Analeptic effects of centrally injected TRH and analogues of TRH in the pentobarbitone-anaesthetized rat

Are thyrotropin-releasing hormone (TRH) and analog taltirelin viable reversal agents of opioid-induced respiratory depression?

Opioid‐induced respiratory depression (OIRD) is a potentially life‐threatening complication of opioid consumption. Apart from naloxone, an opioid antagonist that has various disadvantages, a possible reversal strategy is treatment of OIRD with the hypothalamic hormone and neuromodulator thyrotropin‐releasing hormone (TRH). In this review, we performed a search in electronic databases and retrieved 52 papers on the effect of TRH and TRH‐analogs on respiration and their efficacy in the reversal of OIRD in awake and anesthetized mammals, including humans. Animal studies show that TRH and its analog taltirelin stimulate breathing via an effect at the preBötzinger complex, an important respiratory rhythm generator within the brainstem respiratory network. An additional respiratory excitatory effect may be related to TRH’s analeptic effect. In awake and anesthetized rodents, TRH and taltirelin improved morphine‐ and sufentanil‐induced respiratory depression, by causing rapid shallow breathing. This pattern of breathing increases the work of breathing, dead space ventilation, atelectasis, and hypoxia. In awake and anesthetized humans, a continuous infusion of intravenous TRH with doses up to 8 mg, did not reverse sufentanil‐ or remifentanil‐induced respiratory depression. This is related to poor penetration of TRH into the brain compartment but also other causes are discussed. No human data on taltirelin are available. In conclusion, data from animals and human indicate that TRH is not a viable reversal agent of OIRD in awake or anesthetized humans. Further human studies on the efficacy and safety of TRH’s more potent and longer lasting analog taltirelin are needed as this agent seems to be a more promising reversal drug.

Intravenous and Intratracheal Thyrotropin Releasing Hormone and Its Analog Taltirelin Reverse Opioid-Induced Respiratory Depression in Isoflurane Anesthetized Rats

Thyrotropin releasing hormone (TRH) is a tripeptide hormone and a neurotransmitter widely expressed in the central nervous system that regulates thyroid function and maintains physiologic homeostasis. Following injection in rodents, TRH has multiple effects including increased blood pressure and breathing. We tested the hypothesis that TRH and its long-acting analog, taltirelin, will reverse morphine-induced respiratory depression in anesthetized rats following intravenous or intratracheal (IT) administration. TRH (1 mg/kg plus 5 mg/kg/h, i.v.) and talitrelin (1 mg/kg, i.v.), when administered to rats pretreated with morphine (5 mg/kg, i.v.), increased ventilation from 50% ± 6% to 131% ± 7% and 45% ± 6% to 168% ± 13%, respectively (percent baseline; n = 4 ± S.E.M.), primarily through increased breathing rates (from 76% ± 9% to 260% ± 14% and 66% ± 8% to 318% ± 37%, respectively). By arterial blood gas analysis, morphine caused a hypoxemic respiratory acidosis with decreased oxygen and increased carbon dioxide pressures. TRH decreased morphine effects on arterial carbon dioxide pressure, but failed to impact oxygenation; taltirelin reversed morphine effects on both arterial carbon dioxide and oxygen. Both TRH and talirelin increased mean arterial blood pressure in morphine-treated rats (from 68% ± 5% to 126% ± 12% and 64% ± 7% to 116% ± 8%, respectively; n = 3 to 4). TRH, when initiated prior to morphine (15 mg/kg, i.v.), prevented morphine-induced changes in ventilation; and TRH (2 mg/kg, i.v.) rescued all four rats treated with a lethal dose of morphine (5 mg/kg/min, until apnea). Similar to intravenous administration, both TRH (5 mg/kg, IT) and taltirelin (2 mg/kg, IT) reversed morphine effects on ventilation. TRH or taltirelin may have clinical utility as an intravenous or inhaled agent to antagonize opioid-induced cardiorespiratory depression.

