Interaction between thyrotrophin-releasing hormone and the muscarinic cholinergic system in rat brain

Association between diencephalic thyroliberin and arterial blood pressure in agouti-yellow and ob/ob mice may be mediated by leptin

Leptin, a hormone secreted by the adipose tissue, stimulates anorexigenic peptides and also inhibits orexigenic peptides in hypothalamic arcuate nuclei-located neurons. It also counteracts the starvation-induced suppression of thyroid hormones by up-regulating the expression of preproTRH gene. On the other hand, in addition to its role as a modulator of the thyroid-hypothalamic-hypophysial axis, thyrotropin-releasing hormone (TRH) acts as a modulator of the cardiovascular system. In fact, we reported that overexpression of diencephalic TRH (dTRH) induces hypertension. We have recently shown that, in rats with obesity-induced hypertension, hyperleptinemia may produce an increase of dTRH together with an elevation of arterial blood pressure (ABP) through an increase of sympathetic activity and that these alterations were reversed by antisense oligonucleotide and small interfering RNA against preproTRH treatments. Here we explore the possible role of dTRH as a mediator involved in leptin-induced hypertension in 2 obesity mouse models: agouti-yellow mice, which are hyperleptinemic and hypertensive, and ob/ob mice, which lack functional circulating leptin. These 2 models share some characteristics, but ob/ob mice show lower ABP and plasma catecholamines levels. Then, for the first time, we report that there is a clear association between ABP and dTRH levels in both mouse models, as we have found that dTRH content was elevated in agouti-yellow mice and diminished in ob/ob mice compared with their controls. We also show that, after 3 days of subcutaneous leptin injections (10 microg/12 hours), ABP and dTRH increased significantly in ob/ob mice with no alterations of thyroid hormone levels. These results add evidence to the putative molecular mechanisms for the strong association between obesity and hypertension.

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.

Thyrotropin-releasing hormone in cardiovascular pathophysiology

Thyrotropin (TSH)-releasing hormone (TRH) also known as thyroliberin was the first of a number of peptides exerting several roles as a hormone and as a neuropeptide. Its ubiquitous distribution in the hypothalamus and in the extrahypothalamic regions and its diverse pharmacological and physiological effects are all features of its dual functions. For this reason, TRH has been the subject of much research throughout the past 20 years, work that has examined the structure, function, distribution, and regulation of the tripeptide and it has been extensively reviewed elsewhere [O'Leary R., O'Connor B. Thyrotropin-releasing hormone. J Neurochem. 1995;65:953-963.; Nillni E., Sevarino K. The biology of pro-thyrotropin-releasing hormone-derived peptides. Endocrine Reviews, 1999;20:599-664.]. After a brief overview of its distribution, hypothalamic and extrahypothalamic functions, and receptors involved, this review discusses efforts devoted to support TRH role in cardiovascular regulation with a main focus on hypertension pathophysiology in experimental models and humans.

Thyrotropin-releasing hormone decreases leptin and mediates the leptin-induced pressor effect

Leptin, an adipocyte-released hormone, modifies food intake and energy expenditure regulating hypothalamic-pituitary-thyroid axis function. We previously reported that thyrotropin-releasing hormone (TRH) precursor gene overexpression induces hypertension in the normal rat and that spontaneously hypertensive rats have central TRH hyperactivity with increased TRH synthesis and release and an elevated TRH receptor number. In both models, intracerebroventricular antisense (AS) treatment against the TRH precursor produced a dose-dependent reduction of the increased diencephalic TRH content while normalizing high arterial blood pressure. In this article, we report that male Wistar rats that were made hypertensive by intracerebroventricular injection of a eucaryotic expression plasmid containing the pre-TRH cDNA showed decreased leptin plasma levels and that pre-TRH AS treatment reversed this phenomenon. In addition, male and female spontaneously hypertensive rats showed lower levels of circulating leptin than did sex-matched Wistar-Kyoto control rats. This difference also was abated by the pre-TRH AS treatment. Conversely, 20 microg ICV leptin induced a long-lasting pressor effect (18 +/- 5 mm Hg, n=6, P<0.01, >60 minutes) that was not observed in pre-TRH AS pretreated rats (2 +/- 3 mm Hg, n=6) but persisted in rats used as controls that were treated with inverted oligonucleotide (20 +/- 6 mm Hg, n=4, P<0.01). These data suggest that in rats with TRH-induced hypertension, leptin is decreased, inducing compensatory adiposity. We propose that because leptin produces central TRH synthesis and release, obesity may induce hypertension through TRH system activation and that the TRH-leptin interaction may thus contribute to the strong association between hypertension and obesity.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Antisense inhibition of thyrotropin-releasing hormone reduces arterial blood pressure in spontaneously hypertensive rats

