Effects of metergoline on regional serotonin metabolism in the rat brain

Inhibition of chronic hindlimb flexion in rat: evidence for mediation by 5-hydroxytryptamine

Prolonged high-intensity stimulation of the rat hindlimb produces a persistent unilateral flexion. 5-Hydroxytryptamine (5-HT) has been implicated in the modulation of spinal cord mechanisms. Electrical stimulation across the upper hindlimb was used to induce a persistent hindlimb flexion. The flexion was measured after stimulation and at 72 h, both before and after spinal transection at T7. Transection of the spinal cord typically resulted in an increase in flexion of 3-5 g (rebound). Pretreatment with para-chlorophenylalanine (pCPA) to deplete 5-HT, or the administration of metergoline, a non-specific 5-HT antagonist, had no significant effect on flexion at 72 h in the intact rat but abolished rebound. The 5-HT1A agonist, (+-)-8-hydroxy-2-(di-N-propylamino)tetralin hydrobromide (8-OH-DPAT) and 5-HT1B agonist, m-trifluoromethylphenylpiperazine-HCl (TFMPP), had no effect on flexion at 72 h in the intact rat but reduced rebound. The 5-HT2 agonist, 1-(2.5-dimethoxy-4-iodophenyl)-2-aminopropane-HCl (DOI), suppressed post-stimulation flexion and flexion subsequent to spinal section. Furthermore, ketanserin, a 5-HT2 antagonist, restored flexion suppressed by DOI in the acutely spinalized rat. These results suggest that chronic hindlimb flexion is suppressed in the intact rat by descending, serotonergic fibers which exert an effect through spinal 5-HT2 receptors. Moreover, 5-HT1 agonist suppression of rebound implicates these receptors as well in the modulation of chronic hindlimb flexion.

Chronic treatment with iprindole reduces immobility of rats in the behavioural 'despair' test by activating dopaminergic mechanisms in the brain

Iprindole, 10 mg kg-1 i.p., once daily for 21 days, enhanced the metabolism of dopamine in the frontal cortex and striatum of rats with no effect in the nucleus accumbens 1 h after the last injection. Noradrenaline metabolism in the brainstem and telencephalon was also increased in these conditions. No effect on dopamine or noradrenaline metabolism was seen 24 h after the last injection. The same repeated treatment schedule with iprindole markedly reduced the immobility of rats in the behavioural 'despair' test 1 h after the last injection and the effect was prevented by 0.5 mg kg-1 i.p. haloperidol and 100 mg kg-1 i.p. sulpiride but not by 3 mg kg-1 s.c. prazosin or 5 mg kg-1 i.p. (+/-)-propranolol. The data show that enhanced metabolism of brain dopamine and noradrenaline is associated with the presence of iprindole during repeated treatment and the effect on dopamine mechanism is important in iprindole's ability to reduce rats' immobility in the behavioural 'despair' test.

Changes of serotonin and dopamine metabolism in various forebrain areas of rats injected with morphine either systemically or in the raphe nuclei dorsalis and medianus

The regional brain metabolism of serotonin (5-HT) and dopamine (DA) was studied in rats injected with morphine either systemically or in the nuclei raphe medianus (MR) or dorsalis (DR). A subcutaneous injection of 10 mg/kg morphine significantly raised the levels of 5-hydroxyindoleacetic acid (5-HIAA) in the diencephalon, striatum, nucleus accumbens and cortex with no effect in the hippocampus. Similar changes in 5-HT metabolism were found in animals injected with 5 micrograms/0.5 microliter in the DR whereas morphine injected in the MR raised 5-HIAA levels only in the nucleus accumbens. A subcutaneous or direct injection of morphine in the DR significantly raised the levels of homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC) in the striatum and nucleus accumbens, but injection in the MR was ineffective. All the effects of morphine were blocked by naloxone, injected either intraperitoneally (1 mg/kg) or directly in the raphe nuclei (2 micrograms/0.5 microliter). Pretreatment with parachlorophenylalanine, an inhibitor of serotonin synthesis, significantly reduced the effect of morphine injected in the DR on dopamine metabolism in the striatum and nucleus accumbens. The data suggest that a major mechanism by which morphine increases 5-HT metabolism in the rat forebrain is activation of 5-HT cells in the nucleus raphe dorsalis, and this action may contribute to the increased DA metabolism found in the animal injected with morphine in this brain area.

