[ß-Glu2]TRH Is a Functional Antagonist of Thyrotropin-Releasing Hormone (TRH) in the Rodent Brain.

Selective antagonists of thyrotropin-releasing hormone (TRH; pGlu-His-Pro-NH2), in order to enable a better understanding of this peptide's central functions, have not been identified. Using pGlu-Glu-Pro-NH2 ([Glu2]TRH) as a lead peptide and with modification at its central residue, our studies focused on some of its analogues synthesized as potential functional antagonists of TRH in the rodent brain. Among the peptides studied, the novel isomeric analogue [ß-Glu2]TRH was found to suppress the analeptic and antidepressant-like pharmacological activities of TRH without eliciting intrinsic effects in these paradigms. [ß-Glu2]TRH also completely reversed TRH's stimulation of acetylcholine turnover in the rat hippocampus without a cholinergic activity of its own, which was demonstrated through in vivo microdialysis experiments. Altogether, [ß-Glu2]TRH emerged as the first selective functional antagonist of TRH's prominent cholinergic actions, by which this endogenous peptide elicits a vast array of central effects.

Cardiac Thyrotropin-releasing Hormone Inhibition Improves Ventricular Function and Reduces Hypertrophy and Fibrosis After Myocardial Infarction in Rats.

BACKGROUND: Cardiac thyrotropin-releasing hormone (TRH) is a tripeptide with still unknown functions. We demonstrated that the left ventricle (LV) TRH system is hyperactivated in spontaneously hypertensive rats and its inhibition prevented cardiac hypertrophy and fibrosis. Therefore, we evaluated whether in vivo cardiac TRH inhibition could improve myocardial function and attenuate ventricular remodeling in a rat model of myocardial infarction (MI). METHODS AND RESULTS: In Wistar rats, MI was induced by a permanent left anterior descending coronary artery ligation. A coronary injection of a specific small interfering RNA against TRH was applied simultaneously. The control group received a scrambled small interfering RNA. Cardiac remodeling variables were evaluated one week later. In MI rats, TRH inhibition decreased LV end-diastolic (1.049 ± 0.102 mL vs 1.339 ± 0.102 mL, P < .05), and end-systolic volumes (0.282 ± 0.043 mL vs 0.515 ± 0.037 mL, P < .001), and increased LV ejection fraction (71.89 ± 2.80% vs 65.69 ± 2.85%, P < .05). Although both MI groups presented similar infarct size, small interfering RNA against TRH treatment attenuated the cardiac hypertrophy index and myocardial interstitial collagen deposition in the peri-infarct myocardium. These effects were accompanied by attenuation in the rise of transforming growth factor-ß, collagen I, and collagen III, as well as the fetal genes (atrial natriuretic peptide, B-type natriuretic peptide, and beta myosin heavy chain) expression in the peri-infarct region. In addition, the expression of Hif1α and vascular endothelial growth factor significantly increased compared with all groups. CONCLUSIONS: Cardiac TRH inhibition improves LV systolic function and attenuates ventricular remodeling after MI. These novel findings support the idea that TRH inhibition may serve as a new therapeutic strategy against the progression of heart failure.

Bromocriptine inhibits salsolinol-induced prolactin release in male goats.

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

Thyroxine-induced expression of pyroglutamyl peptidase II and inhibition of TSH release precedes suppression of TRH mRNA and requires type 2 deiodinase.

Suppression of TSH release from the hypothyroid thyrotrophs is one of the most rapid effects of 3,3',5'-triiodothyronine (T(3)) or thyroxine (T(4)). It is initiated within an hour, precedes the decrease in TSHß mRNA inhibition and is blocked by inhibitors of mRNA or protein synthesis. TSH elevation in primary hypothyroidism requires both the loss of feedback inhibition by thyroid hormone in the thyrotrophs and the positive effects of TRH. Another event in this feedback regulation may be the thyroid hormone-mediated induction of the TRH-inactivating pyroglutamyl peptidase II (PPII) in the hypothalamic tanycytes. This study compared the chronology of the acute effects of T(3) or T(4) on TSH suppression, TRH mRNA in the hypothalamic paraventricular nucleus (PVN), and the induction of tanycyte PPII. In wild-type mice, T(3) or T(4) caused a 50% decrease in serum TSH in hypothyroid mice by 5  h. There was no change in TRH mRNA in PVN over this interval, but there was a significant increase in PPII mRNA in the tanycytes. In mice with genetic inactivation of the type 2 iodothyronine deiodinase, T(3) decreased serum TSH and increased PPII mRNA levels, while T(4)-treatment was ineffective. We conclude that the rapid suppression of TSH in the hypothyroid mouse by T(3) occurs prior to a decrease in TRH mRNA though TRH inactivation may be occurring in the median eminence through the rapid induction of tanycyte PPII. The effect of T(4), but not T(3), requires the type 2 iodothyronine deiodinase.

