 C and 41.6 C seen 4 and 3 hours, respectively, after endotoxin injection. The T3 and T4 response, used as an indicator of TRH perfusion of the adenohypophysis, was unchanged after endotoxin or saline solution administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of dose and route of administration of thyroid releasing hormone (TRH) on the concentration of prolactin (PRL) and thyroxine (T4) in cyclic gilts

Our objective was to examine the ability of thyroid releasing hormone (TRH) to stimulate not only the release of the thyroid hormones, but also prolactin (PRL) in the female pig. An experiment was conducted to determine the effect of dose and route of administration of TRH on the concentration of PRL and thyroxine (T4) in cyclic gilts. Six gilts were injected with 0, 5, 25, 125, and 625 micrograms TRH and fed 0, 5, 2.5, 12.5 and 62.5 mg TRH. Gilts received TRH once daily. During the 10-day treatment period, route of TRH administration alternated between i.v. injection and feeding. The dose of TRH progressed from the lowest to the highest. Blood samples were taken prior to TRH injection and thereafter at 15-min intervals for 3 hr. Sampling continued for an additional 3 hr at 30-min intervals when TRH was fed. Concentrations of PRL and T4 were determined by radioimmunoassay. Intravenous injection of gilts with 125 and 625 micrograms TRH resulted in an increase in PRL from 0 to 15 min (P less than .05). All doses of TRH given i.v. elevated T4 over a 2-hr period (P less than .01). TRH failed to increase PRL when TRH was fed (P greater than .5). The feeding of 62.5 mg TRH elevated T4 from 0 to 6 hr (P less than .01). Thus, TRH injection increased PRL rapidly and T4 gradually. When TRH was fed, only a gradual elevation in T4 was observed. We conclude that TRH can elicit the release of both PRL and T4 in the cyclic gilt, but magnitude and duration of the PRL and T4 response depends on the dose and route of TRH administration.     

Effects of Short-term Hyper- and Hypoprolactinaemia on Hormones of the Pituitary, Gonad and -Thyroid Axis and on Semen Quality in Male Beagles

Effects of a short-term hyper- and hypoprolactinaemia on serum concentrations of LH, testosterone and semen quality in six male Beagles were investigated. Blood samples were collected at 3-day intervals for 12 weeks. The time span was divided into five 3-week periods: pre-treatment, metoclopramide (MCP) treatment (0.2 mg/kg orally three times daily), cabergoline (CAB) treatment (5 μg/kg orally once daily), post-treatment 1 and post-treatment 2. In the latter, only semen characteristics were evaluated. Semen parameters were analyzed once per week during the whole 15-week investigation time. At the end of each period, the effects of a single intravenous injection of thyrotropin-releasing hormone (TRH; 10 μg/kg) on the secretion of prolactin (PRL), LH, testosterone, thyroid-stimulating hormone and thyroxine (T4) were investigated. Pre-treatment serum PRL concentration increased under MCP (p < 0.05), followed by a decrease under CAB administration (p < 0.05). Luteinizing hormone and testosterone concentrations were not affected. Except for straight-line sperm velocity, semen quality did not differ between collection periods. A single iv TRH injection induced a significant PRL increase at 20 min in all experimental periods except during CAB treatment. Luteinizing hormone and testosterone did not show clear TRH-related changes. Basic T4 levels were significantly reduced after CAB treatment (p < 0.05). The results of the present study demonstrate that MCP-induced short-term hyperprolactinaemia in male beagles does not seriously affect the hypothalamo-pituitary axis and semen quality.     

Somatotroph and lactotroph function in relation to growth in six-week-old pigs reared in a hot or cool environment

The influences of thermal environment and individual growth rate on somatotroph and lactotroph function were examined in 6-week-old barrows reared entirely in a hot (H: 27–32°C, n = 8) or cool (C: 21°C, n = 10) environment. Growth hormone (GH) and prolactin (PRL) cell contents and responses to growth hormone-releasing hormone (GHRH) or thyrotropin-releasing hormone (TRH) were evaluated in cultured pituitary cells from each animal. Plasma GH, PRL, and insulin-like growth factor-1 (IGF-1) concentrations also were monitored. Thermal environment did not affect in vitro GH secretion, cellular GH content, or plasma GH concentrations. Stimulated in vitro GH release (GHRH-basal) and plasma GH were inversely related to average daily gain (ADG, r = −.76, p < .005 and r = −.51, p < .05, respectively). Cellular GH content also declined as ADG increased (r = −.57, p < .05). Plasma IGF-1 concentrations were not affected by thermal environment and were not related to ADG. Pituitary cells from H animals secreted and contained more PRL than cells from C animals (p < .05). Plasma PRL values were correlated with ADG (r = .54, p < .05), but did not differ between thermal groups. Stimulated in vitro PRL (TRH-vehicle) secretion was positively related with ADG only in the H group (r = .97, p < .001). In contrast, cellular PRL content decreased with ADG in cells from the H barrows (r = −.8, p < .05). Lactotroph function was not related to growth in cells from C pigs. In summary, 1) heat enhanced PRL secretion and cell content; 2) growth and somatotroph function were inversely related; and 3) serum PRL and the PRL response to TRH in cells from H barrows were positively related to growth.     

