Autoradiographic localization of thyrotropin-releasing hormone (TRH) receptors in human spinal cord

Receptor localization in the mammalian dorsal horn and primary afferent neurons

The dorsal horn of the spinal cord is a primary receiving area for somatosensory input and contains high concentrations of a large variety of receptors. These receptors tend to congregate in lamina II, which is a major receiving center for fine, presumably nociceptive, somatosensory input. There are rapid reorganizations of many of these receptors in response to various stimuli or pathological situations. These receptor localizations in the normal and their changes after various pertubations modify present concepts about the wiring diagram of the nervous system. Accordingly, the present work reviews the receptor localizations and relates them to classic organizational patterns in the mammalian dorsal horn.

Immunomodulation of peripheral lymphocytes by hormones of the hypothalamus-pituitary-thyroid axis

The aim of this review is to provide a comprehensive examination of the current literature describing the immunoregulatory effects on the peripheral immune system by the hormones that comprise the hypothalamic-pituitary-thyroid (HPT) axis. This article discusses the effects of the HPT axis hormones on the peripheral lymphoid tissues and the immune responses mediated by the cells that comprise these lymphoid tissues. Neuroendocrine dysfunction in the HPT axis, either naturally or experimentally induced, and the resulting immune dysfunction are also discussed. Emphasis in this article is placed on the most recent study findings and those that provide a unique or novel way of evaluating HPT hormone effects on the immune system. Our knowledge of the immunoregulatory effects of the hormones that comprise the HPT axis has grown tremendously in the last 10 years. As can be seen in this review, the immunoregulatory effects of the HPT axis hormones are quite diverse and influence most, if not all, aspects of immune system physiology. The continued exploration of the bidirectional circuitry between the immune and neuroendocrine systems may allow for development of appropriate prophylactic procedures that prevent dysfunction in both systems.

TRH receptor on immune cells: in vitro and in vivo stimulation of human lymphocyte and rat splenocyte DNA synthesis by TRH

This work examined whether (1) immune cells express thyrotrophin releasing hormone (TRH) receptor mRNA and (2) TRH modulates lymphocyte activation. By Northern blot of RNA extracted from human peripheral blood mononuclear cells (PBMC) and rat splenocytes, a single TRH receptor mRNA band of about 3.8 kb (identical to that obtained from pituitary cells) was obtained, under both basal and stimulated conditions. A significant increase in DNA synthesis was observed in phytohemagglutinin-stimulated PBMC and concanavalin A (Con A) stimulated splenocytes when TRH (10(-6) M-10(-12) M) was added. After 5, 30, 60, 180 min and 24 h of TRH administration in vivo, a significant increase in the rat splenocyte proliferative response to Con A was observed. In vivo administration of anti-rat TSH antibody (1/1000) blocked the increase observed after 30 min of TRH administration on the Con A stimulated splenocyte response. TRH possess immunostimulatory functions directly via its receptor and indirectly via release of other immunostimulatory factors such as thyrotrophin.

Treatment of acute spinal cord injuries: comparison of thyrotropin-releasing hormone and nimodipine

The effects of nimodipine and thyrotropin-releasing hormone (TRH) were compared in a clip-compression model of experimental spinal cord injuries (SCI) in rats. Thirty rats received a 50-g clip-compression injury on the cord at T9. Ten rats were given 0.02 mg/kg nimodipine and dextran 40 (3 ml) i.v. 1 h after injury. Ten rats were given 2 mg/kg TRH and dextran 40 (3 ml) i.v. 1 h after injury followed by 1 mg/kg per hour for 4 h. The remaining ten rats were given only saline. TRH treatment significantly improved somatosensory-evoked potentials (SEPs) and mean arterial blood pressures (MABPs), whereas nimodipine treatment had no effect on these variables (Fisher's exact test (P less than 0.01).

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)

Muscle denervation increases thyrotropin-releasing hormone (TRH) biosynthesis in the rat medullary raphe

To determine whether thyrotropin-releasing hormone (TRH) could exert a trophic role in ventral horn motor neurons, we examined the effect of muscle denervation with botulinum toxin A on TRH mRNA in the rat medullary raphe by in situ hybridization histochemistry. Compared to controls, denervated rats showed a significant increase in the number and silver grain density of hybridized medullary raphe neurons. Increased proTRH gene expression in the medullary raphe in response to motor unit perturbation indicates that TRH may be trophic to lower motor neurons.

