Purines

ATP is present in high concentrations in transmitter vesicles, both for neuropeptides and for small-molecule transmitters. ATP is synthesized in mitochondria ch2— ch2— NH2 + CO2

and is abundant in cytoplasm: it gets pumped into vesicles. Because it is present in vesicles, it is routinely coreleased with neurotransmitters. In some postsynaptic cells, there are receptors that respond to ATP, adenosine, or AMP where they modulate the transmission of other neurotransmitters. Thus, they can inhibit by activating autoreceptors. Receptors are present in the vas deferens, gut, smooth muscle, and neurons of the dorsal horn of the spinal cord. There are two classes of purine receptors, P1 andP2. P1 receptors are sensitive to adenosine and AMP and are all metabotropic. P2 receptors respond to ATP and AMP; a few of these are ionotropic.

Purinergic transmission illustrates an important aspect of chemical synaptic transmission. The abundant release of a substance is necessary, but not sufficient evidence that the substance is a transmitter, even though that substance has been shown to act as a transmitter elsewhere in the body. A postsynaptic receptor must also be present. A substance is not a neurotransmitter if it is released into an area that lacks a receptor for it. It may seem improbable that this occurs frequently, but ATP is actually released at all synapses.

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