Serotonin, but not N-methyltryptamines, activates the serotonin 2A receptor via a ß-arrestin2/Src/Akt signaling complex in vivo.

Hallucinogens mediate many of their psychoactive effects by activating serotonin 2A receptors (5-HT(2A)R). Although serotonin is the cognate endogenous neurotransmitter and is not considered hallucinogenic, metabolites of serotonin also have high affinity at 5-HT(2A)R and can induce hallucinations in humans. Here we report that serotonin differs from the psychoactive N-methyltryptamines by its ability to engage a ß-arrestin2-mediated signaling cascade in the frontal cortex. Serotonin and 5-hydroxy-L-tryptophan (5-HTP) induce a head-twitch response in wild-type (WT) mice that is a behavioral proxy for 5-HT(2A)R activation. The response in ß-arrestin2 knock-out (ßarr2-KO) mice is greatly attenuated until the doses are elevated, at which point, ßarr2-KO mice display a head-twitch response that can exceed that of WT mice. Direct administration of N-methyltryptamines also produces a greater response in ßarr2-KO mice. Moreover, the inhibition of N-methyltransferase blocks 5-HTP-induced head twitches in ßarr2-KO mice, indicating that N-methyltryptamines, rather than serotonin, primarily mediate this response. Biochemical studies demonstrate that serotonin stimulates Akt phosphorylation in the frontal cortex and in primary cortical neurons through the activation of a ß-arrestin2/phosphoinositide 3-kinase/Src/Akt cascade, whereas N-methyltryptamines do not. Furthermore, disruption of any of the components of this cascade prevents 5-HTP-induced, but not N-methyltryptamine-induced, head twitches. We propose that there is a bifurcation of 5-HT(2A)R signaling that is neurotransmitter and ß-arrestin2 dependent. This demonstration of agonist-directed 5-HT(2A)R signaling in vivo may significantly impact drug discovery efforts for the treatment of disorders wherein hallucinations are part of the etiology, such as schizophrenia, or manifest as side effects of treatment, such as depression.

Melatonin receptor-mediated protection against myocardial ischaemia/reperfusion injury: role of its anti-adrenergic actions.

Melatonin has potent cardioprotective properties. These actions have been attributed to its free radical scavenging and anti-oxidant actions, but may also be receptor mediated. Melatonin also exerts powerful anti-adrenergic actions based on its effects on contractility of isolated papillary muscles. The aims of this study were to determine whether melatonin also has anti-adrenergic effects on the isolated perfused rat heart, to determine the mechanism thereof and to establish whether these actions contribute to protection of the heart during ischaemia/reperfusion. The results showed that melatonin (50 microM) caused a significant reduction in both isoproterenol (10(-7) M) and forskolin (10(-6) M) induced cAMP production and that both these responses were melatonin receptor dependent, since the blocker, luzindole (5 x 10(-6) M) abolished this effect. Nitric oxide (NO), as well as guanylyl cyclase are involved, as L-NAME (50 microM), an NO synthase inhibitor and ODQ (20 microM), a guanylyl cyclase inhibitor, significantly counteracted the effects of melatonin. Protein kinase C (PKC), as indicated by the use of the inhibitor bisindolylmaleimide (50 microM), also play a role in melatonin's anti-adrenergic actions. These actions of melatonin are involved in its cardioprotection: simultaneous administration of L-NAME or ODQ with melatonin, before and after 35 min regional ischaemia, completely abolished its cardioprotection. PKC, on the other hand, had no effect on the melatonin-induced reduction in infarct size. Cardioprotection by melatonin was associated with a significant activation of PKB/Akt and attenuated activation of the pro-apoptotic kinase, p38MAPK during early reperfusion. In summary, the results show that melatonin-induced cardioprotection may be receptor dependent, and that its anti-adrenergic actions, mediated by NOS and guanylyl cyclase activation, are important contributors.

Tryptamine induces tryptophanyl-tRNA synthetase-mediated neurodegeneration with neurofibrillary tangles in human cell and mouse models.

