Ectopic activation of GABA B receptors inhibits neurogenesis and metamorphosis in the cnidarian Nematostella vectensis.

The metabotropic gamma-aminobutyric acid B receptor (GABA _B R) is a G protein-coupled receptor that mediates neuronal inhibition by the neurotransmitter GABA. While GABA _B R-mediated signalling has been suggested to play central roles in neuronal differentiation and proliferation across evolution, it has mostly been studied in the mammalian brain. Here, we demonstrate that ectopic activation of GABA _B R signalling affects neurogenic functions in the sea anemone Nematostella vectensis. We identified four putative Nematostella GABA _B R homologues presenting conserved three-dimensional extracellular domains and residues needed for binding GABA and the GABA _B R agonist baclofen. Moreover, sustained activation of GABA _B R signalling reversibly arrests the critical metamorphosis transition from planktonic larva to sessile polyp life stage. To understand the processes that underlie the developmental arrest, we combined transcriptomic and spatial analyses of control and baclofen-treated larvae. Our findings reveal that the cnidarian neurogenic programme is arrested following the addition of baclofen to developing larvae. Specifically, neuron development and neurite extension were inhibited, resulting in an underdeveloped and less organized nervous system and downregulation of proneural factors including NvSoxB(2), NvNeuroD1 and NvElav1. Our results thus point to an evolutionarily conserved function of GABA _B R in neurogenesis regulation and shed light on early cnidarian development.