Dopamine: Mechanism of Action in the CNS and CNS Diseases

1. Dopamine Basics - Synthesis, Storage, Release

• DAT (dopamine transporter) - reuptake back into the presynaptic terminal
• MAO-A/B (monoamine oxidase) - enzymatic degradation to HVA (homovanillic acid)
• COMT (catechol-O-methyltransferase) - methylation degradation

2. Dopamine Receptors - Subtypes and Signaling

• D1-like activation → Gs → ↑adenylate cyclase → ↑cAMP → activates PKA → phosphorylates CREB (transcription factor) → gene expression changes (including BDNF, dynorphin, enkephalin, tyrosine hydroxylase)
• D2-like activation → Gi → ↓cAMP → inhibition of PKA cascade; also opens K+ channels and closes Ca²+ channels → reduced neuronal excitability

3. The Five Classic Dopamine Pathways

(a) Nigrostriatal Pathway

• Origin: Substantia nigra pars compacta (SNpc) → Striatum (caudate-putamen)
• Function: Motor control, movement initiation
• DA receptors: D1 (on direct/"GO" pathway GABAergic neurons) and D2 (on indirect/"STOP" pathway neurons)
  • D1 stimulation activates the direct pathway → facilitates movement ("GO more")
  • D2 stimulation inhibits the indirect pathway → also facilitates movement ("STOP less")
• Disease relevance: Parkinson's disease

(b) Mesolimbic Pathway

• Origin: Ventral tegmental area (VTA) → Nucleus accumbens (ventral striatum) and other limbic structures
• Function: Reward, motivation, pleasure, reinforcement learning
• Regulated by: D3 autoreceptors at both VTA somatodendritic region and striatal terminals; few DATs = wider dopamine diffusion
• Disease relevance: Schizophrenia (positive symptoms), addiction, depression (anhedonia)

(c) Mesocortical Pathway

• Origin: VTA → Prefrontal cortex (PFC)
  • Dorsolateral PFC (DLPFC) - cognition and executive function
  • Ventromedial PFC (VMPFC) - emotions and affect
• Key feature: Very few presynaptic D2/D3 autoreceptors and few DATs in the PFC, so dopamine diffuses widely and is not tightly regulated
• Predominant postsynaptic receptor: D1 (excitatory)
• Disease relevance: Schizophrenia (negative and cognitive symptoms), ADHD, depression

(d) Tuberoinfundibular Pathway

• Origin: Hypothalamus → Anterior pituitary
• Function: Tonically inhibits prolactin release via D2 receptors
• Disease relevance: Blockade by antipsychotics → hyperprolactinemia (galactorrhea, amenorrhea, sexual dysfunction)

(e) Thalamic Pathway

• Less well-characterized; projects from multiple dopamine cell groups to thalamic nuclei; role in sensory gating and arousal

4. Dopamine in CNS Diseases

A. Parkinson's Disease

• The striatum loses dopamine input
• Without D1 activation on the direct pathway, the "GO" signal is lost (akinesia, bradykinesia)
• Without D2 inhibition of the indirect pathway, the "STOP" signal is overactive - excessive subthalamic nucleus (STN) output inhibits the thalamus → hypokinesia, rigidity, resting tremor
• The "funnel" of activity feeding the supplementary motor area (SMA) via the basal ganglia-thalamic circuit is effectively closed

B. Schizophrenia

• Therapeutic antipsychotic effect: ~65-75% D2 occupancy
• EPS (extrapyramidal side effects): >80% D2 occupancy
• Hyperprolactinemia: even modest D2 blockade in tuberoinfundibular pathway

C. ADHD

• ADHD patients have insufficient tonic DA (and NE) firing in the PFC
• Low DA → inadequate D1 and D3 stimulation in the DLPFC → poor working memory, sustained attention, and executive function
• Low DA in the anterior cingulate cortex → poor selective attention
• Phasic DA bursts in the nucleus accumbens, when dysregulated, reinforce impulsive behaviors

D. Depression

• Mesolimbic hypodopaminergia contributes to anhedonia (the inability to feel pleasure) and psychomotor slowing - cardinal features of depression
• Inflammation (proinflammatory cytokines) reduces dopamine synthesis, impairs dopamine release, and downregulates dopamine receptors
• Low dopamine in the nucleus accumbens = reduced motivation, loss of reward, apathy and lack of energy
• Antidepressants with dopaminergic action (bupropion - DAT/NET blocker; aripiprazole - partial D2 agonist) target this pathway

E. Addiction / Substance Use Disorders

• Repeated cocaine/amphetamine → compensatory ↑ CREB in nucleus accumbens → upregulates dynorphin (an endogenous kappa-opioid) → activates KOR on VTA DA neurons → reduces baseline dopamine tone (hypodopaminergic state)
• Reduced basal DA = anhedonia, craving, withdrawal dysphoria - drives compulsive drug-seeking
• ΔFosB accumulates (longer-lasting than CREB changes) → sustained gene expression changes underlying long-term addiction vulnerability

F. Huntington's Disease

• Loss of indirect "STOP" pathway → decreased basal ganglia inhibitory output → thalamus becomes disinhibited → excess thalamocortical drive → hyperkinesia, choreiform movements
• Over time, direct pathway neurons also die → late-stage rigidity and hypokinesia

G. Restless Legs Syndrome (RLS) and Periodic Limb Movement Disorder

H. Tourette Syndrome / Tics

5. Summary Table

Antipsychotics: D2 Receptor Occupancy Comparison

The Therapeutic Window - the Key Framework

• <60% D2 occupancy - sub-therapeutic (insufficient antipsychotic effect)
• 60-80% - therapeutic window (antipsychotic effect, minimal EPS)
• >80% - EPS threshold (parkinsonism, akathisia, dystonia, tardive dyskinesia risk)
• Clozapine exception: effective at only 40-50% due to preferential extrastriatal (temporal-limbic) binding and broad multi-receptor activity

Comparison Table: Antipsychotics by D2 Occupancy Profile

Key Principles Illustrated by the Table

1. The "Atypicality" Rule

2. Fast Dissociation ("Hit-and-Run") Kinetics

• Low EPS despite significant clinical efficacy
• Undetectable striatal D2 occupancy 24h post-quetiapine
• Clozapine's antipsychotic effect correlating more with extrastriatal (temporal-limbic cortex) than striatal D2 binding

3. Partial Agonists - Why High Occupancy Does Not Mean High EPS

4. Prolactin - the Tuberoinfundibular Pathway Proxy

• Have very fast dissociation (clozapine, quetiapine)
• Are partial agonists with residual D2 activity (aripiprazole → actually decreases prolactin in some patients)
• Have low affinity for pituitary D2 receptors

D2 Occupancy at a Glance (Visual Summary)

D2 Occupancy %:
0%   10%   20%   30%   40%   50%   60%   70%   80%   90%   100%
|-----|-----|-----|-----|-----|-----|-----|-----|-----|-----|
                              [CLOZAPINE max striatal ~50%]
                   [QUETIAPINE max striatal ~60-70%]
                              [ZIPRASIDONE ~45-75%]
                                    [OLANZAPINE 70-80%]
                                    [RISPERIDONE 70-80%]
                                    [LURASIDONE 65-80%]
                                    [HALOPERIDOL 75-90%]
                                    |         |
                             Efficacy↑       EPS threshold
                              (>60%)          (~80%)
                                         [ARIPIPRAZOLE ~85-95%*]
                                         [BREXPIPRAZOLE ~85-95%*]
                                         [CARIPRAZINE ~85-95%*]
      * partial agonists - high occupancy without EPS