Amygdala kindling differentially regulates the expression of the elements involved in TRH transmission

Subthreshold electrical stimulation of the amygdala (kindling) activates neuronal pathways increasing the expression of several neuropeptides including thyrotropin releasing-hormone (TRH). Partial kindling enhances TRH expression and the activity or its inactivating ectoenzyme; once kindling is established (stage V), TRH and its mRNA levels are further increased but TRH-binding and pyroglutamyl aminopeptidase II (PPII) activity decreased in epileptogenic areas. To determine whether variations in TRH receptor binding or PPII activity are due to regulation of their synthesis, mRNA levels of TRH receptors (R1, R2) and PPII were semi-quantified by RT-PCR in amygdala, frontal cortex and hippocampus of kindled rats sacrificed at stage II or V. Increased mRNA levels of PPII were found at stage II in amygdala and frontal cortex, and of pro-TRH and TRH-R2, in amygdala and hippocampus. At stage V, pro-TRH mRNA levels increased and those of PPII, decreased in the three regions; TRH-R2 mRNA levels diminished in amygdala and frontal cortex and of TRH-R1 only in amygdala. In situ hybridization analyses revealed, at stage II, enhanced TRH-R1 mRNA levels in dentate gyrus and amygdala while decreased in piriform cortex; those of TRH-R2 increased in amygdala, CA2, dentate gyrus, piriform cortex, thalamus and subiculum and of PPII, in CAs and piriform cortex. In contrast, at stage V decreased expression of TRH-R1 occurred in amygdala, CA2/3, dentate gyrus and piriform cortex; of TRH-R2 in CA2, thalamus and piriform cortex, and of PPII in CA2, and amygdala. The magnitude of changes differed between ipsi and contralateral side. These results support a trans-synaptic modulation of all elements involved in TRH transmission in conditions that stimulate the activity of TRHergic neurons. They show that reported changes in PPII activity or TRH-binding caused by kindling relate to regulation of the expression of TRH receptors and degrading enzyme.     

Chronic ethanol or glucose consumption alter TRH content and pyroglutamyl aminopeptidase II activity in rat limbic regions

Thyrotropin-releasing hormone (TRH), its receptors and inactivating enzyme (PPII) are present in limbic regions. Nutritional changes or acute ethanol administration in male rats differentially modulate TRH or PPII expression. Chronic ethanol effect was studied in male (3, 6 and 8 weeks) and female rats (6 weeks) including naive and pair-fed (glucose) groups. Daily solid food and liquid intake, serum TSH and corticosterone, TRH content and PPII activity in limbic regions, were quantified. Gender differences were found in ethanol and total caloric intake and body weight gain, TSH and corticosterone levels. Ethanol consumption decreased TRH content and PPII activity in frontal cortex of male rats after 3-6 weeks. In contrast, glucose ingestion altered, by the third week, TRH content in amygdala, hippocampus, hypothalamus and nucleus accumbens, PPII activity in hippocampus and frontal cortex; by the sixth week, TRH content in amygdala and n. accumbens of male and females. Withdrawal at 24 h after 3-week ethanol ingestion decreased TRH content in amygdala and PPII activity in n. accumbens, while withdrawal from glucose reverted some of the effects produced by chronic glucose ingestion. Variations in TRH content or PPII activity support a region specific involvement of TRH neurons that depend on the treatment.     

Acute administration of alcohol modulates pyroglutamyl amino peptidase II activity and mRNA levels in rat limbic regions

Released TRH is inactivated by an ectopeptidase, pyroglutamyl aminopeptidase II (PPII). PPII expression and activity are stringently regulated in adenohypophysis, and in rat brain, during kindling stimulation that activates TRHergic neurons. To gain further insight into the possible regulation of PPII, we studied the effect of an acute intraperitoneal ethanol administration that affects TRH content and expression. PPII activity was determined by a fluorometric assay and PPII mRNA levels by semi-quantitative RT-PCR. Activity decreased in frontal cortex 1 h after ethanol injection and, after 6 h, in hippocampus, amygdala and n. accumbens. PPII mRNA levels decreased at 30 and 60 min in frontal cortex and n. accumbens while increased at longer times in these regions and, in hippocampus and hypothalamus. NMDA and GABA(A) receptors' agonists and antagonists were tested at 1 h (+/-ethanol) on PPII activity and mRNA levels, as well as on TRH content and its mRNA. In n. accumbens, PPII mRNA levels decreased by ethanol, MK-801, and muscimol while picrotoxin or NMDA reversed ethanol's inhibition. Ethanol decreased TRH content and increased TRH mRNA levels as MK-801 or muscimol did (NMDA or picrotoxin reverted the effect of ethanol). In frontal cortex, PPII activity was inhibited by ethanol, NMDA and MK-801 with ethanol; its mRNA levels were reduced by ethanol, MK-801 and muscimol (NMDA and picrotoxin reverted ethanol's inhibition). These results show that PPII expression and activity can be regulated in conditions where TRHergic neurons are modulated. Effects of ethanol on PPII mRNA levels as well as those of TRH and its mRNA may involve GABA or NMDA receptors in n. accumbens. Changes observed in frontal cortex suggest combined effects with stress. The response was region-specific in magnitude, tendency and kinetics. These results give further support for brain PPII regulation in conditions that modulate the activity of TRHergic neurons.     