Piracetam and TRH analogues antagonise inhibition by barbiturates, diazepam, melatonin and galanin of human erythrocyte D-glucose transport

1 Nootropic drugs increase glucose uptake into anaesthetised brain and into Alzheimer's diseased brain. Thyrotropin-releasing hormone, TRH, which has a chemical structure similar to nootropics increases cerebellar uptake of glucose in murine rolling ataxia. This paper shows that nootropic drugs like piracetam (2-oxo 1 pyrrolidine acetamide) and levetiracetam and neuropeptides like TRH antagonise the inhibition of glucose transport by barbiturates, diazepam, melatonin and endogenous neuropeptide galanin in human erythrocytes in vitro. 2 The potencies of nootropic drugs in opposing scopolamine-induced memory loss correlate with their potencies in antagonising pentobarbital inhibition of erythrocyte glucose transport in vitro (P<0.01). Less potent nootropics, D-levetiracetam and D-pyroglutamate, have higher antagonist Ki's against pentobarbital inhibition of glucose transport than more potent L-stereoisomers (P<0.001). 3 Piracetam and TRH have no direct effects on net glucose transport, but competitively antagonise hypnotic drug inhibition of glucose transport. Other nootropics, like aniracetam and levetiracetam, while antagonising pentobarbital action, also inhibit glucose transport. Analeptics like bemigride and methamphetamine are more potent inhibitors of glucose transport than antagonists of hypnotic action on glucose transport. 4 There are similarities between amino-acid sequences in human glucose transport protein isoform 1 (GLUT1) and the benzodiazepine-binding domains of GABAA (gamma amino butyric acid) receptor subunits. Mapped on a 3D template of GLUT1, these homologies suggest that the site of diazepam and piracetam interaction is a pocket outside the central hydrophilic pore region. 5 Nootropic pyrrolidone antagonism of hypnotic drug inhibition of glucose transport in vitro may be an analogue of TRH antagonism of galanin-induced narcosis.

Design, synthesis, and biological evaluation of novel, centrally-acting thyrotropin-releasing hormone analogues

Novel, metabolically stable and centrally acting TRH analogues with substituted pyridinium moieties replacing the [His(2)] residue of the endogenous peptide were prepared by solid-phase Zincke reaction. The 1,4-dihydropyridine prodrugs of these analogues obtained after reducing the pyridinium moiety were able to reach the brain and maintain a sustained concentration of the charged, degradation-resistant analogues formed after enzymatic oxidation of the prodrug, as manifested by the analeptic action measured in mice. Among the four analogues reported, compound 2a showed the highest potency and longest duration of action in reducing the pentobarbital-induced sleeping time compared to the parent TRH. No binding to the endocrine TRH-receptor was measured for 2a; thus, this compound emerged as a potent, centrally acting TRH analogue.

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.

Anti-ataxic effects of TRH and its analogue, TA-0910, in Rolling mouse Nagoya by metabolic normalization of the ventral tegmental area

1. The mechanism of the anti-ataxic action of thyrotropin-releasing hormone (TRH) and its analogue. TA-0910, in the Rolling mouse Nagoya (RMN), an ataxic mutant mouse, has been investigated. 2. TRH (30 mg kg-1, i.p.) and TA-0910 (3 mg kg-1, i.p.) reduced the fall index (number of falls/spontaneous motor activity), an index of ataxia, 10-30 and 10-60 min after administration, respectively. 3. Relative local cerebral glucose utilization (LCGU) in the cerebellum and ventral tegmental area (VTA) of the rolling mouse was significantly smaller than that in normal animals. TRH (30 mg kg-1, i.p.) and TA-0910 (3 mg kg-1, i.p.) increased the relative LCGU value of the VTA but not of the cerebellum in rolling mice to the level of normal animals. 4. These results suggest that the ataxia of the rolling mouse may be due to dysfunction of the cerebellum and VTA, and that amelioration by TRH and TA-0910 could result from metabolic normalization of the VTA.

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.

