Stimulation by thyrotropin-releasing hormone of vagal outflow to the thyroid gland

An intracerebroventricular (i.c.v.) injection of TRH to the urethane anesthetized rat stimulates the activity of the superior laryngeal nerve (n.sl) which is a vagal ramus terminating at the thyroid gland and adjacent muscles. The response to TRH, a tonic increase in the n.sl outflow, was dose dependent in the 0.005-5.0 micrograms/100 g B.W. range. In contrast to this, methionine-enkephalin (ENK), neurotensin (NT) and somatostatin (SRIF) (5 micrograms/100 g, i.c.v.) all caused a transient decrease in n.sl activity. SRIF showed the highest attenuating effect when injected alone and was capable of diminishing the increased activity produced by a prior injection of TRH. ENK and NT failed to affect the TRH-induced increased activity. When injected concomitantly with TRH, SRIF blocked the response to TRH while ENK and NT both failed to affect the response to TRH. Pretreatment with triiodothyronine for 5 days strongly inhibited the response of the n.sl outflow to TRH. On the other hand, pretreatment with atropine, haloperidol, propranolol, phenoxybenzamine and p-chlorophenylalanine failed to block the stimulating effect of TRH although the response was diminished by some antagonists. It therefore seemed that TRH transmission is involved in central stimulation and SRIF is antagonistic in this regulation of n.sl outflow to the thyroid gland.     

Neuropeptides as central integrators of autonomic nerve activity: effects of TRH, SRIF, VIP and bombesin on gastric and adrenal nerves

The nerve activity of the gastric ramus of the splanchnic (sympathetic) nerve, gastric ramus of the vagus, adrenal ramus of the splanchnic nerve and the superior laryngeal nerve (laryngeal ramus of vagus) were assessed before and after i.c.v. injection of neuropeptides in the rat. TRH stimulated the vagal branch but attenuated the sympathetic outflow to the stomach. In contrast, the sympathetic outflow to the adrenal was enhanced by TRH. SRIF suppressed the activity of all the nerves studied. VIP did not affect the sympathetic outflow to the stomach while suppressing the gastric branch of the vagus. The adrenal sympathetic branch as well as the superior laryngeal nerve was stimulated by VIP. Bombesin suppressed both vagal and sympathetic outflow to the stomach but markedly stimulated the laryngeal branch of the vagus. The adrenal sympathetic nerve was either stimulated or attenuated slightly by bombesin. These results indicate that centrally administered neuropeptides produce reactions specific for each nerve.     

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.     

脑室内注入TRH对大鼠肝脏无机磷代谢影响的核磁共振分析

采用在大鼠脑室内注入促甲状腺激素释放激素（Ｔｈｙｒｏｔｒｏｐｉｎｒｅｌｅａｓｉｎｇｈｏｒｍｏｎｅ,ＴＲＨ）,并利用３１Ｐ－核磁共振法测定活体大鼠肝脏中的含磷化学物质,并观察ＴＲＨ对肝脏无机磷代谢的影响,研究证明,ＴＲＨ通过中枢神经影响肝脏中无机磷的代谢。由侧脑室注入ＴＲＨ,使肝脏无机磷含量发生显著增加,此作用由于副交感神经的阻断剂阿托品的加入而消失,由此可以认为,ＴＲＨ是经由副交感神经而影响肝脏的代谢。     

Central nervous system action of TRH to stimulate gastric emptying in rats

The effects of intracisternal injection of TRH on gastric emptying of a liquid meal was investigated in 24 h fasted rats using the phenol red method. Intracisternal injection of TRH, RX 77368, or [N-Val2]-TRH, an analog devoid of TSH-releasing activity, 5 min prior to a meal, stimulated gastric emptying measured 20 min later. TRH action was dose dependent (1-100 ng), and rapid in onset. The calculated time for emptying half of the meal was decreased from 16 +/- 3 min (control group) to 4 +/- 1 min (TRH 30 ng). The stable analog, RX 77368, unlike TRH, stimulated gastric emptying when the meal was given 60 min after peptide injection. Intravenous injection of atropine (2.5 micrograms) inhibited and that of carbachol (1 microgram) stimulated gastric emptying whereas i.v. injection of TRH (0.1-1 microgram) had no effect. Vagotomy but not adrenalectomy reversed the increase in gastric emptying induced by intracisternal TRH. Atropine blocked the stimulatory effect of TRH and carbachol. These results demonstrate that TRH acts within the brain to stimulate gastric emptying through vagus-dependent and cholinergic pathways whereas alterations of adrenal and pituitary-thyroid secretion do not play an important role.     

