The effects of 5,7-dihydroxytryptamine and p-chlorophenylalanine on thyrotrophin-releasing hormone in regions of the brain and spinal cord of the rat

Optimization of lipid nanoparticles for the delivery of nebulized therapeutic mRNA to the lungs

Lipid nanoparticles (LNPs) for the efficient delivery of drugs need to be designed for the particular administration route and type of drug. Here we report the design of LNPs for the efficient delivery of therapeutic RNAs to the lung via nebulization. We optimized the composition, molar ratios and structure of LNPs made of lipids, neutral or cationic helper lipids and poly(ethylene glycol) (PEG) by evaluating the performance of LNPs belonging to six clusters occupying extremes in chemical space, and then pooling the lead clusters and expanding their diversity. We found that a low (high) molar ratio of PEG improves the performance of LNPs with neutral (cationic) helper lipids, an identified and optimal LNP for low-dose messenger RNA delivery. Nebulized delivery of an mRNA encoding a broadly neutralizing antibody targeting haemagglutinin via the optimized LNP protected mice from a lethal challenge of the H1N1 subtype of influenza A virus, and delivered mRNA more efficiently than LNPs previously optimized for systemic delivery. A cluster approach to LNP design may facilitate the optimization of LNPs for other administration routes and therapeutics.

Structure-activity studies with high-affinity inhibitors of pyroglutamyl-peptidase II

Inhibitors of PPII (pyroglutamyl-peptidase II) (EC 3.4.19.6) have potential applications as investigative and therapeutic agents. The rational design of inhibitors is hindered, however, by the lack of an experimental structure for PPII. Previous studies have demonstrated that replacement of histidine in TRH (thyrotropin-releasing hormone) with asparagine produces a competitive PPII inhibitor (Ki 17.5 microM). To gain further insight into which functional groups are significant for inhibitory activity, we investigated the effects on inhibition of structural modifications to Glp-Asn-ProNH2 (pyroglutamyl-asparaginyl-prolineamide). Synthesis and kinetic analysis of a diverse series of carboxamide and C-terminally extended Glp-Asn-ProNH2 analogues were undertaken. Extensive quantitative structure-activity relationships were generated, which indicated that key functionalities in the basic molecular structure of the inhibitors combine in a unique way to cause PPII inhibition. Data from kinetic and molecular modelling studies suggest that hydrogen bonding between the asparagine side chain and PPII may provide a basis for the inhibitory properties of the asparagine-containing peptides. Prolineamide appeared to be important for interaction with the S2' subsite, but some modifications were tolerated. Extension of Glp-Asn-ProNH2 with hydrophobic amino acids at the C-terminus led to a novel set of PPII inhibitors active in vitro at nanomolar concentrations. Such inhibitors were shown to enhance recovery of TRH released from rat brain slices. Glp-Asn-Pro-Tyr-Trp-Trp-7-amido-4-methylcoumarin displayed a Ki of 1 nM, making it the most potent competitive PPII inhibitor described to date. PPII inhibitors with this level of potency should find application in exploring the biological functions of TRH and PPII, and potentially provide a basis for development of novel therapeutics.

Involvement of 5-HT receptors in the regulation of food intake in Siberian hamsters

The Siberian hamster provides a physiological model for understanding the hypothalamic control of energy metabolism as it undergoes annual photoperiod-regulated cycles of body weight (i.e. fattening in summer, and catabolism of fat stores in winter). As a first step to investigate whether enhanced serotonergic (5-HT) tone might underlie the catabolic processes in short days, we investigated whether serotonergic stimulation can produce catabolic actions in fat hamsters housed in long days. Acute treatment with the serotonin reuptake inhibitor (+/-) fenfluramine (8 mg/kg, i.p.) produced a prolonged, dose-dependent reduction in food intake in both photoperiods. Behavioural observations and radiotelemetry analyses revealed that this anorectic effect of fenfluramine was associated with short-term increases in locomotor activity and in core body temperature. In a subsequent series of studies, hamsters were pretreated with the 5-HT2C receptor antagonist SB242084 (4 mg/kg, i.p.). This 5-HT2C receptor antagonist completely blocked the anorectic actions of fenfluramine, but did not decrease the hyperthermia or hyperlocomotion induced by fenfluramine; thus, the anorectic actions of fenfluramine probably reflect actions via the 5-HT2C receptor. Consistent with these observations, treatment of hamsters with the 5-HT2C receptor agonist VER 3323 (10 mg/kg, i.p.) or the 5-HT1B/2C receptor agonist mCPP (3 mg/kg, i.p.) reduced food intake. The response to manipulation of serotonergic pathways was not affected by the ambient photoperiod in any of these studies. We conclude that the anorectic actions of fenfluramine are not an indirect consequence of serotonergic actions on arousal pathways, and that its actions on feeding in the Siberian hamster are most likely to be mediated by the 5-HT2C receptor.

Inhibition of pudendal reflexes in spinal rats. Reassessing the role of serotonin

The effects of serotonin (5-HT) and thyrotropin-releasing hormone (TRH) on penile reflexes were investigated in intact and spinally transected male rats. Doses of intrathecal 5-HT (0.0, 1.13, 2.26, 11.3, 22.6, and 113.0 nmol), in a range previously shown to inhibit pudendal reflexes in anesthetized spinal preparations, prolonged the latency to the first penile erection in awake intact rats. However, these doses also provoked hyperreactivity and vocalization. Doses of intrathecal TRH (100 and 500 pmol) that effectively inhibited penile erection in intact animals were less effective in spinalized animals. Finally, a combination of subthreshold doses of TRH (100 pmol) and 5-HT (4.0 nmol) at a ratio known to affect other TRH/5-HT-mediated circuits significantly extended erection latency in animals with spinal transections. These data suggest that 5-HT and TRH are both involved in the inhibitory circuits regulating penile erection, either through corelease onto the same population of cells or through independent release onto different populations of neurons.