Effect of metergoline on human anxiety

In order to assess the role played by serotonin (5-HT) in subjective anxiety, three groups of 12 healthy volunteers were given 12 mg metergoline (MET), 10 mg diazepam (DZ) or placebo (PB), under double-blind conditions, and submitted to a simulated public speaking (SPS) test. MET increased state-anxiety scores, measured by Spielberg's State-Trait Anxiety Inventory. The effect of MET was significantly different from both the PB and DZ groups immediately before the SPS test (prestress) as well as 24 h after medication, and from the DZ group only, 2.5 h after the test (poststress). In contrast, DZ did not significantly affect subjective anxiety. The SPS test significantly increased anxiety in DZ- or PB-treated subjects as compared to prestress scores, whereas the increases in the MET group were not significant, probably because pretest levels were already high. No drug effect on heart rate, skin electrical conductance and quality of sleep during the night following medication was found. In addition, the drugs did not cause bodily symptoms that could secondarily affect mood. Since MET is a 5-HT receptor antagonist, active on the central nervous system, an inhibitory role of 5-HT on subjective anxiety might be suggested.

Stimulation of serotonin synthesis in rat brain after antiepilepsirine, an antiepileptic piperine derivative

Piperine and two of its derivatives, antiepilepsirine (AE or 3,4-methylendioxycynnamoylpiperine) and compound 7448 (N-isopropyl 3 (4 chloro-phenyl) propenoylamide) are very effective in stimulating serotonin (5HT) synthesis. AE raises the ratio of free-to-bound tryptophan (TP) in plasma and induces a long-lasting increase of this aminoacid in brain. At the same time in striatum and limbic area it causes a lasting increase in 5 hydroxyindolacetic acid (5HIAA) a 5HT metabolite and to a lesser extent, an increase in the levels of the monoamine itself. Together with this action on 5HT metabolism we found that AE caused release of 3H-5HT from an in vitro synaptosomal preparation. It thus appears that piperine and its derivatives AE and compound 7148 affect the central serotonergic system.

Evidence of serotonin involvement in the effect of morphine on dopamine metabolism in the rat nucleus accumbens but not in the striatum

Morphine-induced increase of dopamine metabolism in the striatum and nucleus accumbens was studied in rats treated with parachlorophenylalanine, an inhibitor of serotonin synthesis, or metergoline and mianserin, two serotonin antagonists. Morphine's effect in the nucleus accumbens, but not in the striatum, was significantly reduced by all the agents used to reduce serotonin transmission. It thus appears that part of the effect of morphine on dopamine metabolism in the nucleus accumbens is mediated by its ability to activate 5-HT function in this area.

Effects of agents increasing serotonin transmission on the increase of dopamine metabolism caused by morphine in the rat nucleus accumbens

The effect of subcutaneous injection of 10 mg/kg morphine on homovanillic acid (HVA) concentrations in the nucleus accumbens was studied in rats which had received d-fenfluramine, a serotonin releaser, or m-chlorophenylpiperazine, a serotonin agonist, before morphine. 2.5 mg/kg d-fenfluramine or m-chlorophenylpiperazine had no effect on HVA in the nucleus accumbens but significantly reduced the rise in HVA induced by morphine. None of the drugs modified the levels of morphine in the rat brain. The results suggest that agents increasing serotonin transmission inhibit the effect of morphine on dopamine neurons innervating the nucleus accumbens.

