Effect Of TRH Research Articles

Effects of TRH, ethanol, and TRH-ethanol combination on activity in rats with altered monoamine content

Investigations were undertaken with 5,7-dihydroxytryptamine and 6-hydroxydopamine treated rats to see whether activity changes induced by TRH, ethanol and the TRH-ethanol combination would be affected after reduced monoamine function. In keeping with earlier results, TRH increased activity, ethanol reduced activity and the TRH-ethanol combination produced activity counts greater than those for TRH alone. Neither the 5,7-dihydroxytryptamine-induced reduction of brain serotonin nor the 6-hydroxydopamine treatments which reduced brain catecholamines altered the hyperactivity induced by TRH or the TRH-ethanol combination. While reduction of brain serotonin did not affect the ethanol-induced changes in activity, preferential reduction of dopamine as well as reduction of both norepinephrine and dopamine significantly antagonized this measure of ethanol-induced depression. The reduction of dopamine alone produced the greatest effect on this action of ethanol. It can be concluded from the data that the increased locomotion induced by TRH and the TRH-ethanol combination does not depend upon endogenous monoamines, whereas the sedative effects of ethanol are apparently influenced by alterations in brain catecholamine function.

Thyrotropin releasing hormone increases 5-hydroxytryptamine 1 receptors in the limbic brain of the rat

The effect of TRH on 5-HT 1 receptors in the rat brain was investigated. A crude membrane preparation was incubated at 37°C for 15 min with or without TRH prior to [ 3H]5-HT binding assay. TRH at 100 nM increased the number of 5-HT 1 receptors significantly (approximately 20%) in the limbic forebrain and the hippocampus without altering their affinity. As this concentration of TRH is close to its dissociation constant (2 nM and 51 nM in the limbic forebrain, 11 nM in the hippocampus), this effect is probably of physiological relevance. This finding seems to support a pharmacological finding of others that the anti-convulsion effect of TRH may be related to increased serotonergic transmission.

Effects of TRH and a rat TSH preparation on discrete hypothalamic and forebrain catecholamine nerve terminal networks in the hypophysectomized male rat

A rat pituitary TSH preparation in doses of 10 and 100 μg/kg produced rapid and marked increases in dopamine (DA) levels and α-methyltyrosine-induced decline consistent with increased DA synthesis and release in the medial and lateral palisade zones (MPZ, LPZ) of the median eminence, and reduced noradrenaline (NA) turnover in the paraventricular hypothalamic nucleus (PA) of the hypophysectomized male rat. The TSH serum levels measured in these rats 2 h after the injection were within the physiological range after the injection of 10 μg/kg. TRH given intravenously in a dose of 100 μg/kg produced rapid and marked increases of DA release in the MPZ and LPZ of the median eminence and reduction of NA turnover in the PA of the hypophysectomized male rat. The TRH injection did not alter the serum levels of prolactin, TSH, T 3 and T 4. The results indicate that TRH-TSH-DA interactions take place in the local circuits in the median eminence thus supporting the view that a short and an ultrashort feedback action of rTSH and TRH respectively may exist in the median eminence. The rapid action of the rat TSH preparation as well as of TRH further supports this concept. The rat TSH preparation and TRH produced marked reductions in NA turnover in the PA. These results support the possibility that rat TSH and TRH may, via on action on the hypothalamus, influence the facilitatory noradrenergic mechanism operating at the soma-dendritic level of the TRH immunoreactive neurons projecting to the median eminence. Thus, the existence of a neuronal feedback loop from the medio-basal hypothalamus into the paraventricular hypothalamic nucleus is postulated.