The thyrotrophin-releasing hormone analogue MK771 induces tic-like behaviours: the effects of dopamine D1 and D2 receptor antagonists

Thyrotrophin-releasing hormone (TRH) and its analogues induce tic-like behaviours in rodents such as blinking and forepaw licking. Changes in spontaneous blinking frequency are observed in several disease states with dopamine abnormalities and dopaminergic agents modulate blinking. We have therefore investigated the effects of dopamine D1 and D2 receptor antagonists on TRH analogue (1-pyro-2-aminoadipyl-L-histidyl-L-thiazolidine-4-carboxamide; MK771)-induced blinking and bouts of forepaw licking. MK771 (2.5 mg/kg)-induced blinking was not attenuated by the dopamine D1 receptor antagonists (+)-7-chloro-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro(1H)-3-benzazep ine maleate (SCH23390) (0.01, 1.0 and 5.0 mg/kg) and ((-)-trans-6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-N-methyl-5- H-benz[2,1b]azepine (SCH39166; 1.0 and 5.0 mg/kg) or the dopamine D2 receptor antagonists raclopride (3.0 and 5.0 mg/kg) and sulpiride (5.0 and 10.0 mg/kg). D1 but not D2 receptor antagonists attenuated MK771-induced forepaw licking. MK771-induced blinking, therefore, appears not to involve dopamine D1 or D2 receptors and contrary to previously held belief dopamine does not appear to be pivotal in the control of blinking, while MK771-induced forepaw licking is modulated by dopamine D1 but not D2 receptors.

Thyrotropin-releasing hormone-immunoreactive varicosities synapse on rat phrenic motoneurons

The relationship between retrogradely labelled or intracellularly filled phrenic motoneurons and varicosities containing thyrotropin-releasing hormone immunoreactivity was investigated in rats by light and electron microscopy. Phrenic motoneurons were identified via retrograde tracing from the diaphragm with cholera toxin B subunit, which was followed by immunocytochemistry to visualise retrogradely labelled motoneurons and thyrotropin-releasing hormone-immunoreactive nerve fibres in their vicinity. At the light microscopic level, varicose thyrotropin-releasing hormone-immunoreactive nerve fibres were distributed sparsely in the phrenic motor nucleus, with some axons surrounding retrogradely labelled motoneurons. In separate intracellular experiments, four phrenic motoneurons identified by antidromic activation from the C5 phrenic nerve root were subsequently filled with Neurobiotin, and nerve fibres that contained thyrotropin-releasing hormone immunoreactivity were identified by immunocytochemistry. The numbers and locations of thyrotropin-releasing hormone-immunoreactive varicosities that were closely appeared to the intracellularly labelled motoneurons were mapped using a camera lucida technique. Close appositions by thyrotropin-releasing hormone-immunoreactive varicosities were seen on somata as well as on proximal and distal dendrites. The closely apposed varicosities were usually present in tight clusters, which were formed by single varicose axons. However, the distribution was nonuniform, in that some dendrites did not receive any close appositions. Ultrastructural analysis of random ultrathin sections through retrogradely labelled neurons showed that varicosities with thyrotropin-releasing hormone immunoreactivity made 1.8% of all synapses and direct contacts on somata and 2.3% of synapses and contacts with dendrites of the retrogradely labelled phrenic motoneurons. The results of these experiments suggest that thyrotropin-releasing hormone-immunoreactive varicosities provide similar numbers of inputs to both the somata and dendrites of phrenic motoneurons. These thyrotropin-releasing hormone-containing inputs seen via light and electron microscopy could modulate the excitability of phrenic motoneurons.

The serotoninergic bulbospinal system and brainstem-spinal cord content of serotonin-, TRH-, and substance P-like immunoreactivity in the aged rat with special reference to the spinal cord motor nucleus

The 5-hydroxytryptamine (5HT) containing bulbospinal pathway was studied with immunohistochemical (IF) and chemical techniques in 2-3 and 30 months old male Sprague-Dawley rats. The coexisting neuropeptides substance P (SP), thyrotropin-releasing hormone (TRH) and galanin were also analysed. Furthermore, the expression of mRNA encoding aromatic L-amino acid decarboxylase (AADC), prepro-TRH, and preprotachykinin (prepro-SP) was analysed with in situ hybridization (ISH) in the midline raphé nuclei inthe lower brainstem. The results showed a decreased number of axonal 5HT fibers with a normal morphology in the ventral horn of the aged rat lumbosacral spinal cord, and several 5HT immunoreactive (IR) fibers with an aberrant morphology, suggestive of axonal degeneration, were intermingled. This was evident in both the dorsal and ventral horn of the spinal cord. The 5HT-IR fibers with an aberrant morphology usually also contained TRH-and/or SP- and/or galanin-like immunoreactivity (LI) in the ventral horn. These signs of degeneration were clearly less evident in the thoracic and cervical spinal cord segments. Moreover, these changes varied between aged litter-mates. This was in agreement with behavioural signs of motor disturbances, present in about 40% of the aged rats and which in all cases were confined to the hindlimbs. Chemical analyses disclosed significantly lower levels of TRH-LI and, in particular, SP-LI in both the ventral and dorsal quandrants of the spinal cord in the aged rat compared to young adults. The differences were largest in the lumbar regions of the spinal cord. Corresponding analysis of 5HT and 5-hydroxyindoleacetic acid (5HIAA) in the same tissue specimens revealed largely unaltered levels of 5HT and a slight increase in 5HIAA, indicating the possibility of an increased 5HT turnover in the aged rat spinal cord. Neurons in nucleus raphé obscurus and nucleus raphé pallidus were immunoreactive to 5HT, and after pretreatment with colchicine to TRH-, SP-, and galanin-LI as well. There was no obvious difference in number of labeled cells, or labeling intensity, between colchicine-treated young adult and aged rats, although, in the corresponding region of medulla oblongata, chemical analysis disclosed significantly lower levels of 5HT, TRH, and, in particular, SP in untreated aged rats. In contrast, in situ hybridization analysis revealed increased mRNA levels encoding prepro-TRH and prepro-SP in old rats, while mRNA content encoding AADC mRNA was similar in young adult and aged rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonergic regulation of the spinal cord content of thyrotropin releasing hormone in the cerebellar ataxia mutant mouse

The effects of serotonin (5-HT) and various serotonin receptor antagonists on the spinal cord thyrotropin releasing hormone (TRH) content were studied in the rolling mouse Nagoya (RMN) and in the unaffected C3H mouse. TRH was extracted from the cervical, thoracic, lumbar, and sacral cord, at 1 h after the intraperitoneal injection of a serotonin precursor, 2 serotonin agonists, and 5 serotonin receptor antagonists. Administration of 5-hydroxytryptophan and 2-methyl-5-hydroxytryptamine resulted in an increase of the spinal cord TRH content in C3H mice, but not in RMN. Parachlorophenylalanine decreased the spinal cord TRH content in C3H mice, while it increased TRH levels in all regions of the RMN spinal cord. The TRH contents were decreased in all regions of the spinal cord after 5,7-dihydroxytryptamine administration in both C3H mice and RMN. In C3H mice, methysergide, mianserin, ketanserin, and spiperone significantly decreased the TRH content in all regions of the spinal cord, while 3 alpha-tropanyl-1H-indole-3-carboxylic acid ester (ICS205-930) did not affect it. These antagonists paradoxically increased TRH levels in the cervical cord in RMN. The degradation of synthetic TRH by cord homogenates and the number and affinity of spinal cord serotonergic receptors (5-HT1 and 5-HT2) showed no significant difference between C3H mice and RMN. These results suggest that TRH turnover is abnormally regulated by serotonergic neurons in the RMN and that the dysfunction of the serotonergic nerves is attributable to the serotonin autoreceptor.