Regulation of thyrotropin-releasing hormone-expressing neurons in paraventricular nucleus of the hypothalamus by signals of adiposity.

Fasting-induced suppression of thyroid hormone levels is an adaptive response to reduce energy expenditure in both humans and mice. This suppression is mediated by the hypothalamic-pituitary-thyroid axis through a reduction in TRH levels expressed in neurons of the paraventricular nucleus of the hypothalamus (PVN). TRH gene expression is positively regulated by leptin. Whereas decreased leptin levels during fasting lead to a reduction in TRH gene expression, the mechanisms underlying this process are still unclear. Indeed, evidence exists that TRH neurons in the PVN are targeted by leptin indirectly via the arcuate nucleus, whereas correlative evidence for a direct action exists as well. Here we provide both in vivo and in vitro evidence that the activity of hypothalamic-pituitary-thyroid axis is regulated by both direct and indirect leptin regulation. We show that both leptin and α-MSH induce significant neuronal activity mediated through a postsynaptic mechanism in TRH-expressing neurons of PVN. Furthermore, we provide in vivo evidence indicating the contribution of each pathway in maintaining serum levels of thyroid hormone.

Knocking down the diencephalic thyrotropin-releasing hormone precursor gene normalizes obesity-induced hypertension in the rat.

We recently showed that diencephalic TRH may mediate the central leptin-induced pressor effect. Here, to study the role of TRH in obesity-induced hypertension (OIH), we used a model of OIH produced by a high-fat diet (HFD, 45 days) in male Wistar rats. After 4 wk, body weight and systolic arterial blood pressure (SABP) increased in HFD animals. Plasma leptin was correlated with peritoneal adipose tissue. Then, we treated OIH animals with an antisense oligodeoxynucleotide and small interfering (si)RNA against the prepro-TRH. Antisense significantly decreased diencephalic TRH content and SABP at 24 and 48 h posttreatment. Similar effects were observed with siRNA against prepro-TRH but for up to 4 wk. Conversely, vehicle, an inverted antisense sequence and siRNA against green fluorescence protein, produced no changes. SABP decrease seems to be owing to an inhibition of the obesity-enhanced sympathetic outflow but not to an alteration in thyroid status. Using a simple OIH model we demonstrated, for the first time, that central TRH participates in the hypertension induced by body weight gain probably through its well-known action on sympathetic activity. Thus the TRH-leptin interaction may contribute to the strong association between hypertension and obesity.

Regulation of body temperature by thyrotropin-releasing hormone in neonatal chicks.

To understand thermal regulation of neonatal chicks, the contribution of thyrotropin-releasing hormone (TRH), a key regulator of the hypothalamus-pituitary-thyroid axis, was investigated. Intracerebroventricular (i.c.v.) injection of TRH (5 and 20 microg) increased body temperature, but did not change plasma T3 and T4 concentrations. Intraperitoneal (i.p.) injection of triiodothyronine (T3) and thyroxine (T4) did not influence body temperature. Thereafter, the relationships between TRH and the hypothalamus-pituitary-adrenal axis and sympathetic nervous system were further investigated. Central TRH stimulated both corticosterone and epinephrine release. The i.c.v. injection of a corticotropin-releasing factor receptor antagonist attenuated the change in body temperature and corticosterone concentration caused by TRH, but did not influence plasma T3 and T4 concentrations. The i.p. injection of epinephrine did not induce hyperthermia. Therefore, the thermoregulatory response to TRH may differ in neonatal stages being dependent upon the stimulation of the hypothalamus-pituitary-adrenal axis rather than the hypothalamus-pituitary-thyroid axis.