TRH dissociates the aggressivity of vegetative and motor phenomena induced by carbachol in the cat

In these experiments interaction of thyrotropin releasing hormone (TRH) and carbachol injected into the cerebral ventricles of unanaesthetized cats has been investigated. Intracerebroventricular (i.c.v.) carbachol as well as i.c.v. TRH produced emotional behaviour, autonomic and motor phenomena. The most impressive feature of i.c.v. carbachol was the aggressive behaviour, whereas that of i.c.v. TRH the autonomic changes. In cats treated with i.c.v. TRH, the aggressive behaviour, but the autonomic and motor changes of i.c.v. carbachol was potentiated. Since there is evidence that carbachol acts mainly on muscarinic M-2 receptors, the potentiation by TRH of aggressive behaviour, but not the autonomic and motor changes induced by carbachol could indicate heterogeneity of central muscarinic M-2 receptors.     

Thyrotropin-releasing hormone injected intracerebroventricularly in the rat stimulates exocrine pancreatic secretion via the vagus nerve

The effects of intracerebroventricular (i.c.v.) injection of synthetic thyrotropin-releasing hormone (TRH) and its analogue (gamma-butyrolactone-gamma-carbonyl-His-Pro-NH2) were tested in anesthetized rats fitted with pancreatic cannula. TRH injection induced dose-related increases in flow of pancreatic juice, protein output, and amylase output, each reaching a maximum within 10 min. Higher doses of TRH induced longer responses. Injection of the TRH analogue also caused dose-related secretory responses of the exocrine pancreas. The dose-related secretory responses to TRH and the TRH analogue were similar except that the responses to the highest dose of TRH analogue (1600 pmol/100 g b.w.) were significantly higher. Intravenous injection of TRH and the TRH analogue induced little, if any, secretory response of the exocrine pancreas. The effects of i.c.v. injection of TRH and the TRH analogue were completely abolished after bilateral subdiaphragmatic vagotomy. In addition to the secretory effects on the exocrine pancreas, i.c.v. injection of TRH and the analogue caused hyperglycemia, tachycardia, and tear secretion, but the intravenous injection of these peptides had no effect.     

Oral thyrotropin-releasing hormone (TRH) test in acromegaly

The effects of 40 mg oral and 200 microgram intravenous TRH were studied in patients with active acromegaly. Administration of oral TRH to each of 14 acromegalics resulted in more pronounced TSH response in all patients and more pronounced response of triiodothyronine in most of them (delta max TSh after oral TRh 36.4 +/- 10.0 (SEM) mU/l vs. delta max TSH after i.v. TRH 7.7 +/- 1.5 mU/l, P less than 0.05; delta max T3 after oral TRH 0.88 +/- 0.24 nmol/vs. delta max T3 after i.v. TRH 0.23 +/- 0.06 nmol/l, P less than 0.05). Oral TRH elicited unimpaired TSH response even in those acromegalics where the TSH response to i.v. TRH was absent or blunted. In contrast to TSH stimulation, oral TRH did not elicit positive paradoxical growth hormone response in any of 8 patients with absent stimulation after i.v. TRH. In 7 growth hormone responders to TRH stimulation the oral TRH-induced growth hormone response was insignificantly lower than that after i.v. TRH (delta max GH after oral TRH 65.4 +/- 28.1 microgram/l vs. delta max GH after i.v. TRH 87.7 +/- 25.6 microgram/l, P greater than 0.05). In 7 acromegalics 200 microgram i.v. TRH represented a stronger stimulus for prolactin release than 40 mg oral TRH (delta max PRL after i.v. TRH 19.6 +/- 3.22 microgram/, delta max PRL after oral TRH 11.1 +/- 2.02 microgram/, P less than 0.05). Conclusion: In acromegalics 40 mg oral TRH stimulation is useful in the evaluation of the function of pituitary thyrotrophs because it shows more pronounced effect than 200 microgram TRH intravenously. No advantage of oral TRH stimulation was seen in the assessment of prolactin stimulation and paradoxical growth hormone responses.     

