Carrier-mediated serotonin release induced by d-fenfluramine: studies with human neuroblastoma cells transfected with a rat serotonin transporter

N,N-dimethyltryptamine and the pineal gland: Separating fact from myth

The pineal gland has a romantic history, from pharaonic Egypt, where it was equated with the eye of Horus, through various religious traditions, where it was considered the seat of the soul, the third eye, etc. Recent incarnations of these notions have suggested that N,N-dimethyltryptamine is secreted by the pineal gland at birth, during dreaming, and at near death to produce out of body experiences. Scientific evidence, however, is not consistent with these ideas. The adult pineal gland weighs less than 0.2 g, and its principal function is to produce about 30 µg per day of melatonin, a hormone that regulates circadian rhythm through very high affinity interactions with melatonin receptors. It is clear that very minute concentrations of N,N-dimethyltryptamine have been detected in the brain, but they are not sufficient to produce psychoactive effects. Alternative explanations are presented to explain how stress and near death can produce altered states of consciousness without invoking the intermediacy of N,N-dimethyltryptamine.

The reverse operation of Na(+)/Cl(-)-coupled neurotransmitter transporters--why amphetamines take two to tango

Sodium-chloride coupled neurotransmitter transporters achieve reuptake of their physiological substrate by exploiting the pre-existing sodium-gradient across the cellular membrane. This terminates the action of previously released substrate in the synaptic cleft. However, a change of the transmembrane ionic gradients or specific binding of some psychostimulant drugs to these proteins, like amphetamine and its derivatives, induce reverse operation of neurotransmitter:sodium symporters. This effect eventually leads to an increase in the synaptic concentration of non-exocytotically released neurotransmitters [and - in the case of the norepinephrine transporters, underlies the well-known indirect sympathomimetic activity]. While this action has long been appreciated, the underlying mechanistic details have been surprisingly difficult to understand. Some aspects can be resolved by incorporating insights into the oligomeric nature of transporters, into the nature of the accompanying ion fluxes, and changes in protein kinase activities.

Serotonin-transporter mediated efflux: A pharmacological analysis of amphetamines and non-amphetamines

The physiological function of neurotransmitter transporter proteins like the serotonin transporter (SERT) is reuptake of neurotransmitter that terminates synaptic serotoninergic transmission. SERT can operate in reverse direction and be induced by SERT substrates including 5-HT, tyramine and the positively charged methyl-phenylpyridinium (MPP(+)), as well as the amphetamine derivatives para-chloroamphetamine (pCA) and methylene-dioxy-methamphetamine (MDMA). These substrates also induce inwardly directed sodium currents that are predominantly carried by sodium ions. Efflux via SERT depends on this sodium flux that is believed to be a prerequisite for outward transport. However, in recent studies, it has been suggested that substrates may be distinct in their properties to induce efflux. Therefore, the aim of the present study was a pharmacological characterization of different SERT substrates in uptake experiments, their abilities to induce transporter-mediated efflux and currents. In conclusion, the rank order of affinities in uptake and electrophysiological experiments correlate well, while the potencies of the amphetamine derivatives for the induction of efflux are clearly higher than those of the other substrates. These discrepancies can be only explained by mechanisms that can be induced by amphetamines. Therefore, based on our pharmacological observations, we conclude that amphetamines distinctly differ from non-amphetamine SERT substrates.

Pharmacological and genetic characterization of two selective serotonin transporter ligands: 2-[2-(dimethylaminomethylphenylthio)]-5-fluoromethylphenylamine (AFM) and 3-amino-4-[2-(dimethylaminomethyl-phenylthio)]benzonitrile (DASB)

The expression and function of the serotonin transporter (SERT) is important in the regulation of mood and emotion. Determination of SERT alterations in physiological and pathological states is essential for understanding the role of SERT in mood regulation, and in the etiology and therapy of psychiatric disorders. Two SERT ligands, AFM ([(3)H]2-[2-(dimethylaminomethylphenylthio)]-5-fluoromethylphenylamine) and DASB ([(3)H]3-amino-4-[2-(dimethylaminomethylphenylthio)]benzonitrile), have recently been developed for positron emission tomography (PET) imaging. The aim of the present study was to determine the selectivity of these compounds for SERT. Autoradiography of AFM or DASB binding was compared in the brains of mice with genetically normal, diminished, or absent SERT. In addition, the pharmacodynamic profile of [(3)H]AFM was examined in the mouse brain. The distribution of [(3)H]AFM and [(3)H]DASB binding in the normal brains was consistent with that of previously studied serotonin reuptake inhibitors. Both ligands had negligible binding in the brain of SERT knockout mice, and binding was reduced approximately 50% in heterozygote SERT mice. The K(d) of [(3)H]AFM binding in the cortex and midbrain was 1.6 and 1.0 nM, respectively. Competition studies showed that [(3)H]AFM has very low affinity for norepinephrine and dopamine transporters as well as 5-HT receptors, including 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(2C) receptors. In addition, fenfluramine showed a low capability to compete with [(3)H]AFM. The present results suggest that both AFM and DASB are highly selective SERT ligands potentially suitable for use in human PET studies of SERT.