The neuropathological hallmarks of Alzheimer's disease (AD) and other taupathies include neurofibrillary tangles and plaques. Despite the fact that only 2-10% of AD cases are associated with genetic mutations, no nontransgenic or metabolic models have been generated to date. The findings of tryptophanyl-tRNA synthetase (TrpRS) in plaques of the AD brain were reported recently by the authors. Here it is shown that expression of cytoplasmic-TrpRS is inversely correlated with neurofibrillary degeneration, whereas a nonionic detergent-insoluble presumably aggregated TrpRS is simultaneously accumulated in human cells treated by tryptamine, a metabolic tryptophan analog that acts as a competitive inhibitor of TrpRS. TrpRSN- terminal peptide self-assembles in double-helical fibrils in vitro. Herein, tryptamine causes neuropathy characterized by motor and behavioral deficits, hippocampal neuronal loss, neurofibrillary tangles, amyloidosis, and glucose decrease in mice. Tryptamine induced the formation of helical fibrillary tangles in both hippocampal neurons and glia. Taken together with the authors' previous findings of tryptamine-induced nephrotoxicity and filamentous tangle formation in kidney cells, the authors' data indicates a general role of tryptamine in cell degeneration and loss. It is concluded that tryptamine as a component of a normal diet can induce neurodegeneration at the concentrations, which might be consumed along with food. Tryptophan-dependent tRNAtrp aminoacylation catalyzed by TrpRS can be inhibited by its substrate tryptophan at physiological concentrations was demonstrated. These findings indicate that the dietary supplementation with tryptophan as a tryptamine competitor may not counteract the deleterious influence of tryptamine. The pivotal role of TrpRS in protecting against neurodegeneration is suggested, providing an insight into the pathogenesis and a possible treatment of neurodegenerative diseases.

BGC20-761, a novel tryptamine analog, enhances memory consolidation and reverses scopolamine-induced memory deficit in social and visuospatial memory tasks through a 5-HT6 receptor-mediated mechanism.

Inhibition of 5-HT(6) receptors has been shown to improve memory consolidation, thus we tested whether a novel tryptamine analog with high affinity for 5-HT(6) receptors, BGC20-761 (5-methoxy-2-phenyl-N,N-dimethyltryptamine, PMDT), can enhance long-term memory. BGC20-761 (10 mg/kg i.p.) alone had no effect on social recognition in young rats, however, at doses of 5 mg/kg and 10 mg/kg i.p, BGC20-761 dose-dependently reversed a deficit of social recognition induced by scopolamine (0.4 mg/kg i.p.), an anticholinergic drug that impairs memory. BGC20-761 (10 mg/kg i.p.), scopolamine (0.2 mg/kg i.p.) or BGC20-761 + scopolamine had no effects on novel object discrimination in young rats (2 months). In mature rats (6 months), recognition of the novel object was improved following administration of BGC20-761. Scopolamine had no effect in object recognition. However, the addition of scopolamine disrupted the memory-enhancing effect of BGC20-761. Based on the high affinity of BGC20-761 for 5-HT(6) receptors, these cognitive enhancing effects are most likely mediated by 5-HT(6) receptor inhibition. The difference in effects of BGC20-761 in young vs. mature rats may reflect the status of memory consolidation in these different age ranges.

Products of tryptophan catabolism induce Ca2+ release and modulate the cell cycle of Plasmodium falciparum malaria parasites.

Intraerythrocytic malaria parasites develop in a highly synchronous manner. We have previously shown that the host hormone melatonin regulates the circadian rhythm of the rodent malaria parasite, Plasmodium chabaudi, through a Ca2+-based mechanism. Here we show that melatonin and other molecules derived from tryptophan, i.e. N-acetylserotonin, serotonin and tryptamine, also modulate the cell cycle of human malaria parasite P. falciparum by inducing an increase in cytosolic free Ca2+. This occurs independently of the extracellular Ca2+ concentration, indicating that these molecules induce Ca2+ mobilization from intracellular stores in the trophozoite. This in turn leads to an increase in the proportion of schizonts. The effects of the indolamines in increasing cytosolic free Ca2+ and modulating the parasite cell cycle are both abrogated by an antagonist of the melatonin receptor, luzindole, and by the phospholipase inhibitor, U73122.

Pharmacological studies with endogenous enhancer substances: beta-phenylethylamine, tryptamine, and their synthetic derivatives.

