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Aims: Besides acting as gastrointestinal prokinetic agents, 5-hydroxytryptamine(4) (5-HT4) receptor agonists can induce positive inotropism in human isolated atrium, but not in ventricles. We pharmacologically evaluated the gastroprokinetic 5-HT4 receptor agonists tegaserod, prucalopride, R199715, cisapride, the cisapride metabolite norcisapride, and the 5-HT3 receptor agonist MKC773 on human isolated myocardial trabeculae, and compared their effects with those induced by 5-HT and 5-methoxytryptamine (5-MeOT). Main methods: Atrial and ventricular trabeculae were paced and changes in contractile force were studied in the absence or presence of the 5-HT4 receptor antagonist GR113808. Partial agonism was assessed using 5-HT4 receptor agonists as antagonists against 5-HT. To test the contribution of L-type calcium channels, the inotropic responses to 5-HT and 5-MeOT were studied in the absence or presence of verapamil. Key findings: Like 5-HT and 5-MeOT, cisapride and tegaserod, but not prucalopride, R19971 and MKC-733, induced concentration-dependent positive inotropic responses on atrial trabeculae, which were abolished by GR113808. The L-type calcium channel blocker verapamil attenuated inotropic responses to 5-HT and 5-MeOT. None of the agonists affected the contraction of left ventricular trabeculae. Concentration response curves to 5-HT were shifted to the right in the presence of prucalopride, cisapride Significance: We conclude that (i) inotropic responses to 5-HT and 5-MeOT seem to depend on L-type calcium channels, (ii) tegaserod and cisapride behave as partial 5-HT4 receptor agonists, while prucalopride, norcisapride and MKC-733 cause no significant effects on human atrial trabeculae, (iii) R199715 seems to behave as a 5-HT4 receptor antagonist. (C) 2012 Elsevier Inc. All rights reserved.

The lipid peroxidation of triglycerides enriched in polyunsaturated fatty acids was investigated by photoemission techniques and the TBARS assay. Butylated hydroxytoluene, 5-OH-tryptophan and N-acetylserotonin inhibited light emission and TBARS formation in a concentration dependent manner. However, it was enhanced in the presence of melatonin and 5-methoxytryptamine and was dependent on its concentration. The total relative luminic units were found to be lower in those systems incubated in the presence of butylated hydroxytoluene, N-acetylserotonin or 5-OH-tryptophan; this decreased proportionally to the concentration of the compound tested. The order of inhibition was 5-OH-tryptophan>N-acetylserotonin>butylated hydroxytoluene with the following IC50 values: 0.65, 6.5 and 9.0 mM respectively. The free-radical scavenging activity of the indole derivatives was also analyzed by the DPPH method, and the results indicate that 5-OH-tryptophan, and N-acetylserotonin exhibited a dose-dependent free-radical scavenging ability at all of the tested concentrations. Thus, at 10 microM concentration a decrease of 84.71% and 73.50% of initial DPPH was observed, compared to 51.00% of BHT. Melatonin and 5-methoxytriptamine decreased the initial concentration of DPPH only 1.85% and 5.0%, respectively. The possible formation of N(1)-acetyl-N(2) formyl-5-methoxykynuramine (AFMK) during lipid peroxidation of triglycerides enriched in PUFAs with cumene hydroperoxide in the presence of melatonin was also analyzed.

We have previously determined that melatonin modulates hippocampal synaptic transmission in a biphasic way: an initial depression was followed by a recovery/amplification phase. Here we describe the influence of two novel melatonin receptor ligands, BMNEP (N-bromoacetyl-2-iodo-5-methoxytryptamine) and TMPEI (N-[2-(2-Trimethylammoniumethyleneoxy-7-methoxy)ethyl]propionamide iodide), on the population spike (PS) and excitatory postsynaptic potentials (EPSP) recorded from mouse hippocampal slices. BMNEP, which specifically alkylates and constitutively activates the MT2 melatonin receptor, mimicked the first phase of melatonin's action by irreversibly depressing both the PS and EPSP. TMPEI, a charged ligand of plasma membrane melatonin receptors, amplified those potentials in a manner similar to the effect of melatonin observed during the second, recovery phase. Melatonin had no influence on the potentials amplified by the action of TMPEI. Our results suggest that the biphasic, receptor-dependent action of melatonin and its analogs modulates the efficiency of the hippocampal glutamergic synapse and is most likely mediated through two different, sequentially occurring mechanisms.

