Action of 3-hydroxytyramine and some tryptamine derivatives on spinal neurones

The mysterious trace amines: protean neuromodulators of synaptic transmission in mammalian brain

The trace amines are a structurally related group of amines and their isomers synthesized in mammalian brain and peripheral nervous tissues. They are closely associated metabolically with the dopamine, noradrenaline and serotonin neurotransmitter systems in mammalian brain. Like dopamine, noradrenaline and serotonin the trace amines have been implicated in a vast array of human disorders of affect and cognition. The trace amines are unique as they are present in trace concentrations, exhibit high rates of metabolism and are distributed heterogeneously in mammalian brain. While some are synthesized in their parent amine neurotransmitter systems, there is also evidence to suggest other trace amines may comprise their own independent neurotransmitter systems. A substantial body of evidence suggests that the trace amines may play very significant roles in the coordination of biogenic amine-based synaptic physiology. At high concentrations, they have well-characterized presynaptic "amphetamine-like" effects on catecholamine and indolamine release, reuptake and biosynthesis; at lower concentrations, they possess postsynaptic modulatory effects that potentiate the activity of other neurotransmitters, particularly dopamine and serotonin. The trace amines also possess electrophysiological effects that are in opposition to these neurotransmitters, indicating to some researchers the existence of receptors specific for the trace amines. While binding sites or receptors for a few of the trace amines have been advanced, the absence of cloned receptor protein has impeded significant development of their detailed mechanistic roles in the coordination of catecholamine and indolamine synaptic physiology. The recent discovery and characterization of a family of mammalian G protein-coupled receptors responsive to trace amines such as beta-phenylethylamine, tyramine, and octopamine, including socially ingested psychotropic drugs such as amphetamine, 3,4-methylenedioxymethamphetamine, N,N-dimethyltryptamine, and lysergic acid diethylamide, have revitalized the field of scientific studies investigating trace amine synaptic physiology, and its association with major human disorders of affect and cognition.

The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord

Over the past 40 years, much has been learned about the role of serotonin in spinal cord reflex modulation and locomotor pattern generation. This review presents an historical overview and current perspective of this literature. The primary focus is on the mammalian nervous system. However, where relevant, major insights provided by lower vertebrate models are presented. Recent studies suggest that serotonin-sensitive locomotor network components are distributed throughout the spinal cord and the supralumbar regions are of particular importance. In addition, different serotonin receptor subtypes appear to have different rostrocaudal distributions within the locomotor network. It is speculated that serotonin may influence pattern generation at the cellular level through modulation of plateau properties, an interplay with N-methyl-D-aspartate receptor actions, and afterhyperpolarization regulation. This review also summarizes the origin and maturation of bulbospinal serotonergic projections, serotonin receptor distribution in the spinal cord, the complex actions of serotonin on segmental neurons and reflex pathways, the potential role of serotonergic systems in promoting spinal cord maturation, and evidence suggesting serotonin may influence functional recovery after spinal cord injury.

Actions of certain amines on cerebral cortical neurones

A number of derivatives of tryptamine and phenethylamine, and certain other compounds, were tested on neurones in the cerebral cortex of cats by iontophoretic release from micro-pipettes. The characteristic action of many of these compounds was a depression of the neuronal discharge initiated by synaptic activity or by the application of L-glutamate; imidazolylacetic acid, dopamine, ephedrine and ergometrine were particularly effective. Catechol amines, hydroxytryptamines and imidazolylacetic acid had a relatively quick and rapidly reversible action, not unlike that of gamma-aminobutyric acid, whereas ephedrine and derivatives of lysergic acid diethylamide caused a slower and more prolonged depression of the amplitude of spikes, rather like atropine. Several compounds, including 5-hydroxytryptamine, adrenaline and ergometrine, could also excite the same neurone when larger amounts were applied. A few substances, such as dopa and methylergometrine, had a predominantly excitant action.

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: 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.

A pharmacological study of the adrenergic mechanisms involved in the stretch reflex of the decerebrate rat

1 The effects of catecholamine precursors, enzyme inhibitors, and monoamine depletors have been studied on the stretch reflex in the decerebrate rat.2L-DOPA alone had a biphasic effect, facilitation followed by inhibition and recovery of the reflex.3 About 10% of the preparations did not develop decerebrate rigidity. These preparations did not have a stretch reflex. Within 1 to 5 min of the injection of L-DOPA in these inactive preparations, a stretch reflex could be elicited.4 Control experiments indicate that L-DOPA was acting centrally after decarboxylation.5DL-dihydroxyphenylserine had an inhibitory effect, without a preceding facilitation.6 When L-DOPA was preceded by diethyldithiocarbamate (i.v.) or FLA-63 (i.p.), the predominant response was a sustained facilitation, without an accompanying inhibition.7 It is concluded that in the decerebrate rat, increased stretch reflex activity is associated with increased central levels of dopamine, and decreased stretch reflex activity with increased central levels of noradrenaline. The results are discussed with reference to the possibility that dopamine acts by liberating 5-hydroxytryptamine.

