Serotonin

Serotonin, or 5-hydroxytryptamine (5HT), is perhaps the most widely known neurotransmitter in a popular sense. The reason for this is the massive press given to the drug Prozac, a NA that results in an increase in the amount of serotonin available around neurons, a feat accomplished by inhibiting the destruction of serotonin. Serotonin or 5HT was originally discovered in the gastrointestinal mucosa in the 1940s. Its chemical structure is very similar to the amino acid tryptophan, which is, in fact, the dietary precursor of 5HT itself. 5HT is found in virtually all vertebrate organisms, as well as in many invertebrates such as wasps, scorpions, and ocean crustaceans. It is also found in abundance in several plant species, including bananas and pineapples. Although a neurotransmitter of major importance, 5HT is distributed throughout the body. Interestingly, approximately 90% of the total 5HT in the human body is in the gastrointestinal tract, 8% is in blood platelets (where it is involved in the clotting process), and only 2% is in the central nervous system.

5HT is synthesized in the terminal end of the neuron and transported to synaptic vesicles (cystlike structures that exist at the neuronal cell membrane), from which it is released into the synapse for interaction with receptors in adjacent neurons. Once released freely into the synaptic space, molecules of 5HT will either stimulate receptors in other neurons or they will interact with autoreceptors from their original neuron (the neuron that produced them). Interaction with adjacent neurons results in neurotransmission of a nature specific to the type of adjacent neuron. Interaction with autoreceptors results in an important process called re-uptake. Neurotransmitter re-uptake results in a decrease in the amount of the neurotrans-mitter (in this case 5HT) that is available in the synaptic space. Through re-uptake, the 5HT neuron can exert modulatory control over the amount of serotonergic neurotransmission generated from that neuron: net transmission can be reduced through the

Table I

Neurotransmitters in the Brain

Table I

Neurotransmitters in the Brain

Amines

Serotonin (5HT)

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