Cross references: Serotonin Gate (5-HT3R), Rodent 5-HT3R .
Comparing Amphioxus and Lamprey Serotonin
"Serotonin or 5-Hydroxytryptamine (5-HT) is a monoamine neurotransmitter, biochemically derived from tryptophan" "Serotonin is eventually metabolized to 5-HIAA by the liver, and excreted by the kidneys." "In addition to animals, serotonin is also found in fungi and plants. "
Seretonin (5-HT) Receptor (Goog - Wiki)
" With the exception of the 5-HT3 receptor, a ligand gated ion channel, all other 5-HT receptors are G protein coupled seven transmembrane receptors that activate an intracellular second messenger cascade to produce an excitatory or inhibitory response."
Serotonin (5-HT) Receptor (Goog - Wiki)
Of course, it's important to remember that the polarity of the response of the entire animal to the activation of the receptor, i.e., whether it's excitatory or inhibitory, may not be the same as the polarity of the neuron's response to activation of the receptor.
For example, if a 5-HT1 receptor is activated, it will inhibit the neuron on whose membrane it resides. However, if the neuron expressing the 5HT-1 receptor is itself inhibitory in its output, for example, if the neuron is GABAergic, the final outcome of activation of the inhibitory receptor will be excitation of the downstream neuron which was being inhibited by the neuron bearing the 5-HT1 receptor.
Unfortunately, this only tells us what's there, not how it's all hooked together. Until we know the wiring diagram, the polarity of the individual receptors doesn't tell us much about the animal's final behavior.
Serotonin Receptor Subtypes (Wiki)
I haven't copy-and-pasted the table of serotonin receptor subtypes because it's very long. If you want to see it, click on the link.
The "5-HT" in the receptor name stands, of course, for serotonin. The number which follows indicates the family, and the letter which follows the number indicates the subtype. Thus "5-HT1A" is the abbreviation for "serotonin, family 1, subtype A". When you see the letter "R", this is not a subtype but rather is shorthand for "receptor". Thus "5-HT1R" means all subtypes of the 5-HT1 family of receptors, while "5-HT1AR" means the receptor for just the A subtype of the 5-HT1 family of receptors. Note that a receptor is assigned to a family according to its mode of action whereas assignment to a particular subtype depends on four factors: distribution (where it's located in the body), its function, its agonists (which biochemicals activate it) and its antagonists (which biochemicals deactivate it).
"5-HT1A receptors are located both pre- and postsynaptically within the brain, and at either location, their activation leads to neuronal hyperpolarization and reduced firing rate."
"Early pharmacological studies screening for antiaggressive drugs, combined with receptor pharmacology, indicated that the 5-HT1B receptor was a key regulator of aggression."
Metabotropic receptor (Wiki)
eukaryotic cells that acts through a second messenger. It may be located at the surface of the cell or in vesicles."
"They are indirectly linked with ion-channels on the plasma membrane of the cell through signal transduction mechanisms, often G proteins. Hence, they are a type of G protein-coupled receptor."
"This class of receptors includes the metabotropic glutamate receptors, muscarinic acetylcholine receptors, GABAB receptors, and most serotonin receptors, as well as receptors for norepinephrine, epinephrine, histamine, dopamine, neuropeptides and endocannabinoids."
Raphe nuclei (Wiki)
nuclei found in the brain stem. Their main function is to release serotonin to the rest of the brain."
"Overall, the caudal raphe nuclei, including the raphe magnus, pallidus and raphe obscurus, all project towards the spinal cord and brain stem. The more-rostral nuclei, including the raphe pontis, centralis (also called median), dorsal, tend to project towards the brain areas of higher function"
Since the focus of my interest is the subcortical brain, I'll be concentrating on "the caudal raphe nuclei" which "project towards the spinal cord and brain stem". So far, I've only found these discussed in the context of the lamprey.
See: Lamprey Receptors
The "more-rostral nuclei" which "project towards the brain areas of higher function" are discussed in the context of rodents, where most of this research has been done.
See: Rodent 5-HT1R, Vasopressin, Serotonin & Aggression