Norepinephrine and epinephrine are two neurotransmitters that organisms utilize to send chemical signals around the body. Although the two neurotransmitters are very similar, there are a few significant differences.
Chemical structure
Both norepinephrine and epinephrine are catecholamines, meaning they are chemically derived from biosynthesis of a catechol. In the human, norepinephrine is synthesized from dopamine with the help of the enzyme dopamine beta hydroxylase (DBH). This enzyme adds a hydroxyl group from a molecule of O2 and ascorbic acid onto the beta carbon of the dopamine molecule. DBH is usually membrane bound, and remains in the cell membranes of the vesicles.
Epinephrine on the other hand, is downstream of norepinephrine synthesis. Norepinephrine can be processed by the enzyme phenylethanolamine N-methyltransferase, which adds a methyl group onto the terminal amine group.
Location of production
Although both substances are endogenously produced by the body, the location of where they are produced differs.
Norepinephrine functions as both a neurotransmitter and as a neurohormone. In the brain, a small region of cells called the locus coeruleus (LC) produces norepinephrine and releases it broadly to several areas of the brain. In this case, the norepinephrine molecules are stored in vesicles and released via exocytosis, therefore it acts as a neurotransmitter.
The peripheral nervous system also uses norepinephrine, particularly at the sympathetic ganglion. The sympathetic ganglion are a pair of neuronal chains that run from the upper neck down to the coccyx, running just ventral and lateral to the spinal cord. The sympathetic ganglion produces and releases norepinephrine onto the internal organs of the body. This innervation mediates the “fight or flight” response, controlling such effects as changes in blood pressure, respiratory rate, glucose metabolism, and many others.
Some norepinephrine is also synthesized and released into the bloodstream by the adrenal medulla, an endocrine structure that sits above the kidneys. In these circumstances, norepinephrine is signaling as a neurohormonal signal rather than a neurotransmitter.
Epinephrine is predominantly produced by specialized cells in the adrenal medulla. These cells, called chromaffin cells, are present both in the adrenal medulla as well as in a variety of other body parts. The overwhelming majority of the circulating epinephrine in our bloodstream is derived from these adrenal cells.
Receptor action
Both norepinephrine and epinephrine can be produced by the adrenal medulla. Both chemical signals are able to act at one of two classes of receptors, either the alpha adrenergic receptors or the beta adrenergic receptors. The alpha adrenergic receptors are divided into two subtypes, alpha1 and alpha2, while the beta receptors are divided into three subtypes, called beta1, beta2, and beta3.
Compared to epinephrine, norepinephrine has a higher selectivity for the alpha1 and beta1 receptors, and a lower selectivity for beta2. They have a similar binding capacity for both alpha2 and beta3.
Medical effect
Both norepinephrine and epinephrine have useful medical applications. Acute exposure to either substance acts to increased blood pressure, but there are different applications when each substance would be more appropriate.
Epinephrine is best known for the treatment of allergic reactions. The injectable form epinephrine, the Epi-pen, is widely available for people with severe, life-threatening allergies.