Serotonin-2A homodimers are needed for signalling via both phospholipase A and phospholipase C in transfected CHO cells.

Different ligands differentially activate phospholipase A (PLA) and phospholipase C (PLC) signalling pathways that are coupled to the serotonin 2 (5-HT) receptor, a class-A G-protein coupled receptor (GPCR). The serotonin 5-HT receptor has been shown to be expressed as a homodimer displaying some ligands negative cooperativity between protomers in the PLA signalling pathway. We hypothesized that the homodimeric complex is the minimum functional unit required for activation of the PLA and PLC pathways by the serotonin 5-HT receptor. To investigate this hypothesis, we partially blocked the serotonin 5-HT receptors with ritanserin and measured PLA and PLC activity simultaneously. We subsequently added the competitive antagonist spiperone to release the inactivator through a crosstalk mechanism and thus allow the dimer to return to a reactive state. Partial inactivation of the homodimer by ritanserin binding decreased the activity of the receptor by 59±13% and 70±4% in the PLA and PLC pathways respectively (P<0.001), with no difference in the potency of the serotonin (5-HT) was observed. The subsequent binding of spiperone released ritanserin due to the crosstalk between protomers and recovery of the receptor activity to 74±7% and 72±4%. Negative cooperativity between protomers in the dimer was maintained during arachidonic acid (AA) release after blocking ritanserin, as indicated by the biphasic inhibition curves for clozapine over 1μM serotonin (5-HT) in these conditions. These findings provide evidence that serotonin 5-HT receptors must be expressed as homodimers in order to activate both the PLA and PLC signalling pathways.