Thyrotropin-releasing hormone (TRH) plays an important role in central cardiovascular regulation. Recently, we described that the TRH precursor gene overexpression induces hypertension in the normal rat. In addition, we published that spontaneously hypertensive rats (SHR) have central extrahypothalamic TRH hyperactivity with increased TRH synthesis and release and an elevated TRH receptor number. In the present study, we report that intracerebroventricular antisense (AS) treatment with a phosphorothioate oligonucleotide against the TRH precursor gene significantly diminished up to 72 hours and in a dose-dependent manner the increased diencephalic TRH content, whereas normalized systolic blood pressure (SABP) was present in the SHR compared with Wistar-Kyoto (WKY) rats. Although basal thyrotropin was higher in SHR compared with WKY rats and this difference disappeared after antisense treatment, no differences were observed in plasma T4 or T3 between strains with or without AS treatment, indicating that the effect of the AS on SABP was independent of the thyroid status. Because the encephalic renin-angiotensin system seems to be crucial in the development and/or maintenance of hypertension in SHR, we investigated the effect of antisense inhibition of TRH on that system and found that TRH antisense treatment significantly diminished the elevated diencephalic angiotensin II (Ang II) content in the SHR without any effect in control animals, suggesting that the Ang II system is involved in the TRH cardiovascular effects. To summarize, the central TRH system seems to be involved in the etiopathogenesis of hypertension in this model of essential hypertension.

Central overexpression of the TRH precursor gene induces hypertension in rats: antisense reversal

Extrahypothalamic TRH participates in cardiovascular regulation and spontaneous hypertension of the rat. To investigate whether an increase in central TRH activity produces hypertension we studied the effect of the preTRH overproduction induced by I.C.V. transfection with a naked eukaryotic expression plasmid vector which encodes preTRH (pCMV-TRH). Northern blot analysis and RT-PCR showed that pCMV-TRH was transcribed in vitro and in vivo. At 24, 48, and 72 hours, pCMV-TRH (100 microg) in a significant and dose-dependent manner increased 37%, 84%, and 49%, respectively, the diencephalic TRH content and SABP (42+/-3, 50+/-2, and 22+/-2 mm Hg, respectively) with respect to the vector without the preTRH cDNA insert (V[TRH(-)]) as measured by RIA and the plethysmographic method, respectively, in awake animals. In addition, using immunohistochemistry we found that the increase of TRH was produced in circumventricular areas where the tripeptide is normally located. To further analyze the specificity of these effects we studied the actions of 23-mer sense (S), antisense (AS), and 3'self-stabilized sense (Ss) and antisense (ASs) phosphorothioate oligonucleotides against the initiation codon region. Only ASs inhibited the increase of TRH content and SABP induced by pCMV-TRH treatment. In addition, pCMV-TRH-induced hypertension seems not to be mediated by central Ang II or serum TSH. To summarize, central TRH overproduction in periventricular areas induced by I.C.V. transfection produces hypertension in rats which is reversed by specific antisense treatment. This model may help in testing effective antisense oligodeoxynucleotides against other candidate genes.

Distribution of choline acetyltransferase immunoreactivity in the brain of anuran (Rana perezi, Xenopus laevis) and urodele (Pleurodeles waltl) amphibians

Because our knowledge of cholinergic systems in the brains of amphibians is limited, the present study aimed to provide detailed information on the distribution of cholinergic cell bodies and fibers as revealed by immunohistochemistry with antibodies directed against the enzyme choline acetyltransferase (ChAT). To determine general and derived features of the cholinergic systems within the class of Amphibia, both anuran (Rana perezi, Xenopus laevis) and urodele (Pleurodeles waltl) amphibians were studied. Distinct groups of ChAT-immunoreactive cell bodies were observed in the basal telencephalon, hypothalamus, habenula, isthmic nucleus, isthmic reticular formation, cranial nerve motor nuclei, and spinal cord. Prominent plexuses of cholinergic fibers were found in the olfactory bulb, pallium, basal telencephalon, ventral thalamus, tectum, and nucleus interpeduncularis. Comparison of these results with those obtained in other vertebrates, including a segmental approach to correlate cell populations, reveals that the cholinergic systems in amphibians share many features with amniotes. Thus, cholinergic pedunculopontine and laterodorsal tegmental nuclei could be identified in the amphibian brain. The finding of weakly immunoreactive cells in the striatum of Rana, which is in contrast with the condition found in Xenopus, Pleurodeles, and other anamniotes studied so far, has revived the notion that basal ganglia organization is more preserved during evolution than previously thought.