Some regional anatomical relationships of TRH to 5-HT in rat limbic forebrain

It is now a recognized principle that various neuropeptides are neuronally co-localized with biogenic amine or aminoacid neurotransmitters. In the rat CNS it has previously been shown that TRH is co-localized with 5-HT (and also with substance P) in cell bodies of the posterior raphe that project to the spinal cord. Although TRH cell bodies are known to be widely distributed throughout the forebrain there is no other known co-localization with 5-HT. In this study we further specify the anatomical relationship of TRH with 5-HT by use of surgical and neurotoxic lesioning with reference to limbic forebrain regions wherein TRH is greatly increased following seizures. In groups of rats, the fimbria-fornix was lesioned alone, or combined with a lesion of the dorsal perforant path or the ventral perforant path. There was a sham lesioned control group. Additional groups were lesioned with 5, 7 dihydroxytryptamine, 100 g i.v.t., 45 min. after i.p. desipramine, 25 mg/kg. All rats were sacrificed three weeks after lesions. Indoleamines were determined by HPLC in left anterior cortex, left pyriform/olfactory cortex, left dorsal hippocampus and left ventral hippocampus. TRH was determined by specific RIA in the corresponding right brain regions. The modal n was 7 rats. The surgical lesions reduced 5-HT to below the detection limit in dorsal hippocampus in all three groups, and to 31–52% of control in all the ventral hippocampus groups. 5-HIAA was reduced to 19–37% of control in dorsal and to 30–51% of control in ventral hippocampus. TRH was reduced to 44–61% of control in dorsal hippocampus and to 48–53% of control in ventral hippocampus. As was repeatedly observed in our previous reports all TRH levels in ventral hippocampus were higher than in dorsal hippocampus. The 5, 7 dihydroxytryptamine treatment nearly eliminated the indoleamines from all the forebrain regions examined while TRH levels were unchanged. These results can be explained by our previous data showing that immunoreactive TRH is intrinsic and localized to the vicinity of both CA and dentate granule cells of the hippocampus, but about half of hippocampal TRH enters via fibers of the fimbria-fornix. The perforant path appears to contribute no TRH to hippocampus, but, results with the combined lesion groups showed some reduction of 5-HIAA in ventral hippocampus as is expected from the known perforant path contribution of 5-HT. Since the neurotoxic lesion had no effect on TRH, the 5-HT pathway through the fimbria-fornix is probably anatomically separate from a parallel TRH pathway there. This study shows that co-localization of TRH with 5-HT is very unlikely in four specific limbic forebrain regions.Special issue dedicated to Dr. Morris H. Aprison.     

Fluoxetine pretreatment potentiates intracisternal TRH analogue-stimulated gastric acid secretion in rats.

Central injection of TRH or its metabolically stable analogue RX 77368 has been demonstrated to produce a vagal-dependent stimulation in gastric acid secretion. Accumulating evidence exists regarding the interaction of serotonin (5HT) with TRH containing neuronal systems. This study was performed to assess the effect of pretreatment with the 5HT uptake inhibitor fluoxetine on the TRH analogue-induced gastric acid secretory response. Systemic fluoxetine (30 mumol/kg, i.v.) produced a 43-85% increase in the intracisternal RX 77368 (78-780 pmol)-induced gastric acid output, while not affecting the basal acid response. The acid response to a lower dose of RX 77368 (26 pmol) was not altered. In addition, intracisternal fluoxetine (180 nmol) produced a 71% augmentation of the acid secretory response of i.c. RX 77368 (260 pmol). Intracisternal injection of lower doses (60, 120 nmol), or intravenous injection of 180 nmol of fluoxetine was ineffective in altering the intracisternal RX 77368-induced acid response. Pretreatment with the noradrenergic or dopaminergic uptake inhibitor desipramine or GBR 12909 did not alter the RX 77368-stimulated gastric acid secretory response. The results show that fluoxetine pretreatment potentiates the effect of intracisternal RX 77368 on acid secretion. The effect appears to be impulse dependent, and central sites of action are involved. The data suggest an interaction of synaptic serotonin with a RX 77368-elicited event (activation of TRH receptors, second messenger systems and/or firing of the motor vagus) results in potentiation of the RX 77368-induced gastric response.     

