Pharmacological Modification of Chemical Transmission

Introduction

This study guide summarizes the pharmacological mechanisms that modify neurochemical transmission, focusing on cholinergic and adrenergic systems. It covers indirect agonists, false transmitters, feedback control, co-transmitters, and receptor antagonists, providing foundational knowledge for biochemistry and pharmacology students.

Indirectly Acting Agonists

Indirect Cholinergic Agonists (Parasympathomimetics)

Indirect cholinergic agonists enhance cholinergic transmission by inhibiting the enzyme acetylcholinesterase (AChE), which breaks down acetylcholine (ACh) in the synaptic cleft.

• Acetylcholinesterase (AChE) Inhibitors: These drugs prevent the degradation of ACh, increasing its concentration and duration of action at cholinergic synapses.

• Examples: Neostigmine, Physostigmine, Endrophonium, Organophosphates

• Clinical Application: Used in the treatment of Myasthenia gravis, a neuromuscular disorder characterized by muscle weakness.

Mechanism of Action

• AChE inhibitors block the breakdown of ACh, leading to increased stimulation of muscarinic and nicotinic receptors.

• Key reaction: (catalyzed by AChE; inhibited by these drugs)

Indirect Adrenergic Agonists (Sympathomimetics)

Indirect adrenergic agonists increase the activity of adrenergic neurotransmitters (mainly noradrenaline/NE) by various mechanisms.

• Release of Noradrenaline: Tyramine, amphetamine, ephedrine stimulate the release of NE from presynaptic terminals.

• Blockade of Uptake-1: Cocaine and tricyclic antidepressants inhibit the reuptake of NE, increasing its synaptic concentration.

• MAO Inhibitors: Phenelzine, Selegiline inhibit monoamine oxidase, preventing NE breakdown and increasing its availability.

Mechanism of Action

• Indirect agonists either promote release, inhibit reuptake, or prevent degradation of NE.

• Key reaction (MAO):

False Transmitters

Definition and Mechanism

False transmitters are substances taken up by nerve terminals and incorporated into the biosynthetic pathway, forming alternative neurotransmitters that may have different effects.

• Examples: α-methyldopa (antihypertensive drug), α-methylnoradrenaline (acts as a false neurotransmitter)

• Mechanism: α-methyldopa is converted to α-methylnoradrenaline, which acts as a strong agonist at presynaptic and postsynaptic α2-adrenoceptors, reducing sympathetic outflow and lowering blood pressure.

Key Pathway

• Conversion: (via biosynthetic enzymes)

Feedback Control of Neurotransmitter Release (Presynaptic Modulation)

Heterotropic Interactions

One neurotransmitter (NT) affects the release of another NT, either inhibiting or enhancing its release.

• Examples: NA (noradrenaline) and adrenaline can inhibit ACh release; ACh can inhibit NA release.

Homotropic Interactions

A neurotransmitter affects its own release via presynaptic autoreceptors.

• α2-adrenoceptors: Inhibit NA release (negative feedback)

• β-receptors: Stimulate NA release (positive feedback)

Co-Transmitters and Neuromodulation

Definition and Functional Advantage

Neurons can release more than one neurotransmitter or modulator, leading to varied tissue responses and longer-lasting effects.

• One co-transmitter may be removed or inactivated more slowly, producing prolonged effects.

• Release may vary under different conditions (e.g., NPY is released at high stimulation frequencies).

Examples of Co-Transmitters

• ACh: Acetylcholine

• SP: Substance P

• NO: Nitric oxide

• GnRH: Gonadotropin-releasing hormone

• VIP: Vasoactive intestinal peptide

• NE: Nor-epinephrine

• NPY: Neuropeptide-Y

Adrenoceptors and Their Antagonists

Adrenoceptors: Types, Locations, and Effects

Adrenoceptors are G-protein coupled receptors responsive to catecholamines (adrenaline, noradrenaline). They are classified as α and β types, each with subtypes and distinct physiological effects.

Adrenoceptor Antagonists

Alpha-Adrenoceptor Antagonists

• Nonselective: Phentolamine, Phenoxybenzamine (irreversible, used for hypertension in pheochromocytoma)

• Selective α1 antagonists: Prazosin, Terazosin, Alfuzosin (used for hypertension and benign prostatic hypertrophy)

Beta-Adrenoceptor Antagonists

• Non-selective β-blockers: Propranolol, timolol, nadolol (block β1 and β2 receptors)

• Cardio-selective β1 blockers: Atenolol, metoprolol, bisoprolol, esmolol (primarily block β1 receptors)

• Both α and β blocking drugs: Carvedilol, labetalol

Clinical Uses and Side Effects

• Propranolol: Used for angina, hypertension, post-myocardial infarction; side effects include aggravation of asthma, gout, hyperlipidemia, CNS effects.

• Metoprolol, atenolol: Cardio-selective, do not cross blood-brain barrier; used for angina, myocardial infarction, hypertension, arrhythmias.

Summary Table: Mechanisms of Indirect Agonists and Antagonists

Key Terms and Definitions

• Agonist: A substance that activates a receptor to produce a biological response.

• Antagonist: A substance that blocks or dampens the action of an agonist at a receptor.

• Neurotransmitter: Chemical messenger released by neurons to transmit signals across synapses.

• Co-transmitter: A secondary neurotransmitter released alongside the primary neurotransmitter.

• False transmitter: A compound that replaces the natural neurotransmitter in vesicles and may have altered activity.

• Autoreceptor: A receptor located on the presynaptic neuron that regulates its own neurotransmitter release.

Summary Points

1. Acetylcholinesterase enzyme inhibitors are indirect agonists in cholinergic neurotransmission.

2. Amphetamine, tyramine, and ephedrine increase noradrenaline release; cocaine and tricyclic antidepressants block uptake-1 in adrenergic nerve terminals.

3. Alpha-methyl norepinephrine acts as a false neurotransmitter in adrenergic nerve terminals.

4. Neurotransmitter release is regulated by both homotropic and heterotropic mechanisms.

5. Co-transmitters are chemicals released along with the major neurotransmitter during nerve stimulation.

6. There are two types of adrenoceptors: alpha and beta.

7. There are nonselective and selective adrenoceptor antagonists used clinically.

8. Selective alpha-1 and beta-1 blockers are used to treat hypertension.

Additional info: This guide expands on the original notes by providing definitions, mechanisms, and clinical context for each drug class and receptor type, ensuring a comprehensive understanding suitable for biochemistry students.