Cerebral and peripheral blood flow effects of TRH in the rat--a role of vagal nerves

Discovery of a low affinity thyrotropin-releasing hormone (TRH)-like peptide that exhibits potent inhibition of scopolamine-induced memory impairment in mice

TRH-like peptides were synthesized in which the critical N-terminus residue L-pGlu was replaced with various heteroaromatic rings, and the central residue histidine with 1-alkyl-L-histidines. All synthesized TRH-like peptides were evaluated in vitro as agonists in HEK mTRH-R1 and HEK mTRH-R2 cell lines, an expressing receptor binding assay (IC50), and cell signaling assay (EC50). The analeptic potential of the synthesized peptides was evaluated in vivo by using the antagonism of a pentobarbital-induced sleeping time. The peptides 6a, 6c and 6e were found to activate TRH-R2 with potencies (EC50) of 0.002 μM, 0.28 μM and 0.049 μM, respectively. In contrast, for signaling activation of TRH-R1, the same peptides required higher concentration of 0.414 μM, 50 μM and 19.1 μM, respectively in the FLIPR assay. The results showed that these peptides were 207, 178 and 389-fold selective towards TRH-R2 receptor subtype. In the antagonism of a pentobarbital-induced sleeping time assay, peptide 6c showed a 58.5% reduction in sleeping time. The peptide 6c exhibited high stability in rat blood plasma, a superior effect on the scopolamine-induced cognition impairment mice model, safe effects on the cardiovascular system, and general behavior using a functional observation battery (FOB).

Cerebral microvascular effects of nimodipine in combination with soman

Nimodipine, a calcium antagonist, has been shown to increase the detoxification of soman. In this study the cerebral microcirculatory effects of nimodipine and the acetylcholinesterase inhibitor soman was studied. Anaesthetised rats were administered nimodipine, 10 mg kg(-1) or vehicle intra-peritoneally, and 1h later exposed to 45 μg kg(-1) soman intravenously. The regional blood flows were measured using the microsphere method. Nimodipine and soman markedly increased the cerebral blood flow (CBF) and reduced the vascular resistance. Total CBF increased by 146% after nimodipine and by 105% after soman administration. Combined administration of nimodipine and soman caused additional but not fully additive effects on CBF and vascular resistance, indicating possible different mechanisms of the two agents. A part of the nimodipine induced increased detoxification after AChE-inhibition may be associated with this cerebral vasodilation.

Nitric oxide inhibition by L-NAME but not 7-NI induces a transient increase in cortical cerebral blood flow and affects the cerebrovasodilation induced by TRH

The tripeptide thyrotropin releasing hormone (TRH) has multiple interesting and complex physiological effects. One of these is the cerebrovasodilating effect, which has been described under several different conditions. The final mechanism for this effect is unknown. In the present study, we found an initial atropine-resistant cerebral vasodilation (24%) elicited by the NOS inhibitor L-NAME in the rat. D-NAME and 7-NI did not produce this effect. TRH (300 microg kg(-1), i.v.) induced an increase in cerebral blood flow by 62%. L-NAME reduced this effect significantly. The cerebrovasodilating mechanism of TRH, at least in part, is endothelial NO dependent as the neuronal 7-NI NOS inhibitor does not affect the TRH response.

Nimodipine affects the microcirculation and modulates the vascular effects of acetylcholinesterase inhibition

The present investigation was undertaken in order to study whether microvascular effects of the calcium antagonist nimodipine induces changes that can explain an increased detoxification of the highly toxic cholinesterase inhibitor soman. Anaesthetised, tracheotomised and artificially ventilated rats were treated intra-peritoneally (ip) with nimodipine, 10 mg kg(-1) or vehicle followed one hour later by the exposure to 45 microg kg(-1) soman (iv). Nimodipine per se induced a vasodilation in the intestine, myocardium and other muscles. In the abdominal skin soman elicited a significant vasoconstriction that was turned into an increased blood flow after nimodipine pre-treatment. A slight vasoconstriction in diaphragm of soman intoxicated rats was turned into a significant vasodilation by nimodipine pre-treatment. In the intestinal parts no effect of soman was detected. However, in nimodipine pretreated animals soman induced a significant vasoconstriction. The capacity of soman detoxifying processes, i.e. enzymatic hydrolysis and covalent binding to different esterases, is unequally distributed throughout the body. Together with the knowledge of the detoxifying processes of cholinesterase inhibition the results support our theory, that nimodipine alters the peripheral blood flow in a beneficial way resulting in improved detoxification ability.

