Placebo Effects in the Neuroendocrine System: Conditioning of the Oxytocin Responses

Abstract

OBJECTIVE: There is evidence that placebo effects may influence hormone secretion. However, only few studies examined placebo effects in the endocrine system, including oxytocin placebo effects. We studied whether it is possible to trigger oxytocin placebo effects using a classical conditioning paradigm. METHODS: Ninety-nine females were assigned to a conditioned, control or drug-control group. In the two-phase conditioning paradigm, participants in the conditioned and drug-control groups received an oxytocin nasal spray combined with a distinctive smell (conditioned stimulus, CS) during three acquisition days, while the control group received placebo spray. Subsequently, the conditioned and control groups received placebo spray with the CS and the drug-control group- oxytocin spray during three evocation days. Salivary oxytocin was measured several times during each day. Pain sensitivity and facial evaluation tests previously used in oxytocin research were also administered. RESULTS: On evocation day 1, in the conditioned group oxytocin significantly increased from baseline to 5 minutes after CS (B[slope]=19.55, S.E.=5.88, p<.001) and remained increased from 5 to 20 (B=-10.42, S.E.=5.81, p=.071) and 50 minutes (B=-0.70, S.E.=3.37, p=.84). On evocation day 2, a trend for increase in oxytocin was found at 5 minutes (B=15.22, S.E.= 8.14, p=0.062). No placebo effect was found on evocation day 3 (B=3.57, S.E.=3.26, p=0.28). Neither exogenous nor conditioned oxytocin affected pain or facial tasks. CONCLUSIONS: Results indicate that oxytocin release can be conditioned and that this response extinguishes over time. Triggering hormonal release by placebo manipulation offers various clinical possibilities, such as enhancing effects of pharmacological treatments or reducing dosages of medications. TRIAL REGISTRATION: The study was registered as a clinical trial on www.trialregister.nl (number NTR5596).