3,3',5'-triiodo-L-thyronine reduces efficiency of mRNA knockdown by antisense oligodeoxynucleotides: a study with pyroglutamyl aminopeptidase II in adenohypophysis

The impact of hormones on the efficacy of antisense oligodeoxynucleotides (ASOs) is a poorly analyzed subject. We designed, based on the identification of potentially favorable local elements of mRNA secondary structure, eight phosphorothioate ASOs to knock down the expression of an ectopeptidase, pyroglutamyl aminopeptidase II (PPII), in primary cultures of adenohypophysis. Two of the PPII ASOs were very efficient, sequence-specific, and target-specific. Because the expression of PPII is upregulated by 3,3',5'-triiodo-L-thyronine (T3), we studied the impact of varying the protocol of PPII induction on the knockdown efficacy. Hormone removal at transfection increased markedly the ability of (1) PPII ASOs to reduce PPII mRNA levels or PPII activity in adenohypophyseal cells or in C6 rat glioma cells and (2) a thyrotropin-releasing hormone (TRH) receptor-1 (TRH-R1) ASO to reduce TRH-R1 mRNA levels in adenohypophyseal cells. There was no effect of hormone removal on transfection efficacy and no correlation between target mRNA levels and ASO efficacy. These data demonstrated that ASO efficacy could depend on T3 levels; this might be due to regulation of a step generally critical for ASO efficiency.     

Differential response of TRHergic neurons of the hypothalamic paraventricular nucleus (PVN) in female animals submitted to food-restriction or dehydration-induced anorexia and cold exposure

TRH neurons of the hypothalamic paraventricular nucleus (PVN), regulate pituitary-thyroid axis (HPT). Fasting activates expression of orexigenic peptides from the arcuate nucleus, increases corticosterone while reduces leptin, and pro-TRH mRNA levels despite low serum thyroid hormone concentration (tertiary hypothyroidism). TRH synthesis is positively regulated by anorexigenic peptides whose expression is reduced in fasting. The model of dehydration-induced anorexia (DIA) leads to decreased voluntary food intake but peptide expression in the arcuate is similar to forced-food restriction (FFR), where animals remain hungered. We compared the response of HPT axis of female Wistar rats submitted to DIA (2.5% saline solution, food ad libitum, 7 days) with FFR (provided with the amount of food ingested by DIA) and na?ve (N) group fed ad libitum, as well as their response to acute cold exposure. Pro-TRH and pro-CRH mRNA levels in the PVN were measured by RT-PCR, TRH content, serum concentration of TSH and thyroid hormones by radioimmunoassay. DIA rats reduced 80% their food consumption compared to N, decreased PVN pro-CRH expression, serum estradiol and leptin levels, increased corticosterone similar to FFR. HPT axis of DIA animals failed to adapt: FFR presented tertiary hypothyroidism and DIA, primary. Response to cold stimulation leading to increased pro-TRH mRNA levels and TRH release was preserved under reduced energy availability in FFR rats but not in DIA, although the dynamics of hormonal release differed: TSH release augmented only in na?ve; thyroxine in all but highest in DIA, and triiodothyronine in FFR and DIA suggesting a differential regulation of deiodinases.     