Cardiopulmonary effects of HI-6 treatment in soman intoxication

The cardiopulmonary effects of HI-6, together with atropine and soman, were studied in the rat. HI-6 is an effective antidote in acute poisoning with the nerve agent soman. The therapeutic efficiency of HI-6 is still unclear and cannot be explained entirely by the HI-6 reactivating ability of acetylcholinesterase (AChE). Other non-cholinergic factors must be involved. One possible detoxifying process might be an effect of HI-6 on the blood flow to sensitive organs. The purpose of the present study was to investigate 1) whether soman per se induces changes in regional blood flow and 2) whether the blood flow to different organs is affected when HI-6 (50 mg x kg(-1) i.m.) and atropine (10 mg x kg(-1) i.m.) are given either before or immediately after soman intoxication (90 microg x kg(-1) s.c.). For regional blood flow determinations the microsphere method was used with male Wistar rats weighing 300-400 g. The rats were anaesthetised and breathed spontaneously during the experiment. Three different blood flow measurements were made in the same animal and concomitant physiological parameters such as mean arterial blood pressure and respiratory rate were recorded. The blood AChE activity was followed throughout the experiment. Our results show that when HI-6 is given after intoxication with soman, dramatic changes in blood flow occur with a significant decrease in both respiratory rate and blood AChE activity. If HI-6 is given prior to the intoxication, however, all rats are unaffected and none of the parameters measured are changed.

Cerebrovascular effects of the TRH analogues pGlu-3-methyl-His-Pro amide and pGlu-Glu-Pro amide: a comparison with TRH

The goal of the study was to assess whether TRH analogues possess cerebrovascular effects similar to the native peptide. The neuropeptide thyrotropin releasing hormone (TRH) elicits cerebrovasodilation in several species under various conditions. The laser-Doppler method was employed to study the effects of TRH and the analogues pGlu-3-methyl-His-Pro amid (M-TRH) and pGlu-Glu-Pro amide. Intravenous (i.v.) injection of 300 microg kg(-1) of TRH elicited cerebrovasodilation and a 62% increase in blood flow within 1 minute. M-TRH, in a dose of 300 microg kg(-1) i.v., elicited a 80% increase in cerebral blood flow. Even a minute dose of M-TRH (625 ng kg(-1)) caused an increase in cerebral blood flow. No clear difference in effects on the cerebral blood flow was observed between spontaneously and mechanically ventilated animals, pGlu-Glu-Pro amide had no cerebrovascular effect.

Effects of nitric oxide synthase inhibition and endothelin ETA receptor blockade on haemodynamics in hypertensive rats

1. The objectives of the present study were to study regional differences in haemodynamics between spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats induced by the nitric oxide synthase (NOS) inhibitor NG-monomethyl-L-arginine (L-NMMA) and the endothelin ETA receptor antagonist BQ 123 in vivo in tissues known to be important for blood pressure (BP) regulation (heart, kidney and skeletal muscle). Furthermore, the effect of acetylcholine (ACh) infusion (2 micrograms/kg per min) was examined after L-NMMA or BQ 123. The microsphere method was used for determinations of cardiac index (CI) and regional haemodynamics. 2. NG-Monomethyl-L-arginine (20 mg/kg) increased BP (26-48%; P < 0.01) and reduced CI in both rat strains. BQ 123 (1 mg/kg) reduced BP slightly (-4 to 11%; P < 0.05). 3. NG-Monomethyl-L-arginine significantly increased myocardial and skeletal muscle vascular resistance in SHR only; however, in the kidney, L-NMMA reduced blood flow and increased vascular resistance in both rat strains. 4. BQ 123 induced minor changes in regional haemodynamics that were not significantly different between the two strains. 5. Acetylcholine following BQ 123 induced an increase in myocardial blood flow in WKY rats, but decreased blood flow in SHR. Acetylcholine following L-NMMA reduced myocardial blood flow in both strains. 6. Acetylcholine following BQ 123 induced renal vasodilation in WKY rats but, following L-NMMA, ACh did not induce renal vasodilation in either rat strain. In contrast, L-NMMA did not abolish the vasodilation of acetylcholine in skeletal muscle in WKY rats. 7. In conclusion, the contribution of nitric oxide to basal vessel tone was not impaired in the heart, skeletal muscle and kidney in SHR. Antagonism of ETA receptors caused similar haemodynamic responses in both rat strains in these organs. Furthermore, NOS inhibition, but not ETA blockade, blunted the expected ACh-induced vasodilation in the heart and kidney in WKY rats, but not in skeletal muscle in both strains.

