OPRM1 and Bradycardia: It has been shown that perfusion of the fourth cerebral ventricle with the selective peptide μ OR agonist FK 33–824 induces bradycardia in the conscious dogs.25 In anesthetized dogs, [D‐Met2,Pro5]enkephalinamide perfusion through the cerebroventricular system elicited bradycardia, which was accompanied by an increase in the vagal discharge rate.26 It has been shown that intracisternal administration of opioid peptides also evoked bradycardia in unanaesthetized dogs.27 This effect was abolished by pretreatment with atropine.