5-HTia

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. 

The GABAB-receptors, the muscarinic M2- and IVU-receptors for acetylcholine, the dopamine D2-, D3-and D4-receptors, the a2-adrenoceptors for noradrenaline, the 5-HTiA F-receptors for serotonin, and the opioid p-, 8- and K-receptors couple to G proteins of the Gi/o family and thereby lower [1] the cytoplasmic level of the second messenger cyclic AMP and [2] the open probability ofN- andP/Q-type Ca2+ channels (Table 1). The muscarinic Mr, M3- and M5-receptors for acetylcholine and the ai-adrenoceptors for noradrenaline couple to G proteins of the Gq/11 family and thereby increase the cytoplasmic levels of the second messengers inositol trisphosphate and diacylglycerol (Table 1). The dopamine Dr and D5-receptors and the (3-adrenoceptors for noradrenaline, finally, couple to Gs and thereby increase the cytoplasmic level of cyclic AMP. 

The apparent reliance of enzyme activation on phosphorylation and intracellular Ca + gives a clue as to how the rate of 5-HT synthesis might be coupled to its impulse-evoked release. Certainly, the impulse-induced increase in intracellular Ca +, and/or activation of the G protein-coupled receptors that govern synthesis of cAMP, could modify the activity of tryptophan hydroxylase. Indeed, this could explain why activation of either somal 5-HTia autoreceptors in the Raphe nuclei (which depress the firing rate of 5-HT neurons) or terminal 5-HTib autoreceptors (which depress 5-HT release) can reduce the production of cAMP and attenuate 5-HT synthesis. 

Impulse-evoked release of 5-HT, like that of noradrenaline, is subject to fine control by a system of autoreceptors, in particular 5-HTia receptors on the cell bodies of neurons in the Raphe nuclei and 5-HTib/id receptors on their terminals. Because these are all G /o protein-coupled receptors, their activation reduces the synthesis of cAMP so that 5-HTia agonists (or 5-HT itself) decrease neuronal excitability and the firing of Raphe neurons whereas activation of 5-HTib/id receptors seems to disrupt the molecular cascade that links the receptor with transmitter release (see Chapter 4). 

The 5-HT3 receptor is found appropriately in mesocortical areas and while behavioural studies with their antagonists in rodents showed potential antipsychotic activity, they have proved ineffective in patients. 5-HTia agonists may be more useful. They have been found to increase the extracellular concentration of DA in the frontal cortex of rats but diminish apomorphine-induced stereotypy (striatal effect). So they could be of some benefit, especially against negative symptoms, without causing EPSPs (see Chapter 9). 

Beckett, S and Marsden, CA (1997) The effect of central and systemic injection of the 5-HTia receptor agonist 8-OHDPAT and the 5-HTia receptor antagonist WAY100635 on peri-aquaductal grey-induced defence behaviour. J. Psychopharmacol. 11 35 0. 

Cao, BJ and Rodgers, RJ (1997) Influence of 5-HTia receptor antagonism on plus-maze behaviour in mice. II WAY 100635, SDZ 216-525 and NAN-190. Pharmacol. Biochem. Behav. 58 593-603. 

Done, CJ and Sharp, T (1994) Biochemical evidence for the regulation of central noradrenergic activity by 5-HTia and 5-HT2 receptors Microdialysis studies in the awake and anaesthetised rat. Neuropharmacology 33 411-421. 

Dourish, CT, Hutson, PH and Curzon, G (1986) Putative anxiolytics 8-OHDPAT buspirone and TVXQ 7821 are agonists at 5-HTia autoreceptors in the raphe nuclei. Trends Pharmacol. Sci. 1 212-214. 

Przegalinski, E, Chojnacka-Wojcik, E and Filip, M (1992) Stimulation of postsynaptic 5-HTia receptors is responsible for the anticonflict effect of ipsapirone in rats. J. Pharm. Pharmac. 44 780-782. 