The involvement of central cholinergic mechanisms in cardiovascular responses to intracerebroventricular and intravenous administration of thyrotropin-releasing hormone

Intracerebroventricular (i.c.v.) administration of thyrotropin-releasing hormone (TRH) in a range from 0.1 to 100 micrograms induced a dose-related increase in blood pressure in conscious rats, whereas TRH-free acid (TRH-OH) and histidyl-proline diketopiperazine (His-Pro-DKP), metabolites of TRH, did not. The blood pressure responses to intravenous (i.v.) injection of 5 mg/Kg TRH were similar to those induced by TRH (i.c.v.). Pretreatment with atropine (50 micrograms, i.c.v.) significantly reduced the pressor effect of TRH administered through either route. Hemicholinium-3 (50 micrograms, i.c.v.), an inhibitor of choline uptake, also prevented the increase in blood pressure induced by TRH (10 micrograms, i.c.v.). These results indicate that both centrally and peripherally administered TRH have pressor effects that are mediated by central cholinergic mechanisms, probably by activating cholinergic neurons.     

Helodermin, but not cholecystokinin, somatostatin, or thyrotropin releasing hormone, acutely increases thyroid blood flow in the rat

In the present study, we investigated whether peptides located within the thyroid gland, but not directly found in nerve fibers associated with blood vessels, might influence thyroid blood flow. Specifically, we evaluated the effects of helodermin, cholecystokinin (CCK), somatostatin (SRIF) and thyrotropin releasing hormone (TRH) given systemically on thyroid blood flow and circulating thyroid hormone levels. Blood flows in the thyroid and six other organs were measured in male rats using 141Ce-labeled microspheres. Circulating thyrotropin (TSH) and thyroid hormone levels were monitored by RIA. Helodermin (10(-10) mol/100 g BW, i.v. over 4 min) markedly elevated thyroid blood flow (52 +/- 6 vs. 10 +/- 2 ml/min.g in vehicle-infused rats; n = 5). Blood flows to the salivary gland, pancreas, lacrimal gland and stomach (but not adrenal and kidney) were also increased during helodermin infusions. CCK, SRIF, and TRH were without effect on blood flows to the thyroid and other organs even though these peptides were tested at higher molar doses than helodermin. Helodermin, CCK, or SRIF did not affect thyroid hormone or plasma calcium levels. As expected however, plasma TSH and T3 levels were increased at 20 min and 2 h, respectively, following TRH infusions. Since helodermin shares sequence homology with VIP, we next compared the relative effects of these two peptides on thyroid and other organ blood flows. VIP (10(-11) mol/100 g BW, i.v.) was more potent in increasing blood flows to the thyroid, salivary gland, and pancreas than an equimolar dose of helodermin. This study shows that while helodermin, like VIP, has the ability to increase thyroid and other organ blood flows, it appears to be a less potent vasodilator.     

脑室内注入TRH对大鼠肝胆生化影响机理的研究

在大鼠的脑室内注入ＴＲＨ（三肽酰胺）,观察其对大鼠肝胆汁分泌的影响．实验结果表明,侧脑室内注入ＴＲＨ对胆汁分泌量有明显的增强作用,并且随ＴＲＨ剂量加大其作用逐渐增强．而且在侧脑室内注入ＴＲＨ期间,胆汁中Ｋ＋、Ｃｌ－、Ｎａ＋、ＨＣＯ３－离子的排出量亦有增加．其机理在于,侧脑室内注入ＴＲＨ,激活中枢胆碱能系统,并通过送走神经而使肝的新陈代谢发生变化．     

