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I now have comprehensive data from Katzung's Basic and Clinical Pharmacology (16th Edition) across all major CNS drug categories. Let me compile the full detailed answer.
CNS Drugs Pharmacology
Source: Katzung's Basic & Clinical Pharmacology, 16th Edition (with references to Goodman & Gilman's)
1. INTRODUCTION TO CNS PHARMACOLOGY
Drugs acting in the CNS were among the first discovered by humans and remain the most widely used class of pharmacologic agents. They treat neurologic and psychiatric conditions, relieve pain, suppress nausea, and reduce fever.
Key principles:
- Nearly all CNS drugs act on specific receptors that modulate synaptic transmission
- General anesthetics and alcohol may have additional non-specific membrane actions
- Drugs acting on CNS help elucidate disease mechanisms (e.g., dopamine receptor blockade by antipsychotics led to the dopamine hypothesis of schizophrenia)
- GABA receptor drug studies advanced the understanding of anxiety and epilepsy
The CNS contains ~100 billion interconnected neurons. Neurons are classified by location, function, or neurotransmitter type. Key neurotransmitters include glutamate (excitatory), GABA (inhibitory), dopamine, serotonin, norepinephrine, and acetylcholine.
- Katzung's Basic and Clinical Pharmacology, 16th Ed., Ch. 21, p. 577
2. SEDATIVE-HYPNOTIC DRUGS (Chapter 22)
Definition
- Sedative (anxiolytic): reduces anxiety, calming effect with minimal CNS depression
- Hypnotic: produces drowsiness and promotes sleep - more CNS depression than sedation
- A dose-dependent continuum exists: anxiolytic → sedation → hypnosis → general anesthesia → death (especially with barbiturates)
Chemical Classes
A. Benzodiazepines
Mechanism: Positive allosteric modulators of GABA-A receptors. They increase the frequency of chloride channel opening (not duration). Require GABA to be present. Bind at the α-γ subunit interface.
Key drugs:
| Drug | t½ | Notes |
|---|---|---|
| Diazepam | 20-100 h | Active metabolite desmethyldiazepam (t½ >40 h) |
| Lorazepam | 10-20 h | No active metabolites; glucuronide conjugation only |
| Triazolam | 2-3 h | Short-acting hypnotic; α-hydroxylation |
| Midazolam | 1-4 h | IV use; water-soluble; CYP3A4 substrate |
| Alprazolam | 12-15 h | α-hydroxylation |
Biotransformation: Most via microsomal oxidation (CYP3A4) → Phase I metabolites (often active) → Phase II glucuronide conjugation → urine. Lorazepam, oxazepam, and estazolam go directly to glucuronide (preferred in liver disease).
CNS effects: Anxiolysis, sedation, hypnosis, anterograde amnesia, anticonvulsant activity, muscle relaxation.
Adverse effects: Dependence, tolerance, rebound insomnia, respiratory depression (enhanced with alcohol/opioids), anterograde amnesia.
Reversal: Flumazenil (competitive antagonist at GABA-A benzodiazepine site).
B. Barbiturates
Mechanism: Positive allosteric modulators of GABA-A receptors - increase duration of chloride channel opening. At high doses, can directly activate GABA-A without GABA. Also block AMPA receptors.
Dose-response: Linear - increasing dose leads to general anesthesia → respiratory/vasomotor center depression → coma → death. Lower therapeutic index than benzodiazepines.
Key drugs: Phenobarbital (long-acting, antiepileptic), thiopental (ultrashort, IV anesthesia), pentobarbital (short-acting).
Key point: Barbiturates are strong CYP enzyme inducers - many drug interactions.
C. Newer Hypnotics ("Z-drugs")
- Zolpidem, zaleplon, eszopiclone - bind to GABA-A receptors at benzodiazepine site but are structurally non-benzodiazepines. More selective for α1 subunit (sedation > anxiolysis/muscle relaxation). Lower abuse potential.
D. Other Sedative-Hypnotics
-
Buspirone: 5-HT1A partial agonist; anxiolytic without sedation, no physical dependence, delayed onset (1-2 weeks). No cross-tolerance with benzodiazepines.
