CANNABINOID – INDUCED CHANGES IN RAT UTERINE CONTRACTILITY

Abstract

Background: Endocannabinoids are endogenous ligands for cannabinoid receptors. They have been
demonstrated in many mammalian tissues and are widely distributed in the CNS, peripheral nerves, leukocytes,
spleen and testicles. The uterus contains the highest levels of anandamide; the first discovered Endocannabinoid,
suggesting a particular role of anandamide in female reproductive system.
Objective: The present study was designed to demonstrate the effect of anandamide on spontaneous contraction
of pregnant and non pregnant rat uterus and to investigate the possible involvement of cannabinoid CB1
receptors, Nitric Oxide (NO), and small conductance Ca+2 activated K+ channels in anandamide induced effect.
Materials & Methods: The present study was carried out on a total number of 30 adult albino rats (24 females
and 6 males). The male rats were used for induction of pregnancy. The first day of pregnancy was determined by
the presence spermatozoa in the vaginal smear examined microscopically. The female rats were divided into four
equal groups each contains 6 rats. Three groups (non pregnant, day 10 and day 19 of gestation) were used to
study the effects of anandamide (10-6, 10-5 and 10-4 M/ml organ bath fluid) on spontaneous contractile activity of
isolated uterine strips. The fourth group (day 10 of gestation) was used to study the possible mechanisms of
action of anandamide using CB (1) receptor antagonist (AM251, 10-6 M/L), NG-nitro-L-arginine methyl ester (LNAME,
3 x 10-5 M/L), and small conductance Ca++ activated K+ channels Blocker (Apamin, 10-8 M/L).
Results: The present study showed that anandamide exerted a significant dose dependant reduction in frequency
and amplitude of spontaneous contraction of uterine strips isolated from both pregnant and non pregnant rats.
After incubation of uterine strips isolated from pregnant rats on day 10 of gestation with the specific CB1
receptor antagonist AM251, the utero-relaxant effect of anandamide was almost completely abolished. This
finding indicates that CB1 receptors are present in the rat uterus and may be the main receptor subtype involved
in endocannabinoid-induced uterine relaxation. It was also found that pretreatment of uterine strips with NO
synthase inhibitor (L-NAME) and small conductance-Ca+2 activated K+ channel blocker (Apamin) significantly
decreased the anandamide induced utero-relaxant effect. The utero-relaxant effect of anandamide was
significantly more potent in uterine strips isolated from pregnant rats on day 10 of gestation than that in both non
pregnant rats and pregnant rats on day 19 of gestation.
Conclusion: Anandamide exerts a potent relaxant effect in vitro on uterine smooth muscles isolated from
pregnant and non pregnant rat uterus. This relaxant effect is higher in mid-gestation and this may help uterine
quiescence during pregnancy, then diminishes in late pregnancy which may allow effective uterine contraction to
occur during labor. The direct relaxant effect of anandamide is mediated through binding with CB1 receptors.
Moreover, activation of nitric oxide generation and opening of small conductance Ca++ activated K+ channels
play a role in this anadamide induced utero-relaxant effect.