Effects of olfactory stimulation with jasmin and its component chemicals on the duration of pentobarbital-induced sleep in mice

The effect of olfactory stimulation with jasmin and its component chemicals on pentobarbital sleep time was investigated using mice. In the present study we sought to determine which component of jasmin influences pentobarbital sleep time via olfactory stimulation. Sleep time was defined as the time elapsed between intraperitoneal pentobarbital administration and the first time that the animal was able to spontaneously right itself. Sleep time was significantly decreased by olfactory stimulation with jasmin, and also by one of the fractions obtained by fractional distillation at 150 degrees C and 0.1 mmHg. The fraction which influenced the sleep time was found to consist of benzyl benzoate, isophytol, geranyl linalool, phytol and phytyl acetate, which were identified using gas chromatography with mass and infrared spectrometry. In experiments using authentic samples of these components, phytol significantly shortened the pentobarbital sleep time, while the others had no effect. We conclude that phytol is the component of jasmin which reduces the duration of pentobarbital-induced sleep.

Thyrotropin-releasing hormone selectively reverses lorazepam-induced sedation but not slowing of saccadic eye movements

To investigate preliminary reports that benzodiazepine-induced sedation may be reversed by thyrotropin-releasing hormone (TRH), we examined the effect of TRH or saline placebo on two variables which are sensitive to benzodiazepine agonists: changes in sedation and saccadic eye movements. Lorazepam 10 micrograms/kg i.v. increased self-ratings of sedation and reduced self-ratings of alertness and these changes were almost completely reversed by TRH. In contrast the slowing of saccadic eye movements by lorazepam was not reversed by TRH. The effects of TRH do not appear to be due to a direct antagonism at the benzodiazepine receptor, since flumazenil reverses changes in both variables. Moreover ligand binding studies reveal that TRH has very low affinity at this receptor. These clinical data provide the first demonstration that it is possible to distinguish between the effects of benzodiazepines on saccadic eye movements and psychological self-ratings.

Interactions between adrenergic systems, anaesthetic and TRH analogue induced analeptic effects on VBT transmission

Previous behavioural and electrophysiological studies have indicated an antinarcotic action of thyrotropin-releasing hormone (TRH) and its analogues in antagonizing the action of CNS depressant drugs, including baclofen and a variety of anesthetics. While beta-adrenergic receptors are implicated in the level of anaesthesia/arousal, whether the analeptic action of TRH involves adrenergic systems for its expression is uncertain. The object of the present experiments, therefore, was to examine interactions between adrenergic systems and the anti-anaesthetic effects of TRH analogue CG3703. It was found that pretreatment with the beta-block (+/-)propranolol did not abolish or reduce the ability of CG3703 to antagonize urethane-induced depression of VBT transmission. These results suggest therefore, that beta-adrenergic systems are unlikely to be involved in the anti-anaesthetic effect of the tripeptide.

TRH analogue antagonizes anaesthetic induced depression of information transfer through the ventrobasal thalamus of the rat

Previous studies have indicated an anti-narcotic action of TRH and its analogues. A major site of anaesthetic action is in the ventrobasal thalamus (VBT). The present experiments were performed to determine whether there was antagonism between anaesthetics and a TRH analogue on VBT transmission. It was found that the TRH analogue CG3703 reversed the depressant actions of the anaesthetics urethane, sodium pentobarbitone and sodium brietal on ventrobasal transmission. These actions were also observed at the cortical level but cuneate transmission was unaffected. These results are discussed in the context of modulation of information flow through VBT. In particular it is suggested that TRHergic input from the thalamic reticular nucleus may be an important regulator of VBT transmission.

Thyrotropin-releasing hormone has stimulatory effects on ventilation in humans

Thyrotropin-releasing hormone (TRH) stimulates pituitary thyrotropin synthesis and release and also regulates autonomic nervous system functions by acting as a neuromodulator and neurotransmitter. In experimental animals a stimulation of ventilation by thyrotropin-releasing hormone was shown when applied at central nervous system sites that affect respiratory motor output. It was the goal of our study to investigate the respiratory properties of thyrotropin-releasing hormone on basal and stimulated (i.e. CO2-rebreathing) conditions following systemic thyrotropin-releasing hormone application in healthy humans. Thyrotropin-releasing hormone (200 micrograms, 400 micrograms intravenous) initiated a rapid short lasting rise of minute volume, ventilatory air-flow and alveolar oxygen tension under steady state breathing (P less than 0.001). Breathing frequency was less affected, heart rate rose concomitantly (P less than 0.001). While breathing with increasing concentrations of carbon dioxide, minute volume was higher under thyrotropin-releasing hormone than under placebo alone. Further effects (e.g. nausea, dizziness, palpitations) mostly appeared later than respiratory changes and thus may not be responsible for their initiation. Our findings prove systemic thyrotropin-releasing hormone to be a strong respiratory stimulant in man. Response in respiratory output was also accompanied by central nervous system-effects (e.g. dizziness, restlessness, augmented vigilance). The mode of thyrotropin-releasing hormone effects on respiration after peripheral administration is still speculative. An augmented sympathetic output or a direct receptor mediated action at central nervous system sites may be responsible, while a peripheral effect cannot be excluded.

