Immunoelectron microscopic localization of thyrotropin-releasing hormone (TRH) precursor in the rat raphe nuclei

Dorsal periaqueductal gray matter-evoked panic-like behaviors are markedly inhibited by a low peripheral dose of thyrotropin releasing hormone

Stimulation of the dorsal periaqueductal gray matter (DPAG) produces defensive behaviors which are reminiscent of panic attacks. Recent evidence from our laboratory showed that DPAG-evoked defensive behaviors are markedly attenuated in short-term methimazole-induced hypothyroidism. It is not clear, however, whether these effects were due to an increase in thyrotropin releasing hormone (TRH), a decrease in thyroid hormones or to the overall effects of hypothyroidism. Accordingly, here we examined whether the peripheral injection of TRH has any effect either on the panic-like behaviors induced by electrical stimulation of DPAG or anxiety-like behaviors of rats exposed to the elevated plus-maze (EPM). Rats whose stimulation of DPAG produced flight responses (galloping or jumping) with intensities below 60 microA were injected with 1 microg/kg TRH (i.p.) and stimulated 10min after that. The day after, rats were treated with saline and subjected to the same stimulation procedure. Threshold curves were fitted through the logistic model and compared by likelihood-ratio chi(2) tests. TRH and saline effects on EPM performance were appraised in separate groups. Compared to saline-sessions, TRH-injected rats presented thresholds significantly higher for immobility (40%), trotting (33%), galloping (34%), jumping (39%) and exophthalmus (43%). In contrast, TRH had no effects on EPM arm exploration. TRH selective inhibition of DPAG-evoked defensive behaviors adds new evidence that panic attacks may be attenuated by increased levels of this hormone in hypothyroidism.

Peripheral PYY inhibits intracisternal TRH-induced gastric acid secretion by acting in the brain

The site of action of peripheral peptide YY (PYY)-induced inhibition of vagally stimulated gastric acid secretion was studied using immunoneutralization with PYY antibody in urethan-anesthetized rats. Gastric acid secretion (59+/-7 micromol/90 min) stimulated by intracisternal injection of the stable thyrotropin-releasing hormone (TRH) analog RX-77368 (14 pmol/rat) was dose-dependently inhibited by 52%, 69%, and 83% by intravenous infusion of 0.25, 0.5, and 1.0 nmol. kg(-1) x h(-1) PYY, respectively. PYY or PYY(3-36) (2.4 pmol/rat) injected intracisternally also inhibited the acid response to intracisternal RX-77368 by 73% and 80%, respectively. Intravenous pretreatment with PYY antibody (4.5 mg/rat), which shows a 35% cross-reaction with PYY(3-36) by RIA, completely prevented the inhibitory effect of intravenously infused PYY (1 nmol x kg(-1) x h(-1)). When injected intracisternally, the PYY antibody (280 microg/rat) reversed intracisternal PYY (2.4 pmol)- and intravenous PYY (1 nmol x kg(-1) x h(-1))-induced inhibition of acid response to intracisternal RX-77368 by 64% and 93.5%, respectively. These results provide supporting evidence that peripheral PYY inhibits central vagal stimulation of gastric acid secretion through an action in the brain.

TRH in the dorsal motor nucleus of vagus is involved in gastric erosion induced by excitation of raphe pallidus in rats

The influence of excitation of the raphe pallidus neurons on gastric mucosal lesions was examined in urethane-anesthetized rats pretreated with indomethacin (5 mg/kg, i.p., -60 min). Kainic acid (12 ng/30 nl), delivered into the raphe pallidus, increased gastric acid secretion (94.8 +/- 15.9 mumol/60 min) and produced gastric lesions in 2.7 +/- 0.3% of the corpus mucosa. No gastric erosions were observed when kainic acid was injected nearby, but outside, of the raphe pallidus. The gastric lesions induced by microinjection of kainic acid into the raphe pallidus neurons were completely prevented by atropine (0.3 mg/kg, -30 min, s.c.) and bilateral microinjection of TRH antibody # 8964 (1.3 microgram/site) into the dorsal motor nucleus of the vagus (DMN). Microinjection of the TRH antibody into the hypoglossal nucleus or control antibody into the DMN did not modify the mucosal lesions induced by kainic acid into the raphe pallidus. These data suggest that activation of raphe pallidus induced vagal cholinergic mediated gastric erosion through TRH release in the DMN.

