Prejunctional regulation, sympathetic

When stimulated electrically, the sympathetic nerve discharges the contents of part of its vesicles, including norepinephrine, into the extracellular space. Liberated norepinephrine reacts with adrenoceptors located postjunctionally on the membrane of effector cells or prejunctionally on the membrane of varicosities. Activation of pre-syn-aptic a2-receptors inhibits norepinephrine release. Through this negative feedback, release can be regulated. 

Termination of NE actions is mainly due to removal from the neuroeffector junction back into the sympathetic nerve ending via a NE transporter system (uptake 1) (4). At some sympathetic nerve endings, the NE released may activate prejunctional alpha adrenoceptors (5) involved in feedback regulation, which results in decreased release of the neurotransmitter. Diffusion away from the neuroeffector junction may also contribute to termination of actions. NE accumulated into target cells (e.g., via uptake 2) is rapidly inactivated by catechol-O-rnethykransferase (COMT). 

At some sympathetic nerve endings, the NE released may activate prejunctional alpha adrenoceptors (5) involved in feedback regulation, which results in decreased release of the neurotransmitter. Diffusion away from the neuroeffector junction may... 

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