Multihormonal regulation of the pituitary gland binding and secretory responses to hypothalamic neuropeptides in rat GH pituitary strains in culture

The parenchyma of the anterior pituitary gland is constituted of distinct specialized glandular cell types, some of them being able to secrete two, or more, genetically related hormones. The synthesis and the release of each hormone is a discontinuous process controlled by a variety of chemical signals of hypothalamic and peripheral origin. These signals, namely neuropeptides, may cross-react at the level of one cell type and each signal may govern the activity of more than one cell type. Such integrated systems hamper the analysis of binding and secretion coupling of regulatory neuropeptides at the cellular and molecular levels. Prolactin (PRL) and growth hormone (GH) rat clonal pituitary tumor-derived cell lines (GH cells) by contrast offer a reductionist model suitable for such investigations on TRH and somatostatin hypophysiotropic neuropeptides and on their hormonal regulation, as reviewed in this paper. The tetradecapeptide somatostatin inhibits similarly PRL and GH release by GH cells. Using (125)I-Tyr-somatostatin, specific binding sites were characterized; their occupation presents dose dependence and stereospecific requirements similar to the induction of the inhibition of hormone release. In cells cultured in serum-supplemented media, TRH regulates in a biphasic manner somatostatin binding sites: TRH acutely increases and chronically decreases their number without altering their affinity. In cells cultured in serum-free media, TRH competes with (125)I-Tyr-somatostatin and displace it with a similar affinity and nearly identical efficiency as unlabelled somatostatin. These effects of TRH are totally prevented by preculturing the cells in a medium supplemented with tri-iodotyronine, which in addition increases somatostatin binding capacity. The tripeptide TRH stimulates similarly the acute release of PRL and GH by GH cells. By contrast, long-term exposure to TRH inversely regulates the production of both hormones, PRL being augmented and GH inhibited. This results from an opposite regulation of the transcription of the rPRL and rGH genes and of the half-life of their respective encoded mRNA, the stimulation of the rPRL gene being rapid and transient. These effects are mediated by the occupation of specific binding sites, extensively studied using (3)H-TRH, displaying the same dose- and time-dependence, as well as stereospecific requirements, as the regulation of hormone secretion. Factor regulating per se the PRL and GH secretion also alter TRH interaction with GH cells. TRH and tri-iodotyronine both down-regulate the number of TRH binding sites while estrogens or glucocorticoids augment them. TRH binding and secretion coupling are not always regulated in parallel. Evidences for plasma membrane located events were reported for TRH, but also for its internalization. Thus the subcellular localization of the initial signal(s) triggering acute hormone release on the one hand and alteration of specific genes transcription on the other hand is still debated.

Arachidonate stimulates prolactin release in vitro: a role for the fatty acid and its metabolites as intracellular regulator(s) in mammotrophs.

We investigated the involvement of arachidonate in the PRL secretory process using three experimental systems: hemipituitary glands incubated in vitro, cultured pituitary cells, and dispersed anterior pituitary cells perifused in columns. Arachidonate (100 microM) significantly (P less than 0.05) stimulated PRL release in the former system and stimulated PRL secretion in a dose-related manner in cultured cells. In hemipituitary glands, indomethacin, a cyclooxygenase inhibitor, potentiated the arachidonate-mediated stimulation, whereas nordihydroguaiaretic acid or BW755c abolished it. The latter two agents, but not indomethacin, abolished the effect of phospholipase A2 on PRL release in vitro. BW755c also inhibited the stimulatory effect of TRH on PRL release in both experimental systems. Conversely, the stimulation of PRL release by phorbol myristate acetate (PMA), although significantly reduced, was not abolished by either nordihydroguaiaretic acid or BW755c. Quinacrine, a phospholipase A2 inhibitor, also abolished the stimulatory effect of phospholipase A2 or TRH on PRL release. In cultured cells, quinacrine inhibits basal PRL release, but does not affect PRL release induced by arachidonate or (Bu)2 cAMP. These results more firmly establish a role for arachidonate as an intracellular mediator of PRL release and suggest the involvement of an arachidonate metabolic pathway(s) (lipoxygenase and epoxygenase) other than prostaglandin or thromboxane formation. The effect of PMA on PRL release may be attributable only in part to an increase in the production of arachidonate metabolites, and most of PMA's effect on PRL release may relate to its activation of protein kinase C.

