Neuronal Phenotype Dependency of Agonist-Induced Internalization of the 5-HT1A Serotonin Receptor

Selective serotonin reuptake inhibitors (SSRI) are aimed at increasing brain 5-HT tone; however, this expected effect has a slow onset after starting SSRI treatment because of initial activation of 5-HT1A autoreceptor-mediated negative feedback of 5-HT release. After chronic SSRI treatment, 5-HT1A autoreceptors desensitize, which allows 5-HT tone elevation. Because 5-HT1A receptor (5-HT1AR) internalization has been proposed as a possible mechanism underlying 5-HT1A autoreceptor desensitization, we examined whether this receptor could internalize under well controlled in vitro conditions in the LLC-CPK1 cell line and in raphe or hippocampal neurons from rat embryos. To this goal, cells were transfected with recombinant lentiviral vectors encoding N-terminal tagged 5-HT1AR, and exposed to various pharmacological conditions. Constitutive endocytosis and plasma membrane recycling of tagged-5-HT1AR was observed in LLC-PK1 cells as well as in neurons. Acute exposure (for 1 h) to the full 5-HT1AR agonists, 5-HT and 5-carboxamido-tryptamine, but not the partial agonist 8-OH-DPAT, triggered internalization of tagged 5-HT1AR in serotonergic neurons only. In contrast, sustained exposure (for 24 h) to all agonists induced tagged-5-HT1AR endocytosis in raphe serotonergic neurons and a portion of hippocampal neurons, but not LLC-PK1 cells and partial agonist displayed an effect only in serotonergic neurons. In all cases, agonist-induced tagged 5-HT1AR endocytosis was prevented by the 5-HT1AR antagonist, WAY-100635, which was inactive on its own. These data showed that agonist-induced 5-HT1AR internalization does exist in neurons and depends on agonist efficacy and neuronal phenotype. Its differential occurrence in serotonergic neurons supports the idea that 5-HT1AR internalization might underlie 5-HT1A autoreceptor desensitization under SSRI antidepressant therapy.