Cannabinoid type I receptor-mediated inhibition of neuronal noradrenaline reuptake in the mouse heart

Introduction/Background & aims: Activation of cannabinoid type I receptor (CB1) reduces noradrenaline (NAd) release from sympathetic nerve terminals innervating the heart and vasculature (1, 2). In contrast, the effect on NAd clearance is unknown. As the noradrenaline transporter (NAT) removes up to 90% of released NAd from the junctional cleft, here we extend the investigation of CB1-mediated effects to NAT reuptake rate in the cardiovascular system. Method/Summary of work: The left atrium (LA) and mesenteric arteries (MAs) from adult male C57BL/6 mice were isolated and exposed to a fluorescent NAT substrate [Neurotransmitter Transporter Uptake Assay (NTUA)]. The individual noradrenergic nerve terminals were then dynamically imaged with confocal microscopy, a technique we have recently developed (3). NTUA uptake rate was quantified follow- ing tissue pre-treatment with the non-specific CB receptor agonist WIN 55,212–2 and the CB1-specific agonist ACEA. Specificity was tested in the former with a CB1 inverse agonist, SR141716A. Statistical analyses were conducted on the number of nerve terminals (nt = 60/group) from 4 animals/group using a one-way ANOVA followed by a Tukey's post hoc test or Mann–Whitney t-test, where appropriate; significance was deemed at p < 0.05. Results/Discussion: In the mouse LA, WIN 55,212–2 (1 "M) signifi- cantly reduced single-terminal NTUA uptake rate by 35 ± 7% (14± 1%.min-1 vs. control: 22± 1%.min-1; p < 0.0001); this was CB1-specific as the effect was reversed with SR141716A (30 nM) (22± 1%.min-1 vs. control; p > 0.05). This was further demonstrated by a 30 ± 7% reduction in NTUA uptake rate by ACEA (100 nM) (18± 1%.min-1 vs. control: 25± 2%.min-1; p < 0.001). In contrast, neither CB receptor agonists affected NTUA uptake in nerve terminals innervating the MAs compared to their respective controls (control vs. drug; p > 0.05). Conclusion(s): We identify a CB1-mediated reduction in NAT reuptake rate in sympathetic terminals innervating the mouse heart, but not the mesenteric arteries. This reveals a new cardiac effect of can- nabinoids which counterbalances the inhibition of exocytosis to maintain the effects of sympathetic tone.