P2B0124) - Age-related alternation in the effect of TRH in genetically ataxic mice.

P2B0124) - Age-related alternation in the effect of TRH in genetically ataxic mice.

Phospholipid metabolism and prolactin secretion in vitro.

The possible mechanisms by which phospholipid metabolism may be involved in the biochemical events underlying pituitary hormone secretion in basal and stimulated conditions were examined. Particular emphasis was given to the role of changes in the turnover of specific membrane phospholipids, the polyphosphoinositides, in the stimulatory effect of TRH and neurotensin on prolactin release in vitro. Finally, some comments on the involvement of arachidonate and/or its metabolites in the mechanisms of release of the hormone have been reported. In this respect, the possibility that a specific diacylglycerol lipase may represent a link between the 'phosphatidylinositol effect' and the production of arachidonate from mammotroph membranal phospholipids was examined using the rather selective inhibitor of diacylglycerol lipase RHC80267.

Thyrotropin-releasing hormone rapidly stimulates a biphasic secretion of prolactin and growth hormone in GH4C1 rat pituitary tumor cells.

The characteristics of TRH-induced acute PRL and GH secretion were studied in GH4C1 cells, a clonal rat anterior pituitary tumor cell line which secretes PRL and GH. The experiments were carried out both in a flow system in which microcarrier (Cytodex)-attached cells were perifused at a constant rate and in a conventional static culture system. In both systems, cells responded to TRH in a qualitatively similar manner. TRH significantly stimulated PRL and GH secretion within 5 sec without a detectable lag period. The secretion rate was highest during the initial 1 min, declined sharply thereafter despite the continuous presence of TRH, and plateaued at a lower level. The maximum dose of TRH caused 250-700% of basal secretion during the early period (approximately 8 min; first phase) and about 150% of basal secretion thereafter (second phase). The sustained lower secretion (second phase) was maintained as long as cells were exposed to TRH (up to 2.5 h), and the secretion rate returned to the basal level within 30 min of removal of TRH from the medium. The half-maximal doses for the first and second phase secretion were 2-3 and 0.5-1 nM, respectively, in both the perifusion and static culture systems. Over a 2-day period, TRH stimulated PRL synthesis and inhibited GH synthesis. The dose-response curves for these long term effects on hormone synthesis were similar to the dose-response curves for the first phase of release. [N3-methyl-His2]TRH gave similar results, but was more potent than TRH. [N3-methyl-His2]TRH stimulated first phase release with an ED50 of 0.4-0.8 nM, second phase release with an ED50 of 0.1-0.2 nM, and hormone synthesis with an ED50 of 0.7-0.8 nM. Preincubation of the cells with Ca+2-free medium significantly depressed both first and second phase secretion. Preexposure of the cells to cycloheximide (10 micrograms/ml) had little effect on the first phase of secretion, but reduced second phase secretion. The acute effects of TRH on GH and PRL were identical, except that the secretory response tended to be greater for PRL. We conclude that 1) TRH causes hormone secretion very rapidly in a biphasic manner; 2) the first phase of secretion consists primarily of the release of stored hormone, whereas the second phase includes the release of newly synthesized hormone; 3) the dose-response curve of second phase secretion is shifted to the left compared with that of first phase secretion; and 4) both phases of secretion are at least partially dependent on extracellular Ca+2.

Thyrotropin releasing hormone and naloxone attenuate the antihypertensive action of central alpha 2-adrenoceptor stimulation through different mechanisms.

In various forms of shock, TRH is equivalent to naloxone in reversing the hypotension and improving the survival rate. The present findings indicate that in spontaneously hypertensive rats (SHR), TRH has another naloxone-like effect in antagonizing the antihypertensive response to clonidine and alpha-methyldopa. When given during the hypotensive response to alpha-methyldopa, both naloxone and TRH produce a pressor response. While this effect of naloxone is blocked by prazosin, the effect of TRH is not influenced by prazosin or hexamethonium but is inhibited by a vasopressin pressor antagonist. This suggests that the pressor response to naloxone is mediated by the sympathetic nervous system, whereas the similar action of TRH is independent of sympatho-adrenomedullary functions and it is mediated by vasopressin.