Detection of thyrotrophin releasing hormone in rat brain in vivo using novel antibody microprobes: effects of amphetamine

Antibody microprobes of novel design were used to monitor thyrotrophin releasing hormone (TRH) in rat brain before and after-parenteral administration of amphetamine. Specific antibodies to TRH were bound to the outside of glass microprobes by adsorption to a surface of activated charcoal embedded in epoxylite resin. In male Wistar rats anaesthetised with chloral hydrate a series of antibody microprobes were implanted in forebrain. Amphetamine (10 mg/kg i.p.) caused a highly significant decrease in the binding of 125I-TRH to microprobes, indicating an increase in extracellular TRH localised in the lateral septum. There was also evidence of TRH release in the septo-hypothalamic nucleus. Neither saline, nor amphetamine at 2 mg/kg were able to evoke changes in the release of TRH at any sites.

Pre- and post-natal ontogeny of thyrotropin-releasing-hormone in the rat spinal cord: an immunocytochemical study

This work aimed at providing by means of immunocytochemical techniques a detailed study of the ontogeny of thyrotropin-releasing hormone (TRH) in the spinal cord of the rat. We report the first appearance of TRH-immunoreactive fibers in the ventral funiculus of thoracic and lumbar levels at embryonic day 17. At embryonic day 18, fibers penetrated the ventral gray matter towards the central canal. At embryonic day 19, the first immunoreactive fibers were seen in the intermediolateral cell column at upper thoracic levels. This region was invaded at lower thoracic levels on the day of birth. At this time, TRH-immunoreactive axodendritic synapses were observed in the ventral horn and in the intermediolateral cell column. Immunoreactivity increased in these regions until post-natal day 21 when the adult pattern of TRH immunoreactivity was established in the sympathetic nuclei and in the ventral horn. However, a transient TRH-like immunoreactivity was detected in lamina IIi of the dorsal horn between post-natal days 14 and 30: at ultrastructural level, immunoreactive varicosities were seen to establish axodendritic synapses. In conclusion, TRH is one of the earliest peptidergic systems established in the spinal cord and it presents extensive temporal and topographical similarities with the serotonergic system with which it could be colocalized.

The effect of p-chloroamphetamine and p-chlorophenylalanine on the level of thyrotropin-releasing hormone and its receptors in some brain structures and lumbar spinal cord of the rat

The concentration of thyrotropin-releasing hormone (TRH) and the density and affinity of TRH receptors were examined in the ventral and dorsal lumbar spinal cord, nucleus accumbens and striatum of rats with the 5-hydroxytryptamine (5-HT) nerve terminal destroyed with p-chloroamphetamine (PCA), or in animals treated with the inhibitor of 5-HT synthesis p-chlorophenylalanine (PCPA). PCA (2 x 10 mg/kg i.p., 9 and 8 d before killing) and PCPA (3 x 300 mg/kg i.p., 72, 48 and 24 h before killing)--either of them dramatically diminishing the 5-HT and 5-HIAA concentrations in all the examined structures--reduced the TRH level and increased the density of TRH receptors in the ventral lumbar spinal cord. PCPA also reduced the TRH content in the nucleus accumbens. The PCA-induced reduction in the TRH level and increase in the density of TRH receptors in the ventral lumbar spinal cord were significantly attenuated by citalopram (2 x 20 mg/kg i.p., 30 min before PCA), a selective inhibitor of 5-HT uptake. Our results constitute a further proof that coexistence of TRH and 5-HT takes place in the ventral lumbar spinal cord and then indicate that other form(s) of relationship between 5-HT and TRH may exist in some parts of the central nervous system. They also suggest that an up-regulation of TRH receptors occurs in the spinal cord as a result of TRH depletion.

Thyrotropin-releasing hormone (TRH)-like immunoreactivity in the grey monkey (Macaca fascicularis) spinal cord and medulla oblongata with special emphasis on the bulbospinal tract

The distribution of thyrotropin-releasing hormone (TRH)-like immunoreactivity (LI) has been studied in the grey monkey (Macaca fascicularis) spinal cord and medulla oblongata by the use of indirect immunofluorescence and the peroxidase-antiperoxidase (PAP) technique. Furthermore, double-labeling experiments were performed in order to study colocalization of 5-hydroxytryptamine (5-HT)- and substance P-LI. A dense innervation of TRH-immunoreactive (IR) varicose fibers was found in the ventral horn motor nuclei, in the region surrounding the central canal, in the intermediolateral cell column, and in the dorsal horn laminae II and III. In addition, cell bodies harboring TRH-LI were found in the dorsal horn laminae II-IV. In the ventral horn, many of the large cell bodies and their proximal dendrites were totally encapsulated by TRH-IR fibers. From double-labeled sections a high degree of coexistence could be established between TRH-/5-HT-LI, TRH-/substance P-LI, and 5-HT-/substance P-LI in fibers in the motor nuclei; as a consequence, a large proportion of these fibers should harbor TRH-/5-HT-/substance P-LI. A coexistence between TRH-/5-HT-LI could also be demonstrated in the intermediolateral cell column. However, no unequivocal coexistence could be found between TRH-/substance P-LI and 5-HT-/substance P-LI in this region. In the dorsal horn, no clear coexistence could be encountered for any of the above indicated combinations. Electron microscopic analysis of material from the lumbar lateral motor nucleus demonstrated TRH-IR terminals making synapses with large cell bodies and dendrites. In addition, contacts lacking synaptic specializations could also be verified. In the medulla oblongata, with the use of the PAP technique, a large number of cell bodies containing TRH-LI were encountered in the midline raphe nuclei and in nucleus reticularis lateralis. A similar distribution pattern could be found for 5-HT-LI, but no cell bodies containing substance P-LI could be seen in these regions. Chemical analysis of specimens from cervical, thoracic, and lumbar spinal cord revealed higher concentrations of TRH- and 5-HT-LI in the ventral quadrants, whereas substance P-LI dominated in the dorsal quadrants. Thus, the concentrations of TRH-, 5-HT-, and substance P-LI was in accordance with the observed regional variation in density of IR-fibers and varicosities found in the spinal cord. We have shown that TRH-LI has a distribution in the monkey spinal cord and medulla oblongata similar to that previously demonstrated in other species.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of thyrotropin-releasing hormone in the rat spinal cord with special reference to sympathetic nuclei: a light- and electron-microscopic immunocytochemical study

