An anticholinergic agent is a substance that selectively blocks the neurotransmitter acetylcholine from binding to its receptor in the nerve cells of the central and peripheral nervous system, thus inhibiting parasympathetic nerve impulses.
Nerve fibers of the parasympathetic system are responsible for the involuntary movement of smooth muscles in the gastrointestinal and urinary tract, lungs, and other parts of the body. Anticholinergics are divided into categories in accordance to what subtype of receptor they bind to. The most common are antimuscarinic agents which bind to the muscarinic acetylcholine receptors. Others are antinicotinic agents that attach to the nicotinic acetylcholine receptors.
Anticholinergics are used to treat a multitude of conditions, and diseases, including vertigo, motion sickness, extrapyramidal symptoms, gastrointestinal disorders, peptic ulcers, diarrhea, pylorospasm, diverticulitis, ulcerative colitis, nausea, vomiting, genitourinary disorders in a short-term basis, respiratory ailments like asthma, chronic bronchitis and chronic obstructive pulmonary disease, sinus bradycardia due to a hypersensitive vagus nerve. Anticholinergics are also used in situations where a decrease of saliva is advantageous, for example, surgical procedures.
Among recreational users, anticholinergics are commonly referred to as deliriants and are usually considered the least enjoyable class of drugs because of the unpleasant side effects while not generally inducing euphoria. Thus the risk of addiction is low, and recreational use is uncommon.
Side effects, many of which resemble those in delirium, include poor coordination, dementia, decrease mucus production, decreased sweating leading to increased body temperature, pupil dilation and consequent sensitivity to light, decreased ability to focus, blurred vision, cycloplegia, double-vision, increased heart rate, tendency to be easily startled, urinary retention, urinary incontinence while sleeping, decreased bowel movement, ileus, and increased intraocular pressure which is dangerous in people with narrow-angle glaucoma, confusion, agitation, euphoria or dysphoria, respiratory depression, problems with memory, inability to concentrate, wandering thoughts, in inherent speech, wakeful myoclonic jerking, illogical thinking, visual disturbances, periodic flashes of light, periodic changes in visual field, visual snow, restricted or tunnel vision, visual, auditory, other sensory hallucinations, warping or waving of surfaces and edges, textured surfaces, dancing lines, spiders, insects and form constants, lifelike objects indistinguishable from reality, phantom smoking, hallucinated presence of people, seizures, coma, and death, severe drop in systolic blood pressure when suddenly standing up.
Anticholinergic syndrome subsides once all of the causative agents are excreted. Physostigmine can be used and a reversible acetylcholinesterase inhibitor and antidote in life-threatening cases, but come with the side effects associated with cholinergic excess. Racetams and choline activate the cholinergic system and alleviate the cognitive symptoms associated with long-term anticholinergic use. Nicotine also counteracts anticholinergics by activating nicotinic acetylcholine receptors. And caffeine, although an adenosine receptor antagonist, can neutralize the anticholinergic symptoms by reducing sedation and increasing acetylcholine activity.
Plants of the Solanaceae or nightshade family, such as henbane, belladonna, datura, brugmansia, and mandrake, contain various anticholinergic tropane alkaloids, for example, scopolamine, daturine, and atropine. Other plants rich in these compounds are garrya and corkwood.
Opiate-containing drug preparations, such as those containing hydrocodone and codeine are combined with an anticholinergic to deter misuses.