Effects of olfactory stimulation on the sleep time induced by pentobarbital administration in mice

The effect on the pentobarbital sleep time by olfactory stimulation with various odorants was investigated using mice to appraise the physiological or psychological significance of olfactory information. The sleep time was determined as the time elapsed between intraperitoneal pentobarbital administration and the first time that the animal was able to spontaneously right itself. The sleep time was affected by inhalation of some odorants compared to pure air controls, but not by others. The sleep time was prolonged by terpinyl acetate and phenethyl alcohol, and was shortened by lemon oil and jasmin oil. However, neither potentiation nor attenuation of pentobarbital action by odorant inhalation was observed when using anosmic mice produced by intranasal zinc sulphate treatment. In conclusion, olfactory stimulation associated with odorant inhalation influences the pentobarbital sleep time, suggesting that olfactory information may have a more potent influence on the physiological and psychological status than has previously been thought.

Analogs of thyrotropin-releasing hormone (TRH): receptor affinities in brains, spinal cords, and pituitaries of different species

[3H](3-Me-His2) thyrotropin-releasing hormone ([3H]MeTRH) bound to TRH receptors in rodent, rabbit and dog brain and spinal cord (SC), and in rat, sheep, bovine and dog anterior pituitary (PIT) glands, with high affinity (dissociation constants, KdS = 5-9 nM; n = 3-4) but to different densities of these sites (Bmax range 6-145 fmol/mg protein) (rabbit SC greater than sheep PIT much greater than G.pig brain greater than dog brain greater than rat brain greater than bovine and dog PIT). Various TRH analogs competitively inhibited [3H]MeTRH binding in these tissues with a similar rank order of potency: MeTRH greater than TRH greater than CG3703 greater than or equal to RX77368 greater than or equal to MK-771 greater than TRH Glycinamide greater than Glu1-TRH much greater than CG3509 greater than or equal to NVal2-TRH much much greater than TRH free acid much much greater than and cyclo-His-Pro, indicating a pharmacological similarity of CNS and pituitary TRH receptors. While most TRH analogs displaced [3H]MeTRH binding with a similar potency in the different species, TRH exhibited a 2-fold lower affinity in the rat and G.pig brain than in other tissues of other species. Similarly, CG3703 was 2.4-4.5 times more active in the rabbit brain than in the rodent and dog brain, and also more potent in the rabbit brain as compared to the sheep PIT.(ABSTRACT TRUNCATED AT 250 WORDS)

Central ventilatory effects of thyrotropin-releasing hormone in the conscious rat

Thyrotropin-releasing hormone was shown to exert potent ventilatory effects after central administration. These data, however, were derived from studies using anesthetized animal preparations. Since TRH elicits strong arousal reactions, the observed ventilatory effects of TRH under anesthesia may have been due to nonspecific reduction in the anesthetic state of the animals. In order to clarify the extent to which the reversal of anesthesia may change ventilatory parameters after TRH application, we investigated the effect of TRH on ventilation rate, relative tidal volume, relative respiratory minute volume, CO2 production CO2 consumption, and locomotor activity in the conscious, unrestrained rat. Intracerebroventricular application of TRH induced a dose-dependent, sustained increase in ventilation rate, relative tidal volume, and relative respiratory minute volume of maximally 128%, 890%, and 235%, respectively. In addition, CO2 production and O2 consumption were elevated by 4.6 and 11.7 fold, while no significant changes in locomotor activity were observed. The results suggest that TRH stimulates ventilation by a mechanism independent of its analeptic properties.

