Norepinephrine, Epinephrine and Acetylcholine - Synthesis, Storage, Release and Metabolism
Norepinephrine Synthesis and Release
Norepinephrine (NE) is the primary neurotransmitter for postganglionic sympathetic adrenergic nerves. It is synthesized inside the nerve axon, stored within vesicles, then released by the nerve when an action potential travels down the nerve. Below are the details for release and synthesis of NE:
- The amino acid tyrosine is transported into the sympathetic nerve axon.
- Tyrosine (Tyr) is converted to DOPA by tyrosine hydroxylase (rate-limiting step for NE synthesis).
- DOPA is converted to dopamine (DA) by DOPA decarboxylase.
- Dopamine is transported into vesicles then converted to norepinephrine (NE) by dopamine β-hydroxylase (DBH); transport into the vesicle can by blocked by the drug reserpine.
- An action potential traveling down the axon depolarizes the membrane and causes calcium to enter the axon.
- Increased intracellular calcium causes the vesicles to migrate to the axonal membrane and fuse with the membrane, which permits the NE to diffuse out of the vesicle into the extracellular (junctional) space. DBH, and depending on the nerve other secondary neurotransmitters (e.g., ATP), is released along with the NE.
- The NE binds to the postjunctional receptor and stimulates the effector organ response.
Epinephrine Synthesis and Release
Epinephrine is synthesized from norepinephrine within the adrenal medulla, which are small glands associated with the kidneys. Preganglionic fibers of the sympathetic nervous system synapse within the adrenals. Activation of these preganglionic fibers releases acetylcholine, which binds to postjunctional nicotinic receptors in the tissue. This leads to stimulation of NE synthesis within adenomedullary cells, but unlike sympathetic neurons, there is an additional enzyme (phenylethanolamine-N-methyltransferase) that adds a methyl group to the NE molecule to form epinephrine. The epinephrine is released into the blood perfusing the glands and carried throughout the body.
Norepinephrine and Epinephrine Removal and Metabolism
The binding of NE to its receptor depends on the concentration of NE in the vicinity of the receptor. If the nerve stops releasing NE, then the NE concentration in the junctional cleft will decrease and NE will leave the receptor. There are several mechanisms by which the norepinephrine is removed from the intercellular (junctional) space and therefore from the postjunctional receptor:
- Most (~90%) of the NE is transported back into the nerve terminal by a neuronal reuptake transport system. This transporter is blocked by cocaine; therefore, cocaine increases junctional NE concentrations by blocking its reuptake and subsequent metabolism. (This is a major mechanism by which cocaine stimulates cardiac function and raises blood pressure.)
- Some of the junctional NE diffuses into capillaries and is carried out of the tissue by the circulation. Therefore, high levels of sympathetic activation in the body increase the plasma concentration of NE and its metabolites.
- Some of the junctional NE is metabolized within the extracellular space before reaching the capillaries.
- A small amount of NE (~5%) is taken up by the postjunctional tissue (termed "extraneuronal uptake") and metabolized.
NE (and epinephrine) is metabolized by catechol-O-methytransferase (COMT) and monoamine oxidase (MAO). The final product of these pathways is vanillylmandelic acid (VMA). This final product, along with its precursors normetanephrine and metanephrine, is measured in urine and plasma in the diagnosis of pheochromocytoma, which can cause severe hypertension and cardiac arrhythmias.
Acetylcholine Synthesis and Metabolism
Acetyl-CoA is synthesized from pyruvate by mitochondria within cholinergic nerves. This acetyl-CoA combines with choline that is transported into the nerve axon to form acetylcholine (ACh). The enzyme responsible for this is choline acetyltransferase. The newly formed ACh is then transported into vesicles for storage and subsequent release similar to what occurs for NE. After ACh is released, it is rapidly degraded within the synapse by acetylcholineesterase, to form acetate and choline.