Failure of muscarinic blockade to antagonize analepsis induced by thyrotropin-releasing hormone and MK-771 in the rat

Involvement of nicotinergic mechanisms in thyrotropin-releasing hormone-induced neurologic recovery after concussive head injury in the mouse

A behavioral study was performed in an attempt to understand the neuronal mechanisms involved in the thyrotropin-releasing hormone (TRH)-induced improvement of consciousness after concussive head injury in the mouse. Intravenous administration of TRH dose dependently shortened the duration of unconsciousness after concussion in the mouse (ED50 = 3.2 mg/kg). The improvement of recovery evoked by TRH (3 mg/kg i.v.) after concussion was not affected by i.p. pretreatment with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine, alpha-methyl-para-tyrosine, p-chlorophenylalanine, scopolamine or methylscopolamine. However, mecamylamine or hexamethonium i.p. pretreatment completely inhibited the TRH-induced improvement of outcome in traumatic brain injury. The results imply that TRH-induced improvement of recovery after concussion is not associated with increased activity of monoaminergic neurons in the brain. These results suggest that the inhibitory effect of TRH upon unconsciousness after concussion in mice is mainly produced by activation of central cholinergic systems via nicotinic receptors whereas muscarinic receptors seem to be not implicated.

An update on the CNS actions of TRH and its analogs

This brief review will discuss the recent literature on several of the central actions of TRH and its analogs. The most prominent of these actions include: (1) the arousal or analeptic effect in drug narcotized animals or in concussion models; (2) the reversal of cognitive deficits produced by various drugs or procedures, and (3) the improvement of several neurological deficits produced in animal models of spinal and/or cerebellar injury. The mediation of these TRH effects by neurotransmitters is discussed. While little has been published on the human neuropsychopharmacology of TRH, and especially of its analogs, the future holds considerable therapeutic promise for these interesting drugs.

Effects of TRH and atropine on induction and duration of anesthesia with propofol in rats

The effects of IV TRH pretreatment on induction of anesthesia with propofol or pentobarbital were investigated in rats. The effects of IV TRH, administered after induction, on duration of propofol anesthesia and the interaction with atropine were also studied. The doses of propofol or pentobarbital were not influenced by TRH. TRH reduced duration of anesthesia after propofol, with higher brain concentrations of propofol at recovery. Atropine did not block this effect, but given alone prolonged duration of anesthesia. It is concluded that TRH shortens the duration of propofol anesthesia, probably due to a pharmacodynamic effect and not to a pharmacokinetic interaction.

Influence of thyrotropin-releasing hormone and catecholaminergic interactions on CNS ethanol sensitivity

The role of catecholamine neuronal systems in mediating the analeptic and thermogenic effects of thyrotropin-releasing hormone (TRH) was examined in long-sleep (LS) and short-sleep (SS) mice. TRH [0.1 to 40 micrograms, intracerebroventricularly (icv)] was associated with a reduction in the sleep times of LS mice, but no dose of TRH had any effect on sleep times of SS mice. However, TRH (20 micrograms, icv) produced a 1.0 degree to 1.5 degrees C attenuation of the ethanol-induced hypothermia in both LS and SS mice. TRH did not change the rate of ethanol elimination in either line of mice, suggesting that the reduction in LS sleep times and attenuation of LS and SS hypothermia were due to decreased CNS ethanol sensitivity rather than an increase in the rate of ethanol metabolism. TRH (20 micrograms, icv) given alone produced an activation of central and peripheral catecholamine systems in LS, but not SS mice, as reflected by an increase in the in vivo tyrosine hydroxylase (TH) activity in the brain and adrenal gland. TRH, given with ethanol, prevented or attenuated ethanol-induced decreases in the brain and adrenal gland in vivo TH activity in LS mice but not SS mice. Thus, there was an association between the ability of TRH to produce an activation of catecholamine neuronal systems (increased rate of catecholamine biosynthesis) and the analeptic action of TRH to reduce the CNS depressant effects of ethanol (decreased sleep times).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological study of TA-0910, a new thyrotropin-releasing hormone (TRH) analog, (I): Effects on the central nervous system by oral administration

