5-HT, inhibitors

Both histamine and 5-HT have been demonstrated in extracts 5-HT is responsible for some of the bronchoconstrictor activity of cotton dust extracts. Brom-lysergic acid, a specific 5-HT inhibitor, partially reduces activity, suggesting the presence of an "unknown contractor." This does not appear to be acetylcholine or bradykinin. Thus, although histamine, 5-HT, and the "unknown contractor" can cause immediate contractor responses, delayed onset contractions may still be due to secondary release of histamine (105). 

Gastrointestinal motility promotors Antiemetics H.2 antagonists Phenothiazines 5-HT inhibitors... 

Beginning in the 1960s, ben2odia2epiae anxiolytics and hypnotics rapidly became the standard prescription dmg treatment. In the 1980s, buspkone [36505-84-7] (3), which acts as a partial agonist at the serotonin [50-67-9] (5-hydroxytryptamine, 5-HT) type lA receptor, was approved as treatment for generali2ed anxiety. More recently, selective serotonin reuptake inhibitors (SSRIs) have been approved for therapy of panic disorder and obsessive—compulsive behavior. 

Modulation of second-messenger pathways is also an attractive target upon which to base novel antidepressants. Rolipram [61413-54-5] an antidepressant in the preregistration phase, enhances the effects of noradrenaline though selective inhibition of central phosphodiesterase, an enzyme which degrades cycHc adenosiae monophosphate (cAMP). Modulation of the phosphatidyl iaositol second-messenger system coupled to, for example, 5-HT,, 5-HT,3, or 5-HT2( receptors might also lead to novel antidepressants, as well as to alternatives to lithium for treatment of mania. Novel compounds such as inhibitors of A-adenosyl-methionine or central catechol-0-methyltransferase also warrant attention. 

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. 

Sharp, T, Umbers, V and Gartside, SE (1997) Effect of a selective 5-HT reuptake inhibitor in combination with 5-HTlA and 5-HTlB receptor antagonists on extracellular 5-HT in rat frontal cortex in vivo. Br. J. Pharmacol. 121 941-946. 

Monoamine oxidase exists in two forms, MAOa and MAOb. The former is more active against NA and 5-HT than it is against DA, which is a substrate for both, even though, like S-phenylethylamine, it is more affected by MAOb. H seems likely that MAOb is the dominant enzyme in human brain and inhibitors of it, such as selegiline, have some value in the treatment of Parkinson s disease by prolonging the action of the remaining endogenous DA as well as that formed from administered levodopa. 

So far, it has been established from in vitro studies that the enzyme undergoes phosphorylation, a process that changes the conformation of the enzyme protein and leads to an increase in its activity. This involves Ca +/calmodulin-dependent protein kinase II and cAMP-dependent protein kinase which suggests a role for both intracellular Ca + and enzyme phosphorylation in the activation of tryptophan hydroxylase. Indeed, enzyme purified from brain tissue innervated by rostrally projecting 5-HT neurons, that have been stimulated previously in vivo, has a higher activity than that derived from unstimulated tissue but this increase rests on the presence of Ca + in the incubation medium. Also, when incubated under conditions which are appropriate for phosphorylation, the of tryptophan hydroxylase for its co-factor and substrate is reduced whereas its Fmax is increased unless the enzyme is purified from neurons that have been stimulated in vivo, suggesting that the neuronal depolarisation in vivo has already caused phosphorylation of the enzyme. This is supported by evidence that the enzyme activation caused by neuronal depolarisation is blocked by a Ca +/calmodulin protein kinase inhibitor. However, whereas depolarisation... 

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). 

It has been known for many years that aversive stimuli increase serotonergic transmission (reviewed by Chaouloff 1993) and so it was inevitable that exaggerated serotonergic transmission in a hypothetical brain punishment system became linked with anxiety. Unfortunately, much of the evidence for this idea was gleaned from unreliable measures of changes in 5-HT concentration in rodent brain tissue postmortem after experience of moderately severe forms of stress in vivo. Nevertheless, this concept was encouraged by reports that a reduction in 5-HT transmission, following administration of either the 5-HT synthesis inhibitor, p-chloroamphetamine (pCPA),... 

Figure 20.1 Schematic diagram illustrating how antidepressants increase the concentration of extraneuronal neurotransmitter (noradrenaline and/or 5-HT). In the absence of drug (b), monoamine oxidase on the outer membrane of mitochondria metabolises cytoplasmic neurotransmitter and limits its concentration. Also, transmitter released by exocytosis is sequestered from the extracellular space by the membrane-bound transporters which limit the concentration of extraneuronal transmitter. In the presence of a MAO inhibitor (a), the concentration of cytoplasmic transmitter increases, causing a secondary increase in the vesicular pool of transmitter (illustrated by the increase in the size of the vesicle core). As a consequence, exocytotic release of transmitter is increased. Blocking the inhibitory presynaptic autoreceptors would also increase transmitter release, as shown by the absence of this receptor in the figure. In the presence of a neuronal reuptake inhibitor (c), the membrane-bound transporter is inactivated and the clearance of transmitter from the synapse is diminished...

