T. Engelhardt, MD, PhD, FRCA, C. Zaarour, MD, and M. W. Crawford, MBBS, FRCPC, “Plasma cyclic guanosine 3′,5′-monophosphate levels: A marker of glutamate-nitric oxide-guanyl cyclase activity?”, J of Opioid Management, vol. 7, no. 6, pp. 462–466, Nov. 2017.