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There is an increasing interest in the underlying mechanisms of the antidepressant and anxiolytic treatment effect associated with changes in serotonergic neurotransmission after treatment with selective serotonin (5-HT) reuptake inhibitors (SSRIs) in humans. The 5-HT 1A receptor is known to play a crucial role in the pathophysiology of affective disorders, and altered 5-HT 1A receptor binding has been found in anxiety patients. SSRI treatment raises the 5-HT level in the synaptic cleft and might change postsynaptic receptor densities. Therefore, our study in patients suffering from anxiety disorders investigated the effects of long-term treatment with escitalopram on the 5-HT 1A receptor. A longitudinal positrone emission tomography (PET) study in 12 patients suffering from anxiety disorders was conducted. Two dynamic PET scans were performed applying the selective 5-HT 1A receptor antagonist [carbonyl-11 C]WAY-100635. Eight regions of interest were defined a priori (orbitofrontal cortex, amygdala, hippocampus, subgenual cortex, anterior and posterior cingulate cortex, dorsal raphe nucleus and cerebellum as reference). After the baseline PET scan, patients were administered escitalopram (average dose of 11.2 ± 6.0 mg day À1 ) for a minimum of 12 weeks. A second PET scan was conducted after 109±27 days. 5-HT 1A receptor binding potentials in 12 patients were assessed by PET applying the Simplified Reference Tissue Model.There was a significant reduction in the 5-HT 1A receptor binding potential after a minimum of 12 weeks of escitalopram treatment in the hippocampus (P = 0.006), subgenual cortex (P = 0.017) and posterior cingulate cortex (P = 0.034). The significance of the hippocampus region survived the Bonferroni-adjusted threshold for multiple comparisons. These PET data in humans in vivo demonstrate a reduction of the 5-HT 1A binding potential after SSRI treatment.

The present study provides first-time evidence for a specific interrelation between the 5-HT(1A) receptor distribution, sex hormones, and aggression in humans. Our findings point to a reduced down-stream control due to higher amounts or activities of frontal 5-HT(1A) receptors in more aggressive subjects, which is presumably modulated by sex hormones.

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To conclude, when measured in the follicular phase, women do not differ from men in the 5-HT(1A) receptor binding. To explain the higher prevalence of affective disorders in women, further studies are needed to evaluate the relationship between hormonal status and the 5-HT(1A) receptor expression.