The discovery that MAO has two isoenzymes with different distributions, substrate specificity and inhibitor sensitivity has helped to rehabilitate the MAOIs to some extent. These isoenzymes are the products of different genes on the X-chromosome and share about 70% sequence homology. Whereas noradrenaline and 5-HT are metabolised preferentially by MAOa, tyramine and dopamine can be metabolised by either isoenzyme. Selective inhibitors of MAOa (e-g- moclobemide Da Prada et al. 1989) should therefore be safe and effective antidepressants whereas the selective MAOb inhibitor, selegiline, should not have any appreciable antidepressant activity (Table 20.5). 

All TCAs are either secondary- or tertiary-amines of a dibenzazepine nucleus (Fig. 20.3), and they all inhibit neuronal reuptake of noradrenaline and/or 5-HT but are much less potent as dopamine reuptake blockers. A common claim is that secondary amines (e.g. desipramine) are preferential inhibitors of noradrenaline uptake whereas the tertiary derivatives (e.g. imipramine, doxepin and amitryptyline) preferentially inhibit 5-HT uptake. However, when Richelson and Pfenning (1984) actually compared the effects of a wide range of antidepressants on the synaptosomal uptake of [ H]monoamines in vitro, and compared their A s, instead of merely ranking /C50S collected from different studies, they found that tertiary- and secondary-substituted compounds were equi-potent inhibitors of [ H]noradrenaline uptake. Moreover, all the TCAs turned out to be more potent inhibitors of [ H]noradrenaline than of [ H]5-HT uptake. Tertiary amines are even less convincing inhibitors of 5-HT reuptake in vivo, because any such action is diminished by their metabolism to secondary amines (e.g. imipramine to desipramine amitriptyline to nortriptyline). Only clomipramine retains any appreciable 5-HT uptake blocking activity in vivo with (an unimpressive) five-fold selectivity for 5-HT versus noradrenaline. 

Paroxetine is the most potent inhibitor of 5-HT reuptake but, in terms of distinguishing one compound from another, their preferential selectivity for inhibition of 5-HT rather than noradrenaline reuptake is the key criterion. Citalopram is by far the most selective in vitro (1500-3000-fold) and fluoxetine, the most frequently prescribed SSRI in the UK, is the least selective of all these agents (see Stanford 1999). In fact, it is worth questioning whether fluoxetine is a true SSRI at all. 

One of these compounds, venlafaxine (licensed in the UK in 1996), is regarded as an inhibitor of both 5-HT and noradrenaline reuptake but this is based on its actions in vitro. At low doses in vivo, it is a more potent inhibitor of 5-HT (Ki 39 nM) than noradrenaline reuptake (K 210 nM). Moreover, its active metabolite, O-demethylven-lafaxine, is a weaker inhibitor of NA reuptake, and has a longer half-life, than its parent compound. However, at high doses, venlafaxine inhibits reuptake of both these monoamines but has negligible activity at muscarinic, Hi-receptors or ai-adrenoceptors and... 

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 prototype amphetamine enhances release and bloeks reuptake of DA, norepinephrine (ME), and 5-HT and is also a monoamine oxidase inhibitor. As a result of these effeets, drugs in this class are potent indireet agonists at monoaminergie reeeptors. In experimental animals, amphetamine... 

The above conclusion is supported by the results shown in figure 4. Just as inhibitors of the 5-HT uptake carrier can antagonize MDMA-induced [ H]5-HT release in vitro, coadministration of MDMA with an uptake inhibitor such as citalopram can completely block the acute depletion of 5-HT. Although citalopram also antagonized the MDMA-induced decrease in TPH activity, there was still a significant loss of enzyme activity when compared to control. This implies that if MDMA requires access to the interior of the nerve terminals to affect TPH activity, it does not require the activity of the uptake carrier to gain entrance. Hence, these results are consistent with the outcome of synaptosomal uptake experiments with [ HJMDMA (Schmidt et al. 1987), which show that MDMA is not actively concentrated by a carrier system. Furthermore, it is apparent that the loss of enzyme activity alone is not sufficient to reduce 5-HT concentrations, but that release via the carrier must occur simultaneously, to deplete the terminal once synthetic capacity is reduced. 

A final experiment demonstrating the distinetion between the acute and neurotoxic effects of MDMA is shown in figure 12. In this case, the 5-HT uptake inhibitor fluoxetine was administered at various times after MDMA, with all animals being saerifieed 1 week later. The results are shown as a pereentage of eontrol eortieal 5-HT eoneentrations. Simultaneous administration of an uptake inhibitor with MDMA completely blocked the decrease in 5-HT concentrations measured 1 week later. However, administration of the inhibitor 3 hours after MDMA still resulted in complete protection from the neurotoxicity. Approximately 50 percent of the depletion could still be blocked 6 hours after MDMA by 12 hours, the administration of fluoxetine no longer had any effect. Blockade of the neurotoxicity by an uptake inhibitor 3 hours after MDMA clearly differentiates the acute and long-term effects of MDMA, since at this point the acute depletion of 5-HT is already at a maximum. The administration of fluoxetine to MDMA-treated animals... 

FIGURE 12. Timecourse for the antagonism of MDMA-induced neurotoxicity by the 5-HT uptake inhibitor fluoxetine... 

Ross, S.B. Ogren, S.O. and Renyi, L. Antagonism of the acute and longterm biochemical effects of 4-chloroamphetamine on the 5-HT neurons in rat brain by inhibitors of the 5-hydroxytryptamine uptake. Acta Pharmacol Toxicol [Copenh] 39 456-476, 1976. 

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