Because the SSRIs are derived from different chemical groups, their receptor interactions vary from compound to compound but, apart from paroxetine, none of them shows any appreciable binding to muscarinic receptors, a prime objective of their development. However, compared with other SSRIs, fluoxetine binds with moderately high affinity to human 5-HT2A (.K) 280 nM) and 5-HT2C receptors (Aij 55 nM) sertraline is a relatively potent ligand for ai-adrenoceptors, 2-adrenoceptors and Dj receptors and citalopram shows appreciable binding to 5-HTia, oc]-adrenoceptors and Hi receptors (Table 20.6 Stanford 1996). The extent to which any of these receptor interactions affects the efficacy of these compounds is not known. 

HTia (rat) zimel sert fluox > flu vox cital > parox fluox > cital > fluvox > sert... 

There is a good deal of evidence that the therapeutic effects of antidepressants could involve adaptive changes in 5-HTia receptors. Postsynaptic 5-HTia receptor responses became implicated because the hyperpolarisation of hippocampal CA3 pyramidal neurons that follows ionophoretic administration of 5-HT was found to be increased after chronic treatment with most (but not all) antidepressants (Chaput, de Montigny and Blier 1991). Others suggested that antidepressants attenuate postsynaptic 5-HTja responses because the hypothermia, evoked by their activation, is diminished by antidepressants (Martin et al. 1992). 

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)...

More importantly for this discussion is the finding that chronic administration of an antidepressant produces a similar increase in the concentration of extracellular 5-HT in the terminal field together with recovery of neuronal firing. Presumably this is because the prolonged elevation of extracellular 5-HT around the neurons in the Raphe causes progressive desensitisation of the somatodendritic 5-HTia receptors. At this point, inhibition of their firing does not occur and so more 5-HT is released in the cortex (see Hervas et al. 1999). 

A related strategy would be to inactivate the 5-HTib/id autoreceptors which are found on serotonergic nerve terminals and so prevent feedback inhibition of 5-HT release in the terminal field. These drugs would not prevent the impact of indirect activation of 5-HTia receptors, and the reduced neuronal firing, by SSRIs (described above), but they would augment 5-HT release in the terminal field once the presynaptic 5-HTia receptors have desensitised. Selective 5-HTib/id antagonists have been developed only recently but will doubtless soon be tested in humans. 

Arylpiperazines have immensely important effects on various and diverse biological targets, in particular on CNS receptors. In the case of serotonin (5-HT) receptors, compounds containing this arylpiperazine moiety represent the largest systematically studied class of 5-HTia receptor ligands [63]. Structural alterations within long-chain arylpiperazines (LCAPs) occur mainly at the two opposite ends of a molecule and have been described by many authors [64-71]. 

Table 4 SAR data with molecular structures of 5-HTia receptor ligands reported by Palu-chowska et al. (2002) [63]...

Scheme 2 The steps involved in the synthesis of the 5-HTia receptor ligands reported hy Paluchowska et al. (2002) [63], 1 PhsP, NBS, EtsN, CH2CI2, 0-25 °C 11 BOP, EtsN, CH3CN, rt 111 20% K2CO3 - CHCI3, rt IV xylene, reflux V pyridine, reflux... 

While all the achievable conformations of the constrained compounds belong to an extended family—as indicated by molecular modehng studies— the hypothesis that such conformations are responsible for the blockade of postsynaptic 5-HTia receptors has been confirmed by Paluchowska et al. [63]. 

Although initial studies indicated that the various derivatives of 2,5-DMA exhibited low affinity for 5-HTi serotonin receptors (Shannon et al. 1984). it was unclear from these studies what the affinities of the drugs were for the respective subtypes of 5-HTjserotonin sites (i.e., 5-HTia, 5-HTjb, and 5-HTic receptors). In subsequent studies (Titeler et al. 1988), the affinities... 

---