Cholinergic and Anticholinergic Drugs
First, we will take a look at the different divisions of the nervous system.
The Nervous System
The nervous system is made up of the central nervous system (the brain and spinal cord) and the peripheral nervous system (neurons outside the brain and spinal cord). The peripheral nervous system is then divided into the autonomic and somatic nervous system.
The autonomic system is further broken down into the sympathetic and parasympathetic nervous system.
The sympathetic (SNS) and parasympathetic (PSNS) are opposing systems.
The SNS is the “fight or flight” response (AKA – arousing or adrenergic)
The PSNS is the “rest and digest” response (AKA – calming or cholinergic)
When the sympathetic system excites an organ, the parasympathetic system inhibits it. When the parasympathetic system excites an organ, the sympathetic system inhibits the action.
Our focus in this lesson is on how cholinergic and anticholinergic agents affect the parasympathetic nervous system.
Cholinergic drugs are:
- Drugs that stimulate the parasympathetic system
- Also called parasympathomimetics – they mimic the effects of the PSNS neurotransmitter
- Cholinergic agents copy the action of acetylcholine (ACh) – a neurotransmitter released from nerve endings that bind on the receptors of cell membranes of organs, tissues, and glands
There are two types of cholinergic drugs: direct-acting and indirect-acting.
Direct-Acting Cholinergic Drugs
- Bind to cholinergic receptors on specific effector organs, stimulating the organ in a similar way as ACh
- They are synthetic derivatives of choline
- Have widespread systemic effects including cardiac muscle, smooth muscle, exocrine glands, and the eye
Indirect-Acting Cholinergic Drugs
- Inhibit the enzyme ‘acetylcholinesterase,’ resulting in more ACh available at the receptors
- These drugs have the added cholinergic effect of improved skeletal muscle tone and strength
- Indirect-acting cholinergic drugs for Alzheimer’s disease are widely distributed, including to the central nervous system, thus improving cholinergic neurotransmission in the brain
Cholinergic Drug Effects
- CNS – enhanced cognitive functions such as arousal, attention, and memory encoding – treatment for Alzheimer’s disease and dementia
- Eye – pupil constriction – for surgery and treatment of glaucoma
- GI – smooth muscle stimulant – for post-op abdominal distention or paralytic ileus
- GU – urinary bladder stimulant – for post-op or postpartum urinary retention
- Musculoskeletal (indirect acting cholinergic drugs) – improve muscle tone and strength – for myasthenia gravis
Too much cholinergic medication can result in overstimulation of the parasympathetic nervous system, causing unwanted side effects.
The acronym SLUDGE-M will help us remember the adverse effects of cholinergic drugs.
Other Adverse Effects of Cholinergic Drugs
- ↓ HR and BP
- Conduction abnormalities – AV block and cardiac arrest
- Headache, dizziness, convulsions
- ↑ bronchial secretions, bronchospasms
*Overdosing can cause life-threatening problems
*Antidote for cholinergics is the anticholinergic drug atropine
Specific Cholinergic Drugs
- Bethanechol (Urecholine) – ↑ the tone and motility of the bladder and GI tract (should cause urination within 60 min in a pt with urinary retention).
- Pilocarpine (Pilocar) – used to constrict pupils, which ↓ intraocular pressure (glaucoma).
- Neostigmine (Prostigmin) – given for the diagnosis and treatment of myasthenia gravis—it causes skeletal muscle contractions.
- Donepezil (Aricept) – used to treat mild-moderate Alzheimer’s disease—it ↑ ACh in the brain and helps ↑ or maintain memory or learning capabilities (it manages the symptoms, but is not a cure).
Contraindications to using cholinergic drugs
- Peptic ulcer
- Coronary artery disease
*Cholinergic drugs can exacerbate these conditions and should be avoided.
- Drugs that block the action of ACh on the parasympathetic nervous system.
- These cholinergic blocking agents compete with ACh and block it at the receptors in the PSNS, so ACh is unable to bind to the receptor site and cause a cholinergic effect.
- Most anticholinergic drugs interact with muscarinic cholinergic receptors in the brain, secretory glands, heart, smooth muscle, and eye.
Effects of Anticholinergic Drugs
- CNS – ↓ muscle rigidity and muscle tremors (e.g., Parkinson’s disease)
- Eye – pupil dilation – for exams or surgery
- Salivary & lacrimal glands – ↓ secretion
- Heart – ↑ HR
- Respiratory -↓ bronchial secretions, dilate bronchial airways, ↓ airway resistance – COPD, asthma.
- GI – relax smooth muscle tone of GI tract, ↓ intestinal & gastric secretions, ↓ motility & peristalsis – peptic ulcer disease, and irritable bowel.
- GU – antispasmodic effect on smooth muscle – overactive bladder, and incontinence.
Adverse Effects of Anticholinergics
The effect of the drug may be therapeutic, but becomes an adverse reaction if severe or if the drug is given for another purpose or if there is an overdose.
- CNS – excessive stimulation (tremor, restlessness, confusion), followed by excessive CNS depression (respiratory depression, coma)
- Constipation (result of decreased GI motility and muscle tone)
- Dry mouth (result of decreased salivation)
- Urinary retention
- Hot, dry skin (due to decreased sweating)
- Blurred vision, dilation of the pupil (pt may need sunglasses)
*The specific antidote for anticholinergic overdose is
Specific Examples of Anticholinergic Drugs
- Atropine – a naturally occurring belladonna alkaloid, given for bradyarrhythmias (it produces a stimulant effect), also given as an antidote for cholinergic poisoning.
- Ipratropium (Atrovent) – causes bronchodilation, used in asthma and COPD.
- Scopolamine – given for motion sickness, N/V.
- Benztropine (Cogentin) – Also used to treat dystonic reactions caused by antipsychotic drugs
- Oxybutynin (Ditropan)
- Solifenacin succinate (VESIcare)
Contraindications to Using Anticholinergic Drugs
Contraindications include any condition characterized by symptoms that would be aggravated by the drugs (myasthenia gravis, glaucoma, MI).
Cholinergic drugs stimulate the parasympathetic nervous system by copying the action of Ach. They are given for Alzheimer’s disease, glaucoma, paralytic ileus, urinary retention, and myasthenia gravis.
Anticholinergic drugs block the action of ACh on the parasympathetic nervous system. They are given for Parkinsons’s disease, asthma, COPD, and overactive bladder
Thank you for watching this video tutorial on cholinergic and anticholinergic effects on the parasympathetic nervous system!