Characteristics of thyrotropin releasing hormone (TRH) receptors in rat brain

Characteristics of TRH-receptors were studied in the rat central nervous system (CNS). Ion species, pH and temperature importantly influenced TRH-receptor binding. Subcellular distribution of TRH-receptor binding revealed that synaptic membranes had the greatest percentage of total sites. Scatchard analysis suggested that the rat CNS had two distinct TRH binding sites with apparent dissociation constants (Kd) of 5 X 10(09) M and 13 X 10(-8) M. Receptor activity is sensitive to trypsin and phospholipase A digestion, suggesting that protein and phospholipid moieties are essential for the binding of [3H]TRH. Thiol reagents reduced the binding activity of the receptor, suggesting that an intrachain disulfide bond may form an important constituent of the binding site for TRH. The TRH-receptor in the rat brain was successfully solubilized with non-ionic detergent Triton X-100. On gel chromatography with Sepharose 6B column, the solubilized TRH-receptor molecule eluted at the fraction corresponding to an apparent molecular weight of 300,000 daltons, with Stokes' radius of 5.8 nm. The regional distribution of TRH-receptor binding was examined to clarify the site of TRH action. The highest level of binding was in the hypothalamus, cerebral cortex and hippocampus, indicating that TRH affects the CNS function mainly through the limbic system, cerebral cortex and hypothalamus. Moreover, tricyclic anti-depressants and Li+ decreased the binding of [3H]TRH. These findings suggest that endogenous TRH and TRH receptor may play the role of a neurotransmission modulator in the brain to control emotional and mental functions.     

Secretion of TRH in the third cerebral ventricle during acute cold exposure in the unanesthetized rat. Effect of alpha-adrenergic drugs

The concentrations of TRH in the cerebrospinal fluid (CSF) of the 3rd ventricle were measured with push-pull cannulae in 12 conscious rats. In the basal state the level of TRH in 15 min perfusion samples (210 microliters) were low (2.69 +/- 0.05 pg) and mostly undetectable with the RIA available. However, 70 to 80 min after exposure of the rats to cold (4 degrees C) a short lived but significant rise of TRH was measured in all animals. Post cold peaks amounted to 5.15 +/- 0.5 pg/15 min (p less than 0.001 vs baseline levels). This cold response to CSF TRH was influenced neither by pretreatment of rats with the alpha-adrenergic blocker phentolamine, administered i.p. (40 mg/kg) or i. c. v. (10(-5) M) 1 h before cold exposure, nor by i. c. v. infusion of the alpha 1-adrenergic blocker prazosin (10(-5) M). In rats receiving the blockers the post-cold TRH peaks were 6.76 +/- 1.61 pg/15 min and 5.70 +/- 0.70 pg/15 min, respectively. The possible origin of CSF TRH and the resistance of its cold stimulation to alpha-adrenergic blockers, compared to TRH released into the median eminence are discussed.     

Effect of active and passive immunization with TRH on plasma TSH response to propylthiouracil.

The role of thyrotropin-releasing hormone (TRH) in the secretion of TSH from the anterior pituitary was investigated in rats by active and passive immunization with TRH. The plasma TSH response to propylthiouracil (PTU) in TRH-bovine serum albumin (BSA)-immunized rats was significantly lower than that of BSA-immunized or non-immunized rats. Similarly, the increased plasma TSH level following PTU treatment was significantly suppressed after iv injection of antiserum to TRH. However, the decline in plasma TSH levels was not complete. The results of the present study indicate, at least in part, the physiological significance of endogenous TRH in the regulation of pituitary TSH secretion.     

Somatostatin inhibition of growth hormone secretion in an adult bird: the domestic fowl

1. The intravenous (i.v.) infusion of somatostatin (SRIF, 1.0 microgram/kg per min) promptly (within 5 min) reduced the growth hormone (GH) concentration in the plasma of conscious adult chickens. 2. The GH concentration progressively declined throughout a 60-min period of SRIF infusion, but was dramatically increased above pre-infusion levels within 5 min of SRIF withdrawal and maintained at an elevated level for at least 30 min afterwards. 3. Sodium pentobarbitone-anaesthesia lowered the basal GH concentration to levels comparable with those in conscious birds infused with SRIF. When administered to anaesthetized birds, exogenous SRIF was unable to further reduce the GH concentration and unable to induce 'rebound' GH release. 4. While thyrotropin releasing hormone (TRH, 10 micrograms/kg) increased the GH concentration in both conscious and anaesthetized birds, only the GH response in the anaesthetized birds was diminished by SRIF infusion. 5. Rebound GH secretion following the termination of SRIF infusion was observed in both conscious and anaesthetized birds injected with TRH. 6. These results demonstrate that SRIF can inhibit basal and TRH-stimulated GH secretion in adult domestic fowl and indicate that anaesthesia disrupts the normal control of GH releases.     