Annexin 5 inhibits thyrotropin releasing hormone (TRH) stimulated prolactin release in the primary culture of rat anterior pituitary cells.

Annexin 5, a novel calcium-phospholipid binding protein, is thought to be involved in hormone secretion by the anterior pituitary gland. Gonadotropin releasing hormone stimulates annexin 5 synthesis, which, in turn, enhances gonadotoropin secretion. On the other hand, annexin 5 was shown to inhibit prolactin release in vitro. To understand the nature of the opposing effects of annexin 5 on these two major pituitary hormones, the present study examines the inhibitory effect of annexin 5 on prolactin release in relation to thyrotropin stimulating hormone (TRH) using primary cultures of anterior pituitary cells of adult female rats. While recombinant rat annexin 5 was found to have little effect on basal prolactin release, it significantly inhibited TRH-stimulated prolactin release. Addition of specific anti-annexin 5 serum to the culture increased basal prolactin release in a concentration dependent manner, and no further increase in prolactin release was observed following application of TRH in the presence of anti-annexin 5. The enhanced basal prolactin release induced by anti-annexin 5 was reversed by the simultaneous administration of indomethacin, an inhibitor of cyclooxygenase. These results demonstrate that endogenous pituitary annexin 5 exerts an inhibitory effect on prolactin release and suggest that this is attained by suppression of eicosanoid synthesis in vitro.

Stress- as well as suckling-induced prolactin release is blocked by a structural analogue of the putative hypophysiotrophic prolactin-releasing factor, salsolinol.

Prolactin is secreted from the anterior lobe of the pituitary gland in response both to suckling and to stress. We recently observed that 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol), produced in the neurointermediate lobe of the pituitary gland, as well as in the medial basal hypothalamus, can selectively release prolactin from the anterior pituitary. Therefore, it has been proposed that salsolinol is a putative endogenous prolactin-releasing factor (PRF). Here, we report that one structural analogue of salsolinol, 1-methyl-3,4-dihydroisoquinoline (1MeDIQ), can block salsolinol-induced release of prolactin, but does not affect prolactin release in response to thyrotropin releasing hormone (TRH), alpha-methyl-p-tyrosine (alpha MpT) (an inhibitor of tyrosine hydroxylase), domperidone (a D(2) dopamine receptor antagonist), or 5-hydroxytryptophan (5-HTP), a precursor of serotonin). 1MeDIQ profoundly inhibited suckling-, immobilization-, as well as formalin-stress induced prolactin release without any influence on corticosterone secretion. The 1MeDIQ-induced reduction in prolactin response to immobilization stress was dose-dependent. These results suggest that salsolinol can play a pivotal role in the regulation of prolactin release induced by either physiological (suckling) or environmental (stress) stimuli.

Deletion of the Nhlh2 transcription factor decreases the levels of the anorexigenic peptides alpha melanocyte-stimulating hormone and thyrotropin-releasing hormone and implicates prohormone convertases I and II in obesity.

Body weight is controlled by the activation of signal transduction pathways in both the brain and peripheral tissues. Interestingly, although many hypothalamic neuropeptides and receptors have been implicated in the regulation of body weight, the transcriptional and posttranscriptional mechanisms through which these genes are expressed in response to changes in energy balance remain unclear. Our laboratory studies a mouse in which targeted deletion of the neuronal basic helix-loop-helix (bHLH) transcription factor, nescient helix-loop-helix 2 protein (Nhlh2), results in adult-onset obesity. The aim of this work was to use the phenotype of the Nhlh2 knockout mouse and the expression pattern of Nhlh2 to identify genes that are regulated by this transcription factor. In this article, we show that Nhlh2 is expressed throughout the adult hypothalamus. Using dual-label in situ hybridization, we demonstrate that, in the arcuate nucleus of the adult hypothalamus (ARC), Nhlh2 expression can be found in rostral proopiomelanocortin (POMC) neurons, whereas in the paraventricular nucleus (PVN), Nhlh2 is expressed in TRH neurons. In addition, we find that hypothalamic POMC-derived alphaMSH in the ARC and TRH in the PVN are regulated posttranscriptionally via Nhlh2-mediated control of prohormone convertase I and II mRNA levels. This is the first report in which regulation of body weight is linked to the action of a neuronal bHLH transcription factor on prohormone convertase mRNA levels. Furthermore, this work supports a direct role for transcriptional control of neuropeptide processing enzymes in the etiology of adult-onset obesity.