Effects of 5-hydroxytryptamine (5-HT) and related agonists and antagonists on formation of cAMP were determined for enzymatically dissociated ganglia from the myenteric plexus of the guinea-pig small intestine. Formation of cAMP was stimulated by 5-HT in both dose- and time- dependent manners. The stimulatory action of 5-HT was suppressed by the 5-HT 1P antagonist, renzapride, but not by the 5-HT 3 antagonist, tropisetron (formerly ICS 205–930). Neither renzapride nor cisapride increased the levels of cAMP. Levels of cAMP were suppressed by concentrations of tropisetron greater than 1.0 μM. Levels of cAMP were unaffected by the 5-HT 3 agonist, 2-methyl-5-hydroxytryptamine. A putative 5-HT 4 receptor agonist, 5-methoxytryptamine, stimulated formation of cAMP, but to a lesser extent than 5-HT. We conclude that 5-HT acts to stimulate adenylate cyclase and the formation of cAMP in myenteric ganglia. The 5-HT 1P receptor is the most likely subtype involved in 5-HT action on cAMP formation.

The identity of the serotonin (5-HT) receptor(s) that mediate(s) contraction in canine coronary artery and saphenous vein remains controversial. Ring segments of endothelium-denuded coronary artery and helical strips of saphenous vein were suspended in organ chambers for measurement of isometric force. 5-HT, αMe-5-HT and sumatriptan contracted both coronary artery and saphenous vein and the non-selective 5-HT receptor antagonist 1-naphthylpiperazine (100nM) blocked 5-HT-and sumatriptan-induced contraction in both tissues. The agonist rank order potency for contraction (5-HT>sumatriptan>αMe5-HT>5-MeOT>5-MeT) was similar in both tissues and was consistent with that for a 5-HT 1D receptor. Oligonucleotide primers specific for the 5-HT 1D receptor sequence were designed for use in a polymerase chain reaction (PCR). cDNA derived from total RNA or mRNA from canine tissues was used in the PCR. PCR resulted in the amplification of a 632 base pair sequence in both canine coronary artery and saphenous vein; consistent with that expected for the 5-HT 1D receptor. Southern blot analysis, with an oligonucleotide probe internal to the sequence amplified by the PCR primers, confirmed that the sequence amplified by PCR was the 5-HT 1D receptor. Thus, the 5-HT 1D receptor is expressed in canine coronary artery and saphenous vein and taken together with the pharmacological data, supports the possibility that a 5-HT 1D-like receptor mediates contraction in these two tissues.

Recently, a 5-hydroxytryptamine (5-HT) receptor has been described, whose pharmacology was distinct from that of the already known serotonergic receptors, so that it has been called 5-HT 4 . Because the lack of a high affinity radioligand, the identification of this receptor depends entirely on functional pharmacological analysis. Its stimulation leads to an increase in cyclic AMP accumulation in mouse embryo colliculi neurons, in guinea pig hippocampus and in human heart. We studied the effect of two indoleamines, 5-HT and 5-methoxytryptamine (5-MeO-T), and a benzimidazolone derivative, BIMU 8, in stimulating basal adenylyl cyclase activity in human frontal cortex, and characterized the receptor subtype involved. In membranes prepared from this tissue, 5-HT, 5-MeO-T and BIMU 8 dose-dependently stimulated (13–25 %) the basal enzyme activity (220 pmoles cyclic AMP/min/mg protein). 5-MeO-T behaved as a full agonist, BIMU 8 elicited about 60 % of the maximal 5-HT effect. The selective 5-HT 1A agonist 8-OH-DPAT, was devoid of any stimulating activity. ICS 205–930, a low affinity 5-HT 4 receptor antagonist, completely reversed the effect of all three agonists at high concentrations. Therefore, the present data are consistent with the 5-HT-mediated stimulation of adenylyl cyclase in human frontal cortex resulting by the activation of a 5-HT 4 receptor subtype.

