Expression of Cannabinoid Receptors in Mouse and Human Intrauterine Tissues: Insights into the Role of Endocannabinoid Signaling in Late Pregnancy

Abstract Introduction: Endocannabinoid signaling (ECS), mediated primarily by cannabinoid receptors (CNR) 1 and 2, is implicated in embryo implantation, decidualization, and placentation, but less is known about its role in late pregnancy or labour. Reports of elevated serum endocannabinoid concentrations during labour suggest that ECS may modulate uterine function leading up to and during parturition. Effects on uterine function likely vary depending on: the type of cannabinoid present, the intrauterine tissue, and CNR1/2 expression. Study of ECS gene expression in late pregnancy is therefore important to determine the contribution of this pathway to the normal and pathologic physiology of labour, for instance in preterm labour. Examining ECS gene expression in pregnancy may also contribute insight into the effects of exogenous cannabinoid consumption (reported in ~5% of pregnancies) on the late-pregnant uterus. To understand when and where cannabinoid signaling may impact uterine functions, we conducted an observational study on pregnant human and mouse intrauterine tissues. Methods: Human amnion, chorion, decidua, placenta, and upper/lower myometrium (n=6 participants) were biopsied at cesarean delivery (term, non-labour) to determine CNR expression by qPCR. Additional decidua (n=80) and upper/lower myometrium (n=82) samples were similarly obtained with and without labour, at term and preterm, for CNR expression analysis by qPCR. Mouse uteri for CNR expression analysis by qPCR were obtained from timed-mated C57BL6 mice at days 15-20 of pregnancy, in active labour, or post-partum. Human and mouse expression data were analyzed by Student’s t-test, one-way ANOVA (Bonferroni post-hoc), and Pearson’s correlation, as appropriate. Results: Term non-labour chorion, placenta, myometrium and decidua express CNRs, with 2-fold higher expression of CNR1 versus CNR2. In myometrium and decidua, neither CNR1 or CNR2 expression differed with labour status. CNR1 expression correlated positively with gestational age in decidua (r=0.346, p=0.0266) and lower segment myometrium (r=0.3667, p=0.0270) with labour. In mouse uteri, Cnr1 and Cnr2 expression significantly increased post-partum compared to mid/late pregnancy (vs day 19, p<0.0001 Cnr1; vs days 15-18, p<0.05 Cnr2). Conclusion: ECS may occur in the human decidua and myometrium throughout pregnancy and labour as CNR1 and CNR2 expression is maintained throughout. In our mouse model, higher Cnr1 and Cnr2 expression post-partum suggests that ECS may play a role in uterine resolution following delivery. Additional RNAseq analysis of the mouse tissues studied is underway to address whether expression of genes involved in production, transport or metabolism of cannabinoids are altered in late pregnancy or with the initiation of parturition.