Glutamate and GABA receptors in non-neural animals (Placozoa): Preadaptation to neural transmission

Abstract

Origins of neural system is one of the major transitions in planetary evolution. Many details of these transitions are still unknown. In particular, high diversity of neurotransmitters lacks convincing explanation so far. We analyze homologues of neuronal glutamate and gamma-aminobutyric acid (GABA) receptors of Placozoa – animal phyla lacking neurons but displaying motility and complex behaviour. Phylogenetic analysis and comparison of amino acids in ligand-binding pockets show that glutamate and GABA-like receptors of Placozoa are surprisingly numerous, diverse and fast-evolving. All these traits are characteristic of odorant rather than neurotransmitter receptors of higher animals. We argue that chemoreception system was an important source of diverse receptors for emerging nervous system to recruit, and that amino acid neurotransmitters (glutamate, GABA, glycine) were relevant external stimuli for early animals before the emergence of nervous system.