Alpha receptors are divided into subtypes α1 and α2; beta receptors into subtypes β1, β2, and β3. Norepinephrine itself can further be converted into epinephrine by the enzyme phenylethanolamine N-methyltransferase with S-adenosyl-L-methionine as cofactor. L-DOPA is converted into dopamine by the enzyme aromatic L-amino acid decarboxylase (also known as DOPA decarboxylase), with pyridoxal phosphate as a cofactor. Norepinephrine consists of a catechol moiety (a benzene ring with two adjoining hydroxyl groups in the meta-para position), and an ethylamine side chain consisting of a hydroxyl group bonded in the benzylic position. These factors can also cause your body to start producing less epinephrine and norepinephrine. In addition, medications that increase norepinephrine can help with ADHD and depression. Norepinephrine, on the other hand, is used to treat dangerously low blood pressure. Although epinephrine can also be used for this purpose, norepinephrine is preferred due to its pure alpha receptor action. Norepinephrine given through an IV can help constrict blood vessels, increasing blood pressure. Β1Rs preferentially bind epinephrine, along with norepinephrine to a lesser extent and mediates some of their cellular effects in cardiac myocytes such as increased positive inotropy and lusitropy. Norepinephrine, also known as noradrenaline, plays an important role in your body’s fight-or-flight response. People with high norepinephrine levels have a greater risk of heart, blood vessel and kidney damage. Epinephrine is synthesized from norepinephrine by phenylethanolamine-N-methyltransferase in the cytoplasm. Epinephrine is primarily released by the adrenal medulla into the circulation; it is used as a neurotransmitter in only a small number of neurons. The binding of norepinephrine to its receptor activates second messenger signaling cascades that will cause either EPSPs or IPSPs, depending on the receptor subtype. When released from noradrenergic neurons, in the central and sympathetic nervous systems, norepinephrine acts as a neurotransmitter. When released from the adrenal medulla into blood circulation, norepinephrine functions as a hormone. After release, norepinephrine binds to adrenergic receptors on target cells. The release of norepinephrine from the synaptic vesicles is regulated by both stimulatory and inhibitory substances, including presynaptic α-adrenergic and β-adrenergic receptors. They are then absorbed back into the presynaptic cell, via reuptake mediated primarily by the norepinephrine transporter (NET). After an action potential, the norepinephrine molecules quickly become unbound from their receptors. Dopamine is then converted into norepinephrine by the enzyme dopamine β-monooxygenase (formerly known as dopamine β-hydroxylase), with O2 and ascorbic acid as cofactors. Thus the direct precursor of norepinephrine is dopamine, which is synthesized indirectly from the essential amino acid phenylalanine or the non-essential amino acid tyrosine. Stimulants often increase, enhance, or otherwise act as agonists of norepinephrine. Noradrenaline itself is widely used as an injectable drug for the treatment of critically low blood pressure. These results suggest that the increased release of NE from pineal sympathetic nerve endings stimulates testosterone aromatization and inhibits 5 alpha reduction via a beta-adrenoceptor. This can be contrasted with the acetylcholine-mediated effects of the parasympathetic nervous system, which modifies most of the same organs into a state more conducive to rest, recovery, and digestion of food, and usually less costly in terms of energy expenditure. Once back in the cytosol, norepinephrine can either be broken down by monoamine oxidase or repackaged into vesicles by VMAT, making it available for future release. Norepinephrine is stored in these vesicles until it is ejected into the synaptic cleft, typically after an action potential causes the vesicles to release their contents directly into the synaptic cleft through a process called exocytosis. Drugs in this group that are capable of entering the brain often have strong sedating effects, due to their inhibitory effects on the locus coeruleus. Yohimbine acts as a male potency enhancer, but its usefulness for that purpose is limited by serious side-effects including anxiety and insomnia. Drugs such as phentolamine that act on both types of receptors can produce a complex combination of both effects. Epinephrine, similarly to norepinephrine, also goes through reuptake into the presynaptic cell. Epinephrine also binds to α and β adrenergic receptors (described above for norepinephrine) and causes similar activity when bound to these receptors. Epinephrine, also called adrenaline, is a catecholamine, but it is often considered a hormone instead of a neurotransmitter.