This paper deals with the distribution of thyrotropin-releasing hormone-like immunoreactivity in the spinal cord of the rat, and particularly in the sympathetic nuclei, at light and electron microscopic levels. In the dorsal horn, the inner part of laminae II and III displayed thin thyrotropin-releasing hormone immunoreactive profiles. Electron microscopy revealed small immunoreactive varicosities which made synaptic contact with small dendrites or dendritic spines. Dense thyrotropin-releasing hormone-like immunoreactivity was observed in all sympathetic nuclei (nucleus intermediolateralis pars fascicularis and principalis, nucleus intercalatus and dorsal commissural nucleus) except the nucleus intercalatus pars ependymalis. Electron microscopy showed many immunoreactive varicosities which were often in synaptic contact with dendrites (proximal or distal), rarely with perikarya and never with axons. Sometimes, the same immunoreactive varicosity made axodendritic contacts with two dendrites and, conversely one dendrite was sometimes synaptically contacted by two or more immunoreactive varicosities. The ventral horn displayed a diffuse thyrotropin-releasing hormone-like immunoreactivity except for the cremaster nucleus (at lumbar level) which was densely outlined by immunoreactive profiles. Occasionally a large cell body in lamina IX (a putative motoneuron) was outlined by immunoreactive profiles but ultrastructural studies revealed very few immunoreactive axosomatic synapses, while immunoreactive symmetrical or asymmetrical axodendritic synapses were observed. The present study clearly confirms the existence of thyrotropin-releasing hormone immunoreactive synapses, thus substantiating the physiological role of this hormone in the spinal cord.

Serotoninergic denervation suppresses the sympathetic outflow induced by thyrotropin-releasing hormone in conscious rats

Intrathecal administration of thyrotropin-releasing hormone (TRH) resulted in an increase in plasma epinephrine (E) and glucose levels in conscious rats. To elucidate the mechanisms by which TRH stimulates the release of E, the animals were pretreated with 5,7-dihydroxytryptamine, parachlorophenylalanine, methysergide or ketanserin. Serotoninergic denervation suppressed the TRH-induced increase in plasma E and glucose levels, whereas inhibition of serotonin (5-HT) synthesis or blockade of 5-HT receptors did not suppress the responses. These findings suggest that the serotoninergic neurons, but not 5-HT itself, are involved in stimulating the sympathetic outflow by TRH at the spinal level.

Contribution of serotonin neurons to the functional recovery after spinal cord injury in rats

The contribution of serotonin neurons to the functional recovery after spinal cord injury was studied pharmacologically in rats with moderately severe neurologic impairment (complete paraplegia but responsive to tail pinching) 24 h after thoracic spinal cord (T11) compression-induced injury. Fourteen days after cord injury the levels of endogenous norepinephrine (NE, -33%), dopamine (DA, -50%) and serotonin (5-HT, -55%) in the lumbar cord in the injury control rats were decreased and there were significant correlations between the neurologic score and the NE level (rs = 0.562, P less than 0.01) and the 5-HT level (rs = 0.745, P less than 0.001) but not the DA level. Bilateral i.c.v. injection of 5,7-dihydroxytryptamine (200 micrograms/rat) 24 h after cord injury significantly retarded the neurologic recovery during the 14 days after injury, accompanied by a further reduction in the 5-HT level (-86%) but not in the NE or DA level. On the other hand, neither p-chlorophenylalanine (PCPA) (300 mg/kg, i.p., once daily starting 24 h after injury for 13 consecutive days) nor reserpine (1 mg/kg, i.p., 4 times, once 24 h after injury and then every fourth day) had any influence on the time course of the neurologic recovery during the 14 days after injury, although PCPA treatment further reduced the levels of NE (-50%) and 5-HT (-91%), and reserpine treatment further reduced the levels of NE (-95%), DA (-73%) and 5-HT (-85%).(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a radioimmunoassay for RX77368 (pGlu-His-3,3-dimethyl proline amide)--a stable analogue of thyrotrophin releasing hormone (TRH)

The recent interest in RX77368 for the treatment of Motor Neurone Disease (MND) has led to the requirement for an assay (RIA) capable of detecting the peptide at low levels in plasma. Several drug conjugates were prepared in which RX77368 was covalently linked to larger proteins, e.g. bovine serum albumin, keyhole limpet haemocyanin or bovine thyroglobulin, the best yield being obtained with the bis-diazotized benzidine reaction (BDB) linking RX77368 to KLH. The latter conjugate was injected into sheep and ultimately produced an antibody of sufficiently high titre to be used. This combined with an iodinated radiolabel formed the basis of the radioimmunoassay. Cross-reactivity studies using similar analogues and RX77368 metabolites showed that the antibody was specific for RX77368. The greatest cross-reactivity was exhibited by the pGlu-His-monomethylProNH2 peptide (RX74355), but, not being a natural metabolite, this did not interfere with the assay. The RIA was used to measure RX77368 in MND patients in a recent clinical study, where RX77368 was administered both by the intravenous and oral routes. High plasma concentrations of RX77368 were found in the patients given intravenous drug by infusion. The oral route exhibited much lower levels, but had a sustained duration of action of up to 12 h.

The effect of fenfluramine on the thyrotropin-releasing hormone (TRH) content in the rat brain structures and lumbar spinal cord

The effect of fenfluramine (20 mg/kg i.p.) was studied on the thyrotropin-releasing hormone (TRH) content in several brain structures and the ventral part of the lumbar spinal cord of the rat. The effect of fenfluramine on the concentration of 5-hydroxytryptamine (5-HT) in some brain structures and the lumbar spinal cord was also examined. It was found that fenfluramine had no effect on the TRH level in the hypothalamus, hippocampus, occipital cortex, septum, nucleus accumbens and ventral part of the lumbar spinal cord, though the drug produced a profound depletion (by more than 60%) of 5-HT in the hypothalamus, nucleus accumbens, striatum and lumbar spinal cord. On the other hand, fenfluramine significantly increased the TRH content in the striatum, this effect was completely abolished by citalopram (20 mg/kg i.p.) or metergoline (10 mg/kg i.p.) Citalopram also prevented the fenfluramine-induced depletion of the striatal 5-HT. These results indicate a separate neuronal storage of TRH and 5-HT in the structures (ventral part of the lumbar spinal cord, nucleus accumbens, septum) in which the peptide and indoleamine coexist in 5-HT neurons. They also suggest that the fenfluramine-induced increase in the striatal TRH concentration is due to 5-HT release and stimulation of 5-HT receptors.

