PRESYNAPTIC MODULATION OF THE RELEASE OF THE COTRANSMITTERS NORADRENALINE AND ATB

The release of both sympathetic co-transmitters, noradrenaline and ATP , is modulated via presynaptic receptors. However, the degree of the m odulation may differ indicating that the ratio of the released co-tran smitters changes upon presynaptic receptor activation. For example, al pha(2)-autoinhibition affects the release of noradrenaline more marked ly than the release of ATP. Some sympathetic axon terminals possess pr esynaptic P-2-purinoceptors which are activated by endogenous ATP. The se receptors are a novel kind of autoreceptor: they mediate a presynap tic negative feedback mechanism in which released ATP inhibits subsequ ent co-transmitter release.