Caudal raphe-dorsal vagal complex peptidergic projections: role in gastric vagal control

Subpopulations of raphe pallidus (Rpa) and raphe obscurus (Rob) neurons containing TRH, serotonin (5-HT), and substance P contribute projections to the dorsal vagal complex (DVC). Activation of Rpa and Rob neurons induces a vagal cholinergic-dependent stimulation of gastric secretory and motor function and modulates resistance of the gastric mucosa to gastric injury in rats and cats. The caudal raphe nuclei-DVC pathways containing TRH/5-HT are involved in mediating cold-induced vagal stimulation of gastric function and erosion formation. These results suggest that Rpa/Rob-DVC projections containing TRH/5-HT may be an important pathways in the medullary regulation of vagal activity to the viscera.

Cold exposure elevates thyrotropin-releasing hormone gene expression in medullary raphe nuclei: relationship with vagally mediated gastric erosions

The stimulation of thyrotropin release by cold is associated with an increase in thyrotropin-releasing hormone gene expression in the paraventricular nucleus of the hypothalamus. Cold exposure also stimulates autonomic outflow to viscera. There is evidence that caudal raphe nuclei are involved in autonomic regulation through thyrotropin-releasing hormone projections to the dorsal vagal complex and spinal cord. To determine whether cold modulates thyrotropin-releasing hormone gene expression in the caudal raphe nuclei, the effect of cold exposure on thyrotropin-releasing hormone messenger RNA levels in the rat lower brainstem was examined by quantitative Northern blot analysis and thyrotropin-releasing hormone messenger RNA was localized by in situ hybridization. The gastric responses to cold exposure were also assessed in sham or vagotomized rats with pylorus ligation. Thyrotropin-releasing hormone messenger RNA signal was detected in the RNA extracted from the medulla and hypothalamus but not from the amygdala, periaqueductal gray or cerebellum. Cold exposure (4 degrees C) for 1 or 3 h increased thyrotropin-releasing hormone messenger RNA levels in the medulla by 77 +/- 37 and 142 +/- 39% respectively. In situ hybridization histochemistry showed that the increase in silver grain density occurred exclusively in the raphe pallidus and raphe obscurus. Exposure to cold stress for 2 h stimulated gastric acid secretion and resulted in gastric lesion formation in sham but not vagotomized rats. There are established thyrotropin-releasing hormone projections from the raphe pallidus and obscurus to the dorsal vagal complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Prepro-TRH-(160-169) potentiates gastric acid secretion stimulated by TRH microinjected into the dorsal motor nucleus of the vagus

Prepro-TRH-(160-169) (Ps4), one of the predicted connecting peptides of pro-TRH, potentiates TRH-induced TSH release in vivo and in vitro. The influence of Ps4 on TRH in medullary nuclei-induced vagal stimulation of gastric acid secretion was studied in urethane-anesthetized rats with gastric cannula. Ps4 injected into the dorsal motor nucleus of the vagus (DMN) (200 ng) did not influence basal gastric secretion. Ps4 (100, 150 and 200 ng) co-injected with TRH (50 ng) into the DMN potentiated the acid response to TRH by 14%, 76% and 182% respectively while prepro-TRH-(178-199) (Ps5, 438 ng) had no effect. Ps4 (200 ng) co-injected with TRH into the nucleus ambiguus did not modify TRH (50 ng)-induced acid secretion. These results indicate that Ps4 potentiates TRH in the DMN-evoked gastric acid secretion through an action that is peptide and site specific.