Chronic ingestion of ethanol or glucose solutions affects hypothalamic and limbic TRH metabolism in dams and their pups

The effect of chronic ethanol consumption during pregnancy and lactation on thyrotropin releasing hormone (TRH) metabolism was investigated in the hypothalamus and limbic areas of female rats and their weaned pups. Pregnant female rats received ethanol or isocaloric glucose solution during pregnancy either alone, or also during the 3 weeks of lactation. Thyrotropin (TSH) and corticosterone levels were measured in serum; TRH and TRH-gly concentrations were determined in hypothalamus, hippocampus, n.accumbens, frontal cortex and amygdala of dams and pups at 21 days after parturition. Ethanol or glucose consumption during pregnancy and lactation produced a decrease in TSH levels compared with control animals fed at libitum; water replacement during lactation normalized TSH levels only in glucose-fed dams. Pups from ethanol or pair-fed dams showed low weight and increased TSH levels compared with normal rats. Variations in TRH metabolism were detected in limbic areas. Chronic ethanol caused a decrease in the levels of TRH in the hippocampus and frontal cortex of dams. In contrast, glucose chronic ingestion increased TRH content specifically in n.accumbens and amygdala of dams. Most of the variations in TRH content of limbic areas of pups were not specific for glucose or ethanol treatment and correlated with the deleterious effect of the mother's thyroid condition, although some differences were observed depending on pup's gender. These results support the involvement of TRHergic neurons in the limbic system of the female rat exposed to alcohol or glucose during pregnancy and lactation.     

Differential regulation of thyrotropin-releasing hormone receptor mRNA levels by thyroid hormone in vivo and in vitro (GH3 cells).

We studied the effect of thyroid status on thyrotropin-releasing hormone receptor (TRH-R) mRNA levels both in vivo and in vitro (GH3 cells) using a cloned rat TRH-R cDNA by RT-PCR. Experimental hypothyroid rats were produced by total thyroidectomy and were then killed 7 days after the operation. TRH receptor binding in the anterior pituitary and serum TSH level were elevated approximately 2-fold and 8-fold, respectively, in 7 day thyroidectomized rats. TRH-R mRNA levels in hypothyroid rats were also increased significantly compared with those of normal rats. In GH3 cells, however, no significant change of TRH-R mRNA level was observed between cultures treated with triiodothyronine (T3, 10(-9) and 10(-7) M) and the untreated group. The present data indicate that 1) the in vivo effects of thyroid status on TRH-R mRNA levels differ from the in vitro one, and that 2) the down regulation of TRH-R binding by thyroid hormone in GH3 cells may be mediated by translational or post-translational mechanisms.     

Hypothalamic neuronal histamine mediates the thyrotropin-releasing hormone-induced suppression of food intake

We examined the involvement of thyrotropin-releasing hormone (TRH) and TRH type 1 and 2 receptors (TRH-R1 and TRH-R2, respectively) in the regulation of hypothalamic neuronal histamine. Infusion of 100 nmol TRH into the rat third cerebroventricle (3vt) significantly decreased food intake (p < 0.05) compared to controls infused with phosphate- buffered saline. This TRH-induced suppression of food intake was attenuated partially in histamine-depleted rats pre-treated with alpha-fluoromethylhistidine (a specific suicide inhibitor of histidine decarboxylase) and in mice with targeted disruption of histamine H1 receptors. Infusion of TRH into the 3vt increased histamine turnover as assessed by pargyline-induced accumulation of tele-methylhistamine (t-MH, a major metabolite of neuronal histamine in the brain) in the tuberomammillary nucleus (TMN), the paraventricular nucleus, and the ventromedial hypothalamic nucleus in rats. In addition, TRH-induced decrease of food intake and increase of histamine turnover were in a dose-dependent manner. Microinfusion of TRH into the TMN increased t-MH content, histidine decarboxylase (HDC) activity and expression of HDC mRNA in the TMN. Immunohistochemical analysis revealed that TRH-R2, but not TRH-R1, was expressed within the cell bodies of histaminergic neurons in the TMN of rats. These results indicate that hypothalamic neuronal histamine mediates the TRH-induced suppression of feeding behavior.     

Generation of thyrotropin-releasing hormone receptor 1-deficient mice as an animal model of central hypothyroidism

To provide an animal model of central hypothyroidism, mice deficient in the TRH-receptor 1 (TRH-R1) gene were generated by homologous recombination. The pituitaries of TRH-R1-/- mice are devoid of any TRH-binding capacity, demonstrating that TRH-R1 is the only receptor localized on TRH target cells of the pituitary. With the exception of some retardation in growth rate, TRH-R1-/- mice appear normal, but compared with control animals they exhibit a considerable decrease in serum T(3), T(4), and prolactin (PRL) levels but not in serum TSH levels. In situ hybridization histochemistry and real-time RT-PCR analysis revealed that in adult TRH-R1-/- animals TSHbeta-mRNA expression is not impaired whereas PRL mRNA and GH mRNA levels are considerably reduced compared with control mice. The numbers of thyrotropes, somatotropes, and lactotropes, however, are not affected by the deletion of the TRH-R1 gene. The mutant mice are fertile, and the dams nourish their pups well, indicating that TRH is not a decisive factor for suckling-induced PRL release. In situ hybridization and quantitative RT-PCR analysis, furthermore, revealed that, as in control animals, pituitary PRL-mRNA expression in TRH-R1-/- is considerably increased during lactation, albeit strongly reduced as compared with lactating control animals.     

