Serotonin Receptor Evolution

Cross references:    Receptor Evolution   

The molecular evolution of G protein-coupled recep
tors: Focus on 5-hydroxytryptamine receptors (Goog)     Only abstract available online. I got the PDF through the library. 
"Based on an analysis of the percentage of amino acid homology between various species, the rate of molecular evolution of G protein-coupled receptors can be estimated at approx 1% per 10 million years. Based on this assumption, the primordial 5-HT receptor must have evolved more than 700-800 million years ago since the 3 major classes of G protein-coupled 5-HT receptors (i.e. 5-HT1, 5-HT2 and 5-HT6 receptors) are less than 25% homologous. ... The mammalian 5-HT receptor subtypes have differentiated over the past 90 million years ... the first "primordial" 5-HT receptor evolved over 750 million years ago, a date which likely predates the evolution of muscarinic, dopaminergic and adrenergic receptor systems.
My comment
As indicated by the title, this article only deals with the evolution of G-protein, and therefore metabotropic, receptors.  It does not in any way pertain to the ionotropic 5-HT3 receptor. 

Modern views on an ancient chemical: serotonin effects on cell proliferation, maturation and apoptosis (Goog) 
Only abstract available online.  I got the PDF through the library. 
Evolutionarily, serotonin existed in plants even before the appearance of animals. Indeed, serotonin may be tied to the evolution of life itself, particularly through the role of tryptophan, its precursor molecule. Tryptophan is an indole-based, essential amino acid which is unique in its light-absorbing properties. In plants, tryptophan-based compounds capture light energy for use in metabolism ...

The 5-HT1A receptor (5-HT1AR), estimated to be over a billion years old, evolved from the rhodopsin receptor.

Rhodopsin (Wiki) 
"Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light, enabling vision in low-light conditions."

Reconciling the role of central serotonin neurones in human and animal behavior. Behav. Brain Sci. 9: 319–364, 1986.   
Only abstract available online.  I got the 46 page PDF through the library.
from the abstract    
Decreases in serotonin transmission seem to be associated with the increased performance of behaviors that are usually suppressed, though not necessarily because of the alleviation of anxiety, which might contribute to the suppression.
my impression of the PDF from memory
    This is a very long review of more-or-less everything that was known about serotonin 25 years ago.   It hypothesizes that in the strong correlation between elevated circulating cortisol and elevated
Serotonin activity, it is the elevated Serotonin that is the cause and the elevated cortisol that is the effect.  However, it does not offer an explanation of how elevated Serotonin causes elevated cortisol.   
    An alternative hypothesis is that the elevated cortisol is the cause and the elevated serotonin is the effect.  Cortisol suppresses the sensitivity of lutenizing hormone receptors on the testes, thereby reducing the production of testosterone.  Since testosterone inhibits serotonergic receptors, the elevated circulating cortisol causes elevated serotonin activity by reducing the inhibitory effect of testosterone.   

    Full disclosure:  their position is supported by a reference that claims that "Treatment with 8-hydroxy-2(di-n- propylamino)tetralin, a specific 5-HT1A receptor agonist, elevates plasma cortisol concentrations in catheterized rainbow trout in a dosedependent manner (34)."   
    NOTE:  The 'full disclosure', above, doesn't seem to be from the paper.  The paper doesn't use reference numbers like '(34). 

Identification of the prokaryotic ligand-gated ion channels and their implications for the mechanisms and origins of animal Cys-loop ion channels (PubMed) 
Full length HTML and PDF available online for free. 
    "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 ..." 

My comment:   
Note that this article is focused on ionotropic receptors, including 5-HT3R,  whereas most of the articles above are focused on metabotropic receptors.  For a more extensive discussion of ionotropic receptors, including 5-HT3R, see: 
Receptors Evolution Timeline

Comparative anatomy of the serotoninergic systems.