Analysis of structural requirements for TRH-potentiating peptide receptor binding by analogue design

Previous studies established that the [125I-Tyr0]Ps4 derivative of TRH-potentiating peptide (Ps4), Ser-Phe-Pro-Trp-Met-Glu-Ser-Asp-Val-Thr, displays high affinity and selectivity for an orphan membrane receptor in rat anterior pituitary. To identify the sites in Ps4 that determine receptor binding affinity, we have synthesized and screened a panel of 15 single-point substituted analogues for their ability to displace bound [125I-Tyr0]Ps4. The affinity of [Tyr0]Ps4 for rat anterior pituitary membranes [inhibitory constant (Ki), approximately 5 nM] was drastically reduced by the substitution of either Tyr0 with Gly or Asp8 with Asn (Ki approximately 95 and approximately 51 nM, respectively). Deamination of [Tyr0]Ps4 also sharply reduced affinity (Ki approximately 1100 nM). In contrast, Ser1-->Ala, Pro3-->Ala and Thr10-->Val substitutions led to analogues showing a tenfold increase in binding affinity relative to the parent peptide. The change of Phe2-->Leu, Trp4-->Ala, Glu6-->Gln, Val9-->Thr and carboxamidation of Thr10 had no effect on binding affinity. The data suggest that substitutions of the amino-terminal group, Asp8 and Tyr0, have a marked effect on the ability of [Tyr0]Ps4 to compete with [125I-Tyr0]Ps4 for binding to TRH-potentiating peptide pituitary receptor.

Modulation of the biological activity of thyrotropin-releasing hormone by alternate processing of pro-TRH

Thyrotropin-releasing hormone prohormone contains multiple copies of TRH linked together by connecting sequences. Like other plurifunctional prohormone proteins, pro-TRH undergoes differential proteolytic processing in various tissues to generate, beside authentic TRH, several other novel peptides corresponding to C-terminally extended forms of TRH and connecting fragments. The pro-TRH connecting peptides are, together with TRH, predominant storage forms of TRH-precursor related peptides in the rat hypothalamus. Connecting peptides are co-localized with TRH in the median eminence nerve endings and co-released through a mechanism involving voltage-operated Ca2+ channels. The connecting peptide Ps4 is involved in potentiation of the action of TRH on thyrotropin hormone release by pituitary in vitro and in vivo through interactions with a specific pituitary cell receptor coupled to dihydropyridine and omega-connotoxin sensitive Ca2+ channels of the L-type. It also causes dose-dependent increases in the steady state levels of mRNAs of TSH and prolactin through stimulation of the respective gene promoter activities. These findings indicate that Ps4 and TRH, two peptides which originate from a single multifunctional biosynthetic precursor, can function on the same target tissues in a coordinate manner to promote hormonal secretion. This suggests that differential processing of the TRH prohormone may have the potential to modulate the biological activities of TRH.

Distribution of thyrotropin-releasing hormone receptor messenger RNA in the rat brain: an in situ hybridization study

Based on the recent cloning of the mouse thyrotropin-releasing hormone receptor, oligonucleotide probes complementary to the DNA sequence were constructed and used for in situ hybridization studies on the rat brain. Thyrotropin-releasing hormone receptor messenger RNA was found in many areas of the brain, mostly showing high degree of overlap with the distribution thyrotropin-releasing hormone binding sites as previously revealed in autoradiographic studies. Thus, a strong signal was observed in the accessory olfactory bulb, the perirhinal sulcus, the ventral aspects of the hippocampal formation, some amygdaloid nuclei, the diagonal band nucleus, parts of nucleus accumbens, the bed nucleus of the stria terminalis, dorsomedial, lateral and perifornical hypothalamic regions, the septohippocampal nucleus, parts of the vestibular complex, as well as many bulbar motoneurons including the facial, dorsal vagal, ambiguus and hypoglossal nuclei, the superficial layer of the spinal trigeminal nucleus, and motoneurons and dorsal horn neurons in the spinal cord. Cells within one and the same nucleus expressed varying levels of thyrotropin releasing hormone receptor messenger RNA suggesting marked differences in rate of receptor synthesis. Most of these areas receive an input by thyrotropin-releasing hormone-positive nerve endings. Taken together these results suggest that thyrotropin-releasing hormone receptors are mostly localized in the vicinity of the cell bodies which express thyrotropin-releasing hormone receptor messenger RNA and mediate the wide range of actions that have been recorded after administration of exogenous thyrotropin-releasing hormone.

