Which neurotransmitter imbalance most accurately describes the current neurobiological model of OCD, supporting the efficacy of SSRIs as first-line pharmacotherapy?
- A Deficient serotonin transmission in the cortico-striato-thalamo-cortical (CSTC) loop ✓
- B Excess dopamine in the nigrostriatal pathway
- C Excess norepinephrine in the locus coeruleus
- D Deficient GABA in the amygdala
Explanation
The neurobiological model of OCD centres on hyperactivity of the cortico-striato-thalamo-cortical (CSTC) circuit — specifically the orbitofrontal cortex and caudate nucleus, with deficient serotonergic modulation of this loop. SSRIs act by blocking serotonin reuptake, increasing serotonergic tone in the CSTC circuit and reducing loop hyperactivity. PET/fMRI studies show hypermetabolism of the orbitofrontal cortex and caudate at rest in OCD, which normalises with SSRI or CBT treatment. This model explains why SSRIs (in higher doses than for depression) and CSTC-targeting deep brain stimulation are effective in refractory OCD.
Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.
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