Effects of acetylcholine and nitroprusside on systemic and regional hemodynamics in hypertensive rats

This study was performed to investigate the in vivo effects of acetylcholine, a stimulator of endogenous NO production, and nitroprusside, an exogenous NO-donor, on hemodynamics in the normotensive (WKY) and the hypertensive (SHR) rat. Anesthetized rats were given microspheres for the measurement of cardiac index (CI), total vascular resistance (TPRI), regional blood flow and vascular resistance. Infusion of acetylcholine (2 microg/kg/min) caused a marked decrease in TPRI by (-35+/-5%, +/-SEM) in the WKY (n=8), whereas in the SHR (n=8) a less pronounced reduction was seen (-14+/-3%, p<0.01 between groups). CI increased by 27+/-9% in the WKY, but was unaltered in the SHR. Blood pressure decreased similarly (17-20%). Acetylcholine significantly increased blood flow by about 40% in the kidneys and the heart in the WKY, but had no significant effect in the SHR. Other tissues, such as skeletal muscle and cerebral tissues, showed no major changes. Infusion of nitroprusside (1 microg/kg/min) reduced blood pressure by 5 to 10% in the strains. The regional effects of nitroprusside did not differ between the strains. In conclusion, the acetylcholine-induced vasodilation in the kidney and the heart was attenuated in the SHR compared to the WKY. These findings might suggest a difference in the endothelial response between the SHR and the WKY in some, but not in all, tissues.

Thyrotropin-releasing hormone accelerates fetal mouse lung ultrastructural maturation via stimulation of extra thyroidal pathway

Maternal administration of TSH-releasing hormone (TRH) in the euthyroid mouse accelerates fetal lung ultrastructural maturation. However, the mechanism(s) of TRH in fetal lung development remains unclear; it could be due to its neuroendocrine and/or neurotransmitter effects. Although the neuroendocrine effect of TRH is mediated via stimulation of the fetal pituitary-thyroid axis, the neurotransmitter effect is mediated via stimulation of fetal autonomic nervous system activity. In the hyt/hyt mouse there is a point mutation in the beta subunit of the TSH receptor in the thyroid gland of the Balb-c mouse. In these mice TSH does not bind to its receptors, leading ultimately to the development of primary hypothyroidism, which is transmitted as an autosomal recessive trait. A maturational delay in the lung ultrastructure of the hyt/hyt mouse fetus has been observed. This investigation was undertaken to study the effect of maternal TRH treatment on lung ultrastructural maturation in the hyt/hyt mouse fetus. If the effect of TRH is mediated via stimulation of fetal pituitary-thyroid axis, TRH treatment should not enhance lung maturity in the hyt/hyt fetus and vice versa. Adult hyt/hyt mice made euthyroid by triiodothyronine supplementation were mated to carry hyt/hyt pups. Saline or TRH (0.4 or 0.6 mg/kg/dose) was administered to the mother (i.p.) on d 16 and 17 (b.i.d.) and on d 18 of pregnancy 1 h before killing (term, approximately 20 d). The fetal lung electron micrographs were subjected to ultrastructural morphometric analysis of the number of lamellar bodies and glycogen/nuclear ratio in type II cells, and the alveolar/parenchymal ratio by Chalkley point counting with an interactive computerized image analyzer (Optimas, Bioscan). Fetal lungs exposed to the lower dose of TRH (n = 7) showed no significant difference in their ultrastructural maturation when compared with saline-treated controls (n = 5). However, fetal lungs exposed to a higher dose of TRH (n = 6) showed increased numbers of lamellar bodies per type II cell, an increase in the alveolar/parenchymal ratio, larger air spaces, thinner alveolar septa, presence of tubular myelin, and increased numbers of air-blood barriers. We conclude that the effect of TRH in accelerating fetal mouse lung maturation is at least in part mediated via stimulation of extra thyroidal pathways.

