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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.
Epinephrine, also called adrenaline, has powerful effects on the body. Both epinephrine and norepinephrine work on alpha and beta receptors. Epinephrine has slightly more of an effect on your heart, while norepinephrine has more of an effect on your blood vessels. In response to stress, norepinephrine is involved in the fight-or-flight response of the sympathetic nervous system. The resultant increase in vascular resistance initiates a negative feedback mechanism, which in turn decreases heart rate and blood pressure (baroreceptor reflex).
Norepinephrine-producing cells are localized in the pons of the brain stem, a structure called the locus coeruleus. Norepinephrine exerts its effects by binding to α- and β-adrenergic receptors (or adrenoceptors, so named for their reaction to the adrenal hormones) in different tissues. The actions of norepinephrine are vital to the fight-or-flight response, whereby the body prepares to react to or retreat from an acute threat. He demonstrated the presence of norepinephrine in sympathetically innervated tissues and brain, and adduced evidence that it is the sympathin of Cannon and Rosenblueth. Norepinephrine prevents REM sleep, and lack of REM sleep increases noradrenaline secretion as a result of the locus coeruleus not ceasing producing it. The consequence is a massive increase in the amount of norepinephrine and epinephrine released into the bloodstream. It causes a distinctive set of symptoms including aches and pains, rapid heartbeat, elevated blood pressure, sweating, palpitations, anxiety, headache, paleness, and a drop in blood glucose.
Surviving Sepsis Campaign recommended norepinephrine as first line agent in treating septic shock which is unresponsive to fluid resuscitation, supplemented by vasopressin and epinephrine. Norepinephrine is also produced by Merkel cells which are part of the somatosensory system. Norepinephrine released by the locus coeruleus affects brain function in several ways. Noradrenergic cell group A1 is located in the caudal ventrolateral part of the medulla, and plays a role in the control of body fluid metabolism.
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.
In addition to being a hormone and neurotransmitter, epinephrine is also used as a medical treatment in its synthetic form. However, epinephrine has a greater effect on beta receptors compared with norepinephrine. Epinephrine and norepinephrine are very similar neurotransmitters and hormones. Norepinephrine can also be used to preserve blood pressure and maintain organ perfusion in cases of shock due to vasodilation (eg, septic shock and neurogenic shock). In ischemic heart disease, norepinephrine is used to maintain coronary perfusion pressure without increasing heart rate and cardiac output.
Norepinephrine has been reported to exist in a wide variety of animal species, including protozoa, placozoa and cnidaria (jellyfish and related species), but not in ctenophores (comb jellies), whose nervous systems differ greatly from those of other animals. There is also substantial evidence that many people with ADHD show biomarkers involving altered norepinephrine processing. If sympathetic activity is elevated for an extended time, it can cause weight loss and other stress-related body changes. Hyperactivation of the sympathetic nervous system is not a recognized condition in itself, but it is a component of a number of conditions, as well as a possible consequence of taking sympathomimetic drugs.
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.

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