5-HT1B Receptor-Mediated Activation of ERK1/2 Requires Both Gαi/o and β-Arrestin Proteins

5-HT1B receptors modulate synaptic serotonin (5-HT) levels and play a significant role in the regulation of emotional behaviors. These receptors are G alpha(i/o)-coupled and inhibit adenylyl cyclase but have also been reported to activate MAP kinases; however, the details of signaling cascades downstream of 5-HT1B receptor activation remain unclear, particularly in neuronal cells. We generated a stable 5-HT1B receptor-expressing Neuro2A (N2A-1B) neuronal cell line and demonstrate that activation of these receptors by the selective 5-HT1B agonist CP-94253 results in activation of ERK1/2 but not of other closely related MAP kinases. Phosphoproteomics revealed four novel phosphorylation sites on the third intracellular loop of the 5-HT1B receptor, and mutations of serine-256 and serine291 to alanine led to reduced levels of ERK1/2 phosphorylation following receptor activation. Inhibition of Ga vo signaling with pertussis toxin, as well as MEK1/2 inhibition with U0126, also reduced 5-HT1B-mediated ERK1/2 phosphorylation. Finally, we found that knockout of either beta-arrestin 1 or beta-arrestin 2 prevented 5-HT1B-mediated phosphorylation of ERK1/2. Taken together, these results show that 5-HT1B receptor activation selectively induces ERK1/2 activation through both the G alpha(i) subunit and beta-arrestin proteins. This work elucidates the signal transduction pathway of 5-HT1B receptors, as well as key phosphorylation sites within the receptor that modulate ERK1/2 activation, and further characterizes the intracellular mechanisms that underlie 5-HT1B receptor function.