Ortho-substituted polychlorinated biphenyl (PCB) congeners (95 or 101) decrease pituitary response to thyrotropin releasing hormone.

Polychlorinated biphenyl compounds (PCBs) are global environmental contaminants that cause disruption of the endocrine system in humans and wildlife. Recently, we reported that acute exposures to ortho-PCB congeners 95 (2,3,6-2',5') or 101 (2,4,5,-2',5') causes changes in the performance of the hypothalamo-pituitary-thyroid (HPT)-axis in developing rats through mechanism(s) not yet clear. The functionality of the HPT-axis was evaluated by using the thyrotropin releasing hormone (TRH) test following acute exposure to PCBs 95 or 101. Weanling female rats received PCBs 95 or 101 intraperitoneally (ip) at 32 mg/kg for 2 consecutive days and synthetic TRH was given 48 h after the last dose. Serum thyroxine (T4) levels decreased following exposure to both the congeners. In PCB 95-treated rats, serum thyroid stimulating hormone (TSH) levels were elevated in response to TRH, but were only 40% of the control response to TRH. No significant changes were seen in serum prolactin (PRL), hypothalamic dopamine (DA), thyroid gland morphology, or epithelial cell proliferation. It is suggested that these congeners, interfere with the HPT-axis by causing a subnormal response of the pituitary and thyroid to TRH stimulation.

Differential regulation of pituitary hormone secretion and gene expression by thyrotropin-releasing hormone. A role for mitogen-activated protein kinase signaling cascade in rat pituitary GH3 cells.

We examined the possible involvement of mitogen-activated protein (MAP) kinase activation in the secretory process and gene expression of prolactin and growth hormone. Thyrotropin-releasing hormone (TRH) rapidly stimulated the secretion of both prolactin and growth hormone from GH3 cells. Secretion induced by TRH was not inhibited by 50 microM PD098059, but was completely inhibited by 1 microM wortmannin and 10 microM KN93, suggesting that MAP kinase does not mediate the secretory process. Stimulation of GH3 cells with TRH significantly increased the mRNA level of prolactin, whereas expression of growth hormone mRNA was largely attenuated. The increase in prolactin mRNA stimulated by TRH was inhibited by addition of PD098059, and the decrease in growth hormone mRNA was also inhibited by PD098059. Transfection of the cells with a pFC-MEKK vector (a constitutively active MAP kinase kinase kinase), significantly increased the synthesis of prolactin and decreased the synthesis of growth hormone. These data taken together indicate that MAP kinase mediates TRH-induced regulation of prolactin and growth hormone gene expression. Reporter gene assays showed that prolactin promoter activity was increased by TRH and was completely inhibited by addition of PD098059, but that the promoter activity of growth hormone was unchanged by TRH. These results suggest that TRH stimulates both prolactin and growth hormone secretion, but that the gene expressions of prolactin and growth hormone are differentially regulated by TRH and are mediated by different mechanisms.

Involvement of angiotensin II, TRH and prolactin-releasing peptide in the estrogen-induced afternoon prolactin surge in female rats: studies using antisense technology.

The roles of endogenous angiotensin II (AII), thyrotropin-releasing hormone (TRH) and prolactin-releasing peptide (PrRP) on the estrogen-induced prolactin (PRL) surge and the diurnal change of tuberoinfundibular dopaminergic (TIDA) neuronal activity were assessed in this study. Ovariectomized, estrogen-primed rats implanted with intracerebroventricular cannula received daily injection of antisense oligodeoxynucleotide (ODN, 10 microg/3 microl) against the mRNA of AII, TRH or PrRP for two days. Artificial cerebrospinal fluid or the sense ODN were used as the control. In the first experiment, serial blood samples (0.3 ml each) were obtained hourly from each rat through a pre-implanted intraatrial catheter from 1100 to 1700h. Half of the rats pretreated with respective antisense ODN received single injections of AII, TRH or PrRP (1 microg each, i.v.) at 1400h. In the second experiment, groups of rats were decapitated either at 1000 or 1500h. The hypothalamic median eminence tissue of each rat was dissected out and its DOPAC content was used as the index for TIDA neuronal activity. Plasma and serum PRL levels were determined by radioimmunoassay. Pretreatment of antisense ODN against the mRNA of either AII or TRH significantly attenuated the PRL surge; replacement injection of AII or TRH restored the surge. The effect of antisense ODN against PrRP was less significant. None of the treatments significantly affected the diurnal changes of TIDA neuronal activity. In summary, both AII and TRH may play an important role as the PRL-releasing hormone involved in the estrogen-induced afternoon PRL surge.

