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Tural Rajabov.TSMU

Faculity of Medicine 7 th group.

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More than 100 different agents are known to serve as neurotransmitters. Thislarge number of transmitters allows for tremendous diversity in chemical

signaling between neurons. It is useful to separate this panoply of transmittersinto two broad categories based simply on size. Neuropeptides

are relatively large transmitter molecules composed of 3 to 36 amino

acids. Individual amino acids, such as glutamate and GABA, as well as thetransmitters acetylcholine, serotonin , and histamine , are much smaller than

neuropeptides and have therefore come to be called small-moleculeneurotransmitters.

Within the category of small-molecule neurotransmitters, thebiogenic amines (dopamine, norepinephrine, epinephrine, serotonin, and

histamine ) are often discussed separately because of their similar chemicalproperties and postsynaptic actions. The particulars of synthesis, packaging,

release, and removal differ for each neurotransmitter.

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Acetylcholine. Acetylcholine (ACh) was the first substance identified as a

neurotransmitter.

Acetylcholine metabolismin cholinergic nerve terminals.The synthesis of acetylcholinefrom choline and acetyl CoArequires choline cetyltransferase. Acetyl CoA is derived from

pyruvate generated by glycolysis, while choline is transported intothe terminals via a Na+-dependent transporter. Acetylcholine is loaded intosynaptic vesicles via a vesiculartransporter. After release,acetylcholine is rapidlymetabolized byacetylcholinesterase, andcholine is transported back intothe terminal.

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There are three ACh pathways in the CNS:

Pons to thalamus and cortex.Magnocellular forebrain nucleus to cortex.Septohippocampal.

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GABA

GABA is synthesizedfrom glutamate by theenzyme glutamic aciddecarboxylase, which requires pyridoxalphosphate.

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GLYCINE

Glycine can be synthesized by anumber of metabolic pathways;in the brain, the major precursoris serine. High-affinitytransportersterminate the actions ofthese transmitters and returnGABA orglycine to the synapticterminals forreuse, with bothtransmitters beingloaded intosynaptic vesicles via the vesicular inhibitory amino acidtransporter(VIATT)

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EPINEPHRINEThe biosynthetic pathway for the catecholamineneurotransmitters. The amino acid tyrosine is theprecursor for all three catecholamines. The firststep in this reaction athway, catalyzed by tyrosinehydroxylase, is rate-limiting.

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Synthesis of histamine and serotonin. (A)Histamine is synthesized from the aminoacid histidine. (B) Serotonin is derived fromthe aminoacid tryptophan by a two-step process thatrequires the enzymes tryptophan-5-hydroxylase and a ecarboxylase.

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