Norepinephrine

Norepinephrine is a catecholamine which functions to regulate many metabolic processes in the body. Norepinephrine is made from the neurotransmitter dopamine and can be converted into another catecholamine, epinephrine. Dopamine, norepinephrine, [Page 508]and epinephrine are considered catecholamines because they are derived from the same compound, catechol. Despite both being part of the catecholamine class, norepinephrine is classified as a neurotransmitter while epinephrine is classified as a hormone. Norepinephrine is made and released in the brain and the kidneys. It is released from these organs when blood pressure is low and during stressful conditions. During exercise, catecholamine levels increase. Norepinephrine and epinephrine are responsible for the “fight or flight” response during stressful conditions.

Norepinephrine has several effects on metabolic pathways in the body. Once it is released into the body, norepinephrine will bind to specialized receptors, known as adrenergic receptors, which are found on most tissues in the body. There are two classes of adrenergic receptors: alpha and beta. When norepinephrine levels are increased, it can activate enzymes which promote the utilization of stored energy. Glycogen and triglycerides are the storage forms for sugar and fat, respectively, and norepinephrine enables the release of these stored nutrients in metabolic processes called glycogenolysis and lipolysis.

Norepinephrine will also stimulate sugar production in the body, a process known as gluconeogenesis. Norepinephrine can also affect the heart by increasing the frequency and the strength of the heart beat. It will also increase blood pressure. As a result of norepinephrine stimulatory actions, it will increase the metabolic rate of a person. Norepinephrine also works on other systems in the body such as pupil dilation, sodium and potassium balance, and thyroid function.

Norepinephrine has the opposite effect of insulin on many metabolic pathways. Insulin promotes the storage of sugar and fat as glycogenesis and lipogenesis, respectively, while norepinephrine promotes the breakdown of sugar and fat. Within the brain, norepinephrine has many affects. Norepinephrine can affect feeding behavior and mood. Low levels of norepinephrine are associated with depression and increased food intake.

At pharmacological doses (very high), norepinephrine will cause the blood vessels to constrict and thus reduce blood flow to many tissues. This would subsequently lead to a decrease of norepinephrine's effects on tissue (i.e., decreased glycogenolysis and lipolysis). Agonists and antagonists of catecholamines are used as pharmaceutical agents. Agonists are used as appetite suppressants, general stimulants, asthmatic medication, and nasal decongestants. Antagonists are used to treat hypertension, coronary artery disease, and complications of hyperthyroid diseases.