Pharmacological and biochemical comparison of thyrotropin releasing hormone (TRH) and di-methyl proline-TRH on pituitary GH3 cells

1. The binding of [3H]-thyrotropin releasing hormone ([3H]-TRH) and [3H]-RX77368 (di-methyl proline TRH) and the ability of these peptides to stimulate phosphoinositide hydrolysis were investigated in the GH3 pituitary cell line. 2. For both peptides binding was found to be saturable with a single component (Hill slopes were, for TRH, 0.98 and for RX77368, 1.13). TRH bound with greater affinity than RX77368 Kd values were 16 nM and 144 nM respectively. Bmax values were 227 fmol mg-1 protein for TRH and 123 fmol mg-1 protein for RX77368. 3. The rank order of potency of a series of TRH analogues to inhibit binding was the same versus each peptide. However, unlike with saturation analysis, Hill slopes of all displacing ligands were less than 1.0 against both TRH and RX77368 suggesting either multiple binding sites, alteration of affinity state, negative co-operativity or some allosteric interaction. 4. Both peptides stimulated phosphoinositide hydrolysis in a dose-dependent fashion. TRH was more potent than RX77368, EC50 values were 7.9 +/- 1 nM and 96.3 +/- 3 nM respectively. 5. These in vitro data suggest that the greater in vivo potency of RX77368 is not the result of enhanced receptor affinity but is more probably due to its greater metabolic stability.

Norvaline2-TRH: binding to TRH receptors in rat brain homogenates

Norvaline2-thyrotropin-releasing hormone ([Nva2]TRH) has been described as a thyrotropin-releasing hormone (TRH) analog with no thyrotropin (TSH)-releasing capacity but enhanced analeptic activity compared with TRH, as shown by the reversal of haloperidol-induced catalepsy. We have evaluated the receptor-binding properties of [Nva2]TRH in homogenates of rat anterior pituitary, hypothalamus, brainstem and cortex tissue, using [3H]TRH and [3H][3-Me-His2]TRH as radioligands. Apparent Ki values at high affinity TRH-binding sites, labelled predominantly by [3H][3-Me-His2]TRH, ranged from 17.0 to 36.9 microM in all tested regions. Additionally, [Nva2]TRH was shown to compete with [3H]TRH at low affinity TRH-binding sites with similar affinities. It is concluded that the loss of TSH-releasing activity of [Nva2]TRH appears to be due to a drastic reduction in binding affinity to the high affinity TRH receptor subtype. Its analeptic activity, however, may be mediated by low affinity TRH binding sites which are predominantly labelled by [3H]TRH or by yet unidentified mechanisms.

Thyrotropin releasing hormone augments growth of spinal cord transplants in oculo

The effects of thyrotropin releasing hormone (TRH) on spinal cord growth were evaluated using the in oculo transplant model. The growth of fetal spinal cord allografts, placed into the anterior eye chamber of Sprague-Dawley rats, was markedly augmented by acute exposure of the graft and host animal to TRH at the time of transplantation. No significant growth augmentation was seen after equimolar administration of a mixture of the amino acids that comprise the TRH molecule. It is concluded that acutely administered TRH, at the time of grafting, elicits a significant stimulation of the growth of spinal cord tissue. Our data strengthen the rationale for continued clinical trials of this peptide in spinal cord injury.

Controlled acute trial of a thyrotrophin releasing hormone analogue (RX77368) in motor neuron disease

Twenty five patients with motor neuron disease completed a double blind randomised cross over trial of RX77368, a stabilised TRH analogue, iv over 2 hours against saline. Temporary improvement in bulbar symptoms including speech, respiratory parameters, tongue movements and swallowing were seen. Fasciculations increased and spasticity decreased. Change in muscle force with drug was different from placebo but both increase and decrease in force were seen and did not result in detectable changes in function. Side effects were clinically significant in 50% of the patients and cleared within 12 hours. Prolonged rise of thyroxine and an increase in plasma levels of prolactin, thyroid stimulating hormone and growth hormone were seen and followed characteristic patterns.