Neuropeptide Y inhibits spontaneous alpha-melanocyte-stimulating hormone (alpha-MSH) release via a Y(5) receptor and suppresses thyrotropin-releasing hormone-induced alpha-MSH secretion via a Y(1) receptor in frog melanotrope cells.

In amphibians, the secretion of alpha-MSH by melanotrope cells is stimulated by TRH and inhibited by NPY. We have previously shown that NPY abrogates the stimulatory effect of TRH on alpha-MSH secretion. The aim of the present study was to characterize the receptor subtypes mediating the action of NPY and to investigate the intracellular mechanisms involved in the inhibitory effect of NPY on basal and TRH-induced alpha-MSH secretion. Y(1) and Y(5) receptor mRNAs were detected by RT-PCR and visualized by in situ hybridization histochemistry in the intermediate lobe of the pituitary. Various NPY analogs inhibited in a dose-dependent manner the spontaneous secretion of alpha-MSH from perifused frog neurointermediate lobes with the following order of potency porcine peptide YY (pPYY) > frog NPY (fNPY) > porcine NPY (pNPY)-2-36) > pNPY-(13-36) > [D-Trp(32)]pNPY > [Leu(31),Pro(34)]pNPY. The stimulatory effect of TRH (10(-8)6 M) on alpha-MSH release was inhibited by fNPY, pPYY, and [Leu(31),Pro(34)]pNPY, but not by pNPY-(13-36) and [D-Trp(32)]pNPY. These data indicate that the inhibitory effect of fNPY on spontaneous alpha-MSH release is preferentially mediated through Y(5) receptors, whereas the suppression of TRH-induced alpha-MSH secretion by fNPY probably involves Y(1) receptors. Pretreatment of neurointermediate lobes with pertussis toxin (PTX; 1 microg/ml; 12 h) did not abolish the inhibitory effect of fNPY on cAMP formation and spontaneous alpha-MSH release, but restored the stimulatory effect of TRH on alpha-MSH secretion, indicating that the adenylyl cyclase pathway is not involved in the action of fNPY on TRH-evoked alpha-MSH secretion. In the majority of melanotrope cells, TRH induces a sustained and biphasic increase in cytosolic Ca(2+) concentration. Preincubation of cultured cells with fNPY (10(-7) M) or omega-conotoxin GVIA (10(-7) M) suppressed the plateau phase of the Ca(2+) response induced by TRH. However, although fNPY abrogated TRH-evoked alpha-MSH secretion, omega-conotoxin did not, showing dissociation between the cytosolic Ca(2+) concentration increase and the secretory response. Collectively, these data indicate that in frog melanotrope cells NPY inhibits spontaneous alpha-MSH release and cAMP formation through activation of a Y(5) receptor coupled to PTX- insensitive G protein, whereas NPY suppresses the stimulatory effect of TRH on alpha-MSH secretion through a Y(1) receptor coupled to a PTX-sensitive G protein-coupled receptor.

Intrathecal methiothepin and thyrotropin-releasing hormone effects on penile erection.

Intrathecal thyrotropin-releasing hormone (TRH) potently inhibits penile erection at all doses (100, 500, 1000 or 5000 pmol) tested so far. Since the serotonin receptor antagonist methiothepin (MT) inhibits TRH responses in other systems, this study tested the hypothesis that MT-sensitive receptors mediate the effect of TRH on penile erection in rats. When compared to controls, the highest doses of IT TRH (0, 10 or 500 pmol) or MT (5 or 50 nmol) significantly altered penile reflex latency. When coadministered (50 nmol MT/500 pmol TRH), the effect of TRH was reversed, suggesting that the high dose of MT antagonized the inhibitory actions of TRH. The low dose of MT (5 nmol) did not block the 500 pmol TRH inhibition of reflex latency. These data further suggest that MT sensitive receptors are important in (1) mediating normal penile reflexes and (2) mediating the inhibitory response to TRH.