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.     

beta-Endorphin controls vasopressin release during foot shock-induced stress in the rat

This study tested the possibility that beta-endorphin is involved in the regulation of vasopressin release during stress induced by inescapable electric foot shock. To this end, a specific anti-beta-endorphin antiserum or a control serum lacking the specific anti-beta-endorphin antibodies was administered to male rats. Plasma vasopressin concentrations, measured by radioimmunoassay, were not affected by brief foot shock stress in control rats, but were raised significantly by the stress in animals which had received an intracerebroventricular (i.c.v.) injection of the anti-beta-endorphin antiserum. In contrast, when the same volume of the anti-beta-endorphin antiserum was injected into a tail vein, foot shock stress produced only a slight effect on vasopressin release. I.c.v. injection of the antiserum changed neither basal nociceptive threshold nor stress-induced analgesia as revealed by the tail-flick latency. Vasopressin release induced by an osmotic stimulus was not influenced by the anti-beta-endorphin antiserum given i.c.v. The opiate antagonist naloxone or the glucocorticoid dexamethasone raised plasma vasopressin concentration in stressed rats which had received the control serum (i.c.v.); however, after i.c.v. injection of the anti-beta-endorphin antiserum neither naloxone nor dexamethasone elevated the plasma vasopressin concentration beyond the level reached by the anti-beta-endorphin antiserum (i.c.v.) alone. These results suggest that beta-endorphin inhibits the release of vasopressin during foot shock-induced stress in the rat.     

Colchicine Treatment Differently Affects Releasable Thyrotropin-Releasing Hormone (TRH) Pools in the Hypothalamic Paraventricular Nucleus (PVN) and the Median Eminence (ME)

1. Hypophysiotropic thyrotropin-releasing hormone (TRH) is synthesized in the hypothalamic paraventricular nucleus (PVN) and transported to the median eminence (ME) where it enters the hypophyseal portal blood. TRH in the ME is situated exclusively in nerve terminals, whereas TRH in the PVN and septum is of extrinsic (nerve terminals) as well as intrinsic (perikarya) origin. 2. To determine the source and possible differential regulation of TRH release from these structures, we blocked TRH axonal delivery by i.c.v. administration of colchicine into the lateral cerebral ventricle of euthyroid or hypothyroid rats in doses of 7.5 μg or 7.5, 75 and 100 μg, respectively, two days prior to the evaluation of the TRH secretion from the PVN, ME and the septum in vitro. 3. In euthyroid rats a low dose of colchicine did not significantly affect plasma TSH. The secretory response to both ethanol in an isosmolar medium and a high K⁺ in the ME as well as the PVN explants was well preserved. However, colchicine treatment resulted in the significant increase of basal secretion of TRH from the PVN. 4. Hypothyroidism induced by 200 mg/l methimazole in drinking water for two weeks resulted in growth arrest, elevated plasma thyrotropin and decreased TRH content in the PVN and the ME. Colchicine partially decreased elevated plasma thyrotropin and increased the TRH content in the PVN and its basal release in vitro which was independent of extracellular Ca²⁺. Interestingly, a TRH release from the PVN could not be further stimulated either by K⁺ membrane depolarization or by ethanol. TRH responsiveness to the stimulation remained unaffected in the ME. The effect of colchicine on the septal TRH secretion was intermediate between the effect observed in the PVN and the ME. 5. In conclusion, the absence of a TRH secretory response to stimuli in the PVN after colchicine disruption of the microtubules and Golgi system suggests that stimulated TRH release observed from the PVN explants in vitro occurs from nerve terminals projecting to the PVN from other brain regions. The independence from extracellular calcium implies that TRH released under the non-stimulating conditions occurs most likely via the constitutive secretory pathway from dendrites and/or perikarya. Regulation of septal TRH is markedly different from the hypophysiotropic one. An erratum to this article is available at .