The discovery of enhancer regulation in the mesencephalon and the concept that it plays a key role in the operation of innate and acquired drives [Neurochem. Res. 28 (2003) 1187] sets the trace amines (TAs) in their true physiological perspective. The regulation is defined as the existence of enhancer-sensitive neurons in the brain capable of working in a split-second on a high activity level due to endogenous enhancer substances. For the time being, only beta-phenylethylamine (PEA) and tryptamine are the experimentally analyzed examples. (-)-Deprenyl (selegiline), widely used in Parkinson's disease and Alzheimer's disease today, and known as the first selective monoamine oxidase (MAO) type-B inhibitor for decades, was identified as a PEA-derived synthetic mesencephalic enhancer substance. An important and convincing confirmation of the enhancer concept was the recent development of a highly specific and potent tryptamine-derived synthetic mesencephalic enhancer substance, (-)-1-(benzofuran-2-yl)-2-propylaminopentane [(-)-BPAP]. This substance, which is specific and hundreds of times more potent than selegiline, is now the best experimental tool to study the enhancer regulation in the mesencephalon and a promising candidate to significantly surpass the therapeutic efficiency of selegiline in depression, Parkinson's disease, and Alzheimer's disease.

Brain tryptophan/serotonin perturbations in metabolic encephalopathy and the hazards involved in the use of psychoactive drugs.

Several combined pathogenetic factors such as hyperammonemia, different brain tryptophan metabolic disturbances and serotonin physiological/pharmacological alterations not yet defined in all details, will often give rise to the clinical neuropsychiatric condition known as hepatic encephalopathy (HE). Indeed, to this the probable exposure to novel potent CNS-monoamine acting drugs today may put such patients at certain risk for other pharmacodynamic (PD) responses than usually are expected from these "safe" drugs. Moreover, with a compromised liver function in HE, also pharmacokinetic (PK) features for the drugs are likely changed in these patients. Thus, the ultimate clinical outcome by this probable but unknown PD/PK-deviation for such psychoactive drugs when given to HE-patients needs further clarification. Accordingly, delineation of both PD- and PK-effects in experimental HE should shed light on this issue of relevance for monoamine-active drug safety as well as on some further details in the complex tryptophan/monoamine-related pathophysiology that comes into play in HE.

Tryptamine: a metabolite of tryptophan implicated in various neuropsychiatric disorders.

Although early interest in the biomedical relevance of tryptamine has waned in recent years, it is clear from the above discussion that the study of tryptamine is worthy of serious consideration as a factor in neuropsychiatric disorders. The study of [3H]-tryptamine binding sites indicates an adaptive responsiveness characteristic of functional receptors. The question raised by Jones (1982d) on whether tryptamine is acting centrally as a neurotransmitter or a neuromodulator still remains mostly unanswered, although the evidence cited within this review strongly suggests a modulatory role for this neuroactive amine (see also Juorio and Paterson, 1990). The synthesis and degradative pathways of tryptamine, as well as the intricate neurochemical and behavioral consequences of altering these pathways, are now more fully understood. It is not yet clear what the role of tryptamine is under normal physiological [homeostatic] conditions, however, its role during pathological conditions such as mental and physical stress, hepatic dysfunction and other disorders of metabolism (i.e. electrolyte imbalance, increased precursor availability, enzyme induction or alterations in enzyme co-factor availability) may be quite subtle, perhaps accounting for various sequelae hitherto considered idiopathic. The evidence for a primary role for tryptamine in the etiology of mental or neurological diseases is still relatively poor, although the observations that endogenous concentrations of tryptamine are particularly susceptible to pharmacological as well as physiological manipulations serve to reinforce the proposition that this indoleamine is not simply a metabolic accident but rather a neuroactive compound in its own right. Finally, one might wonder what proportion of the data attributed to modifications of 5-HT metabolism might, in fact, involve unrecognized changes in the concentrations of other neuroactive metabolites of tryptophan such as tryptamine.

[Tryptamine as an endogenous modulator of neuronal sensitivity to serotonin]. / Triptamin kak éndogennyi moduliator chuvstvitel'nosti neironov k serotoninu.

Tryptamine has been studied for its effect on the 5-hydroxytryptamine-induced responses of the dorsal root ganglion neurons in rat with intracellular registration of the membrane potential and conductance and application of drugs from micropipettes under pressure. It was found that tryptamine applied in high concentrations acted like 5-hydroxytryptamine; but in the concentration range when it has no effect on the membrane potential and membrane conductance it either enhanced (10(-7) mol/l) or diminished (10(-5) mol/l) 5-hydroxytryptamine responses mediated by 5-HT1A- but not by 5-HT2-receptors. Harmane acted like tryptamine, but its derivatives either only enhanced or only inhibited the 5-hydroxytryptamine effects. The allosterical nature of 5-hydroxytryptamine-modulating action of tryptamine, harmane and its derivatives is discussed.

