Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 9/3/2004
Publication Date: N/A
Technical Abstract: Norepinephrine is a catecholamine and classified in the amine neurotransmitter group. Upon stimulation of the sympathetic nervous system, norepinephrine is synthesized and released from post-ganglionic nerve terminals to bind to alpha and beta adrenergic receptors. Activation of these receptors on or near effector tissues causes smooth muscle constriction or dilation dependent upon the specific tissue. Sympathetic nerve activation occurs in response to stimuli such as fear, pain, anxiety and stress. In a stress response, norepinephrine released from nerve terminals binds to alpha receptors on smooth muscle surrounding the vasculature to cause vasoconstriction in a matter of seconds. Dependant upon the region of vascularization, blood flow can be decreased from 'non-essential' tissues such as the skin or gastro-intestinal tract or blood flow is increased by increasing blood pressure. An increase in blood flow is critical in the stress response and by sympathetic activation, norepinephrine released from neurons in the heart, bind to beta receptors to cause increased force of contraction, increased conduction velocity, and increased heart rate. Not all norepinephrine is released from post-ganglionic sympathetic neurons. Upon stimulation of the sympathetic nervous system the medulla of the adrenal gland is stimulated to produce predominantly epinephrine, however, both epinephrine and norepinephrine are produced at a ratio of approximately 8:1. This source of norepinephrine is then released into the general vascular circulation where it can affect all cells containing adrenergic receptors. Like epinephrine, norepinephrine can be a useful measurement to researchers when assessing animal welfare. This is due to the fact that norepinephrine is consistently elevated in circulating blood when animals are exposed to aversive stimuli such as electric shock, restraint and injury. In contrast to epinephrine which is released directly into the peripheral vasculature, only a minor proportion of norepinephrine is released into the blood and the total norepinephrine measured in the blood is largely influenced by 'spill over' from nerve terminals. The measurement of norepinephrine is faced with the same challenges as that for epinephrine (see entry on Epinephrine).