The effects of thyrotropin releasing hormone on a visual discrimination task in rats

The effects of intracerebroventricular (ICV) administration of thyrotropin releasing hormone (TRH, 1 and 50 micrograms) were assessed on a two-choice visual discrimination task. The data were analysed using signal detection theory techniques in order to test for changes in cognitive and response factors. No significant changes in performance were observed. In a second experiment, the effects of TRH (100 micrograms ICV) on performance were compared with amphetamine (AMP, 1 mg/kg, intra-peritoneally, IP) and a metabolite of TRH, histidyl-proline diketopiperazine (DKP, 100 micrograms ICV). No significant effects on performance as measured by standard indices were observed. However, both TRH and AMP, but not DKP, significantly increased perseverative responding on one lever with respect to saline. In keeping with recent evidence, it is concluded that the traditional non-parametric signal detection parameters of sensitivity and bias are insensitive to certain strategies. Possible mechanisms for the perseveration of responding, and its relationship to stereotypic behaviour, are discussed in the light of the known effects of each compound on dopaminergic systems.

Effect of neurotensin, substance P and TRH on the regulation of dopamine receptors in rat brain

Rats were exposed for one week to either neurotensin (4 μg/h), substance P (3.3 μg/h), thyrotropin-releasing hormone (5 μg/h), or saline administered intracerebroventricularly via mini osmotic-pumps and either haloperidol (2.5 mg/kg i.p., 2 × daily) or vehicle control. The peptide treatments by themselves did not alter [ 3H]spiroperidol binding in either the nucleus accumbens or the striatum. Neurotensin, however, augmented the increase in [ 3H]spiroperidol binding caused by the haloperidol treatment in both the nucleus accumbens and striatum.

Evidence for dopaminergic and serotonergic regulation of prolactin cell activity in the trout Salmo gairdneri

The control of prolactin (PRL) cell activity in Salmo gairdneri was investigated in vivo and in vitro. In some in vivo experiments treatment was followed by estimation of pituitary PRL content by gel electrophoresis or of PRL cell nuclear area by light microscopy. In the remainder, treatment was followed by incubation of the pituitary glands in drug-free medium for estimation of PRL synthesis and release. The dopamine precursor, l-dopa (20 mg/kg), reduced pituitary PRL content. Conversely, the dopamine-receptor blocker, domperidone (10 mg/kg), increased total PRL content and amount released in the subsequent incubation. The initial serotonin precursor, l-tryptophan (75 mg/kg), increased pituitary PRL content and PRL cell nuclear area. 5-HTP (20 mg/kg), the immediate serotonin precursor, increased both percentage PRL release and total PRL levels during subsequent incubation. Pargyline (25 mg/kg) treatment to inhibit serotonin catabolism elevated PRL levels in pituitary and medium during subsequent incubation. The serotonin synthesis blocker, parachlorophenylalanine ( pCPA; 100 mg/kg), nonsignificantly reduced PRL cell nuclear area. When this was followed by incubation, percentage PRL release and total PRL fell significantly. During in vitro incubation, dopamine (2 μg/ml) reduced the release of PRL into the medium, while serotonin (10 −5 M) increased PRL release. These results suggest that both an inhibitory dopaminergic and a stimulatory serotonergic system may be involved in PRL cell regulation in S. gairdneri. The lack of any significant effect of cortisol (1 μg/ml), somatostatin (300 ng/ml, GABA (100 ng/ml) and TRH 100 ng/ml) on PRL release in vitro suggested little or no involvement of these putative regulatory factors in PRL cell regulation.