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 stimulation testing in healthy dogs

Serum triiodothyronine (T3) and thyroxine (T4) concentrations were determined after IV administration of 200 micrograms of thyrotropin-releasing hormone (TRH) to 10 healthy euthyroid dogs. Significant (P less than 0.05) changes were not found in the T3 concentration throughout an 8-hour sampling interval. All dogs had a significant increase (P less than 0.05) in the T4 concentration at 4, 5, 6, 7, and 8 hours after TRH administration. The largest increase in the serum T4 concentration occurred 4 hours after TRH injection. From 4 to 8 hours after TRH administration, the mean increase above basal T4 concentrations was 13.9 +/- 5.4 ng/ml.     

Thyroid stimulating hormone and prolactin secretion after thyrotropin releasing hormone administration to mares: Dose response during anestrus in winter and during estrus in summer

The response of thyroid stimulating hormone (TSH) and prolactin (PRL) concentrations to administration of thyrotropin releasing hormone (TRH) was determined in light-horse mares during the anestrous season (winter) and during estrus (standing heat) in the summer. Within each season, mares (4/group) were treated with either saline (controls) or one of four doses of TRH (80, 400, 2,000 or 10,000 ug) intravenously. Samples of blood were drawn at −15, −.5, 15, 30, 45, 60, 90, 120, 180 and 240 min relative to TRH injection. Concentrations of TSH and PRL in pre-TRH samples were greater (P<.05) in anestrous mares during winter than in estrous mares during summer. Concentrations of TSH increased (P<.05) within 30 min after administration of TRH and remained elevated during the 4-hr sampling period. The maximal net change in TSH concentrations and the area under the response curve were greatest for 2,000 ug of TRH; 80 ug did not produce a significant TSH response. There was no interaction (P >.10) between reproductive state and TRH dose for TSH concentrations. Concentrations of PRL were not significantly affected by any TRH dose during either season. It appears that mares differ from many mammalian species in that they do not respond to an injection of TRH with increases in both TSH and PRL.     

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.     

Fluid accumulation in mouse ligated intestine inoculated with the vascular permeability factor produced by Bacillus cereus.

Partially purified vascular permeability (VP) factor (VPF) of Bacillus cereus induced fluid accumulation in the ligated intestinal loops of mouse (MIL) and rabbit (RIL), suggesting that the VP activity may correlate with fluid accumulation in ligated intestinal loops of these animals. Fluid accumulation was observed at 6-8 hr in 55-67% of mouse intestinal loops inoculated with 40-50 immunodiffusion units (IDU) of partially purified VPF, whereas it was found at 2 hr in all loops with 400-600 IDU of partially purified VPF. In rabbit intestinal loops with 120-190 IDU of partially purified VPF, fluid accumulation was observed at 6 hr. From these findings, VPF produced by B. cereus can be easily detected in both MIL and RIL. The intestinal tissue of mouse intestinal loops was histopathologically damaged at different concentrations of the VPF to induce fluid accumulation. With 50 IDU of partially purified VPF, severe edema was found in the laminia proprial layer and submucosa. With 600 IDU of partially purified VPF, on the other hand, severe necrosis in the surface epithelium of villus and laminia proprial layer, and hyperemia in the submucosa were observed, suggesting that partially purified VPF may be cytotoxic and/or intestinecrotic.     

