Effects of thyrotrophic-releasing hormone (TRH) on thermoregulation in the rat

Thyrotropin-releasing hormone (TRH) in the brain and pituitary of the teleost, Clarias batrachus and its role in regulation of hypophysiotropic dopamine neurons

Thyrotropin-releasing hormone (TRH) regulates the hypothalamic-pituitary-thyroid axis in mammals and also regulates prolactin secretion, directly or indirectly via tuberoinfundibular dopamine neurons. Although TRH is abundantly expressed in teleost brain and believed to mediate neuronal communication, empirical evidence is lacking. We analyzed pro-TRH-mRNA expression, mapped TRH-immunoreactive elements in the brain and pituitary, and explored its role in regulation of hypophysiotropic dopamine (DA) neurons in the catfish, Clarias batrachus. Partial pro-TRH transcript from C. batrachus transcriptome showed six TRH progenitors repeats. Quantitative real-time polymerase chain reaction (qRT-PCR) identified pro-TRH transcript in a number of different brain regions and immunofluorescence showed TRH-immunoreactive cells/fibers in the olfactory bulb, telencephalon, preoptic area (POA), hypothalamus, midbrain, hindbrain, and spinal cord. In the pituitary, TRH-immunoreactive fibers were seen in the neurohypophysis, proximal pars distalis, and pars intermedia but not rostral pars distalis. In POA, distinct TRH-immunoreactive cells/fibers were seen in nucleus preopticus periventricularis anterior (NPPa) that demonstrated a significant increase in TRH-immunoreactivity when collected during preparatory and prespawning phases, reaching a peak in the spawning phase. Although tyrosine hydroxylase (TH)-immunoreactive neurons in NPPa are hypophysiotropic, none of the TRH-immunoreactive neurons in NPPa accumulated neuronal tracer DiI following implants into the pituitary. However, 87 ± 1.6% NPPa TH-immunoreactive neurons were surrounded by TRH-immunoreactive axons that were seen in close proximity to the somata. Superfused POA slices treated with TRH (0.5-2 μM) significantly reduced TH concentration in tissue homogenates and the percent TH-immunoreactive area in the NPPa. We suggest that TRH in the brain of C. batrachus regulates a range of physiological functions but in particular, serves as a potential regulator of hypophysiotropic DA neurons and reproduction.

Administration of Thyrotropin-Releasing Hormone in the Hypothalamic Paraventricular Nucleus of Male Rats Mimics the Metabolic Cold Defense Response

BACKGROUND

Cold exposure increases thyrotropin-releasing hormone (TRH) expression primarily in the hypothalamic paraventricular nucleus (PVN). The PVN is a well-known hypothalamic hub in the control of energy metabolism. TRH terminals and receptors are found on PVN neurons. We hypothesized that TRH release in the PVN plays an important role in the control of thermogenesis and energy mobilization during cold exposure.

METHODS

Male Wistar rats were exposed to a cold environment (4°C) or TRH retrodialysis in the PVN for 2 h. We compared the effects of cold exposure and TRH administration in the PVN on plasma glucose, corticosterone, and thyroid hormone concentrations, body temperature, locomotor activity, as well as metabolic gene expression in the liver and brown adipose tissue.

RESULTS

Cold exposure increased body temperature, locomotor activity, and plasma corticosterone concentrations, but blood glucose concentrations were similar to that of room temperature control animals. TRH administration in the PVN also promptly increased body temperature, locomotor activity and plasma corticosterone concentrations. However, TRH administration in the PVN markedly increased blood glucose concentrations and endogenous glucose production (EGP) compared to saline controls. Selective hepatic sympathetic or parasympathetic denervation reduced the TRH-induced increase in glucose concentrations and EGP. Gene expression data indicated increased gluconeogenesis in liver and lipolysis in brown adipose tissue, both after cold exposure and TRH administration.

CONCLUSIONS

We conclude that TRH administration in the rat PVN largely mimics the metabolic and behavioral changes induced by cold exposure indicating a potential link between TRH release in the PVN and cold defense.