Autoradiographic distribution of TRH binding sites in the human hypothalamus

Using in vitro quantitative autoradiography and [3H]3MeTRH, a selective high affinity radioligand, we examined the rostrocaudal distribution of TRH binding sites in both the infant and the adult human hypothalamus. The saturation curve shows that the [3H]3MeTRH binds with high affinity to a single class of TRH binding sites and is saturable, the apparent constant of dissociation is in the namomolar range. TRH binding sites showed a wide distribution, principally in the anterior and mediobasal levels of the hypothalamus. TRH binding site concentration was highest within the diagonal band of Broca, the lateral preoptic area, the infundibular and the tuberal nuclei. TRH binding site concentration was moderate in the ventromedial nucleus and the medial preoptic area, whereas we observed low densities in the periventricular, paraventricular and mammillary nuclei. The distribution in the infant and the adult is generally similar. However, it is noteworthy that the infant tuberal nuclei displayed a lower binding site density when compared to the adult. On the other hand, the diagonal band of Broca is relatively more labeled in infant. The analysis of the whole hypothalamus allows us to ascertain the absence of lateral asymmetric distribution both in the infant and the adult. No significant difference is noticed when considering as parameters of variation age, sex or post mortem delay.

Prolactin response to growth hormone-releasing hormone during chronic thyrotropin-releasing hormone infusion in the treatment of amyotrophic lateral sclerosis

In six patients suffering from amyotrophic lateral sclerosis we evaluated changes of T4, T3, TSH, PRL, and GH during treatment by continuous iv infusion of TRH for at least 15 days. No clinical improvement was detected. A significant rise of thyroid hormone levels was observed, as well as an upward trend of basal TSH levels and no change of basal PRL and GH levels. TRH acute test-induced TSH and PRL responses became blunted. Treatment provoked also the onset of a responsiveness of PRL to GHRH. The reduced TSH and PRL responses to acute TRH test during treatment could be explained by a down-regulation of TRH pituitary receptors. On the contrary, the onset of PRL responsiveness to GHRH is at present without a satisfactory explanation.

Critical review of gangliosides and thyrotropin-releasing hormone in peripheral neuromuscular diseases

The lack of effective therapy for many of the chronic neuromuscular diseases such as amyotrophic lateral sclerosis, hereditary motor sensory neuropathy (Charcot-Marie-Tooth disease), spinocerebellar degenerations and idiopathic polyneuropathy has led to a search for substances that may stimulate peripheral nerve regeneration. Two such agents that have been proposed are gangliosides (mixed purified bovine brain gangliosides, Cronassial) and thyrotropin releasing factor (TRH). Studies on both of these agents were initially reported with enthusiasm to be successful, but later double-blind controlled studies have failed to confirm these findings. This review provides critical analysis of the designs of studies of potentially effective agents in chronic neuromuscular diseases, and emphasizes the power of the placebo response, and the importance of designing placebos which are indistinguishable from the trial medication other than in the active effect.

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.

TRH and TRH receptors in the spinal cord

Over the past 12 years, substantial progress has been made in delineating the localization of TRH and TRH receptors in spinal cord. High concentrations of both the peptide and its receptor have been observed in the ventral horn in the region of the motoneurons and in the dorsal horn in the substantia gelatinosa. As noted, pharmacological effects of TRH administration on various parameters of spinal cord function have been reported in a number of studies. To date, however, substantial questions remain regarding the physiological role of TRH in the spinal cord. Nevertheless, it is hoped that the extensive information that has been obtained on localization of TRH and TRH receptors in spinal cord will provide a basis for answering these complex questions.

A comparison of the motor behaviours produced by the intrathecal administration of thyrotrophin-releasing hormone and thyrotrophin-releasing hormone analogues in the conscious rat

The behaviours evoked by the intrathecal injection of thyrotrophin-releasing hormone and a variety of analogues (RX77368, CG3509 and CG3703) were examined in conscious rats and the spread of injectate at the peak of the behavioural response was determined using 14C-labelled RX77368. The number of wet-dog shakes observed following intrathecal injection of thyrotrophin-releasing hormone, RX77368, CG3509 and CG3703 was linearly related to log10 dose (0.01-200 micrograms) in the first 6 min with the relative potencies being 1:7:10:60 respectively. The thyrotrophin-releasing hormone analogues also produced a marked forepaw-licking behaviour, but this did not increase with dose. Intrathecal or intraperitoneal pretreatment with prazosin (0.5 microgram and 1 or 2 mg/kg, respectively) attenuated both the wet-dog shake and forepaw-licking behaviours normally produced by the thyrotrophin-releasing hormone peptides. Following intrathecal [14C]RX77368 the radioactivity was principally restricted to the spinal cord with only limited amounts rostral to the rhombencephalon. These results imply that a tonically active bulbospinal noradrenergic pathway facilitates both thyrotrophin-releasing hormone-induced behaviours via alpha 1-adrenoceptors.