The Effect of Adsorbant and Anti-inflammatory Drugs on Secretion in Ligated Segments of Pig Intestine Infected with Escherichia coli

Four adsorbant drug preparations, Kaopectate, colloidal Attapulgite, noncolloidal Attapulgite and Pepto-bismol were investigated for their effects on fluid accumulation in ligated segments of pig intestine inoculated with enteropathogenic Escherichia coli. Two anti-inflammatory drugs. aspirin and methylprednisolone, and two antibiotics, lincomycin and polymyxin B, were also tested. All the drugs except the two anti-inflammatory products were given by injection into the lumen of the intestine. Aspirin was given orally and methylprednisolone was given intramuscularly. The antibiotics were tested at levels at which they had no significant antibacterial effect in in vitro tests. The adsorbant drugs colloidal Attapulgite and Pepto-bismol were shown to be effective in reducing fluid accumulation in ligated segments of pig intestine infected with enteropathogenic E. coli. In the case of Peptobismol this effect was associated with an antibacterial effect as well as an antitoxic effect, probably due to its adsorbant properties. It is possible that an aspirin-like effect in the gut due to the active ingredient bismuth subsalicylate may have contributed to the effectiveness of Pepto-bismol. Colloidal Attapulgite was demonstrated to have an antitoxic effect but did not have an antibacterial effect.

In high doses, the anti-inflammatory drugs acetylsalicylic acid and methylprednisolone were marginally effective in reduction of fluid accumulation in the same test system. Lincomycin was shown to reduce intestinal fluid secretion, whereas polymyxin B had no effect.

     

Hormonal response to thyrotropin-releasing hormone in healthy horses and in horses with pituitary adenoma

Cortisol, triiodothyronine (T3), thyroxine (T4), insulin, and glucose responses to thyrotropin-releasing hormone (TRH) were evaluated in 12 healthy, mature horses and in 7 horses and 4 ponies with clinical signs of pituitary adenoma (PA). Within 1 hour after TRH administration, the increase in T3 and T4 was similar in healthy horses and animals with PA. Plasma cortisol in the group with PA increased (P less than 0.05) within 0.25 hours after TRH administration, and remained increased for 1.5 hours. In the control group, a significant increase in plasma cortisol concentrations did not develop after TRH administration. Plasma glucose and insulin concentrations were higher in animals with PA than in the healthy horses throughout the experiment (6 hours).     

Effect of salicylates on intestinal secretion in calves given (intestinal loops) Escherichia coli heat-stable enterotoxin

The inhibitory effect of salicylates on intestinal secretion in 1- to 5-day-old calves given Escherichia coli heat-stable enterotoxin (ST)-induced intestinal fluid response was investigated. Purified ST was diluted in isotonic saline solution to obtain 1:10, 1:25, 1:50, 1:75, and 1:100 dilutions. Each dilution (1 ml) was inoculated into ligated loops in the distal part of the jejunum of each calf. Acetylsalicylic acid (aspirin) given orally (100 mg/kg) at 4 hours before ST was inoculated did not substantially alter the intestinal fluid response to ST. Sodium salicylate (IV) infusion, begun simultaneously when, or at 1 hour after, ST was inoculated, significantly (P less than 0.05) decreased fluid accumulation in those loops inoculated with ST dilutions of 1:25 or greater. The sodium and potassium concentrations of the accumulated fluid did not differ significantly between or within treatment groups. These results indicate that sodium salicylate infusion may be beneficial in treating enterotoxic colibacillosis in calves. Aspirin given orally at the dose used in the present study, would not have any beneficial effect.     

Effects of Escherichia coli heat-stable enterotoxin b on small intestinal villi in pigs, rabbits, and lambs

Culture supernates from two strains of E. coli were placed into different ligated intestinal sections (loops) of each animal. The two bacterial strains were identical except that one contained a plasmid carrying the heat-stable toxin b (STb) gene, while the other did not. Morphometric techniques were used to assess villous epithelial surface areas and mucosal volumes in both intestinal segments exposed to STb-positive (test) and to STb-negative (control) supernates. In pigs whose intestines were exposed to STb-positive supernatants for 2 hours, both villous epithelial surface area and mucosal volume were significantly smaller in test loops than in control loops (P less than 0.02). In test loops of pigs incubated for 1 hour, and in test loops of lambs incubated for 2 hours, there was a decrease in villous epithelial surface area which approached the test for significance but did not meet it (0.05 less than P less than 0.10). Rabbit test loops did not differ from rabbit control loops in either villous epithelial surface area or mucosal volume. Histological examination of the tissues from all three species revealed epithelial changes in porcine and ovine tissues only. In porcine and ovine tissues, epithelium at villous tips was seen to be cuboidal or squamous, or even to be absent. Villi with similarly altered epithelium were seen in control loops, but were seen much more frequently in test loops. These epithelial changes were seen as early as 30 minutes of incubation in pigs. Intestinal tissues from these pigs were examined by transmission electron microscopy, but no difference between test and control tissues was seen.(ABSTRACT TRUNCATED AT 250 WORDS)     