Effect of neuropeptides on cognitive function

Recent evidence indicates that, in addition to the involvement of cholinergic and other neurotransmitter systems, various neuropeptides that occur in cortical and subcortical brain regions have a role in cognitive behavior. This evidence results largely from behavioral studies in rodents and other animals, following peptide administration and only in a very few cases from similar studies in human subjects. Several neuropeptides studied appear to enhance or produce changes conducive to improvement in cognitive performance and these include vasopressin, corticotrophin-releasing hormone (CRH), somatostatin, substance P, neuropeptide Y, and thyrotrophin-releasing hormone (TRH), while one peptide, galanin, has been reported to inhibit cognitive processes. Of those neuropeptides that improve performance, only TRH has been shown recently to attenuate the memory impairment of human subjects and Alzheimer patients treated with an anticholinergic drug, and this review describes a series of complimentary studies in adult and aged rodents that contribute to our understanding of the possible mechanisms involved in the role of TRH in cognition.

Effects of TRH and atropine on induction and duration of anesthesia with propofol in rats

The effects of IV TRH pretreatment on induction of anesthesia with propofol or pentobarbital were investigated in rats. The effects of IV TRH, administered after induction, on duration of propofol anesthesia and the interaction with atropine were also studied. The doses of propofol or pentobarbital were not influenced by TRH. TRH reduced duration of anesthesia after propofol, with higher brain concentrations of propofol at recovery. Atropine did not block this effect, but given alone prolonged duration of anesthesia. It is concluded that TRH shortens the duration of propofol anesthesia, probably due to a pharmacodynamic effect and not to a pharmacokinetic interaction.

The influence of muscarinic and prostaglandic mechanisms on regional cerebral and peripheral blood flows and on the vascular effects of thyrotropin releasing hormone (TRH)

TRH has pronounced vascular effects. The final transmitter mechanisms of these effects are not fully understood. The present study was conducted in order to elucidate whether these effects are mediated by prostaglandic or muscarinic mechanisms. Muscarinic blockade augmented the vasoconstricting- and pressor effect of TRH; vasodilation in the brain was attenuated only in the caudate nucleus. Indomethacin provoked a decrease in regional cerebral blood flow and in the gastric mucosal blood flow. No effect of indomethacin was observed on the vascular effects of TRH. It is concluded that the cerebral vasodilating and peripheral vasoconstricting effects of TRH are not mediated by prostaglandins. Muscarinic mechanisms are involved in the vasodilating effect of TRH only in the caudate nucleus.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.

Effects of NG-nitro-L-arginine methyl ester on the cardiovascular system of the anaesthetized rabbit and on the cardiovascular response to thyrotropin-releasing hormone

1. The effects of 300 mg kg-1 of the nitric oxide (NO) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on the regional blood flow, on the flow response to 1 mg kg-1 of thyrotropin-releasing hormone (TRH) and on cerebral blood flow autoregulation were studied in urethane anesthetized rabbits subjected to unilateral sectioning of the cervical sympathetic claim. The blood flow measurements were performed by the tracer microspheres method. 2. The cerebral arteriovenous difference in oxygen saturation (CAVOD) was measured before and after the administration of L-NAME and TRH in order to ascertain whether the effects on cerebral blood flow that were observed were secondary to changes in cerebral metabolism. 3. L-NAME caused a significant decrease in blood flow in several cerebral regions; CBFtot decreased to 72 +/- 4% of control (P < 0.001). An increase in blood pressure and a concurrent decrease in heart rate and cardiac output were noted. 4. In the eye, L-NAME caused a reduction in uveal blood flow which was more pronounced on the sympathetically intact side; in the retina the blood flow decreased to 50% of control on both sides. 5. The administration of TRH in animals pretreated with L-NAME caused a significant increase in blood pressure and cerebral blood flow. 6. In L-NAME-treated animals the CBF was not affected when the mean arterial blood pressure was increased by ligation of the abdominal aorta. 7. The CAVOD increased from 56.0 +/- 5.2 to 73.6 +/- 3.5%, 20 min after the administration of L-NAME. In animals given 1 mg kg-1 TRH after L-NAME the CAVOD decreased to 54.6 +/- 4.6%, 5 min after the injection of TRH.8. The results of the present study indicate that endogenous NO is involved in the regulation of regional blood flow and blood pressure in the anaesthetized rabbit. The reduction in cerebral blood flow that was caused by L-NAME was not due to a reduction in cerebral metabolism. An interaction between the NO synthesis/release/effect and the sympathetic nervous system was found in the uvea. There was no evidence for a major involvement of NO in the cardiovascular responses to TRH and autoregulation of cerebral blood flow was not abolished by L-NAME.

