Membranesoluble Messengers

Another type of nonvesicular neurotransmission is mediated by small molecules that are membrane-soluble. The best studied are the gas nitric oxide (NO) and the fatty acid arachidonate and its metabolites. These molecules diffuse through membranes and do not need vesicles to be released. Both NO and arachidonic acid have long been known to play important physiological roles in tissues other than the nervous system: NO primarily as a regulator of contractility of blood vessels and arachidonate in inflammation. NO is formed by the oxidation of arginine catalyzed by the enzyme nitric oxide synthase and an electron donor (FAD, NADPH, or THB) as cofactor. The aminoacid citrulline is produced.

Arachidonic acid is released from membrane phos-pholipids by the receptor-mediated activation of phopholipase A2 through a G-protein. The fatty acid released is then metabolized through two general pathways: (1) lipoxygenase, of which there are several (5-, 8-, 12-, 15-), and (2) cyclo-oxygenase. Metabolites from both of these pathways have been shown to be active in other tissues. Thus far, only 5-, 8-, and 12-lipoxygenase products have been implicated in synap-tic transmission.

It has been proposed that both NO and arachidonic acid act as retrograde messengers. Thus, in LTP, glutamate released from the presynaptic neuron would, in addition to depolarizing the postsynaptic cell, also activate the synthesis of membrane-soluble messengers, which then would diffuse back to the presynaptic element to enhance further release of glutamates.

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