The drop in plasma thyrotropin concentrations in fasted chickens is caused by an action at the level of the hypothalamus: role of corticosterone

Fasting has severe effects on thyroid metabolism in the chicken: plasma thyroxine (T₄) concentrations increase, whereas 3′,5,3-triiodothyronine (T₃) concentrations decrease. In the present report we studied the effect of fasting at the level of: 1) the pituitary (plasma thyrotropin (TSH) concentrations; the sensitivity of thyrotrophs to corticotropin-releasing hormone (CRH) and TSH-releasing hormone (TRH)); and 2) the hypothalamus (TRH content). A regulatory role of corticosterone is discussed. One day of fasting resulted in a drop in plasma TSH concentrations. Fed and nonfed animals were treated with ovine CRH (oCRH) or TRH. The sensitivity of thyrotrophs to the respective hypothalamic hormones was increased when animals were subjected to a 1-d period of fasting. A 75% (TRH) and 50% (oCRH) increase in plasma TSH was recorded in fasted animals, whereas both secretagogues did not evoke any response in their fed counterparts. The drop in plasma TSH cannot, therefore, be attributed to a loss in sensitivity of thyrotrophs to hypothalamic stimulatory control. In an identical experiment, plasma TSH concentrations decreased, whereas hypothalamic TRH content was higher in fasted animals, suggesting a decreased hypothalamic TRH release toward the pituitary. In both fasting experiments, plasma corticosterone concentrations were increased after 1 d of fasting. Because an iv injection of corticosterone-elevated hypothalamic TRH contents and decreased plasma TSH concentrations, a corticosterone-induced TSH decrease during fasting is suggested through an action at the level of the hypothalamus.     

Serum concentrations of pituitary and adrenal hormones in female pigs exposed to two photoperiods

Serum concentrations of pituitary and adrenal hormones were determined in lactating sows and ovariectomized (OVX) gilts exposed to 8 h (8L:16D) or 16 h of light (16L:8D). In addition serum prolactin (PRL) concentrations were determined after a thyrotropin releasing hormone (TRH) challenge. At 103 +/- 2 d of gestation or 3 wk after ovariectomy of nulliparous gilts on d 7 to 9 of the estrous cycle (d - 10), blood samples were collected from jugular vein cannulae at 30-min intervals for 8 h beginning at 0800 h. Immediately after the last sample, 13 sows and five OVX gilts were assigned to 8L:16D and 14 sows and five OVX gilts were assigned to 16L:8D/d and placed in two identical chambers in the farrowing house. Blood sampling was repeated on d 7, 14 and 21 of lactation in the sows and on d 7, 14, 21 and 28 in the OVX gilts. In Exp. 1, serum cortisol (C) concentrations were similar for sows exposed to 8L:16D (n = 7) and 16L:8D (n = 6) treatments, whereas in Exp. 2, serum C concentrations for sows exposed to 8L:16D (n = 6) were lower than those exposed to 16L:8D (n = 6) on d 7, 14 and 21. Photoperiod failed to influence serum concentrations of PRL, luteinizing hormone (LH) and growth hormone in the lactating sows or PRL in the OVX gilts. Photoperiod also failed to affect mean basal serum concentrations, peak height and peak frequency for PRL and LH in the lactating sows or for PRL in the OVX gilts.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of feeding thyrotropin-releasing hormone to lactating sows

