Chapter 32: Cholinergic Agonists

Cholinergic Agonists agents are broadly divided into two groups based on their mechanism of action. Direct-acting cholinergic agonists, including prototypes like bethanechol, carbachol, cevimeline, and pilocarpine, operate by binding directly to and stimulating ACh receptor sites on effector cells. Therapeutic uses for these agents include increasing detrusor muscle tone to treat neurogenic bladder and urinary retention (bethanechol), inducing miosis to alleviate intraocular pressure in glaucoma, and increasing secretions to treat dry mouth associated with Sjögren syndrome. Conversely, indirect-acting cholinergic agonists work by inhibiting the enzyme acetylcholinesterase (AChE), which is responsible for the immediate breakdown of ACh. By preventing this breakdown, they cause ACh to accumulate in the synaptic cleft, prolonging its effect. Indirect-acting agents that do not readily cross the blood–brain barrier, such as neostigmine and pyridostigmine, are used to manage myasthenia gravis, an autoimmune disease characterized by the progressive destruction of ACh receptor sites at the neuromuscular junction, leading to debilitating muscle weakness. Other indirect-acting agents, including donepezil, galantamine, and rivastigmine, are designed to cross the blood–brain barrier to increase ACh levels in the cortex; these are used to manage and slow the progressive memory loss associated with Alzheimer disease. Clinically, the administration of these drugs requires careful monitoring, especially when differentiating between a myasthenic crisis (requiring increased dose) and a cholinergic crisis (requiring drug withdrawal), often achieved diagnostically with edrophonium. Due to the widespread nature of ACh receptors, a major concern is the potential for generalized parasympathetic adverse effects, such as bradycardia, hypotension, increased GI activity leading to diarrhea, involuntary defecation, increased salivation, and urinary urgency, which may limit their therapeutic usefulness. Highly dangerous irreversible AChE inhibitors, such as nerve gas, cause a massive, fatal cholinergic response, necessitating the immediate use of antidotes like pralidoxime, often paired with atropine.