Thyrotropin-releasing hormone receptor binding sites: autoradiographic distribution in the rat and guinea pig brain

Thyrotropin-Releasing Hormone and Food Intake in Mammals: An Update

Thyrotropin-releasing hormone (TRH; pGlu-His-Pro-NH2) is an intercellular signal produced mainly by neurons. Among the multiple pharmacological effects of TRH, that on food intake is not well understood. We review studies demonstrating that peripheral injection of TRH generally produces a transient anorexic effect, discuss the pathways that might initiate this effect, and explain its short half-life. In addition, central administration of TRH can produce anorexic or orexigenic effects, depending on the site of injection, that are likely due to interaction with TRH receptor 1. Anorexic effects are most notable when TRH is injected into the hypothalamus and the nucleus accumbens, while the orexigenic effect has only been detected by injection into the brain stem. Functional evidence points to TRH neurons that are prime candidate vectors for TRH action on food intake. These include the caudal raphe nuclei projecting to the dorsal motor nucleus of the vagus, and possibly TRH neurons from the tuberal lateral hypothalamus projecting to the tuberomammillary nuclei. For other TRH neurons, the anatomical or physiological context and impact of TRH in each synaptic domain are still poorly understood. The manipulation of TRH expression in well-defined neuron types will facilitate the discovery of its role in food intake control in each anatomical scene.

Molecular Profiling of VGluT1 AND VGluT2 Ventral Subiculum to Nucleus Accumbens Shell Projections

The nucleus accumbens shell is a critical node in reward circuitry, encoding environments associated with reward. Long-range inputs from the ventral hippocampus (ventral subiculum) to the nucleus accumbens shell have been identified, yet their precise molecular phenotype remains to be determined. Here we used retrograde tracing to identify the ventral subiculum as the brain region with the densest glutamatergic (VGluT1-Slc17a7) input to the shell. We then used circuit-directed translating ribosome affinity purification to examine the molecular characteristics of distinct glutamatergic (VGluT1, VGluT2-Slc17a6) ventral subiculum to nucleus accumbens shell projections. We immunoprecipitated translating ribosomes from this population of projection neurons and analysed molecular connectomic information using RNA sequencing. We found differential gene enrichment across both glutamatergic projection neuron subtypes. In VGluT1 projections, we found enrichment of Pfkl, a gene involved in glucose metabolism. In VGluT2 projections, we found a depletion of Sparcl1 and Dlg1, genes known to play a role in depression- and addiction-related behaviours. These findings highlight potential glutamatergic neuronal-projection-specific differences in ventral subiculum to nucleus accumbens shell projections. Together these data advance our understanding of the phenotype of a defined brain circuit.

The Edible Seaweed Gelidium amansii Promotes Structural Plasticity of Hippocampal Neurons and Improves Scopolamine-Induced Learning and Memory Impairment in Mice

BACKGROUND

Gelidium amansii has been gaining profound interest in East Asian countries due to its enormous commercial value for agar production and its extensive pharmacological properties. Previous studies have shown that the ethanol extract of Gelidium amansii (GAE) has promising neurotrophic effects in in vitro conditions.

OBJECTIVES

The present study aimed at investigating the protective effects of GAE against scopolamine-induced cognitive deficits and its modulatory effects on hippocampal plasticity in mice.

METHODS

For memory-related behavioral studies, the passive avoidance test and radial arm maze paradigm were conducted. The brain slices of the hippocampus CA1 neurons of experimental mice were then prepared to perform Golgi staining for analyzing spine density and its characteristic shape, and immunohistochemistry for assessing the expression of different pre- and postsynaptic proteins.

RESULTS

Following oral administration of GAE (0.5 mg/g body weight), mice with memory deficits exhibited a significant increase in the latency time on the passive avoidance test and a decrease in the number of working and reference memory errors and latency time on the radial arm maze test. Microscopic observations of Golgi-impregnated tissue sections and immunohistochemistry of hippocampal slices showed that neurons from GAE-treated mice displayed higher spine density and spine dynamics, increased synaptic contact, and the recruitment of memory-associated proteins such as N-methyl-D-aspartate receptors (NR2A and NR2B) and postsynaptic density-95 (PSD-95) when compared with the control group.