Two experiments were conducted to assess the effects of feeding thyrotropin-releasing hormone (TRH) during lactation on sows. In Exp. 1, sows were fed 0, 1, 10, 100 or 1,000 mg TRH on d 10.8 +/- .4 (mean +/- SE) after parturition. Blood samples were taken from sows every 30 min from -2 h to 8 h and at 10, 12 and 18 h from feeding. Consumption of 100 or 1,000 mg TRH increased mean serum concentrations of thyroxine (T4; P less than .001), 1,000 mg TRH increased growth hormone (GH; P less than .06) and 100 or 1,000 mg TRH increased prolactin (PRL; P less than .01), but insulin (INS; P greater than .10) was unaffected by TRH. Serum concentrations of T4 were elevated within 2 to 4 h after feeding TRH and remained elevated for 12 to 18 h. Concentrations of GH and PRL began to increase immediately after feeding 100 or 1,000 mg TRH and remained elevated for 6 and 8 h, respectively. In Exp. 2, sows were fed 0 or 200 mg TRH from d 111 of gestation to weaning at 27.1 +/- .3 d of lactation. Consumption of TRH elevated concentrations of T4 at all stages of lactation and increased respiration rate on d 10 and d 20, heart rate on d 20, and milk production on d 20 of lactation. Consumption of TRH did not influence number of pigs born, number born alive, survival rate during lactation, sow body weight, heartgirth, backfat depth, feed disappearance, or milk production on d 10 of lactation. Piglets nursing sows fed TRH were similar in weight to piglets nursing sows not fed TRH on d 0 and 5 of lactation, but they were heavier on d 10 (P less than .07), 15 (P less than .001), 20 (P less than .001) and 27 (P less than .0001). Sows fed TRH took longer (P less than .001) to return to estrus after weaning than control sows. Results indicated that feeding TRH elevated T4, GH and PRL and that feeding TRH for the duration of lactation increased milk production on d 20 of lactation and increased weaning weights, but it delayed estrus after weaning.     

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.     

Dopaminergic-like activity in toxic fescue alters prolactin but not growth hormone or thyroid stimulating hormone in ewes

Studies were conducted to determine the specificity and cause of altered pituitary hormone secretion when ewes ingest endophyte-infected (Acremonium coenophialum) GI-307 tall fescue (toxic fescue). Plasma concentrations of prolactin (PRL) but not growth hormone (GH) or thyroid stimulating hormone (TSH) in ewes grazing toxic fescue were significantly lower (P < .01) than concentrations measured in ewes grazing orchardgrass (OG). Comparing hormone secretory responses of ewes grazing each grasstype, ewes on toxic fescue released less PRL following thyrotropin releasing hormone (TRH) challenge than ewes on OG. TSH responses to TRH were not affected by grasstype. At this dose of TRH, GH secretion was not significantly affected in either group of ewes. In a separate study, dopamine hydrochloride (DA) was infused into control ewes to define the effect of a pure dopamine agonist on basal and TRH-stimulated secretion of PRL, GH and TSH. DA depressed both basal and TRH-stimulated secretion of PRL without affecting the basal concentrations or responses of GH or TSH. Based on the assumption that the active agent in toxic fescue responsible for the observed hypoprolactinemia was a dopaminergic agonist, haloperidol (HAL), a DA receptor blocking drug, was administered to ewes grazing toxic fescue or OG. HAL evoked significant PRL secretion unaccompanied by any GH or TSH effect in both toxic fescue and OG ewes. Administration of HAL resulted in a gradual increase over 4 hr in PRL in toxic fescue ewes and prolonged the duration of the PRL response to TRH. No differences in circulating plasma concentrations of DA, epinephrine or norepinephrine were measured in ewes on troxic fescue or OG.

Alterations in pituitary hormone secretion due to toxic factors in fescue were confined to PRL. Hormone secretory responses to TRH and HAL suggest that the effects on PRL are mediated through dopamine-like activity in toxic fescue.     

Influence of endophyte-infected tall fescue on serum prolactin and progesterone in gravid mares.

Thirty mares in late gestation were used in a 3-yr study to assess effects of the tall fescue endophyte Acremonium coenophialum on serum prolactin (PRL) and progesterone. Two paddocks of each treatment, 0 or 100% infected 'Kentucky 31' tall fescue, were grazed by the mares for 21 d. Blood was collected three times per week until parturition. At 7-d intervals, mares were challenged with thyrotropin-releasing hormone (TRH) while grazing and blood was collected postinjection. Mares grazing 100% infected tall fescue (E+) had decreased serum PRL compared with mares grazing the 0% infected tall fescue (E-) in 2 of 3 yr. Within 8 d postgrazing, serum PRL for E+ mares equaled or surpassed values of the E- mares. Serum PRL was not different during the 3rd yr. In response to TRH, serum PRL rate of increase was similar between treatments but remained elevated (P less than .01) in the E+ mares at the 180-, 240-, and 300-min sample times. Serum progesterone was lowered (P less than .05) by E+ but increased to control values within 10 d postgrazing. It is concluded that serum PRL and progesterone in the gravid mare were decreased by the presence of A. coenophialum in 'Kentucky 31' tall fescue grass but normal levels were reestablished within 2 to 3 wk.