Effects of i.v. TRH on LH, FSH and GH serum levels in primary hypothyroid children

The occurrence of precocious puberty in children with primary hypothyroidism is a well recognized but poorly understood phenomenon. Several mechanisms have been proposed, but exceptions can be raised for each hypothesis (Lindsay et al, Am J Dis Child 134, 588, 1980). In 9/12 off-therapy primary hypothyroid children and adolescents (aged 1-16 yrs, 9 prepubertal and 3 pubertal) i.v. TRH (5μg/kg) injection elicited an evident increase of serum LH between 15 and 30 min, which exhausted within 90 min. Mean (± SEM) LH serum levels at 15 (2.22±0.45 ng/ml) and 30 min (2.22±0.43) were significantly (p 7 ng/ml)was also recorded in 3/12 hypothyroids. On the contrary FSH serum levels were not substantially modified by TRH bolus, either in the patient or in the control group. An overlap in the pituitary hormonal feed-back mechanism had been hypothesized to explain the occurrence of precocious puberty in primary thyroid failure (Van Wyk and Grumbach, J Pediatr 57, 416, 1960). The nonspecific LH-releasing effect of TRH in our patients especially evident in the pubertal ones supports such hypothesis.

Behavioral effects of TRH, 3-Me-His 2-TRH and naloxone in Hemichromis bimaculatus

Twenty minutes after each injection of TRH (2, 5, 10 micrograms/g bw), Hemichromis bimaculatus displayed frequent chafing (rubbing body on substrate) at rates as high as 65 times in 5 minutes. Chafing continued even after 5 hours. Such frequent displacement activities were not observed in untreated fish. These activities could not be suppressed completely with the opiate-antagonist naloxone. Five minutes after application of 3-Me-His2-TRH (1, 2, 5 micrograms/g bw), H. bimaculatus was chafing at higher rates compared to those injected with TRH. Calling movements, which are regulated by a very high prolactin level, also occurred in some fish. When naloxone (2, 4, 8, 12, 16 micrograms/g body weight) was injected, Hemichromis showed excessive spitting and chewing. When 16 micrograms naloxone was administered, H. bimaculatus started to tremble and tried to escape by the presence of a fish net. The fact that chafing was not completely suppressed after naloxone-application implies that naloxone may mediate opiate and non opiate effects [17].

Evidence for the role of calcium and diacylglycerol as dual second messengers in thyrotropin-releasing hormone action: involvement of Ca+2.

The studies reported here were directed toward establishing the mechanism by which TRH acutely stimulates PRL secretion in GH3 pituitary cells. Studies of TRH stimulation of PRL secretion were conducted on a time scale which enables comparison with other reported rapid effects of TRH on GH3 cells. TRH stimulation of secretion was found to be extremely rapid in onset (less than or equal to 10 sec) and biphasic (phase I, 0-2 min; phase II, 5-60 min). The earliest (phase I) secretory response was observed to be independent of medium Ca+2 concentration or Ca+2 influx, but to be dependent on an intracellular Ca+2 pool. The phase II response to TRH was found to depend, in part, on medium Ca+2. The phase I response to TRH could be mimicked only by agents known to influence Ca+2 translocation in GH3 cells (60 mM K+, A23187, ionomycin, carbonyl cyanide p-trifluoromethoxyphenylhydrazone, and carbonyl cyanide m-chlorophenylhydrazone). These agents failed to promote sustained PRL release characteristic of phase II. It is concluded that the ability of TRH to rapidly stimulate PRL secretion (phase I) is correlated with its ability to rapidly promote a transient cytoplasmic Ca+2 concentration rise from an intracellular Ca+2 pool.