Comparative conformation-activity relationships for hormonally- and centrally-acting TRH analogues

Empirical energy calculations have been applied to a series of thyrotrophin-releasing hormone (TRH) analogues in an attempt to incorporate the conformational parameter into traditional Quantitative Structure-Activity Relationships (QSAR). A search was made of the potential surface of each analogue using a SIMPLEX technique coupled with a model representing solvent effects as a dielectric continuum. Conformers of TRH can be described in terms of the distance between the pyroglutamyl, imidazole and prolyl rings, and presented as two-dimensional descriptor coordinates using multidimensional scaling. This process was repeated for stable and metastable conformers of all the TRH analogues investigated and correlated with experimental data on receptor affinity, relative stability and biological potency of the analogues. The results obtained suggest that modifications to the TRH structure can lead to greater potency in addition to improved stability, and that providing the appropriate theoretical approach is taken, it is possible to apply QSAR-based design procedures to conformationally-flexible drugs.

Regional distribution of in vitro release of thyrotropin releasing hormone in rat brain

To increase our knowledge of the TRH functions in brain and the processes of TRH compartmentalization and release, we studied the in vitro release of endogenous TRH in different brain areas. We also determined the correlation between TRH levels and release under both basal and stimulated conditions. TRH concentration was measured in tissues and media by specific radioimmunoassay. TRH-like material detected in olfactory bulb and hypothalamic incubates (basal or K+ stimulated) were shown to be chromatographically identical to synthetic TRH. Different brain regions showed high variability in the basal release of TRH (1-20% of tissue content). This suggests the existence of different pools. The response to depolarizing stimulus (56 mM K+) was significant only in the following regions: median eminence, total hypothalamus, preoptic area, nucleus accumbens-lateral septum, amygdala, mesencephalon, medulla oblongata and the cervical region of the spinal cord. These regions have been shown to contain a high number of receptors, a high concentration of TRH nerve endings and are susceptible to TRH effects. These results support the hypothesis that TRH functions as neuromodulator in these areas.

Thyrotrophin releasing hormone--5-hydroxytryptamine interactions in the brain studied using chronic immunization and chemical lesioning techniques

This study investigates the effects of chemically lesioning 5-hydroxytryptamine (5HT) neurones and chronic passive immunization of central thyrotrophin releasing hormone (TRH) on 5HT and TRH mediated behavioural responses. 5HT lesions produced by 5,7-dihydroxytryptamine (5,7-DHT) enhanced the behavioural response produced by the 5HT receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MEODMT) while decreasing the locomotor hyperactivity observed following administration of the TRH analogue CG 3509 but having no effect on the reversal of pentobarbitone sleep-time produced by CG 3509. Chronic intracerebroventricular infusion of the purified TRH antibody markedly increased the length of pentobarbitone-induced sleep-time while enhancing the effects of CG 3509 both on locomotor activity and pentobarbitone-induced sleep. TRH antibody infusion also increased the response produced by 5-MEODMT. The results indicate that chronic passive immunization of central TRH induces changes in TRH receptor responsiveness and that there is a functional interaction between TRH and 5HT neuronal systems.

Chronic immunization of endogenous thyrotrophin-releasing hormone (TRH) in brain alters the behavioural response to pentobarbital and a TRH analogue

Rats were infused with purified thyrotrophin-releasing hormone (TRH) anti-serum i.c.v. for two weeks and the reversal of pentobarbital-induced anaesthesia, hypothermia and respiratory depression by central administration of a TRH analogue (CG 3509) was measured. After antibody infusion the anaesthesia time was more than doubled but the responses to CG 3509 were increased, suggesting a role for endogenous TRH in arousal mechanisms which is sensitized following chronic immunological blockade.

Changes in the behavioural response to a TRH analogue following chronic amitriptyline treatment and repeated electroconvulsive shock in the rat

