Effects And Mechanism Of Cannabinoids On Central Respiratory Activity

Natural cannabinoids are the constituents of the marijuana plant(Cannabis sativa),of which the principal psychoactive ingredient isâ–³9-tertrahydrocannabinol(â–³9-THC). There exists endocannabinoid system in mammalian body,including the ligands,the receptors and the enzymes for metabolism of endocannabinoids.Synthetic cannabinoids include non-selective and selective cannabinoid receptor agonists. Previous studies have showed that cannabinoids were able to affect respiratory functions in human and intact animals.However,up to date there has been no evidence whether the effects of cannabinoids on respiratory functions involve respiratory rhythmogenesis centers.The rostral ventrolateral medulla,where respiratory rhythmogenesis centers are located,expresses both CB1R and CB2R, which suggests that cannabinoids may be able to regulate the activity of respiratory rhythmogenesis centers.In 1884,Suzue et al established a model to study central respiratory activity in brainstem slices,and ever since this model has become classic in this field.Thus the first aim of this study is to investigate the effect of cannabinoids on central respiratory activity,and whether the effect is receptor-dependent. Cannabinoids also have regulatory effect on many receptors and ion channels via receptor-independent mechanisms.Recently it was found that an endocannabinoid arachidonoylethanolamide(AEA) is an inhibitor of ATP-sensitive potassium channels (K_atp)- Thus the second aim of this study is to test whether cannabinoids act as K_atp inhibitor to regulate respiratory centers.This second aim was tested by pharmacological interference of K_atp,by comparing the effects of cannabinoids and those of selective K_atpinhibitor glybenclamide on central respiratory activity,and by observing whether the effects of a selective K_atp opener diazoxide on central respiratory activity can be reversed by cannabinoids in receptor-independent manner. Following are the results:1.The effects and receptor-dependent mechanisms of cannabinoids on central respiratory activity(1) Non-selective cannabinoid receptor agonist WIN55212-2,selective CB1R agonist M1438 and CB2R agonist AM 1241 inhibited the frequency and area of central respiratory activity.These results indicate that cannabinoids inhibited central respiratory activity. (2) The effect of Ml438 was not prevented by CB1R antagonist AM251.These results indicate that the effect of Ml 438 is CBlR-independent.(3) The effect of AM 1241 was not prevented by CB2R antagonist AM630.These results indicate that the effect of AM 1241 is CB2R-independent.(4) The effect of AM1241 was not prevented by vanilloid receptorl(VR1) antagonist ruthenium red.These results indicate that the effect of AM1241 is VR1-independent.2.Receptor-independent mechanism of cannabinoids on central respiratory activity(1) The inhibitor of K_atp glybenclamide caused biphasic excitatory-inhibitory effect on central respiratory activity,which resembled the effects of cannabinoids in some slices.(2) Diazoxide caused severe inhibition,and even cessation in some slices,of central respiratory activity.This inhibition was reversed by glybenclamide;and was also reversed by CB1 receptor agonist Ml438 and CB2 receptor agonist AM 1241 in receptor-independent manner.These results indicate that cannabinoids act as K_atp inhibitor in respiratory centers.K_atp has been reported to be opened in some pathological conditions such as hypoxia,to be involved in the genesis of central apnea; and a recent study has found that cannabinoids is able to stabilize sleep respiratory rhythm in an animal model.Thus,it is possible that cannabinoids is also able to stabilize respiratory rhythm via inhibition of K_atp.