Effects of orally administered TA-0910, a new thyrotropin-releasing hormone (TRH) analog, on the central nervous system (CNS) were investigated and compared with those of TRH. TA-0910 shortened the duration of pentobarbital-induced sleep and antagonized reserpine-induced hypothermia at 0.3 mg/kg or more in mice. TA-0910 enhanced the spontaneous motor activity at the higher dose of 30 mg/kg in mice. The drug also activated acute spontaneous EEGs in rabbits at 1 mg/kg. TRH produced these effects at about 100 times higher doses than TA-0910. In antagonizing pentobarbital-induced sleep, the dose ratios of i.v. versus p.o. of TA-0910 and TRH were about 1/10 and 1/100, respectively. The duration of the antagonistic action of TA-0910 on pentobarbital-induced sleep in mice was about 8 times longer than that of TRH when administered orally as well as intravenously. These potent and long-acting TA-0910 effects on the CNS are discussed in connection with its biotransformation.

Segmental synaptic depression caused by diisopropylphosphorofluoridate and sarin is reversed by thyrotropin-releasing hormone in the neonatal rat spinal cord

The organophosphorus compounds diisopropylphosphorofluoridate (DFP) and isopropylmethylphosphonofluoridate (sarin) depressed the monosynaptic reflex (MSR) in spinal cords from 7- to 9-day-old male rats. The concentrations of DFP and sarin which depressed the MSR by nearly 50% were 100 microM and 100 nM, respectively. Simultaneous superfusion of the cords with thyrotropin-releasing hormone (TRH) with either DFP or sarin resulted in a reversal of the depression. The depression caused by DFP was reversed to 95% of control by 100 nM TRH whereas similar reversal of sarin-induced depression required a 10-fold greater concentration of TRH. The potentiating effect of TRH was not affected by atropine even at a high concentration (1 microM) although atropine easily reversed organophosphorus-induced depression of the MSR. It appears that reversal of organophosphorus-induced depression by TRH might occur through a noncholinergic, TRH-sensitive receptor mechanism and may be unrelated to acetylcholinesterase activity. This action represents a possible utility of TRH as an adjunct in organophosphorus toxicity.

Central muscarinic cholinergic antagonists block wet-dog shakes produced by the TRH analog MK-771 in the rat

Thyrotropin-releasing hormone (TRH) is known to elicit wet-dog shakes in rats through a central mechanism of action. In the present study, the ability of muscarinic cholinergic antagonists to inhibit TRH-mediated wet-dog shakes was examined. The longer-acting TRH analog, MK-771, at doses of 1.0, 1.5 and 3.0 mg/kg elicited wet-dog shakes in a dose-dependent manner. The centrally-acting muscarinic cholinergic antagonists, atropine and scopolamine, at doses of 5 and 0.8 mg/kg, respectively, significantly reduced the ability of only the highest dose of MK-771 to elicit wet-dog shakes. When the peripherally-acting antagonists, methylscopolamine and methylatropine, were examined, they were not found to significantly reduce wet-dog shakes produced by MK-771 at doses of 5 and 0.8 mg/kg, respectively. The results of this study suggest that cholinergic antagonists inhibit MK-771-induced wet-dog shakes in a noncompetitive manner and support the view that TRH-mediated wet-dog shakes are modulated by central muscarinic cholinergic systems.

Neuroendocrine mechanisms of stress ulceration: focus on thyrotropin-releasing hormone (TRH)

It is generally accepted that stress ulceration, a multifactorial or pluricausal gastrointestinal disorder, may be the result of mechanistic interrelationships between mucosal, vascular, hormonal and neurogenic factors. The relative importance of each of these independent mechanisms remains unclear. This minireview represents an attempt to interpret many recent studies on certain neurogenic mechanisms and to integrate these observations into the existing body of knowledge. A variety of in vitro techniques and animal models to manipulate actual structures, organ systems, and certain well-defined hormonal influences have been utilized. The peripheral studies have followed, for the most part, the established observation that the stomach is under reciprocal control by sympathetic inhibitory and parasympathetic excitatory autonomic fibers. As a result, several autonomic adrenergic neurotransmitter substances have been found to promote mucosal resistance. Some of these include dopamine, epinephrine, and norepinephrine. Others in contrast, appear to promote vulnerability of the mucosa, and of these, the most well-studied include acetylcholine and histamine.