The affinity of metergoline for 3H-serotonin (5HT) binding sites is regulated by guanine nucleotide

100 microM guanine nucleotide Gpp (NH)p reduces the affinity of the serotonergic antagonist metergoline for 3H-5HT binding sites in rat cerebral cortex. This effect is present both in inhibition binding and in saturation experiments. The hypothesis that the interaction of some serotonergic antagonists with 3H-5HT binding sites is regulated by guanine nucleotides is discussed.

The effect of propranolol on CSF amine metabolites in psychiatric patients

In view of the central side effects of beta-adrenoceptor blocking agents and their alleged antipsychotic action in the absence of DA receptor blockade, it is important to establish which neurotransmitters are likely to be involved. Previous animal and patient studies have, however, produced conflicting data on this point. The changes in CSF HVA, 5HIAA and MHPG during treatment with propranolol (960 mg/day) in chronic schizophrenic patients were monitored on four occasions over 30 days. Clinical changes were monitored by the Wing and Krawiecka Scales and motor activity was assessed by pedometer. CSF HVA levels were significantly increased by propranolol and the rise continued throughout the 30 day period. There were no significant changes in the other metabolites. There was no evidence of any therapeutic benefit from propranolol treatment in these patients. These findings may explain the central side-effects and occasional reports of schizophreniform psychosis after propranolol, since an increase in DA turnover occurs without DA receptor blockade.

Role of 5-hydroxytryptamine in amphetamine effects on punished and unpunished behaviour

In order to assess the contribution of serotonergic (5-HT) mechanisms in the suppressant effect of amphetamine on punished responding, dose-effect curves of amphetamine on key-pecking behaviour of pigeons maintained by food presentation and punished by electric-shock were determined before and after pretreatment with methergoline, a potent and specific 5-HT receptor blocker in the central nervous system. A multiple fixed-interval 5 min, fixed-interval 5 min schedule of reinforcement in which every response, except the reinforced one, was punished in one of the two components (mult FI5 FI5-shock) was used. Effective doses of amphetamine decreased unpunished as well as punished FI response rates. However, the decreases in punished behaviour were more evident and dose-dependent. Methergoline markedly increased FI responding in the punished FI component but only slightly increased or decreased unpunished FI response rates. The most effective dose of methergoline for increasing punished responding was 0.56 mg/kg. Pretreatment with this dose of methergoline unmasked the facilitatory effects of amphetamine on unpunished responding, but did not antagonize its suppressant effect on punished responding. Therefore, although 5-HT seems to mediate punishment-induced response suppression and to inhibit the facilitatory effects of amphetamine on unpunished responding, it is not apparently involved in the suppressant effect of amphetamine on punished behaviour.

Is receptor activation involved in the mechanism by which (+)-fenfluramine and (+)-norfenfluramine deplete 5-hydroxytryptamine in the rat brain?

1 The effects of (+)-fenfluramine, (+)-norfenfluramine and reserpine on the concentrations of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in brainstem and telencephalon were studied in rats treated with methergoline, a 5-HT antagonist. 2 Methergoline significantly reduced the effect of (+)-norfenfluramine (5 mg/kg) on 5-HT levels in telencephalon and brainstem but did not modify the effect of (+)-norfenfluramine (2.5 mg/kg). 3 Neither the effect of (+)-fenfluramine on 5-HT levels nor the decrease of 5-HT metabolism caused by (+)-fenfluramine and (+)-norfenfluramine was significantly modified by methergoline treatment. 4 Methergoline potentiated the effects of reserpine on brain indoles. The effects was particularly evident on 5-HIAA levels in the brainstem, although significant effects were found on 5-HT in the brainstem and 5-HIAA in the telencephalon depending on the dose reserpine used. 5 The results show that postsynaptic receptor activation may partially contribute to the depletion of brain 5-HT caused by (+)-norfenfluramine in the rat. This mechanism does not seem to play a significant role in the effect of (+)-fenfluramine.