Acute ethanol administration induces changes in TRH and proenkephalin expression in hypothalamic and limbic regions of rat brain

Thyrotropin releasing hormone (TRH) present in several brain areas has been proposed as a neuromodulator. Its administration produces opposite effects to those observed with acute ethanol consumption. Opioid peptides, in contrast, have been proposed to mediate some of the effects of alcohol intoxication. We measured TRH content and the levels of its mRNA in hypothalamic and limbic zones 1–24 h after acute ethanol injection. We report here fast and transient changes in the content of TRH and its mRNA in these areas. The levels of proenkephalin mRNA varied differently from those of proTRH mRNA, depending on the time and region studied. Wistar rats were administered one dose of ethanol (intraperitoneal, 3 g/kg body weight) and brains dissected in hypothalamus, hippocampus, amygdala, n. accumbens and frontal cortex, for TRH quantification by radioimmunoassay or for proTRH mRNA measurement by RT-PCR. After 1 h injection, TRH levels were increased in hippocampus and decreased in n. accumbens; after 4 h, it decreased in the hypothalamus, frontal cortex and amygdala, recovering to control values in all regions at 24 h. ProTRH mRNA levels increased at 1 h post-injection in total hypothalamus and hippocampus, while they decreased in the frontal cortex. The effect of ethanol was also studied in primary culture of hypothalamic cells; a fast and transient increase in proTRH mRNA was observed at 1 h of incubation (0.001% final ethanol concentration). Changes in the mRNA levels of proTRH and proenkephalin were quantified by in situ hybridization in rats administered ethanol intragastrically (2.5 g/kg). Opposite alterations were observed for these two mRNAs in hippocampus and frontal cortex, while in n. accumbens and the paraventricular nucleus of the hypothalamus, both mRNA levels were increased but with different kinetics. These results give support for TRH and enkephalin neurons as targets of ethanol and, as possible mediators of some of its observed behavioral effects.     

Thyrotropin-releasing hormone (TRH) and phorbol myristate acetate decrease TRH receptor messenger RNA in rat pituitary GH3 cells: evidence that protein kinase-C mediates the TRH effect.

In a previous report we showed that TRH-induced down-regulation of the density of its receptors (TRH-Rs) on rat pituitary tumor (GH3) cells was preceded by a decrease in the activity of the mRNA for the TRH-R, as assayed in Xenopus oocytes. Here we report the effects of TRH, elevation of cytoplasmic free Ca2+ concentration, phorbol myristate acetate (PMA), and H-7 [1-(5-isoquinolinesulfonyl)2-methylpiperazine dihydrochloride], an inhibitor of protein kinases, on the levels of TRH-R mRNA, which were measured by Northern analysis and in nuclease protection assays using probes made from mouse pituitary TRH-R cDNA, in GH3 cells. These agents were studied to gain insight into the mechanism of the TRH effect, because signal transduction by TRH involves generation of inositol 1,4,5-trisphosphate and elevation of cytoplasmic free Ca2+ concentration, which leads to activation of Ca2+/calmodulin-dependent protein kinase, and of 1,2-diacylglycerol, which leads to activation of protein kinase-C. TRH (1 microM TRH, a maximally effective dose) caused a marked transient decrease in TRH-R mRNA that attained a nadir of 20-45% of control by 3-6 h, increased after 9 h, but was still below control levels after 24 h. Elevation of the cytoplasmic free Ca2+ concentration had no effect on TRH-R mRNA. A maximally effective dose of PMA (1 microM) caused decreases in TRH-R mRNA that were similar in magnitude and time course to those induced by 1 microM TRH. H-7 (20 microM) blocked the effects of TRH and PMA to lower TRH-R mRNA to similar extents.(ABSTRACT TRUNCATED AT 250 WORDS)     

Expression of thyrotropin-releasing hormone receptors in rat testis and their role in isolated Leydig cells

