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Under stress conditions, higher level of epinephrine, released at increased rate from the adrenal medulla, are present in the synaptic cleft between noradrenergic varicosities and effector cells, whence epinephrine is taken up by the amine “pumps” into the varicosities and to its storage vesicles. Epinephrine can be released together with norepinephrine and activate the presynaptic β2-adrenoceptors, thus increasing further transmitter release.
Epinephrine, originally released from the adrenal medulla and taken up into the sympathetic neuron, is released, together with norepinephrine, from the varicosity. While epinephrine triggers an autofacilitatory feedback loop, acting on presynaptic β2-adrenergic receptors, norepinephrine, together with epinephrine, leads to an auto-inhibition of its release from sympathetic nerves mediated by presynaptic α2-adrenoceptors.
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© 2001 Alexander Binder
