Impact of lipid raft integrity on 5-HT3 receptor function and its modulation by antidepressants

In view of a colocalization of the 5-HT3 receptor and antidepressants within raft-like domains we investigated the impact of lipid raft integrity for 5-HT3 receptor function and its modulation by antidepressants. Treatment with methyl-β-cyclodextrine (MβCD) reduced membrane cholesterol levels and caused a disassembly of the raft marker protein flotillin-1, indicating lipid raft impairment. Serotonin evoked cation currents considerably diminished following cholesterol depletion by either MβCD or simvastatin. However, the functional antagonism of desipramine and fluoxetine at the 5-HT3 receptor were retained following cholesterol depletion. Investigating the colocalization of the 5-HT3 receptor and flotillin-1 in Western blots from sucrose density gradients, both proteins were predominantly found in raft-like domains, confirming previous findings. However, immunocytochemistry revealed only a low degree of colocalization between the 5-HT3 receptor and flotillin-1. These results argue for the hypothesis that the modulatory effects of antidepressants at the 5-HT3 receptor are mediated at least in part through non-raft 5-HT3 receptors, which cannot be separated by means of sucrose density gradient centrifugation. In conclusion, lipid raft integrity seems to be important for 5-HT3 receptor function in general, whereas the antagonism of antidepressants at this receptor involve also non-raft 5-HT3 receptors.