Cross references: Parazoa Porifera Parazoa Hormones
Parazoa Nerves Parazoa Genomics Receptors in General Ligand-gated Ion Channel Glutamate Metabotropic Receptor Receptors Evolution Timeline
Origin of neuronal-like receptors in Metazoa (PubMed)
"In the present study, isolated cells from the marine sponge Geodia cydonium are shown to react to the excitatory amino acid glutamate with an increase in the concentration of intracellular calcium [Ca2+]i. ... data suggest that sponge cells contain a Glutamate Metabotropic Receptor (mGluR)-like protein ... that displays marked overall similarity to mGluRs and to gamma-aminobutyric acid B receptors.
The deduced sponge polypeptide, termed putative mGlu/GABA-like receptor, ... shows similarity to mGluR4 and mGluR5. These findings suggest that the earliest evolutionary metazoan phylum, the Porifera, possesses a sophisticated intercellular communication and signaling system, as seen in the neuronal network of higher Metazoa."
I'm very surprised that the identified receptors were metabotropic rather than ionotropic. Maybe I'm mistaken, but it's my impression that ionotropic receptors are much less complicated than metabotropic receptors, and, because of that, I've always assumed that ionotropic receptors evolved first.
However, see below:
Identification of the prokaryotic ligand-gated ion channels and their implications for the mechanisms and origins of animal Cys-loop ion channels (PubMed)
Receptors Evolution Timeline .
"Acetylcholine receptor type Ligand-gated Ion Channels (ART-LGIC; also known as Cys-loop receptors) are a superfamily of proteins that include the receptors for major neurotransmitters such as acetylcholine, serotonin, glycine, GABA, glutamate and histamine, and for Zn2+ ions. They play a central role in fast synaptic signaling in animal nervous systems and so far have not been found outside of the Metazoa."
"Using sensitive sequence-profile searches we have identified homologs of ART-LGICs in several bacteria and a single archaeal genus, Methanosarcina."
So, while not yet identified in the Parazoa, it's a reasonable expectation that Parazoa homologs of ART-LGICs will eventually be found.
"Analysis of phyletic patterns currently suggests that the acetylcholine receptor type Ligand-gated Ion Channels (ART-LGICs) are absent in all other eukaryotic lineages except animals."
If sponges turn out to not have ART-LGICs, would that mean that they aren't animals? See: Receptors Evolution Timeline for further discussion of this reference.