Effects of Endocannabinoids in Acute Cerebral Ischaemia and Whole Blood Platelet Aggregation in the Rat

Cerebral ischaemia causes an increase in the endocannabinoid anandamide and expression of cannabinoid receptors in the brain. Endocannabinoids are known to exert anti-inflammatory effects and may reduce injury in cerebral ischaemia through modulation of the immune response. The aims of this study were to examine the effect of endocannabinoids, anandamide and 2-AG, in acute cerebral ischaemia and investigate the mechanisms involved, including characterising the microglia response. Endocannabinoids exert pro-aggregatory effects in human platelets and may affect cerebral ischaemia through actions on platelets. As such, it was decided to characterise the effects of endocannabinoids on platelet aggregation in rat whole blood. Anaesthetised rats underwent 4 hour permanent middle cerebral artery occlusion and received either: anandamide (10mg/kg, s.c.); 2-AG (6mg/kg, i.v.); metabolism inhibitors, URB597 (0.3mg/kg, s.c.) or JZL184 (10mg/kg, i.v.); or appropriate vehicle. Whole blood aggregometry was performed to examine the aggregatory effect of 2-AG (19-300μM) alone and in the presence of cannabinoid receptor antagonists and metabolism inhibitors. Anandamide and URB597 did not affect total injury volume but modified injury topography with reduced cortical and increased subcortical injury. In contrast, 2-AG and JZL184 increased cortical injury volume (111.7±9.4mm3 and 121.3±10.1mm3 vs. 82.3±3.5mm3) and JZL184 increased total injury (167.9±13.5mm3 vs. 134.7±5.7mm3). Neither anandamide nor 2-AG significantly affected microglia number or activation. The detrimental effect of 2-AG and JZL184 may be related to a reduction in cerebral blood flow (20.2±8.8% and 22.7±6.4 at 4 hours vs. 56.4±12.1% with vehicle). In rat whole blood 2-AG produced agregation at micromolar concentrations through CB2 receptors and COX metabolism to form thromboxane A2. Addition of 2-AG and ADP in combination potentiated the ADP response. Endocannabinoids modified injury development in acute cerebral ischaemia but did not affect the microglia response. The effects of 2-AG on injury may be related to changes in cerebral blood flow and platelet aggregation.