Neurotransmitter Evolution

Cross reference:  Ligands  

The Evolution of the Serotonergic Nervous System (Goo
Only abstract available online.  I got PDF from library. 
"I would propose that the serotonergic raphe system of vertebrates can be traced back to the larval serotonergic system of the common ancestor of phoronids,  brachiopods and chordates." 

This is a really cool diagram that shows the relationship between some of our earliest ancestors, most of which I'd never heard of before.  However, there is no mention of which receptors are present, and this limits its applicability to our effort. 

Serotonergic modulation of behaviour, a phylogenic overview. (PubMed) 
Abstract only.  I got the 35 page PDF from the library.   For the function of serotonin in primates, see:  Primate Dominance Hierarchies  .   
from the abstract   
    "Serotonergic neurons are present in all phyla that possess nervous systems. ... Serotonin exerts its effects by acting in three basic modes: as a classical neurotransmitter, as a neuromodulator, or as a neurohormone."    
from the PDF
    "In the nematode Caenorhabditis elegans, serotonin enhances feeding and egg release. In the mollusc Aplysia californica, serotonergic neurons potentiate biting and enhance a defensive reflex. In the mollusc Tritonia diomedea, serotonin acts as both a classical neurotransmitter and as a neuromodulator in a neural network that generates escape swimming.  In the leech (Hirudo medicinalis, Macrobdella decora, Haementeria ghilianii),  serotonin plays an important role in the initiation and modulation of both appetitive and consummatory components of feeding behaviour. In the lobster Homarus americanus, serotonin promotes the adoption of aflexed or dominant-type posture. In the crayfish Procambarus clarkii, serotonin affects sensory activation of the circuitry controlling tailflips, which may be used defensively or offensively in aggressive encounters; the direction of the effect depends on the dominance status of the animal."  

My comments
1.  This article was written in 1985, long before genomic data mining became possible.  Therefore, there is no information on specific receptors. 
2.  Most of the species considered are protostomes, rather than deuterostomes.  Since the deuterostomes and the protostomes split approximately 600 million years ago, it's risky to put too much weight on the protostome data.  On the other hand, it's also risky to completely ignore the protostome data, and the data shows that, among the protostome larvae, serotonin is universally stimulatory.  It is also stimulatory toward echinoderm larvae, which are the only deuterostomes considered besides the chordates.  Among the chordates, serotonin seems to be generally inhibitory. 
3.  Perhaps this reflects a change in the serotonergic targets.  Among  pre-chordate larvae it may have been motor cells, while among chordates, some of the targets are well established to be GABAergic inhibitory neurons. 
4.  If the effect of serotonin in the adult amphioxus is stimulatory, rather than inhibitory, then my suggested mechanism for amphioxus behavior is proven to be false.      

Evolution and overview of classical transmitters (PubMed) 
Abstract only.  I got the PDF from the library.  
All the classical transmitter ligand molecules evolved at least 1000 million years ago. With the possible exception of the Porifera and coelenterates (Cnidaria), they occur in all the remaining phyla. All transmitters have evolved the ability to activate a range of ion channels, resulting in excitation, inhibition and biphasic or multiphasic responses. All transmitters can be synthesised in all three basic types of neurones, i.e. sensory, interneurone and motoneurone. However their relative importance as sensory, interneurone or motor transmitters varies widely between the phyla. It is likely that all neurones contain more than one type of releasable molecule, often a combination of a classical transmitter and a neuroactive peptide.

Second messengers, i.e. G proteins and phospholipase C systems, appeared early in evolution and occur in all phyla that have been investigated. Although the evidence is incomplete, it is likely that all the classical transmitter receptor subtypes identified in mammals, also occur throughout the phyla. The invertebrate receptors so far cloned show some interesting homologies both between those from different invertebrate phyla and with mammalian receptors. This indicates that many of the basic receptor subtypes ... probably at least 800 million years ago. Overall, the evidence stresses the similarity between the major phyla rather than their differences...

Frontiers | The Evolution of Dopamine Systems in Chordates | Frontiers in Neuroanatomy 
    See:  Amphioxus Dopamine

Comparative Biology of the Neurotransmission   

Comparative anatomy of the serotoninergic systems. - 1981  

The evolution of the serotonergic nervous system. - 2000  

Evolution of neuronal connecting mechanisms: electrical, mixed and chemical synapses (Russian)
    "Investigation of the mechanisms of junctional transmission in the isolated spinal cord in cyclostomes, amphibians, reptilia and mammals reveals the decrease in the number of electrical synapses during evolution from primitive to more advanced vertebrates. Electrical transmission is lacking in reptilian and mammalian cord. On the basis of these data, the analysis of the unitary EPSPs evoked in motoneurons of the lamprey and frog by intracellular stimulation of reticulospinal axons and primary afferent fibers and dendrodendritic interaction between motoneurons, a hypothesis is advanced that neurons of similar type may communicate through pure electrical junctions, whereas successive synaptic articulations between different functional groups of neurons are formed by mixed or chemical synapses. The cellular mechanisms controlling interneuronal communications are discussed."
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