-
Ramelteon: MT1/MT2 melatonin receptor agonist; sleep-onset insomnia.
-
Suvorexant: Orexin receptor antagonist; promotes sleep maintenance.
-
Katzung's, Ch. 22, pp. 597-615
3. ANTIEPILEPTIC (ANTISEIZURE) DRUGS (Chapter 24)
Seizure Classification (ILAE)
| Type | Examples |
|---|---|
| Focal aware (simple partial) | Unilateral motor/sensory symptoms |
| Focal impaired awareness (complex partial) | Automatisms, altered consciousness |
| Focal-to-bilateral tonic-clonic | Secondary generalization |
| Generalized tonic-clonic (grand mal) | Bilateral convulsions |
| Absence (petit mal) | Brief lapses, 3 Hz spike-wave; Lennox-Gastaut atypical |
| Myoclonic | Juvenile myoclonic epilepsy, Dravet syndrome |
| Atonic (drop seizures) | Lennox-Gastaut syndrome |
Mechanisms of Action
1. Sodium channel blockers (use-dependent):
- Bind preferentially to the inactivated state of Na+ channels
- High-frequency seizure discharges more effectively blocked than normal firing (use-dependence + voltage-dependence)
- Drugs: Phenytoin, carbamazepine, lamotrigine, oxcarbazepine, valproate (partial)
- Active in maximal electroshock (MES) model; not in pentylenetetrazol (PTZ) model
2. GABA enhancement:
- Benzodiazepines/Barbiturates - GABA-A allosteric modulators
- Vigabatrin - irreversible GABA-transaminase inhibitor (increases GABA)
- Tiagabine - GABA reuptake inhibitor (blocks GAT-1 transporter)
- Valproate - also enhances GABA synthesis/inhibits degradation
3. T-type calcium channel blockade:
- Ethosuximide - drug of choice for absence seizures; reduces low-threshold T-type Ca2+ currents in thalamic neurons
- Valproate also has T-type Ca2+ channel activity
4. Other mechanisms:
- Levetiracetam - binds SV2A (synaptic vesicle protein 2A); reduces neurotransmitter release
- Gabapentin/Pregabalin - bind α2δ subunit of voltage-gated Ca2+ channels; reduce excitatory neurotransmitter release
- Perampanel - non-competitive AMPA receptor antagonist
Key Antiepileptic Drugs
| Drug | Main Indication | Mechanism | Key Adverse Effects |
|---|---|---|---|
| Phenytoin | Focal, tonic-clonic | Na+ channel block | Gingival hyperplasia, hirsutism, ataxia, zero-order kinetics |
| Carbamazepine | Focal, tonic-clonic; trigeminal neuralgia | Na+ channel block | Diplopia, aplastic anemia, SIADH, CYP inducer |
| Valproate | Broad-spectrum (absence, focal, GTC, myoclonic) | Multiple | Hepatotoxicity, teratogen (NTD), weight gain |
| Ethosuximide | Absence seizures only | T-type Ca2+ block | GI disturbance, drowsiness |
| Lamotrigine | Broad-spectrum | Na+ channel block | Stevens-Johnson syndrome (esp. with valproate) |
| Levetiracetam | Broad-spectrum (adjunct) | SV2A binding | Irritability, behavioral changes |
| Phenobarbital | Focal, tonic-clonic; neonatal seizures | GABA-A (duration) | Sedation, cognitive impairment, CYP inducer |
| Gabapentin | Focal (adjunct); neuropathic pain | α2δ Ca2+ subunit | Somnolence, dizziness |
| Topiramate | Focal, GTC; migraine prophylaxis | Multiple | Cognitive impairment, kidney stones, weight loss |
- Katzung's, Ch. 24, pp. 640-680
4. ANTI-PARKINSON DRUGS (Chapter 28)
Pathogenesis of Parkinson Disease
- Loss of dopaminergic neurons in the substantia nigra pars compacta → reduced dopamine in striatum
- Pathologic hallmark: Lewy bodies (intracellular α-synuclein aggregates)
- Genetic mutations: α-synuclein (4q21), LRRK2 (12cen), Parkin (6q25), UCHL1
- Braak staging: pathology begins in olfactory nucleus/medulla (Stage 1-2) → midbrain/substantia nigra (Stage 3, motor symptoms) → cortex (Stages 4-6)
- Imbalance: reduced dopamine (inhibitory basal ganglia output) + relatively excess acetylcholine
Drug Classes
A. Dopamine Precursor
- Levodopa (L-DOPA): Crosses blood-brain barrier → converted to dopamine by DOPA decarboxylase. Always given with carbidopa (peripheral decarboxylase inhibitor) to reduce peripheral conversion and side effects, increase CNS availability.