CONCLUSION

With these memory-protective functions and a modulatory role in underlying memory-related events, GAE could be a potential functional food and a promising source of pharmacological agents for the prevention and treatment of memory-related brain disorders.

Analysis of the anxiolytic-like effect of TRH and the response of amygdalar TRHergic neurons in anxiety

Thyrotropin-releasing hormone (TRH) was first described for its neuroendocrine role in controlling the hypothalamus-pituitary-thyroid axis (HPT). Anatomical and pharmacological data evidence its participation as a neuromodulator in the central nervous system. Administration of TRH induces various behavioural effects including arousal, locomotion, analepsy, and in certain paradigms, it reduces fear behaviours. In this work we studied the possible involvement of TRHergic neurons in anxiety tests. We first tested whether an ICV injection of TRH had behavioural effects on anxiety in the defensive burying test (DBT). Corticosterone serum levels were quantified to evaluate the stress response and, the activity of the HPT axis to distinguish the endocrine response of TRH injection. Compared to a saline injection, TRH reduced cumulative burying, and decreased serum corticosterone levels, supporting anxiolytic-like effects of TRH administration. The response of TRH neurons was evaluated in brain regions involved in the stress circuitry of animals submitted to the DBT and to the elevated plus maze (EPM), tests that allow to correlate biochemical parameters with anxiety-like behaviour. In the DBT, the response of Wistar rats was compared with that of the stress-hypersensitive Wistar Kyoto (WKY) strain. Behavioural parameters were analysed in recorded videos. Animals were sacrificed 30 or 60min after test completion. In various limbic areas, the relative mRNA levels of TRH, its receptors TRH-R1 and -R2, and its inactivating ectoenzyme pyroglutamyl peptidase II (PPII), were determined by RT-PCR, TRH tissue content by radioimmunoassay (RIA). The extent of the stress response was evaluated by measuring the expression profile of CRH, CRH-R1 and GR mRNA in the paraventricular nucleus (PVN) of the hypothalamus and in amygdala, corticosterone levels in serum. As these tests demand increased physical activity, the response of the HPT axis was also evaluated. Both tasks increased the levels of serum corticosterone. WKY rats showed higher anxiety-like behaviour in the DBT than Wistar, as well as increased PVN mRNA levels of CRH and GR. TRH mRNA levels increased in the PVN and TSH values remained unchanged in both strains although TRH content decreased in the medial basal hypothalamus of Wistar rats only. TRH content was measured in several limbic regions but only amygdala showed specific task-related changes after DBT exposure of both strains: increased TRH content. Expression of TRH mRNA decreased in the amygdala of Wistar, suggesting inhibition of TRHergic neuronal activity in this region. The participation of amygdalar TRH neurons in anxiety was confirmed in the EPM where TRH expression and release correlated with the number of entries, and the % of time spent in open arms, supporting an anxiolytic role of these TRH-neurons. These results contribute to the understanding of the involvement of TRH during emotionally charged situations and shed light on the participation of particular circuits in related behaviours.

Comparisons of dorsal and ventral hippocampus cornu ammonis region 1 pyramidal neuron activity during trace eye-blink conditioning in the rabbit

Previous studies demonstrating a critical role of the hippocampus during trace eye-blink conditioning have focused primarily upon the dorsal portion of the structure. However, evidence suggests that a functional differentiation exists along the septotemporal axis of the hippocampus. In the present study, the activity of 2588 single cornu ammonis region 1 pyramidal neurons of the dorsal hippocampus and ventral hippocampus were recorded during trace and pseudo-eye-blink conditioning of the rabbit. Learning-related increases in dorsal hippocampus neuron firing rates were observed immediately prior to behavioral criterion, and increased over the course of training. Activation of dorsal hippocampus neurons during trace conditioning was also greater than that of ventral hippocampus neurons, including during the trace interval, in well-trained animals. An unexpected difference in the patterns of learning-related activity between hemispheres was also observed. Neurons of the dorsal hippocampus ipsilateral and contralateral to the trained eye, exhibiting significant increases in firing rate [rate increasing neurons], demonstrated the greatest magnitude of activation early and late in training, respectively. Rate increasing neurons of the dorsal hippocampus also exhibited a greater diversity of response profiles, with 69% of dorsal hippocampus rate increasing neurons exhibiting significant increases in firing rate during the conditioned stimulus and/or trace intervals, compared with only 8% of ventral hippocampus rate increasing neurons (the remainder of which were significantly responsive during only the unconditioned stimulus and/or post-unconditioned stimulus intervals). Only modest learning-related activation of ventral hippocampus neurons was observed, reflected as an increase in conditioning stimulus-elicited rate increasing neuron response magnitudes over the course of training. No differences in firing rate between dorsal hippocampus and ventral hippocampus neurons during a 1-day pre-training habituation session were observed. Thus, dorsal hippocampus activation is more robust, suggesting a more substantial role for these neurons in the processing of temporal information during trace eye-blink conditioning.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Changes in thyrotropin-releasing hormone levels in hippocampal subregions induced by a model of human temporal lobe epilepsy: effect of partial and complete kindling