Evidence for tryptaminergic and noradrenergic involvement in the antisecretory action of morphine in the rat jejunum.

Experiments have been performed to determine whether the antisecretory (antidiarrhoeal) effect of morphine in the intestine is mediated by a direct action of morphine on enteric nerves. Rats were pretreated with 6-hydroxydopamine (6-OHDA) or p-chlorophenylalanine (PCPA) to deplete intestinal stores of noradrenaline and 5-hydroxytryptamine (5-HT). Intraperitoneal injection of 6-OHDA (3 doses at 50 mg kg-1) caused a selective reduction in the level of noradrenaline in the jejunum to 7.3% of control. Intraperitoneal injection of PCPA (200 mg kg-1) selectively reduced the jejunal level of 5-HT to 30.5% of control. Groups of rats that had been treated as described above were anaesthetized and then injected intravenously with saline or with blocking doses of either atropine (0.25 mg kg-1), hexamethonium (20 mg kg-1), ketanserin (30 micrograms kg-1), methysergide (30 micrograms kg-1), phentolamine (2 mg kg-1) or propranolol (1 mg kg-1). Following perfusion of the lumen of the jejunum, the rate of glucose absorption was measured to assess the integrity of the mucosa. Glucose absorption was unaltered in animals pretreated with hexamethonium and propranolol but there was a small enhancement in animals pretreated with atropine, PCPA, methysergide, 6-OHDA and phentolamine. The rate of net water absorption from the lumen of the jejunum and the rate of fluid secretion into the lumen following intra-arterial infusion of vasoactive intestinal peptide (VIP, 0.8 microgram min-1) were unaltered by any of the drug treatments. Intravenous injection of morphine (10 mg kg-1) did not alter the levels of noradrenaline or 5-HT in the whole jejunum. However, this dose of morphine did cause a 63.5% decrease in the VIP-induced change in water transport. This antisecretory effect of morphine was unaltered in animals pretreated with atropine, hexamethonium and propranolol. In contrast, methysergide, ketanserin and 6-OHDA abolished the antisecretory effect of morphine. PCPA and phentolamine produced a partial inhibition of morphine's antisecretory effect. It is concluded that morphine produces its antisecretory effect in the jejunum by activation of noradrenergic and tryptaminergic systems.

Relationship of CNS tryptaminergic processes and the action of LSD-like hallucinogens.

Tryptamine produces pharmacologic effects in man and the chronic spinal dog which are similar to those produced by LSD, mescaline, psilocin, DMT, DOM and DOB. These effects include tachycardia, tachypnea, mydriasis, hyperreflexia, behavioral changes and in man, hallucinations. Chronic spinal dogs treated chronically with LSD became tolerant to its ability to produce mydriasis, tachycardia, tachypnea and hyperreflexia, and were cross tolerant to the ability of tryptamine, psilocin, mescaline, DMT, DOM and DOB to produce these same effects. Further, it was found that the brain and spinal cord contained tryptamine and could release it. Further tryptamine levels were higher in the brainstem and spinal cord above the level of transection in the chronic spinal dog that in intact dogs, and the same in the spinal cord below the level of transection. These observations suggested that there were both ascending and descending tryptaminergic pathways. Supporting this hypothesis were the observations that L-tryptophan also produced hyperreflexia in the acute, but not the chronic, spinal dog and cat, and that L-tryptophan hyperreflexia was antagonized by alpha-methyldopa but not pCPA. These observations and others argue that the spinal cord and brain have tryptaminergic mechanisms which are distinct from serotoninergic mechanisms, and that LSD-like hallucinogens act in part through a tryptaminergic mechanism.

A role for an indoleamine other than 5-hydroxytryptamine in the hypothalamic thermoregulatory pathways of the rat.

