CNS Pharmacology - A Simple, Structured Guide

The Big Idea: One Mental Model to Rule Them All

1. Mimics a neurotransmitter (agonist)
2. Blocks a neurotransmitter receptor (antagonist)
3. Increases neurotransmitter availability (blocks reuptake, blocks breakdown, stimulates release)
4. Decreases neurotransmitter availability (blocks synthesis, blocks release, depletes stores)
5. Modulates downstream signaling (second messengers, ion channels)

Step 1: Understand the Basic Wiring

The Neuron and Synapse

1. Action potential arrives at axon terminal
2. Voltage-gated Ca²+ channels open → Ca²+ flows in
3. Vesicles fuse, releasing neurotransmitter into the synaptic cleft
4. Neurotransmitter binds postsynaptic receptor
5. Signal is terminated by reuptake or enzymatic degradation

The Blood-Brain Barrier (BBB) - The Gatekeeper

• Highly lipid-soluble (lipophilic), OR
• Use a specific transporter

Step 2: Understand the Two Receptor Types

Step 3: Learn the 6 Key Neurotransmitter Systems

1. GABA - The Brain's Brake Pedal

• Role: Main inhibitory transmitter. When GABA is active, the neuron is less likely to fire.
• Receptor: GABA-A (ionotropic, Cl⁻ channel) | GABA-B (metabotropic)
• Drugs:
  • Benzodiazepines (diazepam, lorazepam): Enhance GABA-A → more Cl⁻ enters → more inhibition → sedation/anxiolysis/anticonvulsant
  • Barbiturates (phenobarbital): Similar, but higher overdose risk
  • Alcohol: Also enhances GABA-A
  • Vigabatrin: Blocks GABA breakdown → more GABA available (anticonvulsant)
• Memory hook: "GABA = Calm down. Drugs that boost GABA = calm/sedate. Drugs that block GABA = seizures."

2. Glutamate - The Gas Pedal

• Role: Main excitatory transmitter. Fires neurons.
• Receptors: AMPA (fast, Na+), NMDA (slow, Ca²+, needs glycine co-agonist), metabotropic
• Drugs:
  • Ketamine/Memantine: Block NMDA channel pore → anesthesia/anti-Alzheimer
  • Lamotrigine: Blocks Na+ channels, reduces glutamate release → anticonvulsant
• Memory hook: "Glutamate = Go. Block it = less excitation = fewer seizures, less pain, anesthesia."

3. Dopamine - The Reward & Movement System

• Pathways:
  • Nigrostriatal (SN → striatum): movement - loss causes Parkinson's
  • Mesolimbic (VTA → limbic system): reward/pleasure - excess = psychosis
  • Mesocortical (VTA → prefrontal cortex): cognition/motivation
  • Tuberoinfundibular (hypothalamus → pituitary): prolactin regulation
• Drugs:
  • L-DOPA: Dopamine precursor for Parkinson's (restores nigrostriatal dopamine)
  • Antipsychotics (haloperidol, risperidone): Block D2 receptors → reduces psychosis. Block nigrostriatal D2 → extrapyramidal side effects. Block tuberoinfundibular D2 → hyperprolactinemia.
  • Amphetamine/cocaine: Increase dopamine → euphoria, addiction
• Memory hook: "Dopamine - the 4 pathways decide all antipsychotic side effects."

4. Serotonin (5-HT) - Mood, Sleep, Appetite

• Origin: Raphe nuclei in brainstem → projects almost everywhere
• Drugs:
  • SSRIs (fluoxetine, sertraline): Block serotonin reuptake transporter → more 5-HT at synapse → treats depression/anxiety
  • SNRIs (venlafaxine): Block both serotonin AND norepinephrine reuptake
  • Triptans (sumatriptan): 5-HT1B/1D agonists → treat migraine
  • Ondansetron: 5-HT3 antagonist → antiemetic
  • Buspirone: 5-HT1A partial agonist → anxiolytic
• Memory hook: "Serotonin runs from the raphe to everywhere. SSRIs block its reuptake = more available."

5. Norepinephrine (NE) - Arousal & Attention

• Origin: Locus coeruleus → projects broadly
• Drugs:
  • SNRIs/TCAs: Block NE reuptake → antidepressant
  • Atomoxetine: Selective NE reuptake inhibitor → ADHD
  • Clonidine: Alpha-2 agonist → reduces NE release → lowers BP, treats ADHD/withdrawal
  • Beta-blockers (propranolol): Block peripheral beta receptors → reduce performance anxiety

6. Acetylcholine (ACh) - Memory, Arousal, Muscle Control

• Receptors:
  • Muscarinic (mAChR): CNS (memory, arousal), autonomic
  • Nicotinic (nAChR): Neuromuscular junction, autonomic ganglia, CNS reward
• Drugs:
  • Donepezil/Rivastigmine: Block acetylcholinesterase → more ACh → treats Alzheimer's
  • Atropine: Muscarinic antagonist → blocks CNS and peripheral ACh
  • Nicotine: nAChR agonist → stimulant, addiction
• Memory hook: "ACh = Memory. Block ACh in brain = confused/forgetful (anticholinergic side effects in elderly)."

Step 4: Where Drugs Act - Presynaptic vs Postsynaptic

Step 5: Quick Disease-Drug Mapping

Study Tips for CNS Pharmacology

1. Draw the synapse once - label every step (synthesis → storage → release → receptor → reuptake → degradation). Then place drugs at each step. This one diagram explains 80% of CNS drug mechanisms.
2. Group by transmitter, not by drug class - Don't study "antidepressants" as a list. Study "what happens when you increase serotonin?" and the drug names follow naturally.
3. Learn side effects from mechanism - Antipsychotics block D2 everywhere, so: nigrostriatal = EPS, tuberoinfundibular = elevated prolactin, mesolimbic = antipsychotic effect. No memorization needed.
4. Use the BBB as a filter - Always ask "can this drug enter the brain?" Lipophilic = yes. Polar = no. Second-generation antihistamines are polar on purpose to avoid sedation.
5. Ionotropic = fast drugs (acute use), Metabotropic = slow drugs (chronic use) - Benzodiazepines (GABA-A, ionotropic) work in minutes. SSRIs (metabotropic pathway changes) take 2-4 weeks. This explains clinical timing.
6. Retrograde signaling - Remember endocannabinoids go backwards (postsynaptic → presynaptic), which is why cannabis has unique effects. This is pharmacologically distinct from all other transmitters.