Putative transmitter, membrane

Fig. 37. Regression analysis to show the way in which the depolarizations (AV) evoked by cholecystokinin (CCK-8) is related to an associated decrease in membrane resistance R IR). The slope of the line is related to the equilibrium potential of the ionic events which mediate the response of the putative transmitter involved. (From Dodd and Kelly, 1981.)...

Noradrenaline transporters (NAT) are localized in the presynaptic plasma membrane of adrenergic nerve terminals. They belong to a family of proteins with 12 putative transmembrane proteins which are responsible for recycling of released neurotransmitters (noradrena-line/adrenaline, dopamine, serotonin, amino acid transmitters) back into the presynaptic nerve ending. Noradrenaline transporters can be blocked by a number of different antidepressant drags, including tricyclic antidepressants (e.g. desipramine) and selective noradrenaline reuptake inhibitors (e.g. reboxetine). 

The first neurotransmitter to be discovered was acetylcholine, discovered by Loewi in the early 1920s. These studies were conducted with isolated preparations of heart and indicated that a substance inhibitory to the chro-notrophy of the heart was released by inhibitory nerves. Since then, numerous chemicals have been proposed as putative neurotransmitters, and reasonable estimates suggest that there are at least 50 candidates for the role of neurotransmitters. Like acetylcholine, many of the other neurotransmitters that were discovered early were found in peripheral tissues such as the heart or the vertebrate neuromuscular junction. At such sites, there is the release of a neurotransmitter at discrete sites where the pre- and postsynaptic membranes are in intimate contact. However, as research progressed over the past two decades, it became apparent that there were many situations where a nerve terminal was not closely associated with a postsynaptic membrane containing a unique receptor for the transmitter in question, but in fact was so situated that the released chemical could either diffuse into the circulation (i.e., neurohypophysis or adrenal medulla) or into the extracellular space to act on a distant receptor. Chemicals so released are usually not considered neurotransmitters so much as neurohormones or neuromodulators. 

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