Central administration of alpha-melanocyte-stimulating hormone changes lipid metabolism in chicks

Alpha-melanocyte-stimulating hormone (MSH) is well known as an anorexigenic peptide in the brain of mammals. In addition to this, brain alpha-MSH enhances heat production (HP), indicating that the peptide acts as a catabolic factor in the regulation of energy metabolism. The anorexigenic effect of alpha-MSH is also observed in chicks (Gallus gallus), but no information has been available for its effect on HP. The present study was performed to examine whether intracerebroventricular (ICV) injection of alpha-MSH increases HP in chicks. The injection of alpha-MSH (10 and 100 pmol) did not affect oxygen consumption, carbon dioxide production and HP during the 1 h post-injection period. This result was supported by another result that ICV injection of alpha-MSH did not affect locomotion activity in chicks. In contrast, the respiratory quotient was significantly lowered by the ICV injection of MSH. We also found that alpha-MSH significantly increased plasma non-esterified fatty acid concentrations. In summary, brain alpha-MSH appears to exert generally catabolic effects on lipid metabolism in the chick, but does not appear to be involved in the regulation of HP.

Regulation of body temperature by thyrotropin-releasing hormone in neonatal chicks

To understand thermal regulation of neonatal chicks, the contribution of thyrotropin-releasing hormone (TRH), a key regulator of the hypothalamus-pituitary-thyroid axis, was investigated. Intracerebroventricular (i.c.v.) injection of TRH (5 and 20 microg) increased body temperature, but did not change plasma T3 and T4 concentrations. Intraperitoneal (i.p.) injection of triiodothyronine (T3) and thyroxine (T4) did not influence body temperature. Thereafter, the relationships between TRH and the hypothalamus-pituitary-adrenal axis and sympathetic nervous system were further investigated. Central TRH stimulated both corticosterone and epinephrine release. The i.c.v. injection of a corticotropin-releasing factor receptor antagonist attenuated the change in body temperature and corticosterone concentration caused by TRH, but did not influence plasma T3 and T4 concentrations. The i.p. injection of epinephrine did not induce hyperthermia. Therefore, the thermoregulatory response to TRH may differ in neonatal stages being dependent upon the stimulation of the hypothalamus-pituitary-adrenal axis rather than the hypothalamus-pituitary-thyroid axis.

Intracranial administration of transforming growth factor-beta3 increases fat oxidation in rats

The effects of intracranial transforming growth factor (TGF)-beta3 on spontaneous motor activity and energy metabolism were examined in rats. After injection of TGF-beta3 into the cisterna magna of the rat, spontaneous motor activity decreased significantly for 1 h. The intracranial injection of TGF-beta3 produced an immediate decrease in respiratory exchange ratio (RER). No significant changes were observed in energy expenditure. TGF-beta3 induced a significant increase in total fat oxidation and a decrease in total carbohydrate oxidation. Furthermore, the serum substrates associated with fat metabolism were significantly altered in rats injected with TGF-beta3. Both lipoprotein lipase activity in skeletal muscle and the concentration of serum ketone bodies increased, suggesting that the increase in fat oxidation caused by TGF-beta3 may have occurred in the liver and muscle. Intracranial injection of TGF-beta3 appeared to evoke a switch in the energy substrates accessed in energy expenditure. These results suggest that the release of TGF-beta3 in the brain by exercise is a signal for regulating energy consumption.

Thyrotropin-releasing hormone action in the preoptic/anterior hypothalamus decreases thermoregulatory set point in ground squirrels

Earlier work has shown that thyrotropin releasing hormone (TRH) produces dose-dependent decreases in body temperature (Tb) and metabolic rate when microinjected into the dorsal hippocampus (HPC) or preoptic/anterior hypothalamus (PO/AH) of awake ground squirrels. This study employed a behavioral paradigm to investigate the possibility that TRH-induced hypothermia is associated with a decrease in thermoregulatory set point. Six animals were successfully trained to press a bar for radiant heat escape and cool air reinforcement in order to obtain a cooler ambient temperature (Ta). During experimental testing, the animals were microinjected remotely with TRH (10-1000 ng/microliters) or a control solution (sterile saline or TRH-OH) into the PO/AH. The micro-injections were delivered via bilateral injection cannulae inserted through chronic bilateral cannula guides that had been stereotaxically implanted under pentobarbital anesthesia. Cumulative and time-integrated bar presses were obtained on a computer generated display. Tb, measured in the brain via a bead-type thermistor, and chamber Ta were recorded continuously. Following TRH administration, a significant increase in mean bar-press rate was observed during the period in which Tb was falling, when compared to a comparable time period just prior to the microinjection. These findings complement results obtained from four animals that were trained to press a bar for heat reinforcement in a cold (- 10 degrees C) environment. In this alternative behavioral paradigm, microinjection of TRH into the PO/AH or HPC induced a decrease in mean bar-press rate as Tb was falling. The results support the hypothesis that TRH-induced hypothermia in golden-mantled ground squirrels is achieved by lowering thermoregulatory set point.