Treatment of rats with 5-carboxamidotryptamine (5-CT) or 5-methoxy-tryptamine (5-MeOT) induces a hindlimb scratch response. These compounds have high affinity for 5-HT1A and 5-HT1D receptors. The selective 5-HT1A receptor agonist N,N-dipropyl-5-CT (DP-5-CT) also induced hindlimb scratching while the selective 5-HT1D receptor agonist, sumatriptan, did not. 5-CT-induced hindlimb scratching was inhibited dose-dependently by several 5-HT1A antagonists (BMY 7378, NAN-190, MDL 73005EF and pindobind-5-HT1A) as well as the non-selective 5-HT antagonist, methiothepin. Pretreatment of rats with the serotonin (5-HT) synthesis inhibitor, p-chlorophenylalanine (PCPA) or the 5-HT depleting agent, reserpine, markedly attenuated 5-CT-induced hindlimb scratching. These data suggest that hindlimb scratching induced by 5-HT agonists may not be centrally mediated but rather may be mediated by a neuronal 5-HT1A receptor localized outside the blood-brain barrier.

The ability of 18 different tryptophan derivatives to induce gonadal regression in Syrian hamsters when injected daily at either midday or late evening has been examined. The compounds chosen have either been identified within mammalian pineal glands or are thought to be possible metabolic derivatives of melatonin. Of the compounds tested, only melatonin and 5-methoxytryptamine were found to possess antigonadotropic activity. 5-Methoxytryptamine's potency, however, was 1/10th that of melatonin and, like melatonin, 5-methoxytryptamine was effective when injected in the evening but not when injected during midday. In addition to the general survey of tryptophan derivatives for antigonadal capability when injected, 5-methoxytryptamine, melatonin, and 5-methoxytryptophol were compared relative to their abilities to prevent photo-induced gonadal regression when administered within beeswax implants. Again 5-methoxytryptamine and melatonin, but not 5-methoxytryptophol, were effective with 5-methoxytryptamine's potency being less than that of melatonin. These results indirectly support the contention that melatonin is the pineal product which mediates photoperiodic effects upon the Syrian hamster reproductive system.

Zusammenfassung Methylation of the indole OH of serotonine does not suppress its catatonic action after intracisternal injection in the rat. It appears, on the contrary, that methylation lengthens the time of action of the drug.

Pretreatment of rats with clorgyline, a selective inhibitor of MAO-A, significantly inhibited the in vivo deamination of intraventricularly administered serotonin (5-HT) and 5-methoxytryptamine (5-MT), but not phenylethylamine (PEA). Pretreatment with d, l-deprenyl, a selective inhibitor of MAO-B, significantly inhibited the in vivo deamination of all three substrates. Brain and liver homogenates from rats pretreated with clorgyline showed a decreased ability to deaminate ( in vitro ) 5-MT and 5-HT, but not PEA. Homogenates from animals pretreated with d,l-deprenyl showed a decreased capacity to deaminate PEA, but not 5-MT or 5-HT. Clorgyline, when added to brain and liver homogenates, selectively blocked the deamination of 5-MT and 5-HT, but not PEA, whereas, d,l-deprenyl blocked the deamination of PEA without affecting that of 5-MT or 5-HT. In addition, 5-MT was found to be 100 X more potent than PEA at inhibiting the in vitro deamination of 5-HT. These findings suggest that 5-MT and 5-HT are favored substrates for MAO-A in vitro and in vivo . However, in vivo , significant amounts of 5-MT and 5-HT can also be deaminated by MAO-B.

