Functional Analysis of Ligand‐Gated Chloride Channels in a Cnidarian Sheds Light on the Evolution of Inhibitory Signaling

ABSTRACT γ‐aminobutyric acid (GABA) is the predominant inhibitory transmitter in the vertebrate nervous system. Fast inhibitory signaling is mediated by type A GABA receptors (GABA A Rs). While GABA is also present in plants and prokaryotes, it is unknown when it was first used for fast neuronal transmission. Cnidaria represent a sister group to all Bilateria and possess a variety of putative GABA A Rs, none of which has been functionally characterized. In this study, we surveyed putative inhibitory ion channel receptors from four different cnidarians. Phylogenetic analysis reveals a surprising phylogenetic complexity of these receptors. While the majority form a cnidarian‐specific radiation, others cluster with bilaterian receptors. We functionally analyze seven putative Nematostella GABA A Rs of the cnidarian radiation and find that none is activated by GABA or glycine, whereas three are activated by glutamate. Using site‐directed mutagenesis, we identify a lysine residue in the canonical ligand‐binding pocket that is important for activation by glutamate. Our results identify a group of inhibitory ion channel receptors in Cnidaria that use glutamate as a ligand. Moreover, they suggest that inhibitory ion channel receptors in Cnidaria massively diversified, which may have been instrumental in the evolution of complex behaviors and sensory processing by the cnidarian nervous system.