Glutamate and GABA Receptors of Nerveless Animals (Placozoa): Preadaptation to Neurotransmission

The emergence of the nervous system is one of the most important events in the evolution of life on Earth. The details of this event remain poorly understood; in particular, a great variety of known neurotransmitter substances requires an explanation. We analyze the homologs of neuronal glutamate and gamma-aminobutyric acid (GABA) receptors in Placozoa (animals without a nervous system, but possessing the mobility and behavior). Phylogenetic analysis and comparison of amino acid residues responsible for a ligand specificity demonstrated that these Placozoan receptors are unexpectedly numerous, diverse, and evolutionarily dynamic, and by all these traits, they rather resemble the olfactory receptors of higher animals. Our results indicate that the chemoreceptor system was an important source of diverse receptors for the emerging nervous system, and future neurotransmitter amino acids (glutamate, GABA, glycine) were relevant external signals for the first animals that did not yet have a nervous system.