chapter 7- ACETYLCHOLINE

Discussion about cosmetic procedures, specifically wrinkle treatments involving injections.
- Purpose: To reduce the appearance of wrinkles by inhibiting facial muscle contractions.
- Drawbacks: Users lose some emotional expressiveness due to muscle relaxation.

ACh is different from other neurotransmitters due to its clearance method.
- **Key Points:
- Activation:** ACh activates postsynaptic receptors.
- Clearance: Does not get cleared by reuptake.
- Enzymatic Degradation: Main method includes the action of an enzyme called acetylcholinesterase (ACHE).

Speed of Degradation: Acetylcholinesterase operates extremely quickly, degrading ACh in microseconds.
- Significance: Ensures tight regulation of cholinergic transmission in the body and brain, preventing excess ACh accumulation.
- Mechanism of Action: When ACh binds to cholinergic receptors, it is nearby acetylcholinesterase, which then hydrolyzes ACh into two byproducts: acetic acid and choline.

Choline recycling is crucial for ACh synthesis.
- Reuptake Process: Choline transporters take choline back into terminals after hydrolysis.
- Blocking Choline Transporters: Certain drugs, like hemikalinium three, inhibit reuptake, leading to reduced levels of ACh by limiting choline availability for synthesis.

ACh must be tightly regulated, as excessive levels can cause adverse effects.
Balance Aspect: A comparison to standard neurotransmitter reuptake: ACh degradation occurs faster, preventing prolonged activity.

Inhibitors of Acetylcholinesterase: These drugs increase ACh levels in the body.
- Example: Physostigmine is a reversible inhibitor that can be used therapeutically to enhance cholinergic transmission, especially in cognitive dysfunction.
- Alzheimer's Disease: Early treatment sometimes involves cholinesterase inhibitors to mitigate cognitive decline.
Irreversible Inhibitors: Some drugs bind permanently to the ACHE enzyme, such as those used in nerve agents (e.g., Sarin) causing excessive cholinergic activity and potentially causing lethal paralysis.

Overactivation of nicotinic and muscarinic receptors can lead to paralysis through mechanisms including desensitization and depolarization block.
- Desensitization: Receptors may become inactive temporarily after being excessively activated.
- Depolarization Block: Occurs when intense activation prevents the neuron or muscle from generating subsequent action potentials due to ion imbalance.

Nicotinic Receptors' Structure: Comprised of five protein subunits; different combinations lead to various subtypes with distinct functions and sensitivity to ACh.
Mechanism of Activation: ACh binding opens channels allowing sodium and calcium influx, leading to depolarization and excitation.
Functionality Variations: Differences in receptor subtypes affect ACh's excitatory response and duration of activation.

These receptors are metabotropic and have five subclasses (M1 to M5) with varied functions and locations.
- Peripheral Actions:
- M2: Found in the heart, reducing heart rate upon activation.
- M3: Resides in smooth and glandular muscles, promoting secretion and digestive activity.

Agonists: Such as pilocarpine, stimulate muscarinic receptors, enhancing secretory functions but can lead to cholinergic poisoning (SYNDROME: SLUDGE: Salivation, Lacrimation, Urination, Diarrhea, Gastrointestinal distress, Emesis).
Antagonists: Examples include atropine and scopolamine, which block muscarinic receptors; used in cases requiring vasodilation or reduced secretions, with various central nervous system effects.

Basal Forebrain Cholinergic System: Important for cognitive functions such as attention and memory.
Studies have shown that acetylcholine release is crucial in tasks requiring sustained attention.
- Task Example: Signal attention task showing correlation between task demands and acetylcholine release.
A specific neurotoxin targeting cholinergic neurons results in impaired attention, confirming ACh's role in cognitive processes.

ACh regulation is crucial for both peripheral and central nervous system health.
Dysregulation or disruption in ACh can lead to severe physiological and cognitive consequences.
Several pharmacological agents targeting ACh pathways are used for therapeutic benefits but carry potential risks and side effects.