Prejunctional Regulation of Norepinephrine Release

The release of the three sympathetic cotransmitters (catecholamine, ATP, NPY; see Figure 6-5) can be modulated by prejunctional autoreceptors and heteroreceptors. Following their release from sympathetic terminals, all three cotransmitters—NE, NPY, and ATP—can feedback on prejunctional receptors to inhibit the subsequent exocytosis. The a2A and a2C adrenergic receptors are the principal prejunctional receptors that inhibit sympathetic neurotransmitter release; the a2B adrenergic receptors also may inhibit transmitter release at selected sites. Antagonists of this receptor, in turn, can enhance the electrically-evoked release of sympathetic neurotransmitter. NPY, acting on Y2 receptors, and ATP-derived adenosine, acting on P1 receptors, also can inhibit sympathetic

Table 6-5

Characteristics of Transporters for Endogenous Catecholamines

Type of Substrate

Transporter Specificity Tissue Region/Cell Type Inhibitors

Neuronal NET

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