Buhler-notebookLM

Molecular Targets at the Dopaminergic Synapse

1. DA Synthesis (LAADC): L-DOPA is converted to DA by L-amino acid decarboxylase (LAADC)^[6][7]. Enhancing synthesis increases synaptic DA pool.

2. Vesicular Loading (VMAT): Vesicular Monoamine Transporter packages DA into presynaptic vesicles^[6][7]. Inhibiting VMAT depletes vesicular DA storage and reduces release.

3. Exocytotic Release (VG-Ca²⁺ Channels): Voltage-gated calcium channels trigger calcium influx required for exocytosis^[6][7]. Blocking Ca²⁺ channels decreases DA release.

4. Post-Synaptic Receptors (D1-like & D2-like GPCRs): D1-like (mostly stimulatory) and D2-like (mostly inhibitory) metabotropic GPCRs^[6][8]. Agonists activate downstream pathways; antagonists block native DA signaling.

5. Synaptic Breakdown (COMT): Catechol-O-methyltransferase expressed post-synaptically breaks down synaptic DA and NE^[6][7]. COMT inhibitors decrease DA breakdown, enhancing synaptic DA levels.

6. Presynaptic Uptake (DAT): Dopamine Transporter reuptakes DA from the synaptic cleft back into the presynaptic terminal^[6][7]. DAT inhibitors (e.g., bupropion) prolong synaptic DA duration.

7. Presynaptic Breakdown (MAO-B): Monoamine Oxidase B degrades intracellular DA after reuptake^[6][7]. MAO-B inhibitors (e.g., selegiline) prevent cytosolic DA degradation.

Symptoms of Schizophrenia & Synaptic Pathophysiology

• Positive Symptoms (Psychosis):

  • Manifestations: Delusions (paranoid, grandiose), hallucinations (auditory, visual), disorganized speech, and disorganized behavior^[18].

  • Pathology: TOO MUCH / HIGH DA release in subcortical regions (Mesolimbic pathway) acting onto D2 receptors^[19].

• Negative Symptoms:

  • Manifestations: Affective flattening, alogia (paucity of speech), avolition (lack of drive), and anhedonia/apathy^[18].

  • Pathology: TOO LITTLE / LOW DA release in the prefrontal cortex (Mesocortical pathway) acting onto D1 receptors^[18].

• Cognitive Symptoms:

  • Manifestations: Cumulative decline in Executive Function, including impaired attention and working memory^[18][23].

  • Pathology: TOO LITTLE / LOW DA release in the prefrontal cortex (Mesocortical pathway) acting onto D1 receptors^[18].

Role of Dopamine in Motor and Prolactin Pathways & Antipsychotic ADEs

• Nigrostriatal (Basal Ganglia) Pathway:

  • Normal Role: Controls motor movement^[20][24]. Dopamine levels are baseline normal in schizophrenia^[25][26].

  • Antipsychotic ADEs: D2 receptor blockade (especially with 1st-generation typicals) lowers D2 activity below baseline, causing Extrapyramidal Symptoms (EPS) / anti-dopaminergic dyskinesias^[25]:

    • Pseudoparkinsonism: Tremors, rigidity, and gait instability^[27].

    • Acute Dystonia: Involuntary muscle contractions^[27].

    • Akathisia: Severe body restlessness and fidgetiness^[27].

    • Tardive Dyskinesia (TD): Involuntary chewing, lip smacking, tongue movements, and limb jerking^[27][28].

• Tuberohypophyseal / Tuberoinfundibular (Pituitary) Pathway:

  • Normal Role: Dopamine released into the pituitary gland acts on D2 receptors to tonic-inhibit prolactin release^[20][29]. Dopamine levels are baseline normal in schizophrenia^[25][26].

  • Antipsychotic ADEs: Blocking D2 receptors removes prolactin inhibition, causing hyperprolactinemia (resulting in swollen milk glands/galactorrhea and sexual dysfunction)^[17].

1st Generation (Typical) Antipsychotics:

• Drugs: Haloperidol (Haldol), fluphenazine (Prolixin), perphenazine (Trilafon), chlorpromazine (Thorazine)^[11][30].

• Mechanism: Strong D2 receptor antagonists^[11][31].

• Uses: Decreases positive symptoms of schizophrenia; some used as antiemetics^[10][11].

• ADEs: High risk of EPS and hyperprolactinemia; dysphoria/lowered reward; no benefit for negative symptoms^[11].

2nd Generation (Atypical) Antipsychotics

• Drugs: Quetiapine (Seroquel), risperidone (Risperdal), ziprasidone (Geodon), olanzapine (Zyprexa), paliperidone (Invega), iloperidone (Fanapt), asenapine (Saphris), lurasidone (Latuda), clozapine (Clozaril)^[33].

• Mechanism: Moderate D2 receptor antagonists + 5HT2A receptor antagonists^[34][36].

