Cross references: Serotonin Metabotropic Receptor , Rodent 5-HT1R
From: Lamprey Serotonin System
The action of 5-HT on calcium-dependent potassium channels and on the spinal locomotor network in lamprey is mediated by 5-HT1A-like receptors (Goog)
The article was written in 1995. Since then, it has been confirmed that the receptor in question is, in fact, a genuine 5-HT1AR, and not just something similar.
5-HT inhibits N-type but not L-type Ca(2+) channels via 5-HT1A receptors in lamprey spinal neurons (PubMed)
"The primary conclusion ... is that lamprey spinal neurons have 5-HT1A receptors ..."
My comments on the above comparison:
1. Both the amphioxus and the lamprey have 5-HT1AR.
2. 5-HT3R has not been reported in either the amphioxus or the lamprey.
3. Since 5-HT1AR is inhibitory, it's reasonable to suggest that it functions to restrain amphioxus behavior much as it restrains lamprey behavior.
4. Since the lamprey is descended from the amphioxus, I would expect that every receptor that is present in the amphioxus would also be present in the lamprey. If this expectation is valid, then the large number of shared serotonin receptors should result in a large number of shared serotonin activities and responses.