5-HT transporter

Antidepressants Noradrenaline/5-HT transporters Na+, K+ channels l Noradrenaline/ 5-HT reuptake l Na+ currents t K+ currents l Excitability of peripheral and central neurons Cardiac arrhythmia, myocardial infarction, sedation, nausea, dry mouth, constipation, dizziness, sleep disturbance, blurred vision... 

Antidepressants are used in the treatment of neuropathic pain and headache. They include the classic tricyclic compounds and are divided into nonselective nor-adrenaline/5-HT reuptake inhibitors (e.g., amitriptyline, imipramine, clomipramine, venlafaxine), preferential noradrenaline reuptake inhibitors (e.g., desipramine, nortriptyline) and selective 5-HT reuptake inhibitors (e.g., citalopram, paroxetine, fluoxetine). The reuptake block leads to a stimulation of endogenous monoaminer-gic pain inhibition in the spinal cord and brain. In addition, tricyclics have NMDA receptor antagonist, endogenous opioid enhancing, Na+ channel blocking, and K+ channel opening effects which can suppress peripheral and central sensitization. Block of cardiac ion channels by tricyclics can lead to life-threatening arrhythmias. The selective 5-HT transporter inhibitors have a different side effect profile and are safer in cases of overdose [3]. 

Indeed, 5-HT is also a substrate for the 5-HT transporter, itself an important player in the treatment of depression, and more recently for the whole range of anxiety disorders spectrum (GAD, OCD, social and other phobias, panic and post-traumatic stress disorders). It is the target for SSRIs (selective serotonin reuptake inhibitors) such as fluoxetine, paroxetine, fluvoxamine, and citalopram or the more recent dual reuptake inhibitors (for 5-HT and noradrenaline, also known as SNRIs) such as venlafaxine. Currently, there are efforts to develop triple uptake inhibitors (5-HT, NE, and DA). Further combinations are possible, e.g. SB-649915, a combined 5-HTia, 5-HT1b, 5-HT1d inhibitor/selective serotonin reuptake inhibitor (SSRI), is investigated for the treatment of major depressive disorder. 

Finally, the actions of the so-called 5-HT releasing agent , if-fenfluramine, which is well known for its anorectic effects, should be mentioned here. This compound inhibits 5-HT uptake but its metabolite, if-norfenfluramine, increases 5-HT release as do high doses of (i-amphetamine. It is important to realise that this 5-HT release is independent of nerve impulses and the action of such compounds rests on their effects on the 5-HT transporters on the storage vesicles and terminal membrane. Once these drugs have been taken up into 5-HT neurons by the transporter, they cause 5-HT to leak out of its storage vesicles and, ultimately, to be extruded from the neuron by retrotransport (see below and Chapter 4 for further details). 

As with other monoamines, the actions of 5-HT are terminated by its reuptake from the synapse by another member of the family of Na+/CU-dependent transporters. The 5-HT transporter has many features in common with its catecholamine equivalent (described fully in Chapter 8 see Fig. 8.7), including its presumed 12 transmembrane-spanning domains. However, the cloned 5-HT transporter has a for 5-HT of about 450 nM whereas its K for both noradrenaline and dopamine is some ten thousand-fold greater (Povlock and Amara 1997) which means that it is relatively selective for uptake... 

As might be expected, mRNA for the 5-HT transporter is found in high concentrations in the Raphe nuclei but it is also found in other brain regions. Whether this means that non-5-HT neurons can synthesise this protein is unknown but there is some evidence that it is synthesised in astrocytes, at least. One complication is that there are multiple forms of mRNA for the 5-HT transporter, but there is, as yet, no evidence for transporter subtypes in the CNS. However, it must also be remembered that 5-HT transporters are found in the peripheral tissues, notably platelets, mast cells, the placental brush-border and adrenal chromaffin cells and it is possible that these are not all identical. 

Recent evidence indicates that the 5-HT transporter is subject to post-translational regulatory changes in much the same way as neurotransmitter receptors (Blakeley et al. 1998). Protein kinase A and protein kinase C (PKC), at least, are known to be involved in this process. Phosphorylation of the transporter by PKC reduces the Fmax for 5-HT uptake and leads to sequestration of the transporter into the cell, suggesting that this enzyme has a key role in its intracellular trafficking. Since this phosphorylation is reduced when substrates that are themselves transported across the membrane bind to the transporter (e.g. 5-HT and fi -amphetamine), it seems that the transport of 5-HT is itself linked with the phosphorylation process. Possibly, this process serves as a homeostatic mechanism which ensures that the supply of functional transporters matches the demand for transmitter uptake. By contrast, ligands that are not transported (e.g. cocaine and the selective serotonin reuptake inhibitors (SSRIs)) prevent the inhibition of phosphorylation by transported ligands. Thus, such inhibitors would reduce 5-HT uptake both by their direct inhibition of the transporter and by disinhibition of its phosphorylation (Ramamoorthy and Blakely 1999). 

In platelets, depletion of intracellular Ca + reduces 5-HT transport and this points to calmodulin as another endogenous regulator and its antagonists do inhibit 5-HT uptake. In contrast, activation of adenosine (A3) receptors seems to upregulate the transporter, possibly through the PKG, NO/cGGP pathway. 