• Uses: Decreases positive symptoms AND decreases negative symptoms^[34][36].

• ADEs: Reduced risk of EPS and hyperprolactinemia relative to 1st gen; significant metabolic changes (weight gain, elevated triglycerides), sedation (H1 block), orthostatic hypotension (α1 block), anticholinergic effects, QT prolongation, and agranulocytosis (specifically clozapine)^[17][30].

3rd Generation (Atypical) Antipsychotics

• Drugs: Aripiprazole (Abilify), brexpiprazole (Rexulti), cariprazine (Vraylar)^[16][33].

• Mechanism: D2 receptor partial agonists + 5HT2A antagonists + 5HT1A partial agonists^[16].

• Uses: Decreases positive, negative, and cognitive symptoms^[16].

• ADEs: Minimal risk of EPS and hyperprolactinemia due to partial agonist baseline stabilization^[37][38].

Muscarinic Agents (Xanomeline / Trospium - Cobenfy / KarXT)

• Mechanism: No direct D2 blockade! Xanomeline is a central ACh M1 & M4 agonist that upstream-modulates and normalizes dopamine release^[39][40]. Trospium is a peripheral-only M2/M3 antagonist that counteracts peripheral pro-muscarinic side effects without crossing the blood-brain barrier^[39][40].

• Uses: Decreases positive, negative, and cognitive symptoms^[39][40].

• ADEs: Avoids D2-blockade motor/endocrine side effects; peripheral cholinergic ADEs are mitigated by trospium^[39][40].

Parkinson’s Agents & Pro-dopaminergic Drugs

• L-DOPA: Substrate for LAADC to increase DA synthesis^[6][7].

• Bromocriptine (Parlodel): D2 agonist used for Parkinson's and to suppress prolactin (drying up milk supply)^[10].

• Selegiline: MAO-B inhibitor that prevents intracellular DA breakdown^[10].

Antiemetics / GI Motility Drugs:

Metoclopramide (Reglan), Promethazine (Phenergan), Prochlorperazine (Compazine), Droperidol (Inapsine): D2 antagonists that prevent nausea and increase GI motility^[10][12].

NDRIs & ADHD Stimulants:

Bupropion (Wellbutrin): Norepinephrine-Dopamine Reuptake Inhibitor (DAT block) used as an antidepressant and smoking cessation aid^[10].

Specialized Agent — Pimavanserin (Nuplazid)

• Mechanism: Selective 5HT2A/5HT2C inverse agonist/antagonist with no direct D2 blockade^[41][42].

• Use: The only antipsychotic approved for Parkinson’s Disease Psychosis because it reduces mesolimbic hyperactivity without worsening basal ganglia motor function^[41][42]

Class MOA Differences

Antipsychotic Class	Primary Receptor MOA	Circuit-Level Actions	Clinical Effects Produced
1st Gen (Typical)	Strong D2 Antagonism (50–80%+ block)[11]	• Mesolimbic: Blocks excess D2 signaling[11][32] • Mesocortical: No D1 effect[32][43] • Nigrostriatal & Pituitary: Blocks normal baseline D2 signaling[11][32]	• Reduces positive symptoms[32] • No help for negative/cognitive symptoms[32] • High risk of EPS and hyperprolactinemia[11][32]
2nd Gen (Atypical)	Moderate D2 Antagonism + 5HT2A Antagonism[34][36]	• Mesolimbic: Moderate D2 block[34] • Mesocortical: Blocks presynaptic inhibitory 5HT2A receptors, disinhibiting DA release onto D1 receptors[14][34] • Nigrostriatal/Pituitary: Milder D2 block[36]	• Reduces positive symptoms[34] • Improves negative symptoms[34] • Lower risk of EPS and hyperprolactinemia[36]
3rd Gen (Atypical)	D2 Partial Agonism + 5HT2A Antagonism + 5HT1A Partial Agonism[16]	• Mesolimbic (High DA): Acts as an antagonist to lower overstimulation[16][45] • Mesocortical (Low DA): 5HT2A block + 5HT1A agonism elevates/stabilizes DA[16] • Nigrostriatal (Normal DA): Maintains baseline tone[37][38]	• Reduces positive, negative, and cognitive symptoms[16] • Minimal/reduced EPS and hyperprolactinemia[37][38]
Muscarinic Agents (Cobenfy)	ACh M1 & M4 Agonist (xanomeline) + Peripheral M2/M3 Antagonist (trospium)[39][40]	• Upstream Modulation: Selectively decreases DA release in subcortical/striatal regions while increasing DA release in the PFC and hippocampus[39][40] • No Direct D2 Blockade in Basal Ganglia/Pituitary[39][46]	• Treats positive, negative, and cognitive symptoms[39][40] • Avoids D2-blockade EPS and prolactin side effects[39][40]