Thyrotropin-releasing hormone (TRH) was initially discovered as a neuropeptide synthesized in the hypothalamus. Receptors for this hormone include TRH-receptor-1 (TRH-R1) and -2 (TRH-R2). Previous studies have shown that TRH-R1 and TRH-R2 are localized exclusively in adult Leydig cells (ALCs). We have investigated TRH-R1 and TRH-R2 expression in the testes of postnatal 8-, 14-, 21- 35-, 60-, and 90-day-old rats and in ethane dimethane sulfonate (EDS)-treated adult rats by using Western blotting, immunohistochemistry, and immunofluorescence. The effects of TRH on testosterone secretion of primary cultured ALCs from 90-day-old rats and DNA synthesis in Leydig cells from 21-day-old rats have also been examined. Western blotting and immunohistochemistry demonstrated that TRH-R1 and TRH-R2 were expressed in fetal Leydig cells (in 8-day-old rats) and in all stages of adult-type Leydig cells during development. Immunofluorescence double-staining revealed that newly regenerated Leydig cells in post-EDS 21-day rats expressed TRH-R1 and TRH-R2 on their first reappearance. Incubation with various doses of TRH affected testosterone secretion of primary cultured ALCs. Low concentrations of TRH (0.001, 0.01, and 0.1 ng/ml) inhibited basal and human chorionic gonadotrophin (hCG)-stimulated testosterone secretion of isolated ALCs, whereas relatively high doses of TRH (1 and 10 ng/ml) increased hCG-stimulated testosterone secretion. As detected by a 5-bromo-2′-deoxyuridine incorporation test, the DNA synthesis of Leydig cells from 21-day-old rats was promoted by low TRH concentrations. Thus, we have clarified the effect of TRH on testicular function: TRH might regulate the development of Leydig cells before maturation and the secretion of testosterone after maturation. This research was supported by grants from the National Natural Science Foundation of China (nos. 39870109 and 30370750).     

Leptin regulates prothyrotropin-releasing hormone biosynthesis. Evidence for direct and indirect pathways

The hypothalamic-pituitary-thyroid axis is down-regulated during starvation, and falling levels of leptin are a critical signal for this adaptation, acting to suppress preprothyrotropin-releasing hormone (prepro-TRH) mRNA expression in the paraventricular nucleus of the hypothalamus. This study addresses the mechanism for this regulation, using primary cultures of fetal rat hypothalamic neurons as a model system. Leptin dose-dependently stimulated a 10-fold increase in pro-TRH biosynthesis, with a maximum response at 10 nm. TRH release was quantified using immunoprecipitation, followed by isoelectric focusing gel electrophoresis and specific TRH radioimmunoassay. Leptin stimulated TRH release by 7-fold. Immunocytochemistry revealed that a substantial population of cells expressed TRH or leptin receptors and that 8-13% of those expressing leptin receptors coexpressed TRH. Leptin produced a 5-fold induction of luciferase activity in CV-1 cells transfected with a TRH promoter and the long form of the leptin receptor cDNA. Although the above data are consistent with a direct ability of leptin to promote TRH biosynthesis through actions on TRH neurons, addition of alpha-melanocyte-stimulating hormone produced a 3.5-fold increase in TRH biosynthesis and release, whereas neuropeptide Y treatment suppressed pro-TRH biosynthesis approximately 3-fold. Furthermore, the melanocortin-4 receptor antagonist SHU9119 partially inhibited leptin-stimulated TRH release from the neuronal culture. Consequently, our data suggest that leptin regulates the TRH neurons through both direct and indirect pathways.     

Involvement of individual hippocampal signaling protein levels in spatial memory formation is strain-dependent

Although a series of signaling cascades involved in spatial memory have been identified, their link to spatial memory and strain-dependent expression has not been reported so far. Hippocampal levels of the abovementioned signaling proteins were determined in laboratory inbred strain C57BL/6J, the wild-derived inbred strain PWD/PhJ and the wild caught mouse Apodemus sylvaticus (AS) by immunoblotting. The resulting hippocampal protein levels were correlated with results from MWM. Hippocampal signaling protein (hSP) levels were tested also in yoked controls. Within-strain comparison between trained and yoked controls revealed significant differences between levels of Phospho-CaMKII (alpha), Phospho-CREB, Egr-1, c-Src, Phospho-ERK5, Phospho-MEK5 and NOS1 in all of the three strains tested. In addition, the three strains revealed different involvement of individual hSP levels clearly indicating that individual mouse strains were linked to individual hSPs in spatial memory. Phospho-ERK5 levels were not detectable in hippocampi of yoked controls of each strain. We learn from this study that a series of hSPs are associated with spatial memory and that different hSPs are linked to spatial memory in different strains that show different outcome in the MWM. Even correlational patterns in the individual hSPs differed between mouse strains. This is of importance for the interpretation of previous studies on the abovementioned signaling cascades as well as for the design of future studies on these hippocampal proteins. It is intriguing that individual mouse strains, laboratory or wild caught, may use different signaling pathways for spatial memory in the Morris water maze.     