Effects of short-term serotonin depletion on the efficacy of serotonin neurotransmission: electrophysiological studies in the rat central nervous system

The effects of short-term serotonin (5-HT) depletion by p-chlorophenylalanine (PCPA) on the firing activity of dorsal raphe nucleus 5-HT neurons, on the responsiveness of dorsal hippocampus pyramidal neurons to microiontophoretically applied 5-HT and on the efficacy of the electrical stimulation of the ascending 5-HT pathway in suppressing the firing activity of CA3 dorsal hippocampus pyramidal neurons were assessed in chloral hydrate-anesthetized rats. PCPA (250 mg/kg/day i.p. for 2 days) reduced the 5-HT content of the dorsal hippocampus by 90%. However, the number of spontaneously active 5-HT neurons per microelectrode trajectory through the dorsal raphe or their average rate of firing was unaltered. The effect of afferent 5-HT pathway stimulation was reduced in only 40% of treated rats, whereas the sensitivity of CA3 pyramidal neurons to microiontophoretic 5-HT was not modified. The function of the terminal 5-HT autoreceptor was assessed using methiothepin, an autoreceptor antagonist. Methiothepin (1 mg/kg, i.v.) significantly enhanced the efficacy of the stimulation in PCPA-treated rats, although the degree of enhancement was much less than in controls. A greater reduction of the effectiveness of the stimulation was obtained by increasing the dose of PCPA (350 mg/kg/day i.p. for 2 days). This regimen reduced the 5-HT content of the dorsal hippocampus by 95%. In these rats, the sensitivity of the terminal 5-HT autoreceptor was assessed by increasing the frequency of the stimulation from 1 to 5 Hz. This procedure reduced to a similar extent the efficacy of the stimulation in treated and control rats, suggesting that the reduced effectiveness of methiothepin in enhancing 5-HT synaptic transmission in PCPA-treated rats is due to a lower degree of activation of the terminal 5-HT autoreceptor. The present results showing that the 350 mg/kg/day regimen of PCPA, but not the 250 mg/kg/day regimen, reduced the efficacy of the stimulation of the ascending 5-HT pathway suggest that a greater than 90% depletion is required to affect 5-HT neurotransmission significantly. The reduced level of activation of terminal 5-HT autoreceptors in rats treated with the lower dose of PCPA may facilitate the release of the remaining 5-HT per stimulation-triggered action potential, ensuring a virtually unaltered synaptic efficacy.

Thyrotropin-releasing hormone given intrathecally to the rat increases arterial pressure and heart rate

In view of evidence implicating thyrotropin-releasing hormone (TRH) as a chemical mediator of synaptic transmission onto spinal sympathetic neurons, this peptide was administered intrathecally, in a dose of 6.5 nmol, at the T9 and T2 spinal levels in the anesthetized rat. At the lower thoracic level TRH increased arterial pressure and heart rate; these effects peaked at 4-7 minutes and decayed over the next 15-20 minutes. At the upper thoracic level the pressor and cardioacceleratory responses were roughly similar in time course but were smaller in magnitude. Hexamethonium (10 mg/kg i.v.) was tested on the responses from the lower thoracic level; both pressor and cardioacceleratory responses persisted after hexamethonium pretreatment. In addition, intravenous administration of 6.5 nmol of TRH failed to alter arterial pressure or heart rate, suggesting that the effects produced by the intrathecal administration of TRH were due to an action of the peptide in the spinal cord. The results also indicate that the pressor effect and the increase in heart rate may be mediated in the sympathetic ganglia at least partly via nonnicotinic transmission. Our results provide physiological support for the possibility that TRH is a chemical mediator of synaptic transmission onto sympathetic preganglionic neurons. This study indicates that the functional sympathetic pathways utilizing TRH as a chemical mediator include those regulating arterial pressure and heart rate.

Effect of chronic intra-accumbens administration of the TRH analogue CG3509 on histamine-induced behaviour in the rat

1. The present study has investigated the effect of chronic intra-accumbens administration of the thyrotrophin-releasing hormone (TRH) analogue, CG3509, on CG3509- and histamine-induced spontaneous motor activity and brain TRH-like immunoreactive (TRH-LI) levels in the rat. 2. Chronic intra-accumbens administration of CG3509 (5 x 5 micrograms over 3 days) induced: (a) a significant (P less than 0.05) reduction in intra-accumbens CG3509 (0.5 micrograms)-induced hyperactivity, (b) reduced levels of TRH-LI in the nucleus accumbens but not other brain regions, (c) a marked increase (107%, P less than 0.01) in histamine-induced non-locomotor hyperactivity. 3. The present results demonstrate that alteration of central TRH function following treatment with a TRH analogue enhances the effect of intra-accumbens histamine on behavioural hyperactivity, possibly via changes in H1 receptors and suggest that the neuropeptide, TRH and histamine interact in behavioural arousal mechanisms in rat brain.

Thyrotropin-releasing hormone (TRH) effects on spinal cord neuronal excitability

TRH is found in terminals in the dorsal, lateral, and ventral horns of the spinal cord and apparently has at least a weak facilitatory effect on excitability of neurons in all these locations. These findings suggest that TRH may facilitate transmission in somatosensory pathways, enhance sympathetic outflow from the spinal cord, and facilitate somatic motoneuron excitability, at least transiently. All studies that have examined TRH effects on spinal neuronal excitability have used exogenously administered TRH. Virtually nothing is known about how spinal neuronal functioning might be affected by TRH released from terminals after activation of TRH-containing cell bodies. The acquisition of this knowledge awaits the development of specific TRH antagonists. Preliminary experiments suggest that TRH may have prolonged facilitatory effects on the excitability of developing or damaged spinal cord neurons. Further studies are necessary to determine how TRH interacts with other neuroactive peptides and monoamines to affect excitability of neurons in the developing, damaged, and normal adult spinal cord.

