Antipsychotic and Antimanic Drugs

Classification

Typical: low-potency(chlorpromazine, thioridazine) · high-potency(haloperidol, fluphenazine, trifluoperazine). Atypical: clozapine, olanzapine, risperidone, quetiapine, aripiprazole, ziprasidone. Antimanic: lithium, valproate, carbamazepine(latter two = Antiepileptics mechanism).

Mechanism

Typicals: block D2 — efficacy correlates with D2 affinity(dopamine hypothesis evidence base). Therapeutic: mesolimbic blockade. SAME blockade elsewhere = SE profile: nigrostriatal→EPS · tuberoinfundibular→↑prolactin(removes tonic inhibition) · mesocortical→worsens NEGATIVE symptoms(apathy, withdrawal, blunted affect — D2 blockade doesn’t treat these, may worsen). Low-potency(chlorpromazine): weaker D2 affinity→higher dose needed→more off-target(antihistaminic=sedation, antimuscarinic, α1=postural hypotension). High-potency(haloperidol): more D2-selective→more EPS, less sedation/anticholinergic — potency/SE TRADE-OFF, not “better” class.

Atypicals: defining feature = combined 5-HT2A+D2 antagonism, HIGHER 5-HT2A:D2 ratio than typicals(reversed). 5-HT2A block in nigrostriatal pathway offsets D2-driven EPS(serotonin normally inhibits dopamine release there → blocking 5-HT2A restores some dopamine tone) = actual mechanistic basis for “atypical,” not marketing. Clozapine: WEAK D2 affinity + potent 5-HT2A/D4/muscarinic/histaminergic — uniquely effective in treatment-resistant schizophrenia(≥2 failed trials), only agent with this evidence. Aripiprazole: D2 PARTIAL agonist(not full antagonist) → dampens excess mesolimbic tone while preserving some activity where already low(mesocortical) → favourable EPS/prolactin profile.

Lithium: mechanism incompletely understood. Best-supported: ↓inositol monophosphatase(disrupts phosphoinositide 2nd-messenger cycle) + ↓GSK-3β(neuroplasticity/mood). Effective: acute mania + long-term prophylaxis(↓both manic+depressive relapse) + UNIQUE ↓suicide risk in bipolar(distinct from general mood-stabilizing effect).

Adverse Effects — Typicals: EPS Timeline

Acute dystonia(hrs-days): sustained painful spasm, oculogyric crisis/torticollis → Rx: anticholinergics/antihistamines. Akathisia(days-weeks): subjective distressing restlessness, OFTEN MISTAKEN for worsening psychiatric agitation → Rx: dose↓, β-blockers, benzodiazepines. Drug-induced parkinsonism(weeks-months): bradykinesia/rigidity/tremor → Rx: anticholinergics/dose↓ (full detail: Antiparkinsonian Drugs). Tardive dyskinesia(months-years exposure): involuntary, OFTEN IRREVERSIBLE orofacial/limb choreoathetoid movements — striatal D2 receptor UPREGULATION/supersensitivity from chronic blockade. Paradoxically worsened by anticholinergics + by abrupt drug cessation. MOST FEARED long-term complication(irreversibility).

Neuroleptic Malignant Syndrome (NMS)

Rare, life-threatening, idiosyncratic D2-blockade reaction: hyperthermia + severe “lead-pipe” rigidity + autonomic instability + altered consciousness, ↑↑CK(muscle breakdown). Rx: STOP antipsychotic immediately + supportive cooling/hydration + dantrolene(direct skeletal muscle relaxant, ryanodine receptor, ↓Ca2+ release — same drug as malignant hyperthermia) OR bromocriptine(dopamine agonist, restores dopaminergic tone NMS’s D2 block removed) — mirror-image logic of the syndrome.

Adverse Effects — Clozapine

AGRANULOCYTOSIS = defining, potentially fatal SE → MANDATORY regular blood count monitoring(weekly initially) → WHY reserved for treatment-resistant disease despite superior efficacy. Also: seizures(↓threshold more than others, dose-related), myocarditis, marked weight gain/metabolic syndrome, sialorrhoea(excessive salivation — paradoxical given own antimuscarinic activity; separate M4-agonist action).

Adverse Effects — Atypicals General

Much LOWER EPS/tardive dyskinesia risk than typicals BUT greater METABOLIC burden — weight gain, dyslipidaemia, new-onset/worsened diabetes. Most: olanzapine, clozapine. Least: aripiprazole, ziprasidone. GENUINE trade-off(movement vs metabolic risk), not simply “safer.”