Endogenous thyrotropin-releasing hormone has been hypothesized to modulate seizure activity, possibly by subserving an anticonvulsant function in limbic brain. A specific and sensitive radioimmunoassay was utilized to quantitate thyrotropin-releasing hormone levels in dorsoventrally dissected hippocampal subregions after partially (an experimental paradigm of complex partial epilepsy) or fully kindled (repeated generalized) seizures, to define specific seizure-related limbic pathways that may contain thyrotropin-releasing hormone. Samples were taken from electrode controls and 1, 6, 24, 48 and 144 h after a fully kindled seizure or 24 h after the first occurrence of a stage 3-4 (partially kindled) seizure in rats. Thyrotropin-releasing hormone levels were below controls in all subregions taken 1 h after a fully kindled seizure. They resembled control values 6 h after seizure, were substantially elevated at 24 and 48 h, and then returned to control levels by 144h. Low thyrotropin-releasing hormone levels seen shortly after the seizure presumably indicate peptide depletion during the ictus. The higher levels seen at later times occurred during a postictal period coinciding with refraction to additional seizure-generating stimulation. These values probably reflect enhanced synthesis since the largest increases were seen in subregions (dentate gyrus, hilus/CA4, CA3) that contain perforant path terminals, and where previously observed intrinsic hippocampal thyrotropin-releasing hormone messenger RNA increases were seen. The thyrotropin-releasing hormone response was less robust in ventral hilus/CA4 and CA3 areas, leading to speculation that this smaller response could, in part, explain why the ventral (temporal) hippocampus may be more susceptible to seizure-induced damage. No changes in thyrotropin-releasing hormone were detected after partially kindled seizures, suggesting that thyrotropin-releasing hormone is not involved in epileptogenesis or its stereotypic motor behavior. The time-course and distribution of thyrotropin-releasing hormone elevations seen after a fully kindled (repeated generalized) seizure, and the lack of effect of partial kindling (complex partial seizure) are consistent with previous observations concerning postictal thyrotropin-releasing hormone messenger RNA expression. These neurochemical results support the hypothesis that endogenous thyrotropin-releasing hormone can serve an anticonvulsant neuromodulatory function in specific limbic pathways relevant to temporal lobe epilepsy.

Involvement of glutamate and gamma-aminobutyric acid (GABA)-ergic systems in thyrotropin-releasing hormone-induced rat cerebellar cGMP formation

The increase in cyclic guanosine 3',5'-monophosphate (cGMP) caused by subcutaneous injection of thyrotropin-releasing hormone (TRH) tartrate was observed in a region-specific manner in the rat cerebellum. TRH tartrate (TRH-T) (2.8, 7.0 and 17 mg/kg as free TRH, s.c.) produced dose-dependent increases in cGMP levels markedly in the cerebellar superior and inferior vermis, and a smaller but still significant increase in the cerebellar hemispheres and brainstem but no significant increases in other brain regions. The TRH-induced increase in the cGMP level in the cerebellum was suppressed by pretreatment with muscimol, THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3(2H)-one) or MK-801 (dizocilpine maleate) and partially suppressed by atropine but was not suppressed by chlordiazepoxide, oxazepam, phentolamine, propranolol, cyproheptadine, haloperidol, baclofen or DNQX (6,7-dinitroquinoxaline-2,3-dione), suggesting the possible involvement of GABA (gamma-aminobutyric acid)(A)-ergic, N-methyl-D-aspartate (NMDA)-type glutamatergic and cholinergic systems. These results suggest that excitatory amino acids may be involved in the cGMP formation caused by TRH in the cerebellar areas, and that cGMP formation is inhibited by enhancement of GABAA receptor function.

