Pharmacological FMRI: Measuring Opioid Effects on the BOLD Response to Hypercapnia

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Abstract
Opioid binding to the cerebral blood vessels may affect vascular responsiveness and hence confound interpretation of blood oxygen level-dependent (BOLD) responses, which are usually interpreted as neuronal in origin. Opioid binding varies in different brain regions. It is unclear whether opioids alter neurovascular coupling, or whether their effects are purely neuronal. This study used BOLD functional magnetic resonance imaging (FMRI) to investigate the effect of a μ-opioid agonist remifentanil, on cerebrovascular CO2 reactivity (being one component of neurovascular coupling). Hypercapnic challenges were delivered to human volunteers, while controlling potential opioid-induced respiratory depression. The BOLD signal increase to hypercapnia was compared before and during remifentanil administration. Remifentanil was shown not to have a generalised effect on CO2 responsiveness in the cerebral vasculature. However, it caused a significant reduction in the positive BOLD response to hypercapnia in the bilatera...