PROTOTYPIC ANTIDEPRESSANTS ACT AS A G-PROTEIN BIASED LPAR1 AGONIST AND CONTRIBUTE TO ANTIDEPRESSANT EFFECTS

Abstract Background Prototypic antidepressants, such as tricyclic/tetracyclic antidepressants (TCAs), have multiple pharmacological properties and have been considered to be more effective than newer antidepressants, such as selective serotonin reuptake inhibitors (SSRIs), in treating severe depression. However, the clinical contribution of non-monoaminergic effects of TCAs remains elusive. Aims & Objectives In this study, we hypothesized that one of the potential targets of TCAs is the lysophosphatidic acid receptor 1 (LPAR1), a G protein-coupled receptor (GPCR). LPAR1 is one of the six receptors (LPAR1–6) that are activated by lysophosphatidic acid (LPA), a bioactive phospholipid. The receptor is activated by TCAs and LPAR1-deficient mice showed a phenotype with depressive and anxious features. In contrast, the central administration of LPA has been reported to induce depression- like behaviors, which contrasts our hypothesis. Although GPCRs typically activate G proteins and induce receptor desensitization via beta-arrestin-mediated endocytosis, some ligands preferentially activate either signaling pathways of the same GPCR, a phenomenon referred to as biased agonism. LPA may cause functional antagonism of LPAR1 via endocytosis, potentially blocking LPAR1 signaling, as observed in LPAR1-deficient mice. Therefore, we sought to determine whether TCAs activate downstream signals of LPAR1 similar to LPA, or whether they activate selective signals distinct from LPA. Additionally, we examined whether LPAR1 may be involved in mediating the behavioral effects of TCAs. Method The effects of TCA on LPAR1 and downstream signaling were examined using various in vitro biochemical approaches and in vivo behavioral tests. We further investigated the antidepressant effects of LPAR1 ligands and analyzed comprehensive gene expression changes via RNA-seq. Results We discovered that amitriptyline, a typical TCA, directly binds to LPAR1 and activates downstream G protein signaling, while exerting little effect on beta-arrestin recruitment. This suggests that amitriptyline might act as a G protein-biased agonist of LPAR1. This biased agonism is unique to TCAs and has not been observed in other antidepressants, such as SSRIs. LPAR1 was found to be involved in the behavioral effects of amitriptyline. Notably, long-term infusion of the mouse hippocampus with the potent G protein-biased LPAR agonist OMPT, but not the non-biased agonist LPA, induced antidepressant-like behavior, indicating that G protein-biased agonism might be necessary for the antidepressant-like effects. RNA- seq analysis revealed that LPA and OMPT result in opposite patterns of gene expression changes in the hippocampus. Further, pathway analysis revealed that long-term treatment with OMPT activated LPAR1 downstream signaling (Rho and MAPK), whereas LPA suppressed LPAR1 signaling. Discussion & Conclusion Our findings provide insights into the mechanisms underlying the non-monoaminergic antidepressant effects of TCAs and identify the G protein-biased agonism of LPAR1 as a promising target for the development of novel antidepressants.