- Adverse effects: Nausea, orthostatic hypotension, dyskinesias (with long-term use), "on-off" fluctuations, hallucinations, impulse control disorders.
B. Dopamine Agonists
- Pramipexole, ropinirole (non-ergot; D2/D3 agonists) - oral; used as monotherapy or adjunct
- Rotigotine - transdermal patch
- Bromocriptine - ergot derivative; D2 agonist
- Apomorphine - SC injection; for off-period rescue
- Less dyskinesia than levodopa but more hallucinations, somnolence, impulse control disorders
C. MAO-B Inhibitors
- Selegiline, rasagiline - irreversible, selective MAO-B inhibition → reduce dopamine breakdown
- Selegiline metabolizes to amphetamine metabolites (insomnia risk)
D. COMT Inhibitors
- Entacapone, tolcapone - inhibit catechol-O-methyltransferase → reduce levodopa/dopamine peripheral breakdown; used with levodopa to reduce "off" periods
- Tolcapone: risk of hepatotoxicity (restricted use)
E. Anticholinergics
- Benztropine, trihexyphenidyl - block muscarinic receptors; restore dopamine/ACh balance
- Mainly for tremor and drug-induced parkinsonism
- Avoid in elderly (confusion, urinary retention, constipation)
F. Amantadine
-
Antiviral drug with anti-Parkinson effects
-
Mechanism: NMDA antagonist + mild dopamine-releasing/reuptake blocking properties
-
Useful for mild symptoms and levodopa-induced dyskinesia
-
Katzung's, Ch. 28, pp. 773-800
5. ANTIPSYCHOTIC DRUGS (Chapter 29)
Indications
Schizophrenia, bipolar disorder (mania), psychotic depression, dementia-related psychosis, drug-induced psychosis.
Classification
| Generation | Examples | Key Feature |
|---|---|---|
| First-generation (typical/neuroleptic) | Chlorpromazine, haloperidol, fluphenazine, perphenazine | High D2 blockade; high EPS risk |
| Second-generation (atypical) | Clozapine, risperidone, olanzapine, quetiapine, aripiprazole, lurasidone, ziprasidone | Lower EPS; 5-HT2A blockade |
Mechanism of Action
All effective antipsychotics block D2 dopamine receptors. The relationship varies:
- Chlorpromazine: α1 = 5-HT2A > D2 > D1
- Haloperidol: D2 > α1 > D4 > 5-HT2A > D1 > H1
- Clozapine: D4 = α1 > 5-HT2A > D2 = D1
- Olanzapine: 5-HT2A > H1 > D4 > D2 > α1 > D1
- Aripiprazole: D2 = 5-HT2A > D4 (partial D2 agonist)
- Quetiapine: H1 > α1 > M1,2 > D2 > 5-HT2A
Second-generation drugs: equal or greater 5-HT2A blockade vs. D2 → fewer EPS, possible better negative-symptom coverage.