Adrenergic and non-adrenergic cardiovascular effects of thyrotropin-releasing hormone (TRH) in the anaesthetized rabbit

The effects of thyrotropin-releasing hormone (TRH) on regional blood flows were studied in urethane-anaesthetized rabbits. Experiments were performed both with and without adrenergic antagonist pretreatment. The tracer microsphere method was used to measure blood flow. TRH (0.1 mg kg-1) caused an increase in mean arterial blood pressure (MAP) from 9.8 +/- 1 to 11.8 +/- 0.8; a higher dose (1 mg kg-1) increased the blood pressure to 15.2 +/- 1 kPa (P less than 0.001). Total cerebral blood flow (CBFtot) increased to 137 +/- 10% (P less than 0.05) of control at the lower dose and to 214 +/- 16% (P less than 0.001), at the higher dose. A reduction in blood flow at both doses of TRH in several peripheral organs indicates that the pressor effect was mainly due to an effect on the peripheral vascular resistance. In prazosin-pretreated animals in which the MAP was normalized by ligation of the thoracic aorta, TRH elicited an increase in the CBFtot to 131 +/- 12% (P less than 0.05) of control. In the iris, TRH caused vasodilation in prazosin-pretreated animals. In experiments with combined alpha- and beta-adrenergic blockade, a non-adrenergic vasoconstricting effect of TRH was seen in some peripheral organs. The results indicate that TRH activates the sympathetic nervous system thus causing an increased vascular resistance and MAP; these effects are mediated mainly by an alpha 1-adrenergic mechanism. In the spleen, the gastric mucosa and the adrenal glands, the vasoconstriction caused by TRH was partly non-adrenergic. The vasodilation seen in the small intestine and the anterior uvea after TRH treatment and adrenoceptor blockade may be explained by effects on the parasympathetic nervous system. The vasodilating effect of TRH in the brain does not seem to involve alpha 1- or beta-adrenergic mechanisms.

Naloxone and TRH affect regional blood flows in the anesthetized rabbit

The cardiovascular effects of IV naloxone and a subsequent administration of TRH IV were studied in the rabbit. Naloxone caused a vasodilation in the myocardium and adrenal glands. Naloxone elicited an increment in cerebral blood flow in several regions which attenuated the cerebrovasodilating effect of TRH in a few regions. The blockade of endogenous opioids with naloxone did not modify the peripheral vasoconstricting effect of TRH or affect the vascular effects of TRH mediated by the peripheral sympathetic nerves. The results indicate that naloxone has a vasodilating effect in the myocardium and CNS in anesthetized rabbits. The major part of the cardiovascular effect of TRH is not dependent on mechanisms sensitive to naloxone.

Effects of alpha 2-adrenoceptor blockade and thyrotropin-releasing hormone (TRH) on the cardiovascular system in the rabbit

The effects of two different doses of thyrotropin-releasing hormone on regional blood flows were studied in urethane-anaesthetized rabbits pretreated with the alpha 2-adrenergic antagonists yohimbine and idazoxan. The effects of yohimbine were also studied using unanaesthetized rabbits. Blood flow measurements were performed using the tracer microsphere method. Thyrotropin-releasing hormone was injected i.v. at a dose of either 0.1 mg kg-1 or 2.0 mg kg-1. Yohimbine and idazoxan did not modify the effect of thyrotropin-releasing hormone on mean arterial blood pressure. In the anaesthetized animals, blockade of the alpha 2-adrenoceptors resulted in a vasoconstriction in several peripheral organs and the vasoconstriction increased after thyrotropin-releasing hormone administration. Pretreament with yohimbine reduced total cerebral blood flow moderately and in such animals thyrotropin-releasing hormone elicited only minor cerebral blood flow effects. Pretreatment with idazoxan did not reduce the total cerebral blood flow and in such animals it increased from 53 +/- 1 to 75 +/- 4 g min-1 100 g-1 (P less than 0.01) after the administration of the lower dose of thyrotropin-releasing hormone and from 64 +/- 5 to 112 +/- 17 g min-1 100 g-1 (P less than 0.01) after the higher dose. In the conscious animals, yohimbine caused an increase in mean arterial blood pressure and heart rate. Vascular resistance increased in several organs. The cerebral blood flow decreased in white matter (P less than 0.05) and the caudate nucleus (P less than 0.05). The results indicate that there is a yohimbine-sensitive mechanism involved in the cerebrovasodilating effect of thyrotropin-releasing hormone in anaesthetized rabbits.(ABSTRACT TRUNCATED AT 250 WORDS)