Thyrotropin-releasing hormone analog injected into the raphe pallidus and obscurus increases gastric contractility in rats

The present study was performed to investigate the influence of the chemical stimulation of medullary raphe nuclei by the stable TRH (thyrotropin-releasing hormone) analog, RX 77368, on gastric contractility. Urethane-anesthetized rats were acutely implanted with miniature strain gauge force transducers on the corpus of the stomach for continuous recording of gastric contractility. Traces were analyzed by computer. Microinjections of vehicle or RX 77368 into the raphe pallidus or raphe obscurus were performed using pressure injection of 50 nl through glass micropipettes 30 min following basal recording of gastric contractility. RX 77368 (0.7-77 pmol) dose dependently stimulated gastric contractility when microinjected into the raphe pallidus and raphe obscurus. The stimulation of gastric contractions induced by microinjection of RX 77368 (77 pmol) into these raphe nuclei was completely blocked by vagotomy and prevented (raphe obscurus) or reduced (raphe pallidus) by atropine. RX 77368 (7.7-77 pmol) microinjected into the inferior olive, pyramidal tract, medial lemiscus was ineffective. These results demonstrate that chemical stimulation of the raphe pallidus and obscurus by RX 77368 stimulates gastric contractility through vagal and muscarinic pathways. These data suggest a role for medullary raphe nuclei in the central vagal regulation of gastric contractility.

Neuroanatomical connections between corticotropin-releasing factor (CRF) and somatostatin (SRIF) nerve endings and thyrotropin-releasing hormone (TRH) neurons in the paraventricular nucleus of rat hypothalamus

In order to investigate the possible involvement of corticotropin-releasing factor (CRF) and somatostatin (SRIF) on thyrotropin-releasing hormone (TRH) neuronal cell activity in the rat hypothalamic paraventricular nucleus, we have proceeded to the simultaneous localization of CRF or SRIF and TRH. For this purpose, we used a dual immunostaining procedure that employed antibodies to CRF and SRIF and peroxidase-labeled goat anti-rabbit IgG as a first sequence, and antibodies to a cryptic fragment (Phe178-Glu199) of pro-TRH (to label TRH neurons) and alkaline phosphatase-labeled goat anti-rabbit IgG as the second sequence. A rich innervation of the paraventricular nucleus by immunoreactive CRF and SRIF fibers was observed. A large number of CRF and SRIF nerve endings were seen intimate anatomic proximity and often appeared to surround TRH-containing cell bodies. These results strongly suggest that TRH neurons might be regulated by both CRF and SRIF. These interactions might be the neuroanatomical basis for the already observed inhibitory effects of CRF and SRIF on TRH release.

Central thyrotropin-releasing factor analog prevents ethanol-induced gastric damage through prostaglandins in rats

The effects of intracisternal injection of the stable thyrotropin-releasing factor (TRH) analog RX 77368 on gastric lesions induced by 60% ethanol and gastric prostaglandin E2 (PGE2) release were studied in rats. RX 77368 (1.0 and 1.5 ng) injected intracisternally inhibited (by 58% and 78%, respectively) macroscopic gastric damage induced by ethanol. Higher doses (3 and 300 ng) inhibited ethanol-induced gastric injury only in rats pretreated with omeprazole (20 mg/kg SC). Gastric acid output measured in conscious rats 2 hours after pylorus ligation was not modified by intracisternal injection of RX 77368 at 1.5 ng but was significantly increased by 54% at the 3-ng dose. The protective effect of TRH analog (1.5 ng) was completely abolished by indomethacin (5 mg/kg IP) and atropine (2 mg/kg SC) pretreatment. In pylorus-ligated rats, intracisternal RX 77368 (1.5 ng) inhibited ethanol-induced gastric lesions by 64%. Intracisternal injection of RX 77368 (1.5 ng) increased PGE2 levels measured in the effluent of dialysis fibers implanted into the corpus submucosa of urethane-anesthetized rats. Peripheral administration of omeprazole, atropine, indomethacin, or RX 77368 (1.5 ng IV) did not influence gastric damage induced by ethanol. These data show that the stable TRH analog, RX 77368, injected intracisternally at low non-secretory doses acts in the brain to protect against ethanol lesions through prostaglandin and cholinergic pathways. These findings suggest that central vagal activation induced by TRH may play a role in the control of mucosal integrity against ethanol through cholinergic prostaglandin release.