Cardiovascular pharmacology of thyrotropin releasing hormone

Thyrotropin releasing hormone (TRH) and its receptors are present in the cardiovascular nuclei of the brain as well as in the intermediolateral cell column of spinal cord. Anatomical, neurophysiological, functional and pharmacological studies suggest that TRH is a neurotransmitter/neuromodulator in the central nervous system. Administration of TRH to experimental animals or human subjects induces pressor and tachycardic responses and increases plasma levels of catecholamines. These effects are likely to be mediated by a central nervous system activation of the sympathoadrenomedullary system with no involvement of vasopressin or renin-angiotensin system. In the conscious rat, the TRH-induced pressor response is accompanied by an increment in cardiac output and a distinct change in organ blood flow, a hindquarter skeletal muscle vasodilation accompanied by renal and mesenteric vasoconstriction. The role of TRH in hypertension has not been studied. However, the extremely potent pressor and vasoconstrictor properties of TRH makes this tripeptide a candidate for neurotransmitters/modulators involved in the development and/or maintenance of hypertension. The role of TRH in the therapy of shock is at present controversial. Though preliminary experimental work raised hopes and expectations for therapeutic usage of TRH in shock and trauma, the more recent studies have shown no effect or a detrimental effect for TRH in some experimental shock states.

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

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

Effect of spinal cord TRH deficiency on lower motorneuron function in the rat

Thyrotropin-releasing hormone (TRH), present in high concentrations in the mammalian spinal cord, exerts excitatory effects on the alpha-motorneuron (AMN) via axodendritic contacts. We used the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) to deplete TRH from the ventral horn of the spinal cord of adult rats to determine whether the tripeptide may be trophic to the AMN. The rats were studied blindly and sequentially for 11 weeks. Motor performance remained normal by clinical and electrophysiologic assessments. AMN counts were not reduced in the lumbar cord, and gastrocnemius muscle showed no evidence of denervation in treated rats. We conclude that in the adult rat chronic ventral horn TRH deficiency does not lead to AMN degeneration and is not associated with a significant alteration of AMN function.

Thyrotropin-releasing hormone (TRH) in murine motor neuron disease (the wobbler mouse)

Clinical benefits of thyrotropin-releasing hormone (TRH) were tested in wobbler mice, an animal model of motor neuron disease. After the disease was clinically recognized at 3-4 weeks, the animals were divided into two groups, each group consisting of 5 pairs of wobbler mice and normal littermates. TRH (50 mg/kg) and normal saline (NS) were injected intraperitoneally daily, 6 times per week for 9 weeks, in a double-blind study. Weekly assessments consisted of front paw grip strength, push walking, body weight, and semiquantitative grading. At the end of the trial, the brain and spinal cord were sampled to measure TRH and cyclo (His-Pro) concentrations. Progression of motor neuron disease was evident in wobbler mice, regardless of treatment. Descriptive semiquantitative gradings showed the tendency of improvement in TRH-treated wobbler mice. In saline-injected controls, TRH levels in the cervical spinal cord were significantly increased (P less than 0.01) in wobbler mice compared to littermates. However, with TRH treatment, there was no significant difference in TRH and cyclo (His-Pro) levels in any neural tissue between wobbler and controls. The lack of clinical benefits with TRH in wobbler mice may be due to increased TRH levels found in diseased spinal cord in murine motor neuron disease.

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.

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 TRH immunoreactivity in spinal cord after experimental spinal injury

Concentrations of immunoreactive TRH (TRH-ir) in rat spinal cord were examined after traumatic injury. TRH-ir was significantly increased at the injury site (thoracic region) and rostral areas (cervical region), and significantly decreased below the injury site. Changes were delayed and time-dependent. Since TRH may serve as an excitatory neurotransmitter within the spinal cord, these changes may contribute to the functional alterations observed after spinal injury.