Enhancement of brain noradrenaline and dopamine turnover by thyrotropin-releasing hormone and its analogue NS-3 in mice and rats

The effects of intravenous injections of thyrotropin-releasing hormone and its analog NS-3 (montirelin hydrate, CG3703) on the dynamics of brain monoamines were examined in mice and rats. In mice, both NS-3 (0.1-1 mg/kg) and thyrotropin-releasing hormone (10 and 30 mg/kg) increased the concentrations of 4-hydroxy-3-methoxyphenylglycol, 3,4-dihydroxyphenylacetic acid and homovanillic acid. The turnover rates, estimated either by depletion of catecholamines after treatment with alpha-methyl-p-tyrosine or by probenecid-induced accumulation of homovanillic acid, were enhanced by these peptides. In contrast, none of the compounds had any influence on the serotonin turnover. In rats, both NS-3 and thyrotropin-releasing hormone produced a regionally specific increase in the concentrations of the catecholamine metabolites. A microdialysis study demonstrated that NS-3 significantly increased the release of dopamine in the nucleus accumbens as well as the striatum of conscious rats, while thyrotropin-releasing hormone caused a weak but significant enhancement of dopamine release only in the nucleus accumbens. These findings indicate that NS-3 was far more potent than thyrotropin-releasing hormone in facilitating the turnover of catecholamines without affecting serotonin turnover in the mouse and rat brain.

Effect of TA-0910, a novel thyrotropin-releasing hormone analog, on in vivo acetylcholine release and turnover in rat brain

To examine the action of a novel thyrotropin-releasing hormone (TRH) analog, TA-0910 ((-)-N-[(S)-hexahydro-1-methyl-2,6-dioxo-4-pyrimidinylcarbonyl]-L- histidyl-L-prolinamide tetrahydrate), on the cerebral cholinergic systems, the release of acetylcholine (ACh) and choline in freely-moving rats and ACh accumulation in gamma-butyrolactone (GBL, a nerve impulse flow blocker)- and physostigmine-treated rats were examined. TA-0910 (0.1-1 mg/kg, i.p.) caused a marked dose-dependent increase in extracellular ACh levels and a decrease in choline levels in the hippocampus of freely moving rats. These effects were significantly stronger and longer-lasting than similar effects of TRH. TA-0910 (1, 3 mg/kg, i.p.) depressed the ACh accumulation in the cerebral cortex and hippocampus of GBL (1000 mg/kg, i.p.)-treated rats. Moreover, this analog (1, 3 mg/kg, i.p.) increased the accumulation rate of ACh in these regions in physostigmine (1 mg/kg, i.p.)-treated rats. TRH (30 mg/kg, i.p.) affected the ACh accumulation only in the hippocampus of the GBL-treated rats. These results suggest that TA-0910 not only enhances the release of ACh, but also accelerates the ACh turnover, i.e., ACh release and synthesis, at the cholinergic neuronal terminals in normal rats.

NS-3, a TRH-analog, reverses memory disruption by stimulating cholinergic and noradrenergic systems

The effects of a TRH-analog, N[[(3R,6R)-6-methyl-5-oxo-3-thiomorpholinyl]carbonyl]-L-histidyl-L - prolinamide tetrahydrate (NS-3, CG3703, montirelin hydrate) were compared with those of physostigmine on learning and memory disruption in the passive avoidance response (PAR) induced by either electrolytic lesion of the nucleus basalis magnocellularis (NBM) or by treatment with the noradrenergic neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) in rats. a) In NBM-lesioned rats, both NS-3 and physostigmine significantly reversed disruption of memory consolidation examined 15 min after the training session when these drugs were injected IP immediately after the training session. In addition, reversal by NS-3 (0.1 mg/kg) of the disruption of memory was observed even in the retention test conducted 24 h after the training session. b) NS-3 (0.5 mg/kg) significantly reversed the disruption of memory retrieval, when the drug was administered 15 min before the test session. c) DSP4 (50 mg/kg IP) caused memory disruption when the retention tests were conducted between 1 and 48 h after the acquisition session. NS-3 (0.1 mg/kg), but not physostigmine, significantly reversed the disruption of memory induced by DSP4 treatment. These findings suggest that the consistent antiamnestic action of NS-3 is due to the enhancement of both central cholinergic and noradrenergic systems, possibly via facilitation of the release of these transmitters.