Intra-hypothalamic injection of thyrotropin-releasing hormone suppresses brown fat thermogenesis in the anaesthetized rat

Thyrotropin-releasing hormone (TRH) has diverse effects on body temperature in rodents, but the effector mechanisms that mediate its thermoregulatory actions are not well defined. In the present study, microinjection of 10 ng to 5 micrograms of TRH into the anterior hypothalamus (AHy) dose-dependently suppressed heat production in interscapular brown adipose tissue (BAT) in chloral hydrate-anaesthetized rats tested at a room temperature of 23 +/- 2 degrees C. This effect of TRH was mimicked by the structurally related peptides acid-TRH and luteinizing hormone releasing hormone (LH-RH), and by the TRH analog CG 3509, but not by the TRH fragments pGlu-His and His-Pro. The AHy plays a role in the regulation of BAT thermogenic activity, and the present results suggest that some of the effects of TRH on body temperature involve an AHy-mediated inhibitory action on BAT thermogenesis.

Responses of anterior hypothalamic-preoptic thermosensitive neurons to thyrotropin releasing hormone and cyclo(His-Pro)

Effects of local application of thyrotropin releasing hormone (TRH) and its metabolite, histidyl-proline diketopiperazine [cyclo (His-Pro)], on the activity of thermosensitive and thermally-insensitive neurons of the anterior hypothalamic-preoptic area were investigated in urethane-anesthetized rats. Microelectrophoretic application of TRH changed the activity of 126 of 206 neurons tested. Thyrotropin releasing hormone predominantly decreased the activity of warm-sensitive neurons and increased the activity of cold-sensitive neurons. Since it has been generally assumed that warm-sensitive and cold-sensitive neurons in the anterior hypothalamic-preoptic area mediate heat and cold defence responses, respectively, the present results are consistent with previous findings showing hyperthermia after injection of TRH into the hypothalamus in the rat. Cyclo (His-Pro) affected the activity of 59 of 153 neurons tested. In addition, cyclo (His-Pro) did not preferentially affect warm- or cold-sensitive neurons. These results indicate that the previously-determined hypothermic effect of cyclo (His-Pro) cannot be explained by its effects on thermosensitive neurons in the anterior hypothalamic-preoptic area.

Effects of thyrotropin releasing hormone on human sudomotor and cutaneous vasomotor activities

At an ambient temperature of 34-41 degrees C (rh = 40%) forearm sweat rates were measured by capacitance hygrometry in 9 male volunteers. Thyrotropin releasing hormone (TRH) was infused intravenously at 0.1 mg.min-1 for 20 to 30 min. Sweat rate increased rapidly within a minute after initiation of TRH infusion, decreased rapidly after the peak sweat rate was attained in 2-5 min of TRH infusion, and then levelled off in 6-10 min near the level before TRH infusion. Core temperature (Tre, Tty) started to decline at the time of the peak sweat rate and levelled off almost coincidentally with the levelling off in sweat rate. Average values for the rate of sweat expulsions (Fsw), sweat rate and mean body temperature (Tb) were obtained from the data of the last 10 min period of TRH infusion. The regression line for the relationship of Fsw to Tb shifted during the TRH infusion to the left of the line for the control; that of sweat rate to Fsw hardly shifted. At an ambient temperature of 24-27 degrees C TRH produced vasodilation as evidenced by an increase in skin blood flow (measured by means of thermal distribution), an increase in amplitude of the photoelectric plethysmogram and an elevation of skin temperature in the finger tips. It is suggested that TRH may act, either directly or indirectly, on the central thermoregulatory mechanism (or on the thermoreceptive mechanism) to lower the reference temperature for heat dissipation.