大鼠睾丸发育过程中促甲状腺激素释放激素受体mRNA的表达

为研究促甲状腺激素释放激素受体(TRHR)在大鼠睾丸组织中的表达规律和在生殖发育调节中的作用,依据大鼠垂体中的TRH-RcDNA设计引物,采用RT-PCR法从大鼠睾丸组织中获得了TRH-R的cDNA克隆,测序表明其核苷酸序列与大鼠垂体中的TRH-RcDNA序列完全一致.应用非放射性原位杂交(NR-ISH)技术观察TRH-RmRNA在大鼠睾丸中的定位,结果显示杂交信号集中在间质细胞中,生精细胞无杂交信号.利用实时动态定量RT-PCR法观察了TRH-R在不同发育阶段大鼠睾丸中的表达变化,发现在睾丸间质细胞发育的初期阶段(第8天),没有TRH-R的表达,但从第15天起能观察到TRH-R的表达,并且表达量在20天、35天、60天、90天逐渐增加.这些结果表明,大鼠睾丸组织间质细胞能特异性表达TRH-R,并且表达量与发育过程相关.     

Activity-induced and developmental downregulation of the Nogo receptor

The three axon growth inhibitory proteins, myelin associated glycoprotein, oligodendrocyte-myelin glycoprotein and Nogo-A, can all bind to the Nogo-66 receptor (NgR). This receptor is expressed by neurons with high amounts in regions of high plasticity where Nogo expression is also high. We hypothesized that simultaneous presence of high levels of Nogo and its receptor in neurons confers a locked state to hippocampal and cortical microcircuitry and that one or both of these proteins must be effectively and temporarily downregulated to permit plastic structural changes underlying formation of long-term memory. Hence, we subjected rats to kainic acid treatment and exposed rats to running wheels and measured NgR mRNA levels by quantitative in situ hybridization at different time points. We also studied spinal cord injuries and quantified NgR mRNA levels in spinal cord and ganglia during a critical postnatal period using real-time PCR. Strikingly, kainic acid led to a strong transient downregulation of NgR mRNA levels in gyrus dentatus, hippocampus, and neocortex during a time when BDNF mRNA was upregulated instead. Animals exposed to running wheels for 3 and 7, but not 1 or 21, days showed a significant downregulation of NgR mRNA in cortex, hippocampus and the dentate gyrus. NgR mRNA levels decreased from high to low expression in spinal cord and ganglia during the first week of life. No robust regulation of NgR was observed in the spinal cord following spinal cord injury. Together, our data show that NgR levels in developing and adult neurons are regulated in vivo under different conditions. Strong, rapid and transient downregulation of NgR mRNA in response to kainic acid and after wheel running in cortex and hippocampus suggests a role for NgR and Nogo-A in plasticity, learning and memory.     

Thyroid hormone regulation of messenger ribonucleic acid encoding thyrotropin (TSH)-releasing hormone is independent of the pituitary gland and TSH

Cellular levels of mRNA encoding pro TRH in the rostral paraventricular nucleus are reduced by thyroid hormones. To determine whether this regulatory effect of thyroid hormones requires a functional pituitary gland or, specifically, TSH, we examined the effect of T3 on proTRH mRNA in hypophysectomized, thyro-parathyroidectomized male rats with or without bovine TSH replacement. Hypophysectomy plus thyro-parathyroidectomy reduced serum T4 and TSH to undetectable levels in all animals and elevated TRH mRNA in the paraventricular nucleus over that of sham-operated animals. Eleven consecutive daily injections of T3 significantly reduced TRH mRNA levels in both sham controls and thyro-parathyroidectomized rats. However, 11 daily injections of bovine TSH (1 U/day) failed to alter the effect of T3 on TRH mRNA levels. These results demonstrate that the regulatory influence of thyroid hormones on the biosynthesis of TRH within the thyrotropic center of the brain is independent of the pituitary gland and of TSH.