Permeability of NS-3, a thyrotropin-releasing hormone analogue, into the brain after its systemic administration in rats: a microdialysis study

The concentration of NS-3 (montirelin hydrate, CG 3703), a thyrotropin-releasing hormone (TRH) analogue, in the cerebral cortex of urethane-anaesthetized rats was measured after its systemic administration (1 mg kg-1, i.v.), using in-vivo microdialysis coupled with a radioimmunoassay. The concentration in microdialysates was highest (24 nM) during the first 20 min after injection, and it fell below the detection limit (3.5 nM) 100 min after treatment. The maximal interstitial concentration was estimated to be 0.51 microM. From these results, it is suggested that NS-3 can readily penetrate into the brain.

Distribution of thyrotropin-releasing hormone receptor mRNA in rat peripheral tissues

Since the thyrotropin-releasing hormone receptor (TRH-R) cDNA was isolated, the distribution of TRH-R mRNA has been investigated in the central nervous system (CNS) and the pituitary. However, there has been less genetical studies on the distribution of TRH-R mRNA in the peripheral tissues, although TRH exists not only in CNS but also in the peripheral tissues. In this study we investigated the distribution of TRH-R mRNA in rat peripheral tissues by reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis. TRH-R mRNA was detected in almost all of the peripheral tissues tested, although the amount varied considerably depending on the tissues. In the uterus, thymus, ovary, and testis, TRH-R mRNA levels appeared to be relatively high. These results suggest that TRH and its receptor have specific functions in the peripheral tissues as well as in CNS and in the pituitary.

The effect of thyrotropin-releasing hormone (TRH) on limbic status epilepticus in rats

We produced limbic status epilepticus in rats by injecting a combination of dibutyryl-cAMP (db-cAMP) and ethylenediaminetetraacetic acid (EDTA) into the amygdala (AM). Thirty minutes after intra-AM db-cAMP/EDTA injection, thyrotropin-releasing hormone (TRH) was administered intravenously or intracerebroventricularly. Intravenous TRH (3, 25, 50 mg/kg) produced immediate activation of electroclinical seizures, lasting for 25-45 min. In some animals which showed this seizure activation, complete seizure suppression occurred 55-70 min after the TRH treatment. Similar activation of ictal seizures with delayed seizure suppression was obtained after intracerebroventricular TRH (25, 50 micrograms). The findings suggest that the effects of intravenous TRH are due to its central action and that the use of intravenous TRH is not a promising approach for the treatment of status epilepticus.

Site-specific modulation of morphine and swim-induced antinociception following thyrotropin-releasing hormone in the rat periaqueductal gray

Central administration of thyrotropin-releasing hormone (TRH) produces a short-lived antinociceptive response in rats, and also modulates opioid and non-opioid forms of antinociception. Given the presence of TRH cells, fibers and receptors in the periaqueductal gray (PAG), the present study examined the effects of TRH administered into the PAG upon antinociception following either continuous cold-water swims (CCWS, 2 degrees C for 3.5 min) or morphine (0.1-2.5 micrograms) administered into the PAG on the tail-flick and jump tests, and measured changes in core body temperatures as well. Histological examination revealed two groups in which anterior PAG placements were found rostral to the dorsal raphe nucleus, and posterior PAG placements which were at the level of this nucleus. TRH produced brief (5-15 min) but significant increases in latencies and thresholds without altering body temperature in both anterior and posterior PAG placements. Whereas TRH in anterior PAG placements dose dependently (0.1-10 micrograms) decreased CCWS antinociception on both tests, TRH in posterior PAG placements significantly increased CCWS antinociception on the jump test. TRH in both placements reduced the magnitude of CCWS hypothermia. TRH significantly potentiated the magnitude and duration of both morphine antinociception and hyperthermia in both anterior and posterior PAG placements, and shifted mesencephalic morphine's antinociceptive dose-response curve significantly to the left. These data are discussed in terms of the role of the PAG in opioid and non-opioid forms of stress-induced antinociception as well as morphine antinociception, and in terms of the roles of TRH and anterior PAG placements as potential candidates for a collateral inhibition model of antinociceptive responses.