Lack of interaction between thyrotropin releasing hormone and its analogs with 3H-quinuclidinyl benzilate recognition sites in the rat striatum

The effects of thyrotropin releasing hormone (TRH) on the binding of 3H-quinuclidinyl benzilate (QNB) to cholinergic muscarinic receptors of striatal region of rat brain were evaluated. In vitro studies indicate that neither TRH, its two analogs, MK-771 [L-N-(2-oxopiperidin-6-yl-carbonyl)-L-histidyl-L-thiazolidine-4-++ +carboximide] and DN-1417 (gamma-butyrolactone- gamma-carbonyl-L-histidyl-L-prolineamide) nor the metabolite histidyl-proline diketopiperazine [cyclo(His-Pro)] at concentrations ranging from 10(-9) to 10(-3) M affected 3H-QNB binding to striatal muscarinic receptors. On the other hand, p-Glu-His-Pro-OH (TRH-free acid), another metabolite of TRH caused significant inhibition (21%) at 10(-4) M concentration. Intraperitoneal administration of TRH (1 or 10 mg/kg) also failed to elicit any changes in the affinity (Kd) or density (Bmax) of muscarinic receptors in the striatum. The results suggest that TRH does not influence striatal muscarinic receptors and that the known effects of TRH and its analogs on central cholinergic system may be due to mechanisms other than those affecting postsynaptic muscarinic receptors directly.

Effects of TRH, cyclo-(His-Pro) and (3-Me-His2)TRH on identified septohippocampal neurons in the rat

Neurons located in the medial septum-nucleus of the diagonal band of Broca (vertical limb) and antidromically activated by electrical stimulation of the fimbria were recorded in urethane anesthetized rats. Forty-three percent of these septohippocampal neurons (SHNs) were excited by the iontophoretic application of thyrotropin-releasing hormone (TRH). Rhythmically bursting SHNs were more often excited (63%) by TRH than the non-bursting SHNs (35%). The majority of the TRH-sensitive SHNs could also be excited by cholinergic agonists. TRH-induced excitations were not abolished by the simultaneous application of atropine. Potentiation by TRH of acetylcholine, carbachol or glutamate-induced excitations of SHNs were rarely observed. Cyclo (His-Pro) and (3-Me-His2)-TRH were observed to have similar, although less dramatic, effects. These results demonstrate that the SHNs, which are the neurons of origin of the septohippocampal pathway, are readily excited by TRH.

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

The effect of intracerebral injection of TRH and several biologically stable TRH analogues in the pentobarbitone anaesthetized rat was examined. Bilateral injection of TRH (5.0 micrograms total dose) and the analogues RX 77368 (0.01-1.0 microgram), CG 3509 (0.1-1.0 microgram), DN-1417 (1.0 microgram) and MK-771 (1.0 microgram) into the nucleus accumbens reduced the pentobarbitone-induced sleeping time. The TRH metabolite DKP (5 micrograms) had no effect on the sleeping time following intra-accumbens injection. Intra-septal injection of TRH (1.0-5.0 micrograms), RX 77368 (0.1-1.0 microgram) and CG 3509 (0.1-1.0 microgram) also reversed the pentobarbitone-induced sleeping time. In contrast, TRH (5 micrograms) injected into the striatum had no effect on the pentobarbitone-induced sleeping time, and CG 3509 (0.1 microgram) and RX 77368 (0.1 microgram) had weaker effects following intrastriatal injection compared to injection of these analogues into the nucleus accumbens and septum. Measurements of core temperature and respiration rate in rats following intra-accumbens or septal injection of TRH, CG 3509 and RX 77368 showed these peptides to reverse pentobarbitone-induced hypothermia and stimulate respiration rate. However, while intrastriatal injections of CG 3509 and RX 77368 caused an increase in respiration rate they had no effect on core temperature. These results suggest a close association between peptide-induced respiratory stimulation and reversal of pentobarbitone-induced anaesthesia. Since intra-accumbens and septal injection of dopamine (20-100 micrograms) failed to reverse anaesthesia, it is unlikely that the peptide-induced responses are mediated via dopamine release.

