Amphioxus Receptors

Cross reference:  Lamprey Receptors

The amphioxus (Branchiostoma floridae) genome contains a highly diversified set of G protein-coupled receptors (Goog)
Full length PDF and HTML available online for free. 
    It turns out that amphioxus has not one or two but 664 GPCRs.  Incredible! 
  These are quite complicated structures, and I was assuming that an organism as primitive as the amphioxus wouldn't have any.  This study, which involved searching the amphioxus genome, found  1 muscarinic, 2 dopamine, 3 serotonin and one histamine neuroreceptor. 
"...we found orthologs to three serotonin receptors, namely 1, 5 and 6..." 
According to , both 1 and 5 are inhibitory while 6 is excitatory. 
    This paper reports the data mining of the amphioxus genome for GPCRs.  They found genes for:  5-HT1AR, 5-HT1BR, 5-HT1DR, 5-HT1ER, 5-HT1FR, 5-HT2AR, 5-HT2BR, 5-HT2CR, 5-HT4R, 5-HT5AR, 5-HT6R and 5-HT7R. 

Eleven New Putative Aminergic GPCRs ... (Goog) 
"We have identified eleven novel aminergic-like G-protein coupled receptor (GPCRs) sequences (named AmphiAmR1-11) ... The amphioxus genome appears to express a number of D1-like dopamine receptor sequences... It also expresses ... others that resemble vertebrate α-adrenergic receptors. Amphioxus also expresses receptors that resemble vertebrate histamine receptors.

A cannabinoid receptor ortholog in the cephalochordate Branchiostoma floridae (Goog) 
"A gene encoding an ortholog of vertebrate CB1/CB2 cannabinoid receptors was recently identified in the urochordate Ciona intestinalis (CiCBR). Here a cannabinoid receptor ortholog (BfCBR) has been identified in the cephalochordate Branchiostoma floridae. BfCBR is encoded by a single exon and is 410 amino acid residue protein that shares 28% sequence identity with CiCBR and 23% sequence identity with human CB1 and human CB2. The discovery of BfCBR and CiCBR and the absence of cannabinoid receptor orthologs in non-chordate invertebrates indicate that CB1/CB2-like cannabinoid receptors originated in an invertebrate chordate ancestor of urochordates, cephalochordates and vertebrates. Furthermore, analysis of the relationship of BfCBR and CiCBR with vertebrate CB1 and CB2 receptors indicates that the gene/genome duplication that gave rise to CB1 and CB2 receptors occurred in the vertebrate lineage."

AmphiD1/β, a dopamine D1/β-adrenergic receptor from the amphioxus Branchiostoma floridae: evolutionary aspects of the catecholaminergic system during development (Goog)     
Only abstract available online. 
    "Catecholamine receptors mediate wide-ranging functions in vertebrates and invertebrates but are largely unknown in invertebrate chordates such as amphioxus. ... In B. floridae, AmphiD1/β encodes a polypeptide with typical G-protein-coupled receptor features, characterized by highest sequence similarity with D1 dopamine and β-adrenergic receptors. The expression of AmphiD1/β mRNA in different regions of the cerebral vesicle corresponds to that of D1-like receptors in vertebrate homologous structures.

Evolution of monoamine receptors (PubMed) 
"In the cephalochordate Amphioxus, a sister group to vertebrates, a single catecholamine receptor gene was found, which based on molecular phylogeny and functional analysis formed a monophyletic group with both vertebrate dopamine D1 and beta adrenergic receptor classes. In addition, the presence of dopamine but not of noradrenaline was assayed in Amphioxus.

On the origin and evolution of vertebrate olfactory receptor genes: comparative genome analysis among 23 chordate species.

Only abstract available online. 
Among studied vertebrates, mammals have approximately 1,000 olfactory receptor (OR) genes ... the amphioxus has >30 vertebrate-type OR genes though it lacks distinctive olfactory organs ...

Future efforts
PubMed gives several "Related citations" which might be worth coming back to.