The arousal elicited in rats by injection into the nucleus accumbens of the thyrotrophin-releasing hormone analogue CG 3509 (orotyl-histidyl-prolineamide) was used to assess the responsiveness to thyrotrophin-releasing hormone following repeated treatment with amitriptyline or electroconvulsive shock. Fourteen day administration of amitriptyline (15 mg kg-1 i.p. twice daily) reduced the behavioural response to bilateral intra-accumbens injection of CG 3509 (2 X 2.5 micrograms). CG 3509-induced hyperactivity, recovery from pentobarbitone-induced anaesthesia and the reversal of both pentobarbitone-induced hypothermia and decreased respiration, were all significantly reduced compared to either the response of the animals prior to amitriptyline administration or that observed in rats following chronic saline administration. Repeated administration of electroconvulsive shock (5 shocks over 10 days) significantly increased CG 3509-induced hyperactivity and the degree of reversal of pentobarbitone-induced hypothermia and respiratory depression following CG 3509 administration. The results demonstrate that chronic antidepressant treatments alter the central functional responsiveness to thyrotrophin-releasing hormone. These changes are discussed with respect to the effects of antidepressant treatments on 5-hydroxytryptamine receptors and possible thyrotrophin-releasing hormone--aminergic interactions.

Interactions between TRH and ethanol in the medial septum

When rats were pretreated with ethanol (3.0 g/kg, IP), subsequent microinjection of thyrotropin-releasing hormone (TRH) (500 or 1000 ng) into the medial septum, 30 minutes later, significantly shortened the time necessary for the rats to regain their righting reflex. Conversely, microinjection of TRH into the nucleus accumbens (1000 ng/side) or the area of the raphe obscurus (1000 ng) had no effect on ethanol-induced depression, although both of these structures mediate specific TRH effects in the CNS. In order to determine if this antagonism was due to a specific TRH interaction, TRH Fab fragments were microinjected into the medial septum just prior to the microinjection of TRH. Under these conditions, TRH did not alter ethanol's depressant actions. Finally, this TRH antagonism of ethanol-induced depression appears attributable to a net increase in neuronal activity, because electrical stimulation (160 microA, 120 Hz, 1.5 msec duration) of the medial septum antagonized ethanol's impairment of the righting reflex. These results are discussed in relationship to a potential CNS site for the action of ethanol.

Thyrotrophin-releasing hormone inactivation by human postmortem brain

The mechanisms of inactivation of thyrotrophin-releasing hormone (TRH) by peptidases in several areas of normal human postmortem brain have been investigated by radioimmunoassay and high-performance liquid chromatography. Of the several brain regions studied, the cerebral cortex (Brodman's area, BA10) had the highest TRH-degrading activity in both subcellular fractions. Deamidated-TRH (TRH-OH) was the only product formed by the soluble fraction whereas the histidyl-proline diketopiperazine, cyclo(His-Pro), and a small amount of TRH-OH were formed by the particulate fraction. Several centrally acting TRH analogues showed varying degrees of resistance to degradation by the peptidases in the two fractions, the most stable analogue being RX77368 (pGlu-His-3,3'-dimethyl(ProNH2]. Areas of human postmortem brain appear to contain two of the enzymes capable of degrading TRH, a proline endopeptidase forming TRH-OH and a pyroglutamyl aminopeptidase forming cyclo(His-Pro). The use of the assay procedures in further studies on the inactivation of TRH by peptidases from brain areas of patients with neurological disorders may provide complementary information on the dynamics of TRH in these disorders. The stability of the centrally acting TRH analogues may prove useful in examining their therapeutic potential.

A comparison of the locomotor effects induced by centrally injected TRH and TRH analogues

Thyrotrophin-releasing hormone (TRH) and its stable analogues CG3509 and RX77368 were injected directly into the nucleus accumbens, septum and striatum of the rat and locomotor activity was recorded. TRH (5-20 micrograms) caused a dose-dependent increase in locomotor activity when injected into the nucleus accumbens. TRH (20 micrograms) also increased locomotor activity after administration into the septum but not when put into the striatum. Both the TRH analogues (0.1 and 1.0 microgram) produced closely related increases in activity when injected into either the nucleus accumbens or septum but CG3509 was more potent with a longer lasting effect. Also, in contrast with TRH (20 micrograms), both TRH analogues stimulated locomotor activity when injected into the striatum at a dose of 1 microgram but the effect was less marked and delayed in onset compared to the nucleus accumbens and septum response. Dopamine (100 micrograms) injected into the accumbens or septum also produced significant increases in locomotor activity. The locomotor effects of the peptides are discussed in relation to a possible dopamine-mediated mechanism which contrasts with the actions of TRH and the analogues on barbiturate anaesthesia.