Monoamines reverse transporter

The pharmacology of amphetamine is considerably more complex. It does not only block monoamine reuptake, but also directly inhibits the vesicular monoamine transporter, causing an increase in cytosolic but not vesicular dopamine concentration. This may lead to reverse transport of the amines via the membrane-bound transporters. Further mechanisms of amphetamine action are direct MAO inhibition and indirect release of both dopamine and serotonin in the striatum. 

A further complexity of the action of Zn2+ at the hDAT was revealed when we investigated the effect of Zn2+ on reverse transport. Notably, the monoamine transporters are... 

The role of zinc ions in reverse transport mediated by monoamine transporters. J. Biol. Chem. 8, 8. 

MDMA overdose as well as the concomitant consumption of selective serotonin reuptake inhibitors (SSRI) with other dmgs that exert serotoninergic effects (such as inhibitors of monoamine oxidase) can rapidly lead to the serotonin syndrome. Its symptoms, which are reversible upon cessation, of the drug include confusion, muscle rigidity in the lower limbs, and hyperthermia suggesting an acute reaction to serotonin overflow in the CNS. Blocking the function of SERT outside the brain causes side effects (e.g., nausea), which may be due to elevated 5HT however , impairment of transporter function is not equivalent to direct activation of 5HT recqrtors in causing adverse effects such as fibrosis and pulmonary hypertension. 

Several other conditions can provoke this reverse pump type of release. One is when the transmembrane ionic gradient is reversed. Experimentally this is achieved by reducing extracellular Na+. Because the neuronal uptake of monoamines from the synapse by the transporter requires co-transport of Na+ and Cl , reversing the ionic gradient (so that the Na+ concentration is lower outside, than inside, the terminals) will drive the transporter in the wrong direction. Such carrier-mediated release could explain the massive Ca +-independent release of noradrenaline during ischaemia which increases intracellular Na+ concentration and reduces intracellular K+. 

The first two antidepressants, iproniazid and imipramine, were developed in the same decade. They were shown to reverse the behavioural and neurochemical effects of reserpine in laboratory rodents, by inhibiting the inactivation of these monoamine transmitters (Leonard, 1985). Iproniazid inhibits MAO (monoamine oxidase), an enzyme located in the presynaptic neuronal terminal which breaks down NA, 5-HT and dopamine into physiologically inactive metabolites. Imipramine inhibits the reuptake of NA and 5-HT from the synaptic cleft by their transporters. Therefore, both of these drugs increase the availability of NA and 5-HT for binding to postsynaptic receptors and, therefore, result in enhanced synaptic transmission. Conversely, lithium, the oldest but still most frequently used mood stabiliser (see below), decreases synaptic NA (and possibly 5-HT) activity, by stimulating their reuptake and reducing the availability of precursor chemicals required in the biosynthesis of second messengers. 

Dopamine depleting agents. Reserpine, a natural alkaloid that blocks vesicular transport of monoamines, depletes stored monoamines, including DA. DA depletion is associated with the emergence of parkinsonism. This effect of reserpine was among the first clues that PD is the result of DA deficiency (see above). Generally, the parkinsonism resulting from reserpine is reversible. 

Mechanism of action of cocaine and amphetamine on synaptic terminal of dopamine (DA) neurons. Left Cocaine inhibits the dopamine transporter (DAT), decreasing DA clearance from the synaptic cleft and causing an increase in extracellular DA concentration. Right Since amphetamine (Amph) is a substrate of the DAT, it competitively inhibits DA transport. In addition, once in the cell, amphetamine interferes with the vesicular monoamine transporter (VMAT) and impedes the filling of synaptic vesicles. As a consequence, vesicles are depleted and cytoplasmic DA increases. This leads to a reversal of DAT direction, strongly increasing nonvesicular release of DA, and further increasing extracellular DA concentrations. 