Thyrotropin-releasing hormone hyperactivity in the preoptic area of spontaneously hypertensive rats

Thyrotropin-releasing hormone (TRH) plays an important role in central cardiovascular regulation through the activation of different neurotransmitter systems at distinct extrahypothalamic sites. To study possible alterations in the TRH system in the hypertensive state, we measured TRH concentration in cerebrospinal fluid and TRH content of the preoptic area in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) by radioimmunoassay. In addition, we also measured the density of the TRH receptor in this area by a rapid filtration technique using [3H]methyl-TRH. We found a significant increase in both the TRH content (634 +/- 61 versus 350 +/- 26 pg/mg protein, SHR versus WKY; P < .01, n = 5) and density of TRH receptors without changes in affinity (Bmax, 5.0 +/- 0.1 versus 3.3 +/- 0.1 fmol/mg protein, P < .01, n = 4). An increase in TRH concentration was also found in the cerebrospinal fluid of SHR (30 +/- 3 versus 21 +/- 2 pg/mL, P < .01, n = 5), suggesting increased TRH release in the central nervous system. Northern blot analysis indicated a threefold augmented abundance of TRH precursor mRNA in the preoptic area of SHR. A polyclonal antibody raised against TRH injected peripherally or intracerebroventricularly lowered arterial blood pressure in SHR but not in WKY. In addition, long-term treatment with enalapril (5 mg/kg twice daily), which was effective in inhibiting serum angiotensin-converting enzyme activity by more than 50%, decreased arterial blood pressure and preoptic area TRH content of SHR, whereas another vasodilator, diltiazem (10 mg/kg every 8 hours), failed to produce a similar change.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin-releasing hormone release is enhanced in hippocampal slices after electroconvulsive shock

Hippocampal thyrotropin-releasing hormone (TRH) release was examined after seizures were induced by electroconvulsive shock (ECS). Rat hippocampal slices taken 12, 24, or 48 h after 3 days of alternate-day ECS treatment or sham-ECS treatment were stimulated with potassium with or without calcium in a superfusion system containing in-line charcoal adsorbent to concentrate TRH. Released TRH and tissue TRH were measured by radioimmunoassay. The TRH content of hippocampal slices was increased fivefold over sham-ECS levels 12, 24, and 48 h after ECS, but this was not associated with an increase in basal TRH release. Potassium-stimulated TRH release was significantly elevated over basal release 12, 24, and 48 h after ECS. Potassium-stimulated calcium-dependent TRH release increased linearly after ECS, reaching its highest level 48 h after seizure. Thus, although enhanced calcium-dependent TRH release was associated with elevated tissue levels, this relationship was not proportional in that tissue TRH was elevated to the same extent at all times after ECS, whereas potassium-evoked calcium-dependent TRH release increased gradually over time after seizure. These results suggest that postictal elevations in TRH are associated with an enhanced capacity for release that develops as a result of a time-dependent shift of TRH from a storage compartment ot a readily releasable pool. The observed elevation in stimulated TRH release may be relevant to seizure-induced modulation of TRH receptors in vivo.

Behavioural effects of scopolamine and the TRH analogue RX77368 on radial arm maze performance in the rat

Effects of repeated intracerebroventricular administration of the thyrotrophin-releasing hormone (TRH) analogue, RX77368 (3,3'-dimethyl-TRH, 2 μg, once daily), on a scopolamine-induced performance deficit in an eight-arm radial maze were evaluated in adult rats. Scopolamine (0.3 mg/kg i.p.-30 min) pre-treatment produced a significant deficit in the number of unrepeated arm entries and total arm entries and increased the percentage of incorrect arm entries and the total time on the maze, compared with saline-treated controls. Prior treatment with RX77368 (40 min before maze testing) produced a partial but significant attenuation of the scopolamine-induced performance deficit on the maze during the first five trials but RX77368 also enhanced maze performance during the same period when given alone. These results suggest that the observed scopolamine-induced performance deficit on the radial arm maze partly results from a reduction in locomotion and maze exploration rather than solely impairment of memory, and that RX77368 treatment may improve radial maze performance by increasing arousal and exploratory behaviour in rats rather than directly enhancing cognition.