Activity of the hypothalamic-pituitary-thyroid axis during exposure to cold

It seems clear from the studies reviewed here that there is adequate evidence to support the concept of a biphasic response of the thyroid gland to cold as first postulated by Moll et al. (1972). The initial response to acute exposure to cold begins at the level of the hypothalamus as a result of either neural stimuli from skin and other areas and/or blood of somewhat lower than normal temperature reaching the hypothalamus (Andersson et al., 1963). As a result, the secretion of norepinephrine and/or dopamine may increase, and serotonin and/or somatostatin may decrease. The net result of these is an increase in the release of TRH from the hypothalamus. This, in turn, stimulates the cascade for the release of TSH from the anterior pituitary gland and thyroid hormone from the thyroid gland. Moll et al. (1972) postulated the lack of a feedback limb in this acute phase, and, indeed, this may be the case. It is possible, however, that certain hormones, such as somatostatin, norepinephrine, T3, and T4 could act in the capacity of feedback inhibitors. Additional experiments will be required to assess this possibility. The transitional link between the acute (less than 1 day) and chronic (greater than 1 day) phases of the response of the thyroid gland to cold could be T4 itself. An increase in the concentration of T4 in plasma has been reported to increase peripheral deiodination of T4 to T3 by kidneys and liver of rats. There are no studies at present to indicate that hepatic conjugation can be increased by elevation of plasma levels of T4 and T3. If it can, these responses would provide adequate reasons as to why peripheral metabolism of thyroid hormones increases during chronic exposure to cold. The time-course for these changes to occur needs to be studied in greater detail to establish the sequence of events following acute exposure to cold. The latter may also increase urinary excretion of T4 and T3 in man, but not the rat. This suggests that another aspect of exposure to cold needing additional study is measurement of the binding affinities of T4 and T3 for their transport proteins during exposure to cold as compared to affinities prior to exposure to cold. If binding affinities are reduced, the amount of free hormones would increase and, consequently the likelihood of being excreted into urine and conjugated by the liver would also increase.(ABSTRACT TRUNCATED AT 400 WORDS)

Biphasic effects of intra-accumbens histamine administration on spontaneous motor activity in the rat; a role for central histamine receptors

1. The effect of intra-accumbens injection of histamine and related compounds on the spontaneous motor activity of the rat has been investigated. 2. Microinjections of histamine (1-200 micrograms) induced dose-dependent, biphasic changes in rat activity consisting of an initial brief hypoactivity response followed by a marked hyperactivity phase. The histamine metabolite, n-tele-methylhistamine was without effect. 3. Pretreatment with the H1-receptor antagonist mepyramine (10 micrograms) blocked the hypoactivity response and markedly attenuated histamine-induced hyperactivity. In contrast, pretreatment with the H2-receptor antagonist SKF93479 had no effect on histamine-induced behaviour. 4. Microinjection of the H1-receptor agonist 2-thiazolylethylamine induced a marked hyperactivity response, but unlike the response to histamine, there was no initial hypoactivity. The H2-receptor agonist dimaprit had no apparent behavioural effects following intra-accumbens injection. 5. Intra-accumbens injection of the non-selective histamine agonists n alpha-methylhistamine or n alpha, n alpha-dimethylhistamine induced both marked hypoactivity and hyperactivity responses which were comparable with the effects of histamine. 6. The present results demonstrate a histamine, H1-receptor-mediated arousal in the nucleus accumbens which follows transitory hypoactivity, possibly due to activation of presynaptic H3-receptors.

Inhibition of p-chlorophenylalanine-induced muricide behavior following TRH microinjection into the limbic structures in the rat

The effect of central microinjections of thyrotropin releasing hormone (TRH) on muricide behavior was investigated in rats with chronically implanted cannulas into one of the limbic structures: the amygdala, the hippocampus or the nucleus accumbens. The rats were made aggressive by p-chlorophenylalanine administration. Saline injection did not inhibit muricide, whereas TRH (10 micrograms in 0.5 microliter per side) significantly suppressed mouse-killing reaction upon injection into each of three regions. It is suggested that the observed antiaggressive effect of TRH may involve stimulation of the central noradrenergic and/or serotonergic transmission.

The spinal reflex of chronic spinal rats is supersensitive to 5-HTP but not to TRH or 5-HT agonists

The effects of thyrotropin-releasing hormone (TRH), 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) and L-5-hydroxytryptophan (5-HTP) were studied on the monosynaptic reflex (MSR) and the polysynaptic reflex (PSR) in acute and chronic spinal rats. Radioimmunoassay showed that while chronic spinal transection (for 2 weeks) caused the complete depletion of TRH in the ventral lumbar enlargement a certain level of TRH was maintained in the dorsal lumbar enlargement. This result suggested the existence of TRH-containing neurons in the dorsal horn other than the medullary raphe neurons descending to the spinal cord. The latency to the start of the MSR was shortened in chronic spinal rats and the amplitudes of the MSR and PSR were significantly greater than those in acute spinal rats. There was no obvious difference in the effects of TRH and 5-MeODMT on spinal reflexes of acute and of chronic spinal rats although marked supersensitivity to 5-HTP was observed in both the MSR and the PSR in chronic spinal rats. The supersensitivity to 5-HTP was considered to be due to a lack of 5-hydroxytryptamine (5-HT) uptake into 5-HT-containing nerve terminals rather than to a change in 5-HT receptors. It is suggested that TRH and 5-HT do not show any mutual requirement for each other in their effects on the spinal reflex since co-depletion of TRH and 5-HT did not change the effects of TRH and 5-MeODMT in chronic spinal rats. The coexistence of 5-HT and TRH in the descending spinal pathway is not considered to be significant for the control of spinal reflexes at the postsynaptic level.

Inhibition of affective aggression and dominance in rats after thyrotropin-releasing hormone (TRH) microinjection into the nucleus accumbens

The effect of 10 micrograms TRH injected bilaterally into the nucleus accumbens septi on two models of affective aggression and on dominance in a water-competition task was investigated in pairs of male Wistar rats. TRH significantly suppressed affective shock-induced and apomorphine-induced fighting. It also decreased dominance when administered to dominant rats while no effect was noted upon injection into subordinate animals. The peptide influenced neither water consumption in thirsty rats nor the pain threshold in a hot plate test.