A second way of increasing synaptic concentrations of noradrenaline and 5-HT is to block their neuronal reuptake. Several groups of compounds act in this way and can be classified according to their relative selectivity for the noradrenaline and 5-HT transporters. 

It is perhaps not surprising that, even after taking into account pharmacokinetic differences between these drugs, the therapeutic doses of the SSRIs do not parallel their Ki for inhibition of 5-HT reuptake. For instance, citalopram is about a thousand times more selective than fluoxetine for inhibition of 5-HT uptake, and yet their clinically effective doses are similar. In short, not only is their selectivity for the 5-HT transporter in vitro a poor predictor of their efficacy in vivo but it has to be questioned whether any of these compounds actually work by blocking 5-HT uptake alone. 

Figure 20.6 Schematic representation of the effects of 5-HT reuptake inhibitors on serotonergic neurons, (a) 5-HT is released at the somatodendritic level and by proximal segments of serotonergic axons within the Raphe nuclei and taken up by the 5-HT transporter. In these conditions there is little tonic activation of somatodendritic 5-HTia autoreceptors. At nerve terminals 5-HTib receptors control the 5-HT synthesis and release in a local manner, (b) The blockade of the 5-HT transporter at the level of the Raphe nuclei elevates the concentration of extraneuronal 5-HT to an extent that activates somatodendritic autoreceptors (5-HTia). This leads to neuronal hyperpolarisation, reduction of the discharge rate and reduction of 5-HT release by forebrain terminals, (c) The exposure to an enhanced extracellular 5-HT concentration produced by continuous treatment with SSRIs desensitises Raphe 5-HTia autoreceptors. The reduced 5-HTia function enables serotonergic neurons to recover cell firing and terminal release. Under these conditions, the SSRI-induced blockade of the 5-HT transporter in forebrain nerve terminals results in extracellular 5-HT increases larger than those observed after a single treatment with SSRIs. (Figure and legend taken from Hervas et al. 1999 with permission)...

The brain 5-HT transporter (5-HTT) is the principal site of action of many antidepressants. This transporter takes up 5-HT into the presynaptic neuron, thus... 

McGregor, I.S., Clemens, K.J., Van der Plasse, G., Li, K.M., Hunt, G.E., Chen, F., and Lawrence, A.J., Increased anxiety 3 months after brief exposure to MDMA ( Ecstasy ) in rats association with altered 5-HT transporter and receptor density, Neuropsychopharmacology 28(8), 1472-1484, 2003. 

Staley et al. (2001) 21 Smokers 21 nonsmokers P-CIT SPECT No overall binding difference between smokers and nonsmokers f brainstem 5-HT transporters in male smokers... 

Overall, a limited number of experiments on rodents suggest that the upregulation of nicotinic receptors by nicotine treatment is observed in males but not females. On the other hand, nicotine self-administration has the same upregulation effect on both male and female rats. In human smokers, DA and 5-HT transporter availabihty appears to be higher in females than males although the difference is not substantial. 

Over 95% of the body s serotonin (5-HT) is found in the gastrointestinal tract in enterochromaffin cells and neurons. There are 18 known serotonin receptor sub-types, of which 5-HT, 5-HT3, and 5-HT are located in the gut and modulate gut secretion, motility, and sensation (57). The 5-HT in the synaptic spaces stimulates these receptors until it is actively cleared by a 5-HT transporter protein located on the presynaptic neuronal endings. 

Both clinical and experimental studies have shown that a number of transmitter receptors and amine transport processes show circadian changes. It is well established that depression is associated with a disruption of the circadian rhythm as shown by changes in a number of behavioural, autonomic and neuroendocrine aspects. One of the main consequences of effective treatment is a return of the circadian rhythm to normality. For example, it has been shown that the 5-HT uptake into the platelets of depressed patients is largely unchanged between 0600 and 1200 hours, whereas the 5-HT transport in control subjects shows a significant decrease over this period. The normal rhythm in 5-HT transport is only reestablished when the depressed patient responds to treatment. Thus it may be hypothesized that the mode of action of antidepressants is to normalize disrupted circadian rhythms. Only when the circadian rhythm has returned to normal can full clinical recovery be established. 

Additional evidence for a role of 5-HT in the development of neonatal rodent SSC derives from the transient barrel-Hke distribution of 5-HT, 5-HTib, and 5-HT2A receptors, and of the 5-HT transporter (Lebrand et al. 1996 Mansour-Robaey et al. 1998).The transient barrel-Hke 5-HT pattern visualized in layer IV of the SSC of neonatal rodents stems from 5-HT uptake and vesicifiar storage in thalamocortical neurons, transiently expressing at this developmental stage both 5-HT transporter and the vesicular monoamine transporter (VMAT2) despite their later glutamatergic phenotype (Lebrand et al. 1996). 

Fabre V, Rioux A, Lesch KP, Murphy DL, Lanfumey L, Hamon M, Marres MP (2000) Altered expression and coupling of the serotonin 5-HTlA and 5-HTlB receptors in knock-out mice lacking the 5-HT transporter. Eur J Neurosci 12 2299-2310 Finn DA, Rutledge-Gorman MT, Crabbe JC (2003) Genetic animal models of anxiety. Neurogenetics 4 109-135... 

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