Cocaine (Fig. 13—3) has two major properties it is both a local anesthetic and an inhibitor of monoamine transporters, especially dopamine (Fig. 13—4). Cocaine s local anesthetic properties are still used in medicine, especially by ear, nose, and throat specialists (otolaryngologists). Freud himself exploited this property of cocaine to help dull the pain of his tongue cancer. He may have also exploited the second property of the drug, which is to produce euphoria, reduce fatigue, and create a sense of mental acuity due to inhibition of dopamine reuptake at the dopamine transporter. Cocaine also has similar but less important actions at the norepinephrine and the serotonin transporters (Fig. 13—3). Cocaine may do more than merely block the transporter—it may actually release dopamine (or norepinephrine or serotonin) by reversing neurotransmitter out of the presynaptic neuron via the monoamine transporters (Fig. 13—4). 

Classical neurotransmitters and monoamines may rarely be secreted by neurons, not by exocytosis, but by transporter reversal. This mechanism involves the transport of neurotransmitters from the cytosol to the extracellular fluid via transporters that normally remove neurotransmitters from the extracellular fluid. This mechanism appears to account for the burst of dopamine released by amphetamines (Fleckenstein et al., 2007), but its physiological occurrence remains unclear. 

Psychostimulants generally act by increasing the level of extracellular monoamine. The mechanisms responsible include inhibition of plasma membrane monoamine transport and, in particular, stimulation of flux reversal, which may occur by... 

The alternating access transport model has been used to describe the mechanism by which substrates are transported across the membrane via the monoamine transporters (Forrest et al., 2008). This model postulates that the transporter can exist in at least two conformations. These conformations include an extracellularly facing form that is open to the extracellular environment and can bind substrate and Na" and CF ions (Forrest et al., 2008). An intracellularly facing form allows the release of substrate into the cell and the binding of the countertransported ion to reverse the conformation of the transporter (Forrest et al., 2008). The alternating access model is supported by recent crystal structures of other transporters (Weyand et al., 2008 Faham et al., 2008). Two additional conformations of these transporters have also been described. A closed-closed conformation is predicted that prevents accessibility of substrate and ions from either side of the transporter. This closed-closed conformation was observed in the crystal structure of a leucine transporter... 

Psychostimulants exert their effects by increasing levels of extracellular neurotransmitter. Psychostimulants are classified as uptake inhibitors or releasers. Cocaine is an example of an uptake inhibitor (Table 2). Cocaine exerts its effects by binding to DAT, NET, or SERT. This binding prevents the transport of neurotransmitter, resulting in increased synaptic neurotransmitter levels. Amphetamines such as MDMA are classified as releasers. They are substrates of the monoamine transporters. Releasers reverse the direction of transport from inward to outward, leading to an increase in the levels of neurotransmitter in the synaptic cleft (Fleckenstein et al., 2007 Rothman and Baumann, 2003). 

The second mechanism is the one more relevant to the action of amphetamine and related agents. This mechanism is illustrated in Fig. 4.1. Amphetamine, and other small molecular weight compounds with similar structures, are substrates at the monoamine uptake carriers and are transported into the neuron. The uptake carrier has an extracellular and intracellular face, and after transporting a substrate (amphetamine, etc.) into the neuron, the intracellular carrier face can bind to dopamine and transport it back to the extracellular face. This exchange diffusion mechanism is calcium independent, and is capable of robustly increasing synaptic transmitter levels. This process is often described as a "reversal" of the normal uptake carrier process. 

Emetine (48) is an isoquinoline derived alkaloid from Cephaelis species (Rubiaceae) [104]. It was first isolated in 1817 [167] and was the lead compound for the development of the drug tetrabenazine (49). Tetrabenazine was originally developed as an antipsychotic drug in the 1960s, but in 2008 was approved by the US Food and Drug Administration for use in Huntington s disease chorea [168, 169]. The mode of action of tetrabenazine is reversible inhibition of the human vesicular monoamine transporter 2 to deplete monoamines, including DA, from nerve terminals [168]. Consequently, adverse effects can include extrapyra-midal dysfunction and parkinsonism [109] (Scheme 42.12). 

One primary neurochemical mode of action of MDMA involves an indirectly mediated release of monoamines via reversed plasma membrane monoamine transport function and disruption of vesicular storage. Increased 5-HT release appears to be the dominating feature and DA, NA and acetylcholine release are also observed to be involved, although to a lesser degree. Additional mechanisms include inhibition of tryptophan hydroxylase and MAO-A, as well as involvement of S-ffTjA receptors. 

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