The regulation of TRH and serotonin receptors: chronic TRH and analog administration in the rat

Thyrotropin releasing hormone (TRH) and serotonin (5-HT) are co-transmitters in spinal and medullary neurons. To study the functional significance of this relationship and the regulation of TRH receptors, we chronically administered TRH and analogs MK-771 and CG-3509 to rats at a dose which evoked behavioral abnormalities. TRH reduced specific binding at spinal (24%) and hypothalamic (31%) TRH receptors and decreased TRH stimulated motor behaviors, such as rearing and cage crossing (locomotion). 5-HT1 and 5-HT2 receptors were unaffected except for an 11% increase in specific binding at spinal 5-HT1 sites. 5-HT and NE concentrations measured by HPLC were not altered in brainstem or hippocampus. In contrast, spinal TRH receptor specific binding increased 11% in rats treated intracisternally with 5,7-dihydroxytryptamine, but the effect was not significant. In competition studies in vitro, MK-771, TRH, and CG-3509 had no activity at 5-HT2 receptors in neocortex, brainstem, or spinal cord, and little activity at 5-HT1 sites (IC 50s greater than 100 uM). A mixed competitive and non-competitive binding profile at 5-HT1 sites resulted in the presence of 100 uM TRH. Conversely, 5-HT agonists had minimal effect at TRH receptors in spinal cord. These data suggest reciprocal or independent regulation of 5-HT1 and TRH receptors in co-transmitter and non-cotransmitter regions, respectively, in response to chronic TRH administration.

The comparative pharmacology of the behavioral syndromes induced by TRH and by 5-HT in the rat

1. The relationship of the behavioral syndromes induced by the co-transmitters thyrotropin releasing hormone (TRH) and serotonin (5-HT) has not been previously studied with drugs selective for 5-HT receptor subtypes. 2. Both the TRH analog MK-771 (in naive rats) and 5-hydroxytryptophan (in rats with 5,7-dihydroxytryptamine [DHT] lesions) evoked reciprocal forepaw tapping, Straub tail, hunching, hindlimb abduction, and shaking behavior. Sniffing and rearing were features of the MK-771 but not the 5-HT syndrome. 3. 5-HTP potentiated MK-771-induced hyperthermia. 4. MK-771 evoked two types of shaking behavior, head shakes (HS) and wet-dog shakes (WDS). Neither independently was dose-related, unlike total shaking behaviors. 5. MK-771-induced shaking behavior was pharmacologically dissociated from other MK-771-evoked behaviors. A 5-HT1A agonist (8-OH-DPAT) blocked WDS, but like putative 5-HT1B (RU 24969) and 5-HT2 (DOI) agonists and the 5-HT antagonists methysergide (non-selective), ritanserin (5-HT2 selective), and l-propranolol (5-HT1 selective), it did not block other antagonists behavioural effects of MK-771. 6. Ipsapirone, a 5-HT1A-active drug purported both as an agonist and as an antagonist, inhibited MK-771-evoked WDS, like 8-OH-DPAT, but did not induce the serotonin syndrome, unlike 8-OH-DPAT. 7. DHT-treated rats were behaviorally supersensitive to 10 mg/kg MK-771 as indicated by a significantly shortened latency of onset of WDS and greater frequency of abnormal forepaw movements. The same rats were also supersensitive to 50 mg/kg 5-HTP to a significantly greater degree. 8. These data suggest behavioral relatedness of the TRH and 5-HT syndromes, but distinctive pharmacologic features and presumed mechanisms of action.

Measurement of catecholamines, indoleamines, thyrotrophin releasing hormone and substance P in rat and human spinal cord using a common extraction method

An extraction method is described that allows the measurement of 5-hydroxyindoleacetic acid, dopamine, noradrenaline, substance P and thyrotrophin releasing hormone in the same sample of rat or human spinal cord. The substances were extracted using a mixture of 45% ethanol/0.1 M acetic acid and 0.02% sodium metabisulphite. The amines were then measured using high performance liquid chromatography with electrochemical detection and the peptides by radioimmunoassay. The common extraction method makes maximum use of limited postmortem material, reduces assay time and allows study of the interrelationships between the amines and peptides in rat or human spinal cord.

Regional distribution of substance P- and thyrotrophin-releasing hormone-like immunoreactivity and indoleamines in the rabbit spinal cord

The distribution of thyrotrophin-releasing hormone (TRH), substance P, and the indoleamines [5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA)] has been examined in selected regions of the thoracic and lumbar spinal cord of the rabbit using sensitive radioimmunoassays for the first two and HPLC with electrochemical detection for the indoleamines. The levels of TRH- and substance P-like immunoreactivity (TRH-I and SP-I, respectively) were greatest in the ventral and dorsal grey matter, respectively. The level of TRH-I in most thoracic regions was greater than that in equivalent lumbar regions, but the only segmental difference in SP-I was in the ventral grey matter, where the lumbar segment contained more immunoreactivity. 5-HT and 5-HIAA were more evenly distributed than either peptide and showed no segmental variation in levels in equivalent regions, but the ventral grey matter contained significantly higher levels of 5-HT and had a greater 5-HT/5-HIAA ratio than all other regions. The absolute levels and the overall distribution of SP-I, TRH-I, and indoleamines in the thoracolumbar cord of the rabbit was very similar to that previously reported in both rats and humans, and the possible functional role of the peptides and indoleamines in spinal neurones is discussed.

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.

A thyrotropin-releasing hormone-containing system in the rat dorsal horn separate from serotonin

In this study, we report the identification of a thyrotropin-releasing hormone (TRH)-containing system in the dorsal horn of the rat spinal cord. This system is distinct from the TRH and serotonin (5-hydroxytryptamine, 5-HT) cotransmitter supraspinal system that has projections to the intermediolateral (IML) and ventral columns. Spinal cord sections from untreated rats, and those treated with colchicine or 5,7-dihydroxytryptamine (5,7-DHT) were processed using peroxidase-antiperoxidase (PAP) immunocytochemistry with nickel intensification. Results of the 5,7-DHT treatment were verified by quantifying TRH and 5-HT by radioimmunoassay (RIA) and high performance liquid chromatography (HPLC), respectively. Prominent immunocytochemical staining for TRH in the dorsal horn was seen in varicose fibers mainly in lamina II and superficial lamina III of the dorsal horn of the spinal cord of control rats. A few fibers were seen ascending into lamina I. A moderate number of fibers that were immunoreactive for 5-HT were primarily in laminae I and II. The distribution of TRH- and 5-HT-containing neurites in the IML and the ventral horn agreed with previously published reports. Rats treated with colchicine showed many small round TRH immunoreactive cells that were limited to laminae II/III of the dorsal horn. TRH immunoreactivity in the dorsal horn and IML was resistant to the effects of the selective serotonin neurotoxin, 5,7-DHT, while the ventral horn was depleted of TRH staining. Serotonin was almost completely eliminated in all spinal cord laminae. Quantitative biochemical studies showed significant, but non-parallel reductions of TRH and 5-HT in cervical, thoracic and lumbar spinal cord. These studies demonstrate the existence of TRH-containing cell bodies and terminals in the dorsal horn of the rat spinal cord. These findings provide evidence that a TRH-containing system exists in the dorsal horn of the rat and that it is distinct from the descending medullary raphe system that contains 5-HT; suggest that a population of TRH-containing fibers that project to the IML may not contain 5-HT; and confirm previously published results that 5-HT and TRH coexist in terminals in the ventral horn of the spinal cord.