Adverse Effects — Lithium

NARROW therapeutic index → regular plasma level monitoring. Tremor, polyuria/polydipsia(interferes with ADH at collecting duct → nephrogenic-DI-like picture), hypothyroidism(↓thyroid hormone release). Toxic: coarse tremor, ataxia, confusion, seizures. Toxicity POTENTIATED by hyponatraemia/dehydration/thiazides(lithium handled renally like Na+ → ↑proximal Na+ reabsorption→↑lithium reabsorption too) → counsel CONSISTENT salt/fluid intake, not avoidance of one trigger.

Clinical Relevance

Four-pathway framework(mesolimbic/nigrostriatal/tuberoinfundibular/mesocortical) makes the ENTIRE antipsychotic SE profile predictable, not memorized separately: same D2 blockade treating psychosis in one pathway causes EPS+hyperprolactinaemia+worsened negative symptoms in the other three, purely by which pathway. Atypicals’ advantage = genuine SPECIFIC mechanism(5-HT2A protecting nigrostriatal pathway), not vague “newer/gentler” claim.

Classification

Typical (first-generation) antipsychotics
- Low potency: chlorpromazine, thioridazine
- High potency: haloperidol, fluphenazine, trifluoperazine
Atypical (second-generation) antipsychotics: clozapine, olanzapine, risperidone, quetiapine, aripiprazole, ziprasidone
Antimanic/mood-stabilizing agents: lithium, valproate, carbamazepine (the latter two covered under Antiepileptics for their mechanism)

Mechanism of Action

Typical antipsychotics: block D2 dopamine receptors, with antipsychotic efficacy correlating closely with D2 affinity across the class (the original evidence base for the dopamine hypothesis of schizophrenia) — efficacy is attributed to blocking excess dopaminergic activity in the mesolimbic pathway. The same D2 blockade, however, occurs non-selectively across all four major dopamine pathways, and blockade at the other three explains essentially the entire adverse-effect profile: nigrostriatal pathway blockade produces extrapyramidal symptoms (drug-induced parkinsonism, as covered under Antiparkinsonian Drugs), tuberoinfundibular pathway blockade removes dopamine’s normal tonic inhibition of prolactin release (hyperprolactinaemia), and mesocortical pathway blockade is thought to worsen the negative symptoms of schizophrenia (apathy, social withdrawal, blunted affect) that D2 blockade does nothing to treat and may aggravate. Low-potency agents (chlorpromazine) have weaker D2 affinity requiring higher doses, which brings along more off-target receptor blockade (antihistaminic — sedation; antimuscarinic — anticholinergic effects; α1-adrenergic — postural hypotension); high-potency agents (haloperidol) are more D2-selective at their effective dose, producing comparatively more extrapyramidal symptoms and less sedation/anticholinergic burden for an equivalent antipsychotic effect — this potency/side-effect trade-off, not one class being simply “better,” is the actual organizing distinction.

Atypical antipsychotics: the defining pharmacological feature is combined 5-HT2A and D2 receptor antagonism, with most atypicals showing relatively higher 5-HT2A affinity than D2 affinity (the reverse ratio from typicals) — serotonergic antagonism in the nigrostriatal pathway is thought to counteract/offset the D2-blockade-driven extrapyramidal effects there (serotonin normally inhibits dopamine release in this pathway, so blocking 5-HT2A partially restores dopamine tone despite D2 blockade), explaining atypicals’ substantially lower extrapyramidal symptom burden — the actual mechanistic basis for the “atypical” label, not merely a marketing distinction. Clozapine is mechanistically distinct even within this class — comparatively weak D2 affinity (among the weakest of any effective antipsychotic) alongside potent 5-HT2A, D4, muscarinic, and histaminergic activity — and is uniquely effective in treatment-resistant schizophrenia (failure of at least two adequate trials of other antipsychotics), the only agent with this specific evidence base, though its use is constrained by its most serious adverse effect (below). Aripiprazole is mechanistically unique again — a D2 partial agonist rather than a full antagonist, which stabilizes dopaminergic tone (dampens it where excessive in mesolimbic pathways, while providing enough intrinsic activity to avoid fully blocking it where dopamine tone is already low, as in the mesocortical pathway) rather than uniformly blocking it — the pharmacological basis for its comparatively favourable extrapyramidal and prolactin side-effect profile.

Lithium: mechanism remains incompletely understood despite decades of clinical use — the best-supported components are inhibition of inositol monophosphatase (disrupting the phosphoinositide second-messenger cycle, reducing downstream signal amplification in neurons) and inhibition of glycogen synthase kinase-3β (GSK-3β), a kinase implicated in neuroplasticity and mood regulation. Effective for both acute mania and long-term prophylaxis of bipolar disorder (reducing both manic and depressive relapse), and carries a specific, well-established reduction in suicide risk in bipolar disorder distinct from its mood-stabilizing effect generally — a genuinely unique property among mood stabilizers.

