Thyrotropin releasing hormone: neurochemical evidence for the potentiation of imipramine effects on the metabolism and uptake of brain catecholamines

Studies on thyrotropin-releasing hormone-induced micturition in cats

In unanaesthetized cats micturition produced by thyrotropin-releasing hormone (TRH) was investigated after its injection into the cerebral ventricles through chronically implanted cannulae. TRH in doses from 0.1 to 1.0 mg evoked dose-dependent micturition. In cats treated with intracerebroventricular (i.c.v.) reserpine and 6-hydroxydopamine, but not with i.c.v. 5,6-dihydroxytryptamine and hemicholinium, the micturition caused by i.c.v. TRH was abolished. Chlorpromazine and antazoline injected into the cerebral ventricles prevented the micturition induced by i.c.v. TRH. On the other hand, mecamylamine, yohimbine, propranolol, atropine and methysergide injected i.c.v. had virtually no effect or partially antagonized the micturition evoked by TRH similarly injected. It is apparent therefore that centrally induced TRH micturition could be related to central catecholaminergic mechanisms.

Effects of acute and long-term treatments with thyrotropin-releasing hormone on locomotor activity and jumping behavior in mice

The acute and chronic effects of thyrotropin-releasing hormone (TRH) on ambulation and, in combination with apomorphine, on jumping behavior were investigated in mice. A single administration of TRH (1-10 mg/kg SC) produced an initial hyperactivity in a dose-dependent manner. Following administration of TRH (1-10 mg/kg SC) for 21 successive days, the stimulatory effect on locomotion progressively increased. Haloperidol exerted a biphasic action on hyperlocomotion induced by acute and repeated TRH, i.e., stimulation at lower doses (0.01-0.02 mg/kg SC) and inhibition at higher doses (0.05-1 mg/kg SC). When TRH was administered in combination with low doses of apomorphine, locomotor activity was inhibited but jumping behavior occurred. The inhibitory effect of low doses of apomorphine on locomotion was shifted from doses of 0.1-0.25 mg/kg SC of apomorphine for acute TRH (10 mg/kg) to 0.25-0.35 mg/kg for repeated TRH (10 mg/kg), whereas the stimulatory effect of higher doses of apomorphine (0.5-1 mg/kg SC) on locomotion tended to decrease with repeated TRH. Jumping behavior induced by the combined treatment of TRH and apomorphine was proportional to the dose of TRH but exhibited an inverted-U response to the dose of apomorphine. Chronic TRH (10 mg/kg) in combination with apomorphine (0.1-1 mg/kg SC) also produced jumping behavior, but the dose-response curve for apomorphine was shifted to the right. The present results suggest that repeated treatment with TRH in mice produces hyperlocomotion, despite attenuation of both pre- and postsynaptic receptor activity, and that the inhibitory effect of repeated TRH on presynaptic receptors may be more potent than that on postsynaptic receptors.

Potentiation by TRH of the effect of imipramine on the forced-swimming test

Discovery of the potentiation of thyrotropin releasing hormone (TRH)-induced hyperthermia in mice by antidepressants which activate alpha-adrenergic systems instigated investigation of other relations between TRH and antidepressants. For this study the forced-swimming test using mice was chosen since this test is more sensitive for selection of antidepressants which modify catecholaminergic systems than for those affecting 5-hydroxytryptaminergic systems. The effects of imipramine were potentiated by TRH. The involvement of alpha-adrenergic systems was then investigated in this effect since it is already known that these systems are directly implicated in the potentiation of TRH-induced hyperthermia by some antidepressants. Then the involvement of opiate systems was investigated since endogenous opiates are implicated in the action of some antidepressants, and some interactions between TRH and opiate systems are known to exist. TRH made effective a completely inactive dose of imipramine as small as 2 mg kg-1 (i.p.) or 1 microgram per mouse (i.c.v.). Pretreatment by both alpha 1- and alpha 2-adrenoceptor antagonists (phenoxybenzamine, 8 mg kg-1 i.p.; phentolamine, 4 mg kg-1 i.p.) or by a alpha 1-adrenoceptor antagonist (prazosin, 2 mg kg-1 i.p.) did not prevent this potentiation. In contrast the alpha 2-adrenoceptor antagonist (Yohimbine, 2 mg kg-1 i.p.) blocked the TRH effect. The imipramine potentiation by TRH was blocked by pretreatment with an opiate antagonist (naloxone, 1 mg kg-1 i.p.) and the potentiation was decreased in morphine-tolerant mice. 5 These data indicate that potentiation of the effects of imipramine on the forced-swimming test does not seem to be associated with an increase of effective levels of noradrenaline in the synaptic clefts and suggest an interaction between TRH and the opiate systems.

Behavioral effects of thyrotropin releasing hormone in frontal decorticated rats

A low dose intracerebroventricular injection of thyrotropin releasing hormone (TRH, 100 ng) changed many behavioral responses in the rat. TRH increased locomotion, scratching, body shaking, piloerection, and rearing, but decreased sniffing, and resting. Ablation of frontal neocortex further enhanced the TRH effects on locomotion and resting. A dose effect of TRH (0, 5, 10, 50, 100 ng) to increase general activity was established and the effect was further enhanced by decortication. In our test situations decortication had no effect by itself. Since the TRH effects became much more pronounced without the frontal neocortex it appears that the cortex exerts a powerful inhibitory effect to moderate the TRH effects. The TRH effect does not depend upon the frontal cortex, actually a cortical function is to dampen the TRH effects on various behavioral responses.