1. Intrahypothalamic injection of either 5-hydroxytryptamine (5-HT) (20 mug) or tryptamine (1 mug) caused hypothermia and hyperthermia respectively in lightly restrained rats maintained at an ambient temperature of 20 +/- 1 degrees C.2. Both the 5-HT- and the tryptamine-sensitive sites were located within the same region of the preoptic area.3. When rats were tested at different ambient temperatures (4, 20 and 29 degrees C), intrahypothalamic injection of 5-HT caused a marked fall in core temperature (-1.3 degrees C) in rats maintained at 4 degrees C, but smaller responses were obtained at 20 and 29 degrees C (-0.9 and -0.5 degrees C respectively). Tryptamine caused a significant hyperthermia in rats kept at 20 degrees C, but had no significant effect in rats maintained at either 4 or 29 degrees C.4. The hypothermic effect of 5-HT was selectively antagonized by systemic pre-treatment with cyproheptadine (2.5 mg/kg), but not by methergoline (0.625 mg/kg) and methysergide (0.2 mg/kg). In contrast, the hyperthermic effect of tryptamine was blocked by methergoline and methysergide, but not by cyproheptadine.5. Cyproheptadine (2.5 mg/kg) reduced the ability of rats to cope with a heat load but had no effect on the response to cold. In contrast, methergoline (0.625 mg/kg) and methysergide (0.2 mg/kg) reduced the ability to cope with cold but the rats' ability to cope with a heat load remained intact.6. These results suggest the existence of two indoleamine pathways within the preoptic anterior hypothalamus involved in the control of body temperature: a serotonergic pathway mediating heat loss and a non-serotonergic pathway mediating heat gain. The non-serotonergic system may exert its effects by modulating the activity of a central serotonergic system.

Further studies on the role of indoleamines in the responses of cortical neurones to stimulation of nucleus raphe medianus: effects of indoleamine precursor loading.

Responses of cortical neurones to stimulation of nucleus raphe medianus (RM) were determined before and after parenteral administration of l-tryptophan or l-5-hydroxytryptophan (5HTP). The short latency inhibitory effects of stimulation and the longer latency excitation were enhanced by l-tryptophan. 5-Hydroxytryptophan, on the other hand, enhanced the excitatory effects but had no effects on the initial inhibition. Both precursors were able to induce a third type of response, a long-latency inhibition which succeeded the excitation. Inhibition of tryptophan hydroxylase abolished the facilitatory effects of tryptophan on the excitatory and long-latency inhibitory effects of stimulation of the raphé medianus but the enhancement of the short-latency inhibition remained intact. Inhibition of tryptophan hydroxylase failed to alter any of the effects of 5HTP on evoked responses to stimulation of the raphé medianus. Finally, inhibition of aromatic amino acid decarboxylase abolished the effects of both indoleamine precursors on all response phases. The results are consistent with a previous suggestion that the short-latency inhibition may be tryptamine-mediated while the other response phases are mediated by 5-hydroxytryptamine.

Nerve endings and pharmacological receptors in cerebral vessels.

1. The cerebral vessels possess adrenergic, cholinergic, serotonergic and peptidergic innervations. 2. The cerebral vessels have alpha- and beta-adrenergic, cholinergic, serotonergic, histaminergic H1 and H2 and dopaminergic receptors whose activation by different agents causes vasomotor responses. 3. The induced effects by noradrenaline, adrenaline and serotonin are characterized by a cerebral vasoconstriction clearly manifested in man, the awake or anesthetized animal and isolated vessels. The vasoconstrictor response generally obeys an activation of specific receptors. 4. Under experimental conditions acetylcholine, histamine, dopamine and isoproterenol relax these vessels, in which cholinergic, H2-histaminergic, dopaminergic and beta-adrenergic (beta 1) receptors are implicated, respectively.

Tryptamine: a neuromodulator or neurotransmitter in mammalian brain?

Tryptamine synthesized by decarboxylation of L-tryptophan occurs as an endogenous constituent of mammalian brain albeit at very low concentrations (low ng/g range). It is primarily metabolized by oxidative deamination by MAO and possesses an extremely rapid turnover and half-life. Subcellular localization appears to be in nerve terminals and it is releasable by electrical or potassium evoked depolarization. Neuropharmacological and electrophysiological data strongly suggest the existence of post-synaptic receptors for tryptamine independent of those for 5HT. There may exist a rostrally projecting neuronal tryptamine containing system arising from cell bodies in or close to the nucleus raphé medianus. The demonstration of specific receptors for tryptamine in the CNS strongly indicates a transmitter role, although a strong case can be made for a role as a modifier of central 5HT systems. The possibility also exists that 5HT and tryptamine may be mediators of functionally opposite neuronal pathways. Whatever the role of tryptamine in the CNS it is clear that it not simply present as an accident of metabolism or a "biological artefact." The indications are that it possesses important functions in central neurotransmission.