As with the other monoamines, 5-HT is found primarily in storage vesicles (30-35 nm diameter) where 'serotonin-binding proteins' (SBPs) have also been identified. These seem to form a macromolecular complex with 5-HT. In fact, three such proteins have now been characterised, but only one of them, 45kDa SBP, appears to be secreted into the synapse along with 5-HT. Whether they serve any role other than forming an osmotically inert storage matrix for 5-HT is unknown.
In other respects the storage of 5-HT resembles that of noradrenaline with its uptake by vesicles resting on energy-dependent, 'vesicular monoamine transporters' (VMATs) (see Chapter 8). Functional disruption of this transporter, either through competitive inhibition (e.g. by methylenedioxymethamphetamine (MDMA, 'Ecstasy')) or dissipation of the pH gradient across the vesicle membrane that drives the uptake of 5-HT (e.g. by MDMA, reserpine and ^-chloroamphetamine) explains why these compounds deplete the vesicular pool of 5-HT.
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