Animal studies suggest that N-methyl-d-aspartate receptor (NMDAR) hypofunction and subsequent decline in intracellular nitric oxide (NO) are responsible for development of ketamine-induced psychedelic symptoms. To examine this mechanism in humans, we administered the NO donor sodium nitroprusside during infusion of racemic ketamine (RS-ketamine), containing equal amounts of S(+)- and R(–)-ketamine isomers, or esketamine, containing just the S(+)-isomer.
In this randomised, double blind, placebo-controlled crossover study, healthy volunteers were treated with sodium nitroprusside 0.5 μg kg−1 min−1 or placebo during administration of escalating doses of RS-ketamine (total dose 140 mg) or esketamine (70 mg). Drug high, internal and external perception, obtained using the Bowdle questionnaire, were scored over time on a visual analogue scale. The area-under-the-time-effect-curve (AUC) was calculated for each end-point.
Sodium nitroprusside significantly reduced drug high AUC [mean (standard deviation); placebo 9070 (4630) vs sodium nitroprusside 7100 (3320), P=0.02], internal perception AUC [placebo 1310 (1250) vs nitroprusside 748 (786), P<0.01] and external perception AUC [placebo 4110 (2840) vs nitroprusside 2890 (2120), P=0.02] during RS-ketamine infusion, but was without effect on any of these measures during esketamine infusion.
These data suggest that NO depletion plays a role in RS-ketamine-induced psychedelic symptoms in humans. The sodium nitroprusside effect was observed for R(–)- but not S(+)-isomer-induced psychedelic symptoms. Further studies are needed to corroborate our findings and assess whether higher sodium nitroprusside doses will reduce esketamine-induced psychedelic symptoms.