Changes in body temperature after administration of amino acids, peptides, dopamine, neuroleptics and related agents: II

This survey begins a second series of compilations of data regarding changes in body temperature induced by drugs and related agents. The information listed includes the species used, the route of administration and dose of drug, the environmental temperature at which experiments were performed, the number of tests, the direction and magnitude of change in body temperature and remarks on the presence of special conditions, such as age or brain lesions. Also indicated is the influence of other drugs, such as antagonists, on the response to the primary agent. Most of the papers were published since 1978, but data from many earlier papers are also tabulated.

Involvement of adrenergic receptor mechanisms within hypothalamus in the fever induced by amphetamine and thyrotropin-releasing hormone in the rat

The mechanisms underlying the thermal effects induced by intrahypothalamic administration of either d-amphetamine or thyrotropin-releasing hormone (TRH) has been investigated in conscious rats. Direct administration of d-amphetamine (1-10 micrograms in 1 microliter) or TRH (1-4 micrograms in 1 microliter) into the preoptic anterior hypothalamus caused hyperthermia or fever at the ambient temperature (Ta: 8, 22 and 30 degrees C) studied. The fever induced by d-amphetamine or TRH was due to increased metabolic heat production at Ta 8 degrees C, while at Ta 30 degrees C the fever was due to cutaneous vasoconstriction in the rat. At Ta 22 degrees C, the fever was due to both increased metabolism and cutaneous vasoconstriction. Furthermore, the fever induced by intrahypothalamic administration of TRH was greatly reduced by pretreatment with intrahypothalamic administration of either yohimbine (a blocking agent of alpha-adrenergic receptors), phentolamine (a blocking agent of alpha-adrenergic receptors) or DL-propranolol (a blocking agent of beta-adrenergic receptors) in the rat. However, the fever induced by d-amphetamine was antagonized by pretreatment with yohimbine or phentolamine, but not with DL-propranolol in the rat. These observations indicate that the adrenergic receptor mechanisms within the hypothalamus are involved in the fever induced by both d-amphetamine and TRH.

Involvement of both opiate and catecholaminergic receptors in the behavioural excitation provoked by thyrotropin-releasing hormone: comparisons with amphetamine

Following direct administration of thyrotropin-releasing hormone (TRH), but not thyroid-stimulating hormone (TSH) or vehicle solution, into the lateral cerebral ventricle in rats, three main categories of behaviour were provoked: activity of the normal type--stimulation of forward locomotion, head and body rearing (as shown by an enhancement in gross movements); stereotyped activity--increased grooming and head swaying (as shown by an enhancement in fine movements); abnormal behaviour--tail elevation and piloerection (as observed grossly). The behavioural excitation caused by TRH was antagonized by pretreatment of the rats with either a narcotic receptor antagonist, naloxone, an alpha-adrenergic receptor antagonists, yohimbine, or a dopaminergic receptor antagonist, haloperidol, but not with a beta-adrenergic receptor antagonist, propranolol. Intraventricular administration of amphetamine to rats caused stimulation of forward locomotion, head and body rearing, increased grooming and sniffing. Unlike TRH, amphetamine did not produce wet-dog shakes, tail elevation and piloerection. Furthermore, the amphetamine-induced excitation was antagonized by pretreatment with a dopaminergic receptor antagonist, haloperidol, but not with either naloxone, yohimbine or propranolol. The data indicate that both opiate and catecholaminergic receptors are involved in the TRH-induced behavioural excitation, whereas dopaminergic receptors are involved in amphetamine-induced excitement in the rat.

Is thyrotropin releasing hormone an endogenous ergotropic substance in the brain?

The neuropharmacological profile for thyrotropin releasing hormone (TRH) has been compared with various other types of central nervous system stimulants, and the results of the comparison, considered together with the results obtained by other workers, suggest that TRH may function as an endogenous ergotropic substance. Its ergotropic properties suggest that TRH analogues with better biological stability than the parent compound may be useful agents for treating some psychiatric disorders; for hastening the recovery of consciousness and respiration after anaesthesia; for treating narcotic overdosage or dependence, narcolepsy, minimal brain damage, and various neuromuscular disorders; and for aiding the diagnosis of "brain death".