Distribution of thyrotrophin-releasing hormone receptor messenger RNA in rat pituitary and brain

The distribution sites of messenger RNA encoding for the thyrotrophin-releasing hormone receptor have been studied in rat pituitary and brain. A specific 35S-labelled riboprobe generated from a rat thyrotrophin-releasing hormone receptor complementary DNA clone was used to perform in situ hybridization experiments on brain and pituitary sections. A positive hybridization signal was found in the anterior lobe of the pituitary gland, the intermediate and posterior lobes were negative. Hybridization was also detected in different areas of the brain. These areas include distinct regions in the olfactory system, septal area, amygdaloid complex, cerebral cortex, hypothalamus, hippocampus, basal ganglia and the motor nuclei of cranial nerves in brainstem. This study has shown for the first time the exact site of thyrotrophin-releasing hormone receptor expression in the central nervous system. These results correlate well with regions thought to possess thyrotrophin-releasing hormone recognition sites.

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

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

Identification of neurons expressing thyrotropin releasing-hormone receptor mRNA in spinal cord and lower brainstem of rat

The distribution of mRNA coding for a pituitary thyrotropin releasing-hormone (TRH) receptor was examined on sections of spinal cord and lower brainstem of rat using in situ hybridization. Hybridization signals were observed over large neurons in the ventral horn in cervical, thoracic, and lumbar segments of spinal cord, and over neurons in the motor nuclei of the lower brainstem. Although significant thyrotropin-releasing hormone binding has been reported in the superficial dorsal horn, only background levels of hybridization were observed over neurons in this region. These findings suggest that mRNA coding for thyrotropin-releasing hormone receptor is expressed in some spinal and brainstem motor neurons. Since many of these neurons are innervated by TRH-containing afferents, TRH may exert a direct effect upon at least some of these cells.

Thyrotropin-releasing hormone causes direct excitation of dorsal vagal and solitary tract neurones in rat brainstem slices

The effect of thyrotropin-releasing hormone (TRH) on neurones in the dorsal motor nucleus of the vagus and the nucleus of the solitary tract was studied using extracellular single-unit recordings from brainstem slices of the rat. About one third of vagal neurones were excited by TRH. The remaining neurones were unaffected. The lowest effective peptide concentration was around 10 nM and a half maximal effect was achieved at about 100 nM. The action of TRH persisted in a low-calcium, high-magnesium solution which blocks synaptic transmission. The biologically inactive compound, TRH-free acid, was without effect. In the nucleus of the solitary tract, one fourth of the neurones were excited by TRH; none were inhibited by this peptide. Part of the vagal TRH-responsive neurones were also excited by oxytocin and some of the solitary tract neurones sensitive to TRH also responded to vasopressin. We conclude that a fraction of neurones located in the dorsal motor nucleus of the vagus and the nucleus of the solitary tract possess functional TRH receptors. TRH may thus act as a neurotransmitter or neuromodulator in the dorsal brainstem and may participate in the regulation of autonomic functions.

Autoradiographic localization of thyrotropin-releasing hormone and substance P receptors in the rat dorsal vagal complex

We utilized quantitative autoradiography to localize receptors for thyrotropin-releasing hormone (TRH) and substance P in individual subnuclei of the rat nucleus tractus solitarii (NTS) and the dorsal vagal complex. Within the NTS, TRH receptor concentrations were highest within the gelatinosus and centralis subnuclei and the medial subnucleus rostral to the area postrema, moderate within the intermediate subnucleus and the medial subnucleus adjacent to the area postrema, and low within the ventrolateral and commissural subnuclei and the medial subnucleus caudal to the area postrema. In contrast, substance P receptor concentrations were high throughout the medial subnucleus, moderate in all other subnuclei medial to the tractus solitarius, and relatively low in subnuclei lateral to the tractus solitarius. The dorsal motor nucleus of the vagus contained high concentrations of both TRH and substance P receptors, whereas we observed low TRH and moderate substance P receptors in the area postrema. High TRH and moderate substance P receptors were observed in the adjacent hypoglossal nucleus. In addition, we compared the concentrations of TRH receptors between chloroform-defatted and nondefatted tissue sections, and noted little effect of white matter tritium quench upon the observed TRH receptor concentrations. These results suggest that neurotransmitter receptors within the rat dorsal vagal complex are organized in a manner consistent with previous cytoarchitectural and hodological partitioning of the NTS and that the distribution of an individual neurotransmitter receptor in the NTS may correspond to the role of that transmitter in modulating autonomic function.

