The sympathetic nervous system regulates cardiovascular function by activating adrenergic receptors which are known to have a critical role in regulating transmitter release from sympathetic nerves and from adrenergic neurons in the central nervous system. alpha 2A adrenergic receptor knockout mouse (alpha 2A-KO) showed an increase in sympathetic activity with resting tachycardia, depletion of cardiac tissue norepinephrine concentration. Knockout mouse study reveals that the alpha 2B adrenoceptor (alpha 2B AR) mediates the hypertensive response, and the alpha 2A AR mediates the hypotensive response, induced by alpha 2 agonists. Both the alpha 2A and alpha 2C AR subtypes are required for normal presynaptic control of transmitter release from sympathetic nerves. alpha 2A AR subtype inhibits transmitter release at high stimulation frequencies, whereas the alpha 2C AR subtype modulates neurotransmission at lower levels of nerve activity. Salt-induced hypertension experiment using alpha 2B-KO and alpha 2C-KO revealed that alpha 2B AR subtype is necessary to raise BP in response to dietary salt loading. Lack of adequately functional renal alpha 2B AR is thought to preclude reabsorption of sodium. alpha 1B-KO study revealed that alpha 1B AR is a mediator of the blood pressure response and the aorta contractile response induced by alpha 1 agonists.