Adverse Effects

Typical antipsychotics — extrapyramidal symptoms, a spectrum worth distinguishing by time course since management differs by which one is present: acute dystonia (hours to days — sustained, painful muscle spasm, often oculogyric crisis/torticollis, managed with anticholinergics or antihistamines); akathisia (days to weeks — subjective, distressing motor restlessness, often mistaken for worsening psychiatric agitation rather than a drug effect, managed with dose reduction, β-blockers, or benzodiazepines); drug-induced parkinsonism (weeks to months — bradykinesia, rigidity, tremor, managed with anticholinergics or dose reduction, covered fully under Antiparkinsonian Drugs); tardive dyskinesia (months to years of exposure — involuntary, often irreversible orofacial/limb choreoathetoid movements, thought to reflect striatal D2 receptor upregulation/supersensitivity from chronic blockade, paradoxically sometimes worsened by anticholinergics and by simply stopping the causative drug abruptly — the single most feared long-term extrapyramidal complication given its potential irreversibility).

Neuroleptic malignant syndrome (NMS): a rare but life-threatening idiosyncratic reaction to D2 blockade — hyperthermia, severe muscle rigidity (“lead-pipe”), autonomic instability, and altered consciousness, with markedly elevated creatine kinase from muscle breakdown. Managed by immediately stopping the antipsychotic, supportive cooling/hydration, and specific pharmacological treatment with dantrolene (a direct skeletal muscle relaxant, acting on the ryanodine receptor to reduce Ca²⁺ release, the same drug used for malignant hyperthermia) or bromocriptine (a dopamine agonist, restoring the dopaminergic tone that NMS’s D2 blockade removed) — the mechanistic mirror-image logic of the syndrome itself.

Clozapine: agranulocytosis — the specific, defining, potentially fatal adverse effect requiring mandatory regular blood count monitoring throughout therapy (weekly initially, tapering to less frequent with sustained normal counts) — this monitoring requirement is the reason clozapine is reserved for treatment-resistant disease rather than used more broadly despite its superior efficacy in that population. Also: seizures (dose-related, lowers seizure threshold more than other antipsychotics), myocarditis, marked weight gain/metabolic syndrome, and sialorrhoea (excessive salivation, a distinctive, almost paradoxical anticholinergic-adjacent effect given the drug’s own antimuscarinic activity, attributed to a separate muscarinic M4-agonist action).

Atypicals generally: substantially lower extrapyramidal/tardive dyskinesia risk than typicals (mechanism above), but a correspondingly greater burden of metabolic adverse effects — weight gain, dyslipidaemia, and new-onset/worsened diabetes mellitus, most pronounced with olanzapine and clozapine, comparatively less with aripiprazole and ziprasidone — a genuine trade-off (movement disorder risk versus metabolic risk) rather than atypicals being simply “safer” across the board.

Lithium: narrow therapeutic index requiring regular plasma level monitoring — tremor, polyuria/polydipsia (lithium interferes with ADH action at the collecting duct, producing a nephrogenic diabetes insipidus-like picture — a specific, examined mechanism), hypothyroidism (lithium inhibits thyroid hormone release), and at toxic levels, coarse tremor, ataxia, confusion, and seizures. Toxicity is potentiated by hyponatraemia/dehydration/thiazide diuretics (lithium is handled renally very similarly to Na⁺, so anything that increases proximal Na⁺ reabsorption increases lithium reabsorption alongside it, raising levels) — a frequently tested interaction/precipitant, and the reason patients on lithium are counselled to maintain consistent salt/fluid intake rather than to simply “avoid” any one triggering scenario.

Clinical Relevance

The mesolimbic/nigrostriatal/tuberoinfundibular/mesocortical four-pathway framework is what makes the entire antipsychotic adverse-effect profile predictable rather than memorized as an unrelated list: the same D2 blockade that treats psychosis in one pathway produces extrapyramidal symptoms, hyperprolactinaemia, and worsened negative symptoms in the other three, purely as a function of which pathway is affected — and atypical antipsychotics’ comparative advantage follows directly from a genuine, specific additional mechanism (5-HT2A antagonism partially protecting the nigrostriatal pathway), not from a vaguer claim of being a newer or gentler drug class.

Four dopamine pathways (mesolimbic, nigrostriatal, tuberoinfundibular, mesocortical) mapped to therapeutic effect vs each adverse effect of D2 blockade.
Extrapyramidal symptom timeline (acute dystonia → akathisia → parkinsonism → tardive dyskinesia) against duration of exposure.
NMS pathophysiology and the mirror-image logic of dantrolene/bromocriptine treatment.