Effect of social isolation on CB1 and D2 receptor and fatty acid amide hydrolase expression in rats.

Rearing rats in isolation has been shown to produce behavioral and neurochemical alterations similar to those observed in psychoses such as schizophrenia. Also, a dysregulation in both the endocannabinoid and dopaminergic systems has been implicated in schizophrenia. The aim of this study was to determine if there are differences in CB1 receptor and fatty acid amide hydrolase (FAAH) protein expression, as well as D2 dopamine receptor expression in different brain regions in rats reared in different environmental conditions. Twenty-one-day-old male Sprague-Dawley rats were either reared in individual cages (isolated rats) or in group cages of six per cage (group-housed rats) for 8 weeks. Quantitative fluorescence immunohistochemistry was performed on brain slices using antibodies specific to the CB1 or D2 receptor, or the enzyme FAAH. Raising rats in isolation led to a significant decrease in CB1 receptor expression in the caudate putamen and the amygdala, a significant increase in FAAH expression in the caudate putamen and the nucleus accumbens core and shell, and no significant change in D2 receptor expression in any region studied. These results indicate that the endocannabinoid system is altered in an animal model of aspects of psychosis. This implies that rearing rats under different housing conditions may provide new insight into the role of the endocannabinoid system in the development of psychoses.

Effects of CB(1) cannabinoid receptor activation on cerebellar granule cell nitric oxide synthase activity.

Cerebellar granule cells (CGCs) express the CB(1) subtype of cannabinoid receptor. CB(1) receptor agonists Win 55212-2, CP55940 and HU210 inhibit KCl-induced activation of nitric oxide synthase (NOS) in CGCs. Win 55212-2 has no effect on either basal NOS activity or on activation by N-methyl-D-aspartate and its effect is abolished by pre-treatment of the cells with pertussis toxin. The CB(1) receptor antagonist/inverse agonist SR141716A both reverses the effects of Win 55212-2 and produces an increase in NOS activity that is additive with KCl. These results support the hypothesis that activation of the CB(1) receptor in CGCs results in a decreased influx of calcium in response to membrane depolarization, resulting in a decreased activation of neuronal NOS.

Relationships between ligand affinities for the cerebellar cannabinoid receptor CB1 and the induction of GDP/GTP exchange.

The hypothesis of these studies is that ligand efficacy at the neuronal CB1 receptor is dependent on the ratio of ligand affinities for the active and inactive states of the receptor. Agonist efficacy was determined in rat cerebellar membranes using agonist-induced guanosine 5'-O-(3-[35S]thiotriphosphate) binding; efficacy was variable among the CB1 agonists examined. Ligand affinities for the active and inactive state of the CB1 receptor were determined by competition with [3H]CP55940 and [3H]SR141716A in the presence of 5'-guanylylimidodiphosphate, respectively. All of the agonists investigated had a higher affinity for the active state than the inactive state. The fraction of CB1 receptors in the active state at a maximally effective concentration was calculated for each agonist and was found to correlate significantly with agonist efficacy. These studies demonstrate that the CB1 receptor of the cerebellum can assume an active conformation in the absence of agonist and that the variability in efficacy among CB1 receptor agonists can be explained by the relative affinities of these ligands for the CB1 receptor in the active and inactive states.

Human platelets and polymorphonuclear leukocytes synthesize oxygenated derivatives of arachidonylethanolamide (anandamide): their affinities for cannabinoid receptors and pathways of inactivation.

Arachidonylethanolamide (AEA), the putative endogenous ligand of the cannabinoid receptor, has been shown to be a substrate for lipoxygenase enzymes in vitro. One goal of this study was to determine whether lipoxygenase-rich cells metabolize AEA. [14C]AEA was converted by human polymorphonuclear leukocytes (PMNs) to two major metabolites that comigrated with synthetic 12(S)- and 15(S)-hydroxy-arachidonylethanolamide (HAEA). Human platelets convert [14C]AEA to 12(S)-HAEA. 12(S)-HAEA binds to both CB1 and CB2 receptors with approximately the same affinity as AEA. 12(R)-HAEA, which is not produced by PMNs, has 2-fold lower affinity for the CB1 receptor and 10-fold lower affinity for the CB2 receptor than 12(S)-HAEA. 15-HAEA has a lower affinity than AEA for both receptors, with Ki values of 738 and >1000 nM for CB1 and CB2 receptors, respectively. The addition of a hydroxyl group at C20 of AEA resulted in a ligand with the same affinity for the CB1 receptor but a 4-fold lower affinity for the CB2 receptor than AEA. 12(S)-HAEA and 15-HAEA are poor substrates for AEA amidohydrolase and do not bind to the AEA uptake carrier. In conclusion, the addition of a hydroxyl group at C12 of the arachidonate backbone of AEA does not affect binding to CB receptors but is likely to increase its half-life. The addition of hydroxyl groups at other positions affects ligand affinity for CB receptors; both the position of the hydroxyl group and the configuration of the remaining double bonds are determinants of affinity.