We have developed an assay for serotonin (5-HT) stimulation of adenylate cyclase activity in membranes from adult guinea pig hippocampus. The response to 5-HT is concentration-dependent, with an EC50 of 0.01 microM, a shallow slope, and mean maximal stimulation of 90% over basal activity. The response to 5-HT is GTP-dependent and additive to the maximal stimulation by histamine. Micromolar concentrations of the known 5-HT receptor agonists, tryptamine and 5-methoxytryptamine, also stimulate cAMP production in this system, and their effect is not additive to that elicited by a maximal concentration of 5-HT. These results are consistent with the hypothesis that the response to 5-HT is elicited through a distinct receptor coupled to adenylate cyclase; the magnitude and the reproducibility of the 5-HT response in this system should make it useful for receptor classification. To examine the effect of prior exposure to endogenous 5-HT on the responsiveness of the system, we assayed 5-HT stimulation of enzyme activity in membranes prepared from animals 25-27 hrs after treatment with a single injection of reserpine (5 mg/kg, i.p.). The mean maximal stimulation of adenylate cyclase by 5-HT was increased to 150% over basal activity with no effect on the EC50 or slope of the 5-HT concentration-response curve. Reserpine pretreatment did not affect basal activity or histamine-stimulated adenylate cyclase activity. These results are discussed in the context of a hypothesis that endogenous 5-HT normally exerts a desensitizing effect on its receptors in situ.

Rat brain cortex slices preincubated with 3H-5-hydroxytryptamine (3H-5-HT) were superfused with physiological salt solution containing paroxetine, an inhibitor of 5-hydroxytryptamine (5-HT) uptake. The effects of various indolethylamines on the electrically evoked tritium overflow (containing 66.3% unmetabolized 3H-5-HT) were investigated (the percentage of unmetabolized 3H-5-HT was not altered by the indolethylamines or metitepin). 6,7-Dihydroxytryptamine (6,7-DHT) did not affect the stimulation-evoked tritium overflow, whereas the latter was inhibited by the other tryptamine derivatives investigated; when the compounds were compared to each other on the basis of their inhibitory potencies the following rank order was obtained: unlabelled 5-HT greater than 5-methoxytryptamine greater than 4-HT greater than 6-HT greater than 5,6-DHT greater than tryptamine greater than 7-HT greater than 5,7-DHT. The inhibitory effects of these compounds were antagonized by metitepin. It is concluded that the indolethylamines inhibit the stimulation-evoked 3H-5-HT release by activating the presynaptic 5-HT autoreceptors on the 5-HT neurones of the rat brain cortex. Similarities may exist between these receptors and the postsynaptic 5-HT1 binding sites of this brain area.

5-HT receptors are present on many types of neurone in the peripheral nervous system (PNS), e.g. sympathetic, parasympathetic, enteric and sensory cells, and mediate complex effects. These include depolarization, cell discharge and facilitation or depression of transmission. If 5-HT receptors can be classified according to the membrane mechanism associated with them, following the system adopted for mollusc neurones, such a classification would have to take into account two kinds of presynaptic and at least four kinds of postsynaptic action. Recent work suggests that a small number of analogues of 5-HT (tryptamine, 5-MOT, LSD) and antagonists (cocaine, methysergide, quipazine) may be useful in differentiating the various kinds of 5-HT receptor in the PNS. It is suggested that no single feature should be relied upon to characterize the receptors; classification might be based on consideration of function, evidence of tachyphylaxis, sensitivity to methysergide, cocaine, etc. On this basis, it is tentatively concluded that there are two kinds of 5-HT receptor mediating excitation in the PNS, neither of which can sensibly be termed an ‘M’ receptor. An interim form of terminology is proposed which makes use of an acronym of the distinctive features. A receptor mediating (E)xcitation, which shows (T)achyphylaxis, is (M)ethysergide (I)nsensitive but is blocked by (C)ocaine might be designated a 5-HT ETMIC receptor, while a second which differs because it is insensitive to cocaine but activated by (F)ive-methoxytryptamine might be designated a 5-HT ETMIF receptor. Amongst receptors mediating (I)nhibition, the best characterized is one mediating decreased transmitter release and activated by (L)SD. The term 5-HT IL receptor is proposed. A second, post-synaptic inhibitory receptor is likely, but has not been adequately characterized at present.