Glucose stimulates and insulin inhibits release of pancreatic TRH in vitro.

OBJECTIVE: Pancreatic TRH is present in insulin-producing B-cells of the islets of Langerhans. There is fragmentary evidence that it may be involved in glucoregulation. The aim of our present study was to analyze how glucose and insulin affect TRH secretion by the pancreatic islets. DESIGN: Isolated pancreatic islets were incubated with different concentrations of glucose, insulin and glucagon, and TRH release was measured. RESULTS: In the present study, 6 and 12mmol/l d-glucose caused significant TRH release from isolated adult rat pancreatic islets when compared with that in the presence of the same concentrations of biologically ineffective l-glucose. Thirtymmol/l d-glucose was also ineffective, but this was not due to depression of secretion by hyperosmolarity since isosmotic compensation for the high glucose addition did not restore its stimulatory effect. Five micromol/l dibutyryl cyclic 3',5'-adenosine monophosphate (db-cAMP) increased both basal and glucose-stimulated TRH release, but this effect was not seen with 50micromol/l db-cAMP. Stimulation of phosphodiesterase by imidazole resulted in decreased basal but not glucose-stimulated release of TRH. Glucagon (10(-7)mol/l) did not affect either basal or glucose-stimulated release of TRH, while insulin (10(-7) and 10(-6)mol/l) inhibited both. CONCLUSION: Our present data showing that glucose stimulates and insulin inhibits pancreatic TRH release are compatible with the possibility that this substance may play a role in glucoregulation.

Overexpression of the G protein G11alpha prevents desensitization of Ca2+ response to thyrotropin-releasing hormone.

Doubly transfected human embryonal kidney cells (clone E2M11 of the HEK 293 cell line) expressing both thyrotropin-releasing hormone (TRH) receptors and G11alpha protein in high amounts were used to analyze the desensitization phenomenon of the Ca2+-mobilizing pathway. Quite unexpectedly, we did not observe any significant desensitization of the [Ca2+]i response to TRH in these cells after repeated or prolonged incubation with the hormone (up to 5 h). Under the same conditions, the TRH-induced [Ca2+]i response was completely desensitized in the parent cell line (293-E2 cels) expressing TRH receptors alone. In both cell lines, inositol phosphate response was desensitized after TRH exposure, although basal levels of inositol phospates in TRH-pretreated cells were much higher than in "naive" TRH-unexposed cells. These data suggest a significant role of the G protein G11alpha in desensitization of the Ca2+-mobilizing pathway occuring after repeated or long-term exposure of target cells to TRH-receptor agonists.

Hypo-osmolarity stimulates and high sodium concentration inhibits thyrotropin-releasing hormone secretion from rat hypothalamus.

The hypothalamic paraventricular nucleus, representing cell bodies in which thyrotropin-releasing hormone is synthesized, and the median eminence, representing nerve terminals, were incubated in vitro. Various hypo- and hyperosmotic solutions were tested to determine osmotic sensitivity of thyrotropin-releasing hormone secretion. High KCl (56 mM) causing membrane depolarization was used as a non-specific control stimulus to induce thyrotropin-releasing hormone secretion. A 30% decrease of medium osmolarity (from 288 to 202 mOsmol/l) increased thyrotropin-releasing hormone secretion from both the paraventricular nucleus and median eminence. A 30% decrease of medium NaCl content by its replacement with choline chloride did not affect basal thyrotropin-releasing hormone secretion. Increasing medium osmolarity with biologically inactive L-glucose did not affect basal or KCl-induced thyrotropin-releasing hormone secretion from either structure. Medium made hyperosmotic (350-450 mOsmol/l) by increasing the NaCl concentration resulted in a dose-dependent decrease of basal thyrotropin-releasing hormone secretion and abolished KCl-induced thyrotropin-releasing hormone secretion. If an osmotically equivalent amount of choline chloride was substituted for NaCl, there was no effect on thyrotropin-releasing hormone secretion, indicating a specific action of Na+. This study indicates a specific sensitivity to high concentrations of Na+ ions of both thyrotropin-releasing hormone-producing parvocellular paraventricular neurons and thyrotropin-releasing hormone-containing nerve terminals in the median eminence.