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.

Segmental distribution of thyrotropin releasing hormone in rat spinal cord

The segmental and laminar distribution of thyrotropin releasing hormone (TRH) was determined in the rat spinal cord using radioimmunoassay (RIA) and immunocytochemistry (ICC). Immunoreactive TRH was found in sensory, autonomic, and motor spinal columns. Dorsal horn TRH-containing fibers and cell bodies in lamina II and along the lamina II/III border were seen by ICC in all spinal cord segments. ICC showed dense TRH immunoreactivity in the sympathetic areas of the thoracic cord. Densely staining TRH-containing fibers were seen in the ventral horn of all spinal segments. RIA of whole segment extracts showed high concentrations in C6-T1 and T12-L6. Low levels were seen in C2-C4 and T5-T6. Other segments were intermediate in concentration. RIA and ICC results were comparatively evaluated.

Regional distribution of immunoreactive-thyrotrophin-releasing hormone and substance P, and indoleamines in human spinal cord

The regional distributions of thyrotrophin-releasing hormone (TRH) and substance P in postmortem human spinal cord were determined by radioimmunoassay in fresh tissue taken from 22 patients who died without known neurological disease. Dorsal, ventral, and intermediolateral spinal cord regions were obtained from different segmental levels (lumbar L1, 2, 3, and 4; thoracic groups T1-3, T4-6, T7-9, and T10-12) together with selective regions of grey matter of lumbar spinal cord. The effects on peptide levels of the age of the patient, the postmortem time interval, and freezing the tissue samples prior to assay were assessed. Levels of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were determined in regional lumbar and thoracic tissue using HPLC with electrochemical detection. Substance P was found in the highest concentration in the dorsal spinal cord, with no significant segmental differences. In contrast, TRH was present in higher levels in the ventral rather than the dorsal spinal cord, with segmental differences. There was a significant difference in the 5-HT/5-HIAA ratio between dorsal and ventral spinal cord, with the highest ratio in the ventral spinal cord. There were no significant differences in substance P, TRH, or 5-HT levels in spinal cords between 5 and 20 h postmortem or from patients aged between 65 and 90 years. Freezing the tissue (-80 degrees C for 24 h) prior to assay significantly reduced TRH and substance P levels compared to samples assayed immediately without prior freezing.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

In vivo evaluation by differential pulse voltammetry of the effect of thyrotropin-releasing hormone (TRH) on dopaminergic and serotoninergic synaptic activity in the striatum and nucleus accumbens of the rat

In vivo differential pulse voltammetry was used to determine the effect of thyrotropin-releasing hormone (TRH) on dopaminergic and serotoninergic synaptic activity in the striatum and nucleus accumbens of the rat. Thyrotropin releasing hormone (TRH) produces marked stimulatory effects on behaviour, which have been attributed to the release of dopamine in the nucleus accumbens. Other studies indicate a close relationship between the peptide and serotonin in the brain. We have thus used an improved differential pulse voltammetry technique to evaluate the effects of TRH on the extracellular content of the dopamine and serotonin metabolites. Dihydroxyphenylacetic acid (DOPAC) and 5-Hydroxy-indoleacetic acid (5HIAA) in the nucleus accumbens and striatum of the rat in vivo. TRH rapidly increased extracellular DOPAC, reaching a maximum after 60 min in the nucleus accumbens, and after 40 min in the striatum. There was also a slower increase in extracellular 5-HIAA content in both areas, reaching a plateau after 100 min. The delayed time course of the increase in 5-HIAA suggested that the increase in 5-HIAA content might be secondary to the increase in dopamine turnover produced by TRH. These results suggest that doses of TRH which produce behavioural stimulation increase the release of both dopamine and serotonin in the nucleus accumbens and striatum.

Early postnatal administration of 5,7-dihydroxytryptamine: effects on substance P and thyrotropin-releasing hormone neurons and terminals in rat brain

Substance P, thyrotropin-releasing hormone (TRH) and serotonin are putative neurotransmitters which have been proposed to co-exist in some brain neurons. Our previous immunocytochemical and biochemical studies have demonstrated that 85-100% of all serotonin neurons are destroyed following neonatal 5,7-dihydroxtryptamine (5,7-DHT) treatment. In this study, we have determined the effect of neonatal 5,7-DHT and desipramine (DMI) treatment on the biochemical content and immunocytochemical localization of substance P and TRH throughout the brain. Interestingly, we have observed that virtually all substance P- and TRH-immunoreactive cells in the ventral pons-medulla are destroyed by the neurotoxin. However, peptide-containing neurons in other regions were not affected. Additionally, we measured the peptide content and found that TRH is significantly decreased in the spinal cord (-50%) and pons-medulla (-20%), but not in other brain regions. Substance P content was not significantly altered in any region, even after a greater than 90% reduction of serotonin. These data indicate that the co-localized substance P and TRH forms a small proportion of the total peptide in brain.

Role of spinal cord neuropeptides in pain sensitivity and analgesia: thyrotropin releasing hormone and vasopressin

The existence of a wide variety of neuropeptides within the spinal cord dorsal horn raises the question of their possible roles in sensory processing. The present series of behavioral experiments examined the effects of intrathecal (IT) administration of two such neuropeptides, thyrotropin-releasing hormone (TRH) and vasopressin (VAS), on pain sensitivity and antinociception. TRH exerted no marked effect on basal pain sensitivity over the dose range examined (0.25 ng-2.5 micrograms). However, a U-shaped dose-response effect on morphine antinociception (3 micrograms, IT) was observed, wherein potent attenuation, moderate attenuation, or enhancement of morphine-induced antinociception was observed following the various doses tested. In contrast, VAS produced non-opiate antinociception at the highest doses tested (25 ng and 250 ng) and none of the VAS doses (0.25 ng-250 ng) appeared to interact with IT morphine (3 micrograms) antinociception. Lastly, IT TRH was not observed to interact with IT VAS antinociception. These data provide evidence that these neuropeptides exert strikingly different effects on pain sensitivity and opiate antinociception, and provide initial evidence that TRH may be included in the growing list of neuropeptides that can act like endogenous opiate antagonists within the central nervous system.