Synaptically released 5-HT modulates the activity of tonically discharging neuronal populations in the rostral ventral medulla (RVM)

There is substantial evidence for an important modulating role of monoamines (catecholamines and serotonin, 5-HT) in the rostral ventral medulla (RVM), a region which plays an important role in cardiovascular and nociceptive functions. We investigated in slices the role of endogenous monoamines in the synaptic control of the activity of rat RVM neuronal populations using intracellular recordings in the lateral RVM plus lateral aspect of nucleus paragigantocellularis lateralis. A triple-labelling protocol allowed us to identify the location of impaled neurons and their eventual monoaminergic phenotype within the serotonergic and catecholaminergic populations of the RVM. Focal electrical stimulation revealed the existence of a functional monoaminergic input onto RVM neurons which was mediated by endogenous 5-HT acting at inhibitory 5-HT1A receptors but did not involve noradrenergic neurotransmission. The slow 5-HT-mediated inhibitory postsynaptic potential (IPSP) was only observed in the regularly discharging neurons, which were found to be neither catecholaminergic nor serotonergic. The synaptic release of 5-HT was, itself, under an inhibitory control involving GABAA (gamma-aminobutyric acid) receptors. Moreover, we characterized the effect of the 5-HT-releasing agent fenfluramine on this functional 5-HT-mediated synaptic transmission. Our results show that the effect of fenfluramine is biphasic consisting of an initial prolongation of the serotonergic IPSP followed by a decrease in amplitude. Our data provide a basis for the previously reported inhibitory effects of exogenously applied serotonin agonists/antagonists on the autonomic functions controlled by the RVM. This 5-HT pathway, which functionally links the serotonergic and catecholaminergic regions, might play an important role in cardiovascular and nociceptive functions.

Inhibition of plasma membrane monoamine transporters by beta-ketoamphetamines

Methcathinone and methylone, the beta-ketone analogues of methamphetamine and 3,4-methylenedioxymethamphetamine (MDMA), respectively, were tested for neurotransmitter uptake inhibition in vitro. The beta-ketones were threefold less potent than the nonketo drugs at inhibiting platelet serotonin accumulation, with IC(50)'s of 34.6+/-4.8 microM and 5.8+/-0.7 microM, respectively. Methcathinone and methylone were similar in potency to methamphetamine and MDMA at catecholamine transporters individually expressed in transfected glial cells. For dopamine uptake, IC(50)'s were 0.36+/-0.06 microM and 0.82+/-0.17 microM, respectively; for noradrenaline uptake, IC(50) values were 0.51+/-0.10 microM and 1. 2+/-0.1 microM, respectively. In chromaffin granules, IC(50)'s for serotonin accumulation were 112+/-8.0 microM for methcathinone and 166+/-12 microM for methylone, 10-fold higher than the respective values for methamphetamine and MDMA. Our results indicate that methcathinone and methylone potently inhibit plasma membrane catecholamine transporters but only weakly inhibit the vesicle transporter.

Phosphorylation and sequestration of serotonin transporters differentially modulated by psychostimulants

Many psychotropic drugs interfere with the reuptake of dopamine, norepinephrine, and serotonin. Transport capacity is regulated by kinase-linked pathways, particularly those involving protein kinase C (PKC), resulting in transporter phosphorylation and sequestration. Phosphorylation and sequestration of the serotonin transporter (SERT) were substantially impacted by ligand occupancy. Ligands that can permeate the transporter, such as serotonin or the amphetamines, prevented PKC-dependent SERT phosphorylation. Nontransported SERT antagonists such as cocaine and antidepressants were permissive for SERT phosphorylation but blocked serotonin effects. PKC-dependent SERT sequestration was also blocked by serotonin. These findings reveal activity-dependent modulation of neurotransmitter reuptake and identify previously unknown consequences of amphetamine, cocaine, and antidepressant action.

Carrier-dependent and Ca(2+)-dependent 5-HT and dopamine release induced by (+)-amphetamine, 3,4-methylendioxymethamphetamine, p-chloroamphetamine and (+)-fenfluramine

1. The mechanism underlying 5-hydroxytryptamine (5-HT) and/or dopamine release induced by (+)-amphetamine ((+)-Amph), 3,4-methylendioxymethamphetamine (MDMA), p-chloroamphetamine (pCA) and (+)-fenfluramine ((+)-Fen) was investigated in rat brain superfused synaptosomes preloaded with the 3H neurotransmitters. 2. Their rank order of potency for [3H]-5-HT-releasing activity was the same as for inhibition of 5-HT uptake (pCA > or = MDMA > or = (+)-Fen > > (+)-Amph). Similarly, their rank order as [3H]-dopamine releasers and dopamine uptake inhibitors was the same ((+)-Amph > > pCA = MDMA > > (+)-Fen). We also confirmed that the release induced by these compounds was prevented by selective transporter inhibitors (indalpine or nomifensine). 3. [3H]-5HT and/or [3H]-dopamine release induced by all these compounds was partially (31-80%), but significantly Ca(2+)-dependent. Lack of extracellular Ca2+ did not alter uptake mechanisms nor did it modify the carrier-dependent dopamine-induced [3H]-dopamine release. (+)-Amph-induced [3H]-dopamine release and pCA- and MDMA-induced [3H]-5-HT release were significantly inhibited by omega-agatoxin-IVA, a specific blocker of P-type voltage-operated Ca(2+)-channels, similar to the previous results on (+)-Fen-induced [3H]-5-HT release. 4. Methiothepin inhibited the Ca(2+)-dependent component of (+)-Amph-induced [3H]-dopamine release with high potency (70 nM), as previously found with (+)-Fen-induced [3H]-5-HT release. The inhibitory effect of methiothepin was not due to its effects as a transporter inhibitor or Ca(2+)-channel blocker and is unlikely to be due to its antagonist properties on 5-HT1/2, dopamine or any other extracellular receptor. 5. These results indicate that the release induced by these compounds is both 'carrier-mediated' and Ca(2+)-dependent (possibly exocytotic-like), with the specific carrier allowing the amphetamines to enter the synaptosome. The Ca(2+)-dependent release is mediated by Ca(2+)-influx (mainly through P-type Ca(2+)-channels), possibly triggered by the drug interacting with an unknown intracellular target, affected by methiothepin, common to both 5-HT and dopamine synaptosomes.