Neurotransmitter receptors functional types

The concept of chemical transmission in the nervous system arose in the early years of the century when it was discovered that the functioning of the autonomic nervous system was largely dependent on the secretion of acetylcholine and noradrenaline from the parasympathetic and sympathetic nerves respectively. The physiologist Sherrington proposed that nerve cells communicated with one another, and with any other type of adjacent cell, by liberating the neurotransmitter into the space, or synapse, in the immediate vicinity of the nerve ending. He believed that transmission across the synaptic cleft was unidirectional and, unlike conduction down the nerve fibre, was delayed by some milliseconds because of the time it took the transmitter to diffuse across the synapse and activate a specific neurotransmitter receptor on the cell membrane. 

G protein-coupled receptors function as adapters between the virtually boundless multitude and variety of extracellular hormone and neurotransmitter signals and the lower (yet still considerable) number of intracellular G-proteins. Therefore, it is very common to have receptors for multiple transmitters or hormones converge onto the same type of G protein and thus trigger the same response. E g., glucagon and epinephrine both activate adenylate cyclase in the liver, through separate receptors but the very same G protein. 

Proteins or protein complexes of the cell surface involved in cell excitation and the resultant cell-cell communication are grouped here as excitability proteins. They include ion channels, neurotransmitter receptors which influence the excitability of cells, and ion pumps that transport ions across cell membranes to establish ionic concentration gradients that make excitation of cells possible. Such proteins are found in various combinations on membranes of neurons of the central and peripheral nervous system, and on muscle cells that contract in response to neuronal excitation. They are also found on other cell types which specialize in functions such as secretion. They play vital roles in the specialized functions of these cells. 

Neuroactive steroids affect neurotransmission through their action at the membrane ion-gated and other neurotransmitter receptors.Recent studies have demonstrated that pregnane-type neuroactive steroids, such as pregnenolone (PREG) and its downstream conversion products, play an important role in the homeostatic mechanisms that counteract the inhibitory effect of stress on the 7-aminobutyric acid type A receptor function and... 

G0 was isolated as an other PTx-ribosylated G-protein which co-purifies with G, but which does not inhibit adenylate cyclase. There are two main isoforms (G0l and Go2), with additional splice-variants. G0 is particularly abundant in the nervous system, comprising up to 1% of membrane proteins. Its main function is to reduce the opening probability of those voltage-gated Ca2+ channels (N- and P/Q-type) involved in neurotransmitter release. Hence, it is largely responsible for the widespread auto-inhibition of transmitter secretion by presynaptic receptors and this effect is mediated through released py subunits. 

The neurotransmitters of the ANS and the circulating catecholamines bind to specific receptors on the cell membranes of effector tissue. Each receptor is coupled to a G protein also embedded within the plasma membrane. Receptor stimulation causes activation of the G protein and formation of an intracellular chemical, the second messenger. (The neurotransmitter molecule, which cannot enter the cell, is the first messenger.) The function of intracellular second messenger molecules is to elicit tissue-specific biochemical events within the cell that alter the cell s activity. In this way, a given neurotransmitter may stimulate the same type of receptor on two different types of tissue and cause two different responses due to the presence of different biochemical pathways within each tissue. 

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