Comparing Amphioxus and Lamprey Serotonin
Serotonin Gate (5-HT3R) ,
Serotonin Metabotropic Receptor ,
Rodent 5-HT1R , Rodent 5-HT3R
Amphioxus Serotonin System.
This section resulted from an attempt to justify the assumption that the 5-HT3R is present in the amphioxus. The consequences of 5-HT3R activation are quite clear. It's quickly stiumulatory, but since most of its input is to inhibitory GABA neurons, its effect on the overall behavior of the animal is inhibitory. This fits well into my hypothesis of amphioxus behavior presented in Boys without Fathers.
However, I did an exhaustive search for evidence of the 5-HT3 receptor in amphioxus and, at first, found nothing. All the research seemed to have been done on humans and rodents. I found absolutely nothing specifically about 5-HT3R on anything below the mammals on the evolutionary scale.
Further reading showed that the article, below, strongly suggests that the amphioxus does, in fact, have 5-HT3R. It just doesn't specify it by that name.
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 for a discussion of this article.
Immunocytochemical Localization of Serotonin in Embryos, Larvae and Adults of the Lancelet, Branchiostoma Floridae (Goog)
states "Comparing the distribution of GABA and serotonin, it seems at least that both neurotransmitters coexist in some nerve cell bodies and fibres."
which at least opens the door to the possibility that serotonin inhibits the overall behavior the amphioxus by exciting inhibitory GABA neurons.
See: Amphioxus Serotonin System for further discussion of this reference.
As described below, there are three other 5-HT receptors in amphioxus, and this certainly complicates any analysis of the role of serotonin in amphioxus behavior.
The amphioxus (Branchiostoma floridae) genome contains a highly diversified set of G protein-coupled receptors (Goog)
"...we found orthologs to three serotonin receptors, namely 1, 5 and 6..."
See: Amphioxus Serotonin System.
According to Serotonin Metabotropic Receptor, both 1 and 5 are inhibitory while 6 is excitatory. The full table is at: Amphioxus Serotonin System.
Do I need to know any more than that?
Well, it would be nice to know on which neurons these receptors are located, but, as far as I can tell, that information is not yet available.
This reverse engineering effort was initiated by:
Inhibition by steroids of [14C]-guanidinium flux through the voltage-gated sodium channel and the cation channel of the 5-HT3 receptor of N1E-115 neuroblastoma cells.
This is fairly technical, but, in plain English, steroids inhibited the effect of serotonin at the cation channel of the 5-HT3 receptor on mouse neuroblastoma cells. Influx of [14C]-guanidinium ... was concentration-dependently inhibited.
the guanidinium cation, [CH6N3]+