Effects of bombesin, vasoactive intestinal peptide and neurotensin on TRH-induced body shaking in rats

Bombesin, vasoactive intestinal peptide (VIP) and neurotensin were found to suppress body shaking behavior caused by intracerebroventricular injection of TRH.

The spectrum of actions of thyrotropin-releasing hormone on the CNS: a polygraphic study on rabbits

The effects of i.c.v. administered thyrotropin-releasing hormone (TRH) on EEG, rectal temperature and behaviour were studied on rabbits in basal conditions and after different pretreatments. The effects of TRH on the EEG and behaviour do not seem to involve serotoninergic, catecholaminergic and cholinergic systems. A possible involvement of the GABAergic system is hypothesized.

Minireview. Peptides and the blood-brain barrier

Most neuropeptides are known to occur both in the central nervous system and in blood. This, as well as the occurrence of central nervous peptide effects after peripheral administration, show the importance of studying the relationships between the peptides in the two compartments. For many peptides, such as the enkephalins, TRH, somatostatin and MIF-1, poor penetration of the blood-brain barrier was shown. In other cases, including beta-endorphin and angiotensin, peptides are rapidly degraded during or just after their entry into brain or cerebrospinal fluid. Some peptides, such as insulin, delta-sleep-inducing peptide, and the lipotropin-derived peptides, enter the cerebrospinal fluid to a slight or moderate extent in the intact form. Many peptide hormones, such as insulin, calcitonin and angiotensin, act directly on receptors in the circumventricular organs, where the blood-brain barrier is absent. Oxytocin, vasopressin, MSH, and an MSH-analog alter the properties of the blood-brain barrier, which may result in altered nutritient supply to the brain. In conclusion, the diffusion of most peptides across the brain vascular endothelium seems to be severely restricted. There are, however, several alternative routes for peripheral peptides to act on the central nervous system. The blood-brain barrier is a major obstacle for the development of pharmaceutically useful peptides, as in the case of synthetic enkephalin-analogs.

Neuropharmacological evaluation of RX 77368--a stabilised analogue of thyrotropin-releasing hormone (TRH)

L-Pyroglutamyl-L-histidyl-L-2,3-dimethylprolineamide (Pyr-His-Dmp . NH2; RX 77368) a stabilised analogue of thyrotropin-releasing hormone (TRH) has been examined for neuropharmacological effects in animal tests. The compound was more potent than either TRH or clinically established drugs in four animal tests of antidepressant potential (reserpine reversal, clonidine antagonism, tremorine reversal and learned immobility). RX 77368 also antagonised barbiturate sleeping time. Given by itself to rats the peptide produced arousal as characterised by EEG and EMG measurements and delayed the onset of sleep. The arousal induced was not accompanied by increases in locomotor activity. The profile of pharmacological activity for RX 77368 did not correspond to the profiles of tricyclic antidepressants, psychic-stimulants or analeptic drugs. The possible clinical uses for such a molecule are discussed.

Jumping behavior induced by thyrotropin releasing hormone in combination with apomorphine in mice

Neuronal mechanisms involved in the jumping produced by thyrotropin releasing hormone (TRH) plus apomorphine were studied in mice. TRH (5--20 mg/kg IP) administered after apomorphine (0.25--2.0 mg/kg IP) elicited jumping behavior, the frequency of jumping being proportional to the dose of TRH but inversely related to the dose of apomorphine. Jumping behavior was also induced by TRH (20 mg/kg IP) administered after atropine (5 mg/kg IP) or clonidine (0.5 mg/kg IP). The jumping elicited by TRH (20 mg/kg IP) in combination with apomorphine (0.25 mg/kg IP) was decreased by pretreatment with haloperidol (1 mg/kg IP), physostigmine (0.2 mg/kg IP) or phentolamine (10 mg/kg IP), unaffected by propranolol (10 mg/kg IP), and markedly increased by atropine (5 mg/kg IP) or clonidine (0.5 mg/kg IP). The results suggest that TRH in combination with apomorphine, atropine or clonidine elicits jumping in which dopaminergic, cholinergic and noradrenergic functions are concomitantly involved.

Thyrotropin releasing hormone potentiates the effects of imipramine on brain serotonergic system

Daily administration of thyrotropin-releasing hormone (TRH) (20 mg/kg) for 10 days in 2 equally divided doses increased 5-hydroxytryptamine and/or 5-hydroxyindoleacetic acid levels in certain brain areas. Chronic imipramine treatment (10 mg/kg) for 10 days significantly decreased the levels of brain 5-hydroxytryptamine and 5-hydroxyindoleacetic acid. When TRH was administered concurrently with imipramine (10 mg/kg), this neuropeptide significantly potentiated the effects of imipramine on lowering of endogenous levels of 5-hydroxytryptamine (striatum, hypothalamus) and 5-hydroxyindoleacetic acid (striatum, midbrain, pons-medulla). Our data provide a possible neurochemical basis for the reported potentiation of tricyclic antidepressant action by TRH.