Regulation by cannabinoid receptors of anandamide transport across the blood-brain barrier and through other endothelial cells.

The endocannabinoid anandamide (AEA) has many neurovascular activities. However, it is not yet clear how AEA can be metabolized at the neurovascular interface, and how it can move through the vascular and the cerebral compartments. The results reported in this article show that isolated bovine brain microvessels, an ex vivo model of the blood-brain barrier, have detectable levels of endogenous AEA and possess the biochemical machinery to bind and metabolize it, i.e. type-1 and type-2 cannabinoid receptors (CB1R and CB2R), a selective AEA membrane transporter (AMT), an AEA-degrading fatty acid amide hydrolase, and the AEA-synthesizing enzymes N-acyltransferase and N-acyl-phosphatidylethanolamines-specific phospholipase D. We also show that activation of CB1R enhances AMT activity through increased nitric oxide synthase (NOS) activity and subsequent increase of NO production. AMT activity is instead reduced by activation of CB2R, which inhibits NOS and NO release. In addition, binding experiments and immunoelectronmicroscopy demonstrate that different endothelial cells vary in the expression of CB1R and CB2R on the luminal and/or abluminal sides. The different localization of CBRs can lead to a diverse effect on AMT activity on the luminal and abluminal membranes, suggesting that the distribution of these receptors may drive AEA directional transport through the blood-brain barrier and other endothelial cells.

The expression of native and oxidized LDL receptors in brain microvessels is specifically enhanced by astrocytes-derived soluble factor(s).

Ex vivo rat brain microvessels express receptors for native as well as for oxidized low-density lipoproteins. In brain microvessels-derived endothelial cells, the expression levels of both receptors were enhanced by co-cultivation with rat astrocytes, even in the absence of actual contact between the two cell types, suggesting a soluble factor(s)-based mechanism of induction. No modulation effect could be evidenced in a heterologous cellular system. Since both receptors were found to be expressed also in astrocytes, these cells are likely to contribute substantially to the lipoprotein management at the blood-brain barrier and in the brain compartment.

Expression of OP4 (ORL1, NOP1) receptors in vascular endothelium.

Endothelial cells from rat brain microvessels, human aortic artery and human umbilical vein were examined, together with ex vivo rat brain capillaries and rat aortic ring sections, for the expression of opioid receptor-like OP-4 mRNA and protein. High levels of mRNA expression and an immunopositive reaction for the receptor protein were detected in the endothelial cells from primary and from established in vitro cultures, as well as in the intima of ex vivo rat aortic rings, where the signal was limited to the endothelial layer. Interaction of the OP4 receptor with its physiological ligand nociceptin caused, in cultured endothelial cells, the activation of a mitogen-activated protein (MAP) kinase cascade. Taken together, these results show that the OP4 receptor is synthesised and functionally expressed in endothelial cells, presumably as a starting point for some vasoactive mechanism(s).