Comparing Amphioxus and Lamprey Serotonin

Cross references:  Serotonin Metabotropic Receptor Rodent 5-HT1R 
Serotonin References    

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 present results taken together with previous data indicates that the receptor responsible for the effects of 5-HT ... on the locomotor pattern generator in lamprey shares certain features, but is not identical to the mammalian 5-HT1A receptor."   
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) 
"5-HT is a potent modulator of locomotor activity in vertebrates. In the lamprey, 5-HT dramatically slows fictive swimming."
"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.