Modification of the responses of brain stem neurones to transmitter substances by anaesthetic agents

1. The effects of microiontophoretic applications of acetylcholine (ACh), (-)-noradrenaline ((-)-NA) and 5-hydroxytryptamine (5-HT) have been investigated on spontaneously active brain stem neurones in decerebrate unanaesthetized rats and in rats anaesthetized with either tribromoethanol, urethane or pentobarbitone.2. Four types of responses to both (-)-NA and 5-HT were seen. These were: simple excitation; excitation preceded by a short-lasting inhibition; short-lasting inhibition and prolonged inhibition. Three types of responses to ACh were seen: an excitation with long latency of onset; excitation with short latency of onset, resembling the response to an excitant amino acid, and a short-lasting inhibitory response.3. The types of responses to microiontophoretically applied ACh, (-)-NA or 5-HT in anaesthetized and unanaesthetized animals were similar.4. The number of ACh excitatory responses with short latency of onset were significantly reduced in the pentobarbitone-anaesthetized group and a small but significant increase in the number of 5-HT inhibitory effects were observed in each anaesthetized group of animals.5. A significantly greater proportion of slower firing neurones (less than 10 spikes/s) were found in the pentobarbitone-anaesthetized animals.6. The effects of microiontophoretically applied and i.v. administered pentobarbitone were studied on spontaneously active neurones which responded consistently to ACh and a control agonist.7. Pentobarbitone administered by either route reduced the firing rate of most neurones studied and was shown to antagonize specifically the excitation of neurones by exogenously applied ACh.8. It is suggested that postsynaptic antagonism of endogenously released ACh may be a contributing factor in the mechanism of action of pentobarbitone.

Excitation and depression of cortical neurones by 5-hydroxytryptamine

1. 5-Hydroxytryptamine (5-HT) and various 5-HT antagonists have been applied micro-electrophoretically from multibarrelled micropipettes into the environment of single neurones in the post-sigmoid and suprasylvian gyri of the cat cerebral cortex.2. In unanaesthetized animals (encéphale isolé) a high proportion of neurones (30%) were excited by 5-HT. This excitation usually had a rapid onset and was seen both in spontaneously active neurones and in otherwise quiescent neurones in which firing was induced by L-glutamate. Some neurones were so sensitive that the uncontrolled diffusion from micropipettes was sufficient to excite them. More cells were excited by 5-HT applied as a cation from solutions of the bimaleate salt than when solutions of the creatinine sulphate salt were used.3. In a high proportion of cells (33%) spontaneous firing or amino acid excitation was depressed by 5-HT.4. A mixed effect was seen in a small proportion (6%) of the cells tested; usually 5-HT caused an excitation initially which was followed by a depression. In other cells, desensitization occurred, and the excitatory effect of 5-HT was diminished or lost.5. When glutamate was used to excite otherwise quiescent cells, there was a significant increase in the number of cells excited by 5-HT and a significant decrease in the number of cells unaffected compared with spontaneously active cells.6. The micro-electrophoretic application of D-lysergic diethylamide (LSD 25), 2-brom LSD (BOL 148), methysergide (UML 491), or 2'- (3-dimethylaminopropylthio)cinnamanilide (SQ 10643) temporarily prevented excitation by 5-HT in half the cells tested. LSD and SQ 10643 were particularly potent in this respect. This antagonism of 5-HT excitation could still be seen when excitation of the cell by L-glutamate or acetylcholine (ACh) was unaffected.7. The depression induced by 5-HT was not prevented by the application of known 5-HT antagonists in the majority of the cells tested (93%). In two cells, however, the depression was reversibly prevented by these antagonists.8. Some cells tested with 5-HT were also tested with ACh or (-)-noradrenaline. The response of a cell to ACh was not significantly related to its response to 5-HT. The degree of correlation between the responses to noradrenaline and 5-HT was large, but not statistically significant with the small number of cells studied.9. The effects of 5-HT on cells in animals anaesthetized with alpha-chloralose did not differ significantly from its effects in unanaesthetized preparations. It is suggested that the use of this anaesthetic may prove a useful alternative to unanaesthetized preparations.10. The systemic injection of small quantities of thiopentone sodium selectively and reversibly reduced the sensitivity of some units to excitation by 5-HT at a time when the response to glutamate was unaffected. On other occasions, the 5-HT excitation was unaffected, though the response to glutamate was reduced.11. These results are discussed in relation to the possible nature of the 5-HT receptors in the cerebral cortex, and the interfering effects of anaesthesia on the response of brain cells to potential transmitter substances.