Receptor Effects → Toxicities
| Receptor Blocked | Clinical Effect |
|---|---|
| D2 (nigrostriatal) | EPS, tardive dyskinesia |
| D2 (tuberoinfundibular) | Hyperprolactinemia |
| α1 | Orthostatic hypotension |
| H1 | Sedation, weight gain |
| M1 (muscarinic) | Dry mouth, urinary retention, constipation |
| 5-HT2C | Weight gain, metabolic syndrome |
Extrapyramidal Side Effects (EPS)
- Acute dystonia - sustained muscle contractions (1-5 days); treat with benztropine/diphenhydramine
- Akathisia - motor restlessness (days-weeks); treat with propranolol/benzodiazepine
- Parkinsonism - bradykinesia, rigidity, tremor (weeks); treat with anticholinergics
- Tardive dyskinesia - involuntary orofacial movements (months-years); treat with valbenazine/deutetrabenazine (VMAT2 inhibitors)
- Neuroleptic Malignant Syndrome (NMS) - fever, rigidity, autonomic instability, elevated CK; treat with dantrolene + bromocriptine
Clozapine - Special Considerations
-
Only antipsychotic for treatment-resistant schizophrenia
-
Risk of agranulocytosis (1-2%) - mandatory WBC monitoring (ANC monitoring program)
-
Mechanism: D4 > D1 and α1 > 5-HT2A, low D2 - explains low EPS
-
Katzung's, Ch. 29, pp. 801-832
6. ANTIDEPRESSANT DRUGS (Chapter 30)
Pathophysiology of Depression
- Monoamine hypothesis - reduced serotonin (5-HT), norepinephrine (NE), dopamine
- Neurotrophic hypothesis - reduced BDNF (brain-derived neurotrophic factor), hippocampal atrophy
- HPA axis hypothesis - elevated cortisol, glucocorticoid receptor abnormalities
- Glutamate/NMDA hypothesis - basis for ketamine's rapid antidepressant effect
The delay in antidepressant action (2-4 weeks) is explained by the time needed for receptor down-regulation and neurotrophic factor synthesis (BDNF), not simply by immediate monoamine changes.
Drug Classes
A. SSRIs (Selective Serotonin Reuptake Inhibitors)
- Mechanism: Inhibit SERT (serotonin transporter) → increased synaptic 5-HT
- Drugs: Fluoxetine, sertraline, paroxetine, citalopram, escitalopram, fluvoxamine
- Escitalopram = S-enantiomer of citalopram (more selective)
- Indications: MDD, GAD, PTSD, OCD, panic disorder, PMDD, social anxiety disorder, bulimia
- Adverse effects: Sexual dysfunction, GI disturbance, insomnia/agitation, serotonin syndrome (with MAOIs), QTc prolongation (citalopram)
- Fluoxetine has the longest t½ (~2-6 days; active metabolite norfluoxetine ~1-2 weeks); fewer withdrawal symptoms
B. SNRIs (Serotonin-Norepinephrine Reuptake Inhibitors)
- Mechanism: Dual SERT + NET inhibition
- Drugs: Venlafaxine, desvenlafaxine, duloxetine, levomilnacipran
- Duloxetine: also approved for neuropathic pain, fibromyalgia, stress urinary incontinence
- Adverse effects: Hypertension (NE component), similar to SSRIs; venlafaxine withdrawal syndrome
C. TCAs (Tricyclic Antidepressants)
- Mechanism: Block SERT and NET; also block H1, muscarinic, α1 receptors
- Drugs: Amitriptyline, nortriptyline, imipramine, clomipramine, doxepin
- Clomipramine: most serotonergic; first-line for OCD
- Adverse effects: Anticholinergic (dry mouth, urinary retention), antihistaminic (sedation), α1 blockade (orthostatic hypotension), cardiac toxicity in overdose (Na+ channel block → QRS widening → fatal arrhythmia)
- Contraindicated after MI; increased risk in patients with cardiac disease
D. MAOIs (Monoamine Oxidase Inhibitors)
- Mechanism: Inhibit MAO-A (NE, 5-HT) and/or MAO-B (DA) → prevent monoamine breakdown
- Drugs:
- Irreversible non-selective: Phenelzine, tranylcypromine (tyramine cheese reaction risk)
- Reversible MAO-A inhibitor: Moclobemide (reversible → less tyramine reaction)
- Selective MAO-B: Selegiline (low dose; anti-Parkinson at higher dose)
- Critical drug interactions:
- Tyramine reaction (hypertensive crisis) with aged cheese, cured meats, wine
- Serotonin syndrome with SSRIs, TCAs, tramadol, meperidine, triptans
E. Atypical Antidepressants