Autoradiographic distribution of thyrotropin-releasing hormone receptors in the African lungfish Protopterus annectens

We used quantitative autoradiography to examine the distribution of thyrotropin-releasing hormone (TRH) receptors in the central nervous system (CNS) of the African lungfish Protopterus annectens. We found that the distribution of TRH receptors throughout the CNS of the lungfish was heterogeneous with the highest concentrations (500-800 fmol/mg protein) in the olfactory bulb and telencephalon, moderately high concentrations (200-500 fmol/mg protein) in the diencephalon, and moderate (50-200 fmol/mg protein) to low (less than 50 fmol/mg protein) concentrations in the brainstem and spinal cord. Except for the motor nuclei of the cranial nerves and spinal cord, TRH receptors were concentrated in the acellular regions. In the telencephalon and diencephalon, the receptor density was inversely related to cellular density. These results provide a neuroanatomic and neuropharmacologic basis for further investigations of TRH in the African lungfish.

Quantitative autoradiography of TRH receptors in discrete brain regions of different mammalian species

The results clearly show marked heterogeneity and ubiquity of the CNS distribution of TRH receptors across several mammalian species including man. The use of high resolution autoradiography coupled with image analysis has permitted the visualization and quantification of TRH receptor density in even very small regions and nuclei of the CNS. This technique will undoubtedly help elucidate the other areas of TRH receptor localization that have thus far escaped detection in mammals and that are yet to be studied in lower vertebrates. Although an attempt has been made to correlate the presence of the peptide, its receptors, and its possible physiological functions, only further detailed physiological/behavioral investigations will ultimately unravel and support the diverse neurotransmitter and trophic roles of TRH in CNS and endocrine function.

Acute effects of high-dose thyrotropin releasing hormone infusions in Alzheimer's disease

Thyrotropin releasing hormone (TRH) was administered intravenously to ten patients with Alzheimer's Disease (AD) in a high-dose paradigm, thought to maximize central nervous system effects and potentially produce facilitation of cholinergic function, a known property of the neuropeptide. Acute effects of TRH on behavioral, cognitive and physiologic measures were assessed after patients received 0.1 mg/kg TRH, 0.3 mg/kg TRH and placebo, the higher TRH dose and placebo being given in a randomized, double-blind fashion. Patients showed statistically significant increases in arousal and improvement in affect, as well as a modest improvement in semantic memory, all after receiving the higher TRH dose. Both TRH doses produced transient rises in systolic blood pressure, with no effect on diastolic blood pressure, heart rate or temperature. This study suggests that high-dose TRH can be safely administered to AD patients and is neurobehaviorally active; further studies are needed to determine the extent and mechanism of the cognitive and psychobiological properties of this peptide in AD and other neuropsychiatric disorders.

Chemical and surgical lesions of rat olfactory bulb: changes in thyrotropin-releasing hormone and other systems

Stereotaxic injection of kainic acid (15 micrograms) into rat olfactory bulbs was accompanied by a 53% (n = 4; p less than 0.02) depletion of endogenous thyrotropin-releasing hormone (TRH) as compared to sham-operated controls 2 weeks postlesion. TRH levels remained unaltered in three other caudal regions. Bulbar kainate lesions produced a 58% (n = 5; p less than 0.001) decrease in TRH receptor binding capacity without affecting the receptor affinity. Kainate lesions also reduced bulbar muscarinic and benzodiazepine receptors by 60% and 48%, respectively. Again, no changes in TRH receptors were apparent in six other brain areas after bulbar kainate treatment. Injection of the dopaminergic neurotoxin, 6-hydroxydopamine (8 micrograms), into rat bulbs decreased TRH receptors by 35% (n = 4; p less than 0.05) 1 week postlesion. One month after surgical bulbectomy, TRH and TRH receptor levels in a number of brain areas were unaltered compared to those of control animals. These studies suggest that TRH in the olfactory bulb originates intrinsically and may be produced predominantly for local use. Secondly, TRH receptors in the bulb appear to be postsynaptically localized on intrinsic neurons, although a small proportion are also associated with presynaptic elements of dopaminergic noradrenergic neurons. Bulbar TRH receptors exhibited nanomolar affinity and a pharmacological selectivity akin to that of the pituitary gland and other brain regions.