Behavioral effects of thyrotropin releasing hormone in frontal decorticated rats

A low dose intracerebroventricular injection of thyrotropin releasing hormone (TRH, 100 ng) changed many behavioral responses in the rat. TRH increased locomotion, scratching, body shaking, piloerection, and rearing, but decreased sniffing, and resting. Ablation of frontal neocortex further enhanced the TRH effects on locomotion and resting. A dose effect of TRH (0, 5, 10, 50, 100 ng) to increase general activity was established and the effect was further enhanced by decortication. In our test situations decortication had no effect by itself. Since the TRH effects became much more pronounced without the frontal neocortex it appears that the cortex exerts a powerful inhibitory effect to moderate the TRH effects. The TRH effect does not depend upon the frontal cortex, actually a cortical function is to dampen the TRH effects on various behavioral responses.

Thyrotropin-Releasing Hormone Stimulates Body Weight Gain and Increases Thyroid Hormones and Growth Hormone in Plasma of Cockerels

The effect of TRH on growth in 4-week-old cockerels was examined in two separate experiments. Daily injection of TRH via the brachial vein stimulated growth in 4-week-old cockerels over 17 days of treatment in the first experiment and over 25 days in the second. In the first experiment, TRH at 1.0 micrograms and 10.0 micrograms/bird caused significant (P less than 0.05) increases of 12.0% and 12.4%, respectively, in growth rate, whereas in the second experiment, only the 1.0 micrograms/bird level of TRH caused an increase (P less than 0.05). In each experiment, the increase in body weight gain was not TRH dose dependent, and neither feed consumption nor feed efficiency was affected. Possible involvement of pituitary hormones in TRH-stimulated growth in cockerels was studied in a separate experiment, and the effects of TRH on plasma T3, T4, and GH were examined. TRH was given iv at 0.1, 1.0, and 10.0 micrograms/bird daily for 5 days, and plasma T3, T4, and GH concentrations were measured 15, 60, and 180 min postinjection by RIA on days 1, 3, and 5. The responses of T3 and T4 to TRH were greatest on day 1, were diminished by days 3 and 5, and were not dose-related. Significant, not dose-related, elevations of plasma GH concentrations were obtained at all doses of TRH. Based on these results, we suggest that TRH has the ability to promote a significant increase in body weight gain in 4-week-old cockerels, and the stimulatory effects may be mediated through GH and/or thyroid hormones.

Effects of TRH and somatostatin on releases of prolactin and growth hormone in vitro by the pituitary of Poecilia latipinna. II. Electron-microscopic morphometry using automatic image analysis.

Pituitary glands from a teleost fish were incubated in the presence of the synthetic hypophysiotropic peptides, thyrotrophin-releasing hormone and somatostatin, in two media of different osmotic pressure. The effects on prolactin and growth hormone cells were detected by electron-microscopic morphometry with the aid of an image analyser. Thyrotrophin-releasing hormone caused changes in prolactin cell ultrastructure consistent with stimulated hormone release and, in the low osmotic pressure medium, appeared to increase synthetic activity. There was no effect on growth hormone cells. After somatostatin treatment, both synthesis and release in prolactin cells appeared to be inhibited, and there was an obvious inhibition of synthesis and release in growth hormone cells. The response of both cell types to somatostatin did not appear to be dependent on the osmotic pressure of the medium.

Enrichment of rat anterior pituitary cell types by metrizamide density gradient centrifugation.

Anterior pituitary cells derived from hypothyroid rats were enzymatically dispersed and then fractionated by equilibrium sedimentation through a metrizamide density gradient. Populations relatively enriched in thyrotrophs, somatotrophs, mammotrophs, or gonadotrophs were obtained. The thyrotroph-enriched fraction was put into short term culture and tested for hormone responsiveness. TRH stimulated TSH release with an ED50 of 8 X 10(-10) M, and this TRH effect was inhibited by T3 with an ED50 of 1.6 X 10(-11) M free T3. Metrizamide density gradient centrifugation is a useful technique for obtaining relatively enriched populations of viable anterior pituitary cell types.

TRH effect on PRL, LH and GH release in the young Turkey, [formula omitted] and [formula omitted

TRH effect on PRL, LH and GH release in the young Turkey, [formula omitted] and [formula omitted