Tryptophan depletion does not modify response to CCK-4 challenge in patients with panic disorder after treatment with citalopram

Rationale Data by [Bell et al. J Psychopharmacol (2002) 16:5–14] suggest that a decrease in 5-HT neurotransmission predisposes to panic attacks and that the antipanic effect of SSRIs depends upon the availability of 5-HT in the brain.Objectives Our aim was to assess the effect of acute tryptophan depletion (TD) on cholecystokinin-tetrapeptide (CCK-4)- induced symptoms in patients with panic disorder (PD) who had responded to a 10-week treatment with a selective serotonin (5-HT) reuptake inhibitor (SSRI), citalopram.Materials and methods A total of 18 patients (6 males and 12 females, mean age 34.5 years) received a tryptophan-free amino acid drink and a control drink, each followed by a CCK-4 challenge (25 μg), 1 week apart in a double-blind crossover design.Results The results showed no significant differences in response to the CCK-4 challenge between the TD and the control conditions. Panic rate after the CCK-4 challenge was 27.8% after depletion and 33.3% after control drink (χ ²=0.13, p=0.72). No significant effects of TD were observed in panic intensity scores, subjective anxiety, or cardiovascular indices.Conclusions This study demonstrates that an acute lowering of brain 5-HT availability with TD does not affect response to a CCK-4 challenge in PD patients successfully treated with citalopram. Thus, the reduction of CCK-4 sensitivity following SSRI-treatment in patients with PD may be related to mechanisms other than 5-HT availability in the brain, possibly to a reduction in brain cholecystokinin receptor sensitivity.