Guanine nucleotide regulation of receptor binding of thyrotropin-releasing hormone (TRH) in rat brain regions, retina and pituitary

Guanine nucleotides inhibited the specific binding of [3H](3-Me-His2)thyrotropin-releasing hormone ([3H]MeTRH) to receptors for TRH in washed homogenates of rat anterior pituitary gland in a dose-related manner. The order of potency (at 100 and 500 microM final) was Gpp(NH)p (a stable analog of GTP) greater than GTP much greater than GDP much greater than cGMP (with the adenine nucleotides being inactive) in the pituitary and various brain regions. Gpp(NH)p at 1 mM caused 17-35% inhibition of [3H]MeTRH binding to different tissues including the pituitary, hypothalamus, retina and nucleus accumbens. A statistically significant nucleotide effect was not observed, however, in the olfactory bulb and medulla/pons membranes. Gpp(NH)p (1 mM) increased the dissociation constants for [3H]MeTRH binding by 1.9- to 2.4-fold in the pituitary, n. accumbens and retinal preparations without altering the apparent binding capacity. These data suggest that TRH receptor binding can be allosterically regulated by guanine nucleotides and provide further evidence for the existence of guanine nucleotide binding protein(s) coupled to the TRH receptor.

Regulatory peptide receptors: visualization by autoradiography

The receptors for regulatory peptides have been extensively characterized using radioligand binding techniques. By combining these binding techniques with autoradiography it is possible to visualize at the light and electron microscopic levels the anatomical and cellular localization of these receptors. In this review we discuss the procedures used to label peptide receptors for autoradiography and the peculiarities of peptides as ligands. The utilization of autoradiography in mapping peptide receptors in brain and peripheral tissues, some of the new insights revealed by these studies particularly the problem of 'mismatch' between endogenous peptides and receptors, the existence of multiple receptors for a given peptide family and the use of peptide receptor autoradiography in human tissues are also reviewed.

Thyrotropin-releasing hormone reduces myo-inositol content in rat cerebellum pretreated with lithium

The effect of thyrotropin-releasing hormone (TRH) and lithium on myo-inositol metabolism has been assessed in rat cerebral cortex, cerebellar cortex, and sciatic nerves. Sprague-Dawley male rats were injected subcutaneously with 10 mEq/kg of LiCl and intraperitoneally with 10 mg/kg of TRH-tartrate, alone or in combination. Either lithium or TRH alone had little effect on the myo-inositol concentration in cerebellar cortex, whereas the combination of lithium and TRH significantly lowered the level. The myo-inositol level of cerebellar cortex reached its nadir (70% of values in untreated control rats) 30 min after addition of TRH and then returned to the control level at 90 min. In cerebral cortex, both lithium alone and lithium plus TRH significantly reduced the myo-inositol level. No effect was seen on the myo-inositol concentration in sciatic nerves with these regimens. These results suggested that the pharmacological dose of TRH activated phosphatidylinositol turnover in rat cerebellar cortex and subsequently reduced the myo-inositol level in the presence of lithium.

Modulation of dopamine receptors by thyrotropin-releasing hormone in the rat brain

Nanomolar concentration of thyrotropin-releasing hormone (TRH) in vitro caused a significant reduction of [3H]apomorphine binding sites (70% of the control) in the rat striatum and the limbic forebrain. [3H]Spiperone binding was not affected by TRH. On the other hand, dopamine and apomorphine displaced [3H]TRH binding partially, suggesting the presence of a TRH receptor subpopulation that has a high affinity for dopamine agonist. Most of the neuroleptics displaced [3H]TRH binding dose-dependently in the micromolar range. (-)-Sulpiride had no affinity to TRH receptors. These findings suggest that one of the important roles of TRH as a neuromodulator is to modulate receptors for classical neurotransmitters, and this receptor-receptor interaction may be of importance in explaining the well known stimulating effects of TRH on the dopaminergic system.