Placebo Effects Are Small on Average in the 7.5% CO2 Inhalational Model of Generalised Anxiety.

BACKGROUND: Anxiety disorders are highly prevalent and socio-economically costly. Novel pharmacological treatments for these disorders are needed as many patients do not respond to current agents or experience unwanted side-effects. However, a barrier to treatment development is the variable and large placebo response rate seen in trials of novel anxiolytics. Despite this, the mechanisms that drive placebo responses in anxiety disorders have been little investigated, possibly due to low availability of convenient experimental paradigms. We aimed to develop and test a novel protocol for inducing placebo anxiolysis in the 7.5% CO2 inhalational model of generalised anxiety in healthy volunteers. METHODS: Following a baseline 20-minute CO2 challenge, 32 healthy volunteers were administered a placebo intranasal spray labelled as either the anxiolytic 'lorazepam' or 'saline'. Following this, participants surreptitiously underwent a 20-minute inhalation of normal air. Post-conditioning, a second dose of the placebo was administered, after which participants completed another CO2 challenge. RESULTS: Participants administered sham 'lorazepam' reported significant positive expectations of reduced anxiety (p = 0.001) but there was no group-level placebo effect on anxiety following CO2 challenge post-conditioning (p's > 0.350). Surprisingly, we found many participants exhibited unexpected worsening of anxiety, despite positive expectations. CONCLUSIONS: Contrary to our hypothesis, our novel paradigm did not induce a placebo response, on average. It is possible that effects of 7.5% CO2 inhalation on prefrontal cortex function, or behaviour in line with a Bayesian predictive coding framework, attenuated the effect of expectations on subsequent placebo response. Future studies are needed to explore these possibilities.

Pharmacological Treatment of Generalised Anxiety Disorder: Current Practice and Future Directions.

INTRODUCTION: Generalized Anxiety Disorder (GAD) is a common psychiatric condition, characterized by the presence of general apprehensiveness and excessive worry. Current management consists of a range of pharmacological and psychological treatments. However, many patients do not respond to first-line pharmacological treatments and novel anxiolytic drugs are being developed. AREAS COVERED: In this review, the authors first discuss the diagnostic criteria and epidemiology of GAD. The effective pharmacological treatments for GAD and their tolerability are addressed. Current consensus guidelines for treatment of GAD are discussed, and maintenance treatment, the management of treatment resistance, and specific management of older adults and children/adolescents are considered. Finally, novel anxiolytics under development are discussed, with a focus on those which have entered clinical trials. EXPERT OPINION: A range of effective treatments for GAD are available, particularly duloxetine, escitalopram, pregabalin, quetiapine, and venlafaxine. There is a limited evidence base to support the further pharmacological management of patients with GAD who have not responded to initial treatment. Although many novel anxiolytics have progressed to clinical trials, translation from animal models has been mostly unsuccessful. However, the potential of several compounds including certain psychedelics, ketamine, oxytocin, and agents modulating the orexin, endocannabinoid, and immune systems merits further study.