- Bupropion: NE + dopamine reuptake inhibitor; no sexual dysfunction; also smoking cessation; contraindicated in seizure disorders/eating disorders
- Mirtazapine: α2 presynaptic antagonist + 5-HT2/3 antagonist; strong sedation and weight gain (H1 antagonism); useful in anorexic/depressed patients
- Trazodone: 5-HT2A antagonist + weak SERT inhibitor; mainly used for insomnia; risk of priapism
- Vortioxetine: SERT inhibitor + 5-HT receptor modulator; improves cognition
F. NMDA Receptor Antagonists (Novel)
- Esketamine (S-enantiomer of ketamine): intranasal; FDA-approved 2019 for treatment-resistant depression; rapid onset within 24 hours; dissociative side effects; short duration (5-7 days)
- Dextromethorphan + Bupropion (Auvelity): FDA-approved 2022; moderate NMDA antagonist + CYP2D6 inhibitor combination
G. GABA Modulators
-
Brexanolone (allopregnanolone): GABA-A positive allosteric modulator; approved for postpartum depression (2019); IV infusion over 60 hours
-
Katzung's, Ch. 30, pp. 833-870
7. SEDATIVE-HYPNOTIC AND ETHANOL (Chapter 23) - Key Points
Alcohol (Ethanol) Pharmacology
- Enhances GABA-A activity + inhibits NMDA receptors
- Acute tolerance and physical dependence (withdrawal = GABA-A underactivity + NMDA overactivity → seizures, DTs)
- Disulfiram: inhibits aldehyde dehydrogenase → acetaldehyde accumulates → aversive reaction (flushing, nausea, tachycardia)
- Naltrexone (μ-opioid antagonist): reduces alcohol craving
- Acamprosate: NMDA antagonist; reduces alcohol cravings
8. GENERAL ANESTHETICS (Chapter 25)
Mechanisms
General anesthetics produce: analgesia, amnesia, unconsciousness, and suppression of autonomic reflexes.
- GABA-A potentiation (most IV + volatile agents): propofol, barbiturates, etomidate, benzodiazepines, volatile agents (halothane, isoflurane, sevoflurane, desflurane)
- NMDA antagonism: Ketamine, nitrous oxide
- Meyer-Overton rule: Anesthetic potency correlates with lipid solubility (MAC inversely related to lipid solubility)
Key Agents
| Agent | Type | Key Points |
|---|---|---|
| Propofol | IV | Rapid onset/offset; TIVA; inhibits CYP; propofol infusion syndrome |
| Thiopental | IV (barbiturate) | Rapid onset; redistributes; cumulative with repeat dosing |
| Ketamine | IV | NMDA antagonist; dissociative anesthesia; preserves airway reflexes; bronchodilation; increases BP/HR; emergence delirium |
| Etomidate | IV | Minimal cardiovascular effects; adrenocortical suppression |
| Midazolam | IV benzo | Amnesia; premedication |
| Nitrous oxide | Inhaled | NMDA antagonist; weak anesthetic (MAC >100%); must use with other agents |
| Halothane | Inhaled | Hepatotoxicity risk (halothane hepatitis) |
| Sevoflurane | Inhaled | Low blood:gas partition → rapid induction/recovery; renal fluoride concern |
| Desflurane | Inhaled | Lowest blood:gas partition (fastest recovery); airway irritant |
- Katzung's, Ch. 25, pp. 450-480
9. LOCAL ANESTHETICS (Chapter 26)
Mechanism: Block voltage-gated Na+ channels from the intracellular side in the inactivated state (use-dependent block). Prevent nerve impulse generation and propagation.
Chemistry: Weak bases (pKa 8-9); require ionized form for binding + neutral form for membrane penetration.
- Inflamed tissue (lower pH) → more ionized form → reduced effectiveness
Order of nerve block: Small, unmyelinated fibers (pain/temperature) blocked first → larger myelinated fibers (touch, pressure) last.
Agents
| Drug | Duration | Notes |
|---|---|---|
| Lidocaine | Moderate | Most versatile; also antiarrhythmic |
| Bupivacaine | Long | Cardiotoxic in overdose; blocks cardiac Na+ channels; obstetric use |
| Ropivacaine | Long | Less cardiotoxic than bupivacaine |
| Procaine | Short | Ester; PABA metabolite (allergy risk) |
| Tetracaine | Long | Ester; spinal anesthesia |
Epinephrine added to prolong duration (vasoconstriction reduces absorption) - avoid in digits, nose, ear, penis.