Minireview. Thyrotropin releasing hormone and CNS cholinergic neurons

The centrally mediated pharmacological effects of thyrotropin releasing hormone (TRH), their mechanistic basis and therapeutic implications, along with the possible physiological significance of extrahypothalamic TRH, have been the subject of numerous investigations for over a decade. Despite this effort a holistic perspective on these issues and considerations does not exist. However, with continued research employing multiple and diverse experimental approaches, many interactions of TRH and related peptides with central cholinergic mechanisms have been revealed. These interactions are documented in this review and it is proposed that they can account for several of the more prominent pharmacological actions of these peptides. Additionally, it is suggested that a function of endogenous YHR, throughout the neuroaxis, may be to regulate the excitability of central cholinergic neurons.

Thyrotropin-releasing hormone (TRH) and its analog (DN-1417): interaction with pentobarbital in choline uptake and acetylcholine synthesis of rat brain slices

TRH or its analog DN-1417 (gamma-butyrolactone-gamma-carbonyl-L-histidyl-L-proliamide) given 15 min after intravenous (i.v.) administration of pentobarbital (30 mg/kg) markedly shortened the pentobarbital-induced sleeping time in rats. This effect was almost completely abolished by intracerebroventricular pretreatment with atropine methylbromide (20 micrograms/rat), thereby suggesting the involvement of cholinergic mechanism. The action mechanism was investigated using rat brain slices. TRH (10(-6)-10(-4)M) or DN-1417 (10(-7)-10(-5)M) caused significant increases in the uptake of [3H]-choline into striatal slices. TRH(10(-4)M) or DN-1417(10(-5)M) also stimulated the conversion of [3H]-choline to [3H]-acetylcholine in striatal slices. A 30% reduction of acetylcholine synthesis from [3H]-choline in hippocampal slices and a 40% reduction of [3H]-choline uptake in slices of cerebral cortex, hippocampus and hypothalamus were observed in rats pretreated with pentobarbital (60 mg/kg, i.v.). TRH or DN-1417 (20 mg/kg, i.v.) given 15 min after the administration of pentobarbital markedly reversed both of the pentobarbital effects. Direct application of pentobarbital (5 X 10(-4)M) to slices in vitro also caused a 20-40% reduction of [3H]-choline uptake of cerebral cortex, hippocampus and diencephalon. A concomitant application of TRH(10(-4)M) or DN-1417(10(-5)M) and pentobarbital abolished the pentobarbital effect. These results provide neurochemical evidence that the antagonistic effects of TRH and DN-1417 on pentobarbital-induced narcosis are closely related to alterations in the rat brain choline uptake and acetylcholine synthesis, which are considered to be measures of the activity of cholinergic neurons.

TRH and its novel analog (DN-1417): antipentobarbital action and involvement of cholinergic mechanisms

Possible neuroanatomical loci and the mode of action of thyrotropin-releasing hormone (TRH) or its analog, gamma-butyrolactone-gamma-carbonyl-histidyl-prolinamide citrate (DN-1417), in reducing the pentobarbital-induced sleeping time were investigated by using an intracerebral microinjection technique in rats. Intravenous, intraperitoneal or intracerebroventricular (ICV) injection of TRH or DN-1417 produced a dose-related reduction of the sleeping time induced by pentobarbital. TRH or DN-1417 given into the posterior hypothalamic regions including the dorsal premammillary nucleus, lateral hypothalamic area and posterior nucleus of hypothalamus had a significant pentobarbital sleep shortening action in low doses. Injection of these peptides into the dorsomedial nucleus of thalamus, mesencephalic reticular formation, medial septal nucleus or hippocampus was also effective, in comparatively low doses. However, higher doses were required to elicit the effect when the injections were made into the nucleus accumbens, lateral preoptic area or caudate nucleus. In this respect, the parietal cortex was insensitive to TRH or DN-1417. The pentobarbital sleep shortening action of TRH or DN-1417 injected peripherally or into the hypothalamic regions was markedly antagonized by ICV or intrahypothalamic pretreatment with atropine methyl bromide. On the contrary, ICV injection of atropine methyl bromide had a weak or no antagonizing action on the effect of TRH injected ICV or into the reticular formation, medial septal nucleus or hippocampus. These results suggest that possible neuroanatomical sites mediating the pentobarbital sleep shortening action of TRH or DN-1417 may be posterior hypothalamic regions, dorsomedial nucleus of thalamus, reticular formation, medial septal nucleus or hippocampus. A cholinergic mechanism may also be involved in the effect of TRH on the hypothalamus.