ADRENERGIC NEURON BLOCKING DRUGS act to prevent the release of noradrenaline from nerves in the sympathetic nervous system, which is involved in controlling involuntary autonomic functions including blood pressure, heart rate and the activity of muscles of internal organs (e.g. blood vessels, gastrointestinal tract, urogenital tract). Noradrenaline is the main neurotransmitter of the sympathetic nervous system, so adrenergic neuron blocker drugs act like other ANTISYMPATHETIC AGENTS to cause an overall fall in blood pressure. Their therapeutic action normally takes some weeks to develop, and their mechanisms of action result in some initial release of noradrenaline. The main use of such drugs is in ANTIHYPERTENSIVE therapy, but side-effects limit their use. Examples include bethanidine, bretylium, debrisoquine and guanethidine.