Toxicity: CNS (tinnitus, seizures) > cardiovascular (arrhythmias, cardiac arrest)
- Katzung's, Ch. 26, pp. 481-500
10. OPIOID ANALGESICS (Chapter 31)
Receptor Types
| Receptor | Endogenous Ligands | Effects |
|---|---|---|
| μ (mu, MOP) | Endorphins, enkephalins | Analgesia, euphoria, respiratory depression, constipation, miosis, dependence |
| κ (kappa, KOP) | Dynorphins | Analgesia, sedation, dysphoria, miosis |
| δ (delta, DOP) | Enkephalins | Analgesia, mood modulation |
Mechanism: Opioid receptors are Gi/Go-coupled GPCRs → decreased cAMP, increased K+ conductance (hyperpolarization), decreased Ca2+ influx → reduced neuronal excitability and neurotransmitter release.
Key Opioid Drugs
| Drug | Notes |
|---|---|
| Morphine | Gold standard; active metabolite morphine-6-glucuronide (M6G); accumulates in renal failure |
| Codeine | Prodrug; CYP2D6 converts to morphine; ultra-rapid metabolizers at risk |
| Oxycodone | Oral; moderate-strong; abuse potential |
| Fentanyl | 100x more potent than morphine; IV/transdermal; rapid onset/offset |
| Sufentanil | Most potent opioid (500-1000x morphine); IV anesthesia |
| Methadone | Long t½; NMDA antagonist component; used in opioid dependence and chronic pain |
| Meperidine (pethidine) | Normeperidine metabolite → seizures; avoid in renal failure/MAOIs |
| Tramadol | Weak μ agonist + SNRI; lower abuse risk; risk with MAOIs/serotonin syndrome |
| Buprenorphine | Partial μ agonist + κ antagonist; ceiling effect on respiratory depression; opioid dependence |
| Naloxone | Pure opioid antagonist; IV reversal of opioid overdose; t½ 60-90 min (shorter than most opioids) |
| Naltrexone | Oral opioid antagonist; opioid/alcohol dependence treatment |
Adverse effects: Respiratory depression (main concern), constipation (no tolerance develops), nausea/vomiting, miosis, urinary retention, pruritus (histamine release with morphine).
Opioid dependence: Physical dependence - withdrawal includes anxiety, restlessness, lacrimation, rhinorrhea, piloerection, muscle aches, insomnia, diarrhea.
Summary Table: CNS Drug Categories
| Category | Primary Mechanism | Key Drugs |
|---|---|---|
| Sedative-Hypnotics | GABA-A enhancement | Benzodiazepines, barbiturates, zolpidem |
| Antiepileptics | Na+ channel block / GABA enhancement / T-Ca2+ block | Phenytoin, carbamazepine, valproate, ethosuximide, levetiracetam |
| Anti-Parkinson | Dopamine replacement/enhancement, ACh antagonism | Levodopa/carbidopa, dopamine agonists, MAO-B inhibitors, anticholinergics |
| Antipsychotics | D2 receptor antagonism (+5-HT2A for atypicals) | Haloperidol, clozapine, olanzapine, risperidone, aripiprazole |
| Antidepressants | Monoamine reuptake inhibition / MAO inhibition / NMDA antagonism | SSRIs, SNRIs, TCAs, MAOIs, bupropion, esketamine |
| General Anesthetics | GABA-A potentiation / NMDA antagonism | Propofol, ketamine, sevoflurane, thiopental |
| Local Anesthetics | Na+ channel block | Lidocaine, bupivacaine, ropivacaine |
| Opioid Analgesics | μ/κ/δ opioid receptor agonism (Gi-coupled) | Morphine, fentanyl, buprenorphine, naloxone |
Primary Sources:
- Katzung's Basic and Clinical Pharmacology, 16th Ed. - Chapters 21-31 (CNS section)
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th Ed.
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