Expression of cannabinoid CB 1 R–GPR55 heteromers in neuronal subtypes of the Macaca fascicularis striatum

Local activation of CB1 receptors by synthetic and endogenous cannabinoids dampens burst firing mode of reticular thalamic nucleus neurons in rats under ketamine anesthesia

The reticular thalamic nucleus (RTN) is a thin shell that covers the dorsal thalamus and controls the overall information flow from the thalamus to the cerebral cortex through GABAergic projections that contact thalamo-cortical neurons (TC). RTN neurons receive glutamatergic afferents fibers from neurons of the sixth layer of the cerebral cortex and from TC collaterals. The firing mode of RTN neurons facilitates the generation of sleep-wake cycles; a tonic mode or desynchronized mode occurs during wake and REM sleep and a burst-firing mode or synchronized mode is associated with deep sleep. Despite the presence of cannabinoid receptors CB1 (CB1Rs) and mRNA that encodes these receptors in RTN neurons, there are few works that have analyzed the participation of endocannabinoid-mediated transmission on the electrical activity of RTN. Here, we locally blocked or activated CB1Rs in ketamine anesthetized rats to analyze the spontaneous extracellular spiking activity of RTN neurons. Our results show the presence of a tonic endocannabinoid input, since local infusion of AM 251, an antagonist/inverse agonist, modifies RTN neurons electrical activity; furthermore, local activation of CB1Rs by anandamide or WIN 55212-2 produces heterogeneous effects in the basal spontaneous spiking activity, where the main effect is an increase in the spiking rate accompanied by a decrease in bursting activity in a dose-dependent manner; this effect is inhibited by AM 251. In addition, previous activation of GABA-A receptors suppresses the effects of CB1Rs on reticular neurons. Our results show that local activation of CB1Rs primarily diminishes the burst firing mode of RTn neurons.

Heteromers Formed by GPR55 and Either Cannabinoid CB1 or CB2 Receptors Are Upregulated in the Prefrontal Cortex of Multiple Sclerosis Patients

Multiple sclerosis (MS) is an autoimmune, inflammatory, and neurodegenerative disease of the central nervous system for which there is no cure, making it necessary to search for new treatments. The endocannabinoid system (ECS) plays a very important neuromodulatory role in the CNS. In recent years, the formation of heteromers containing cannabinoid receptors and their up/downregulation in some neurodegenerative diseases have been demonstrated. Despite the beneficial effects shown by some phytocannabinoids in MS, the role of the ECS in its pathophysiology is unknown. The main objective of this work was to identify heteromers of cell surface proteins receptive to cannabinoids, namely GPR55, CB1 and CB2 receptors, in brain samples from control subjects and MS patients, as well as determining their cellular localization, using In Situ Proximity Ligation Assays and immunohistochemical techniques. For the first time, CB1R-GPR55 and CB2R-GPR55 heteromers are identified in the prefrontal cortex of the human brain, more in the grey than in the white matter. Remarkably, the number of CB1R-GPR55 and CB2R-GPR55 complexes was found to be increased in MS patient samples. The results obtained open a promising avenue of research on the use of these receptor complexes as potential therapeutic targets for the disease.

GPR55 regulates the responsiveness to, but does not dimerise with, α1A-adrenoceptors

Emerging evidence suggests that G protein coupled receptor 55 (GPR55) may influence adrenoceptor function/activity in the cardiovascular system. Whether this reflects direct interaction (dimerization) between receptors or signalling crosstalk has not been investigated. This study explored the interaction between GPR55 and the alpha 1A-adrenoceptor (α_1A-AR) in the cardiovascular system and the potential to influence function/signalling activities. GPR55 and α_1A-AR mediated changes in both cardiac and vascular function was assessed in male wild-type (WT) and GPR55 homozygous knockout (GPR55^-/-) mice by pressure volume loop analysis and isolated vessel myography, respectively. Dimerization of GPR55 with the α_1A-AR was examined in transfected Chinese hamster ovary-K1 (CHO-K1) cells via Bioluminescence Resonance Energy Transfer (BRET). GPR55 and α_1A-AR mediated signalling (extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation) was investigated in neonatal rat ventricular cardiomyocytes using AlphaScreen proximity assays. GPR55^-/- mice exhibited both enhanced pressor and inotropic responses to A61603 (α_1A-AR agonist), while in isolated vessels, A61603 induced vasoconstriction was attenuated by a GPR55-dependent mechanism. Conversely, GPR55-mediated vasorelaxation was not altered by pharmacological blockade of α_1A-ARs with tamsulosin. While cellular studies demonstrated that GPR55 and α_1A-AR failed to dimerize, pharmacological blockade of GPR55 altered α_1A-AR mediated signalling and reduced ERK1/2 phosphorylation. Taken together, this study provides evidence that GPR55 and α_1A-AR do not dimerize to form heteromers, but do interact at the signalling level to modulate the function of α_1A-AR in the cardiovascular system.

Expression of GPR55 and either cannabinoid CB1 or CB2 heteroreceptor complexes in the caudate, putamen, and accumbens nuclei of control, parkinsonian, and dyskinetic non-human primates

Endocannabinoids are neuromodulators acting on specific cannabinoid CB₁ and CB₂ G-protein-coupled receptors (GPCRs), representing potential therapeutic targets for neurodegenerative diseases. Cannabinoids also regulate the activity of GPR55, a recently "deorphanized" GPCR that directly interacts with CB₁ and with CB₂ receptors. Our hypothesis is that these heteromers may be taken as potential targets for Parkinson's disease (PD). This work aims at assessing the expression of heteromers made of GPR55 and CB₁/CB₂ receptors in the striatum of control and parkinsonian macaques (with and without levodopa-induced dyskinesia). For this purpose, double blind in situ proximity ligation assays, enabling the detection of GPCR heteromers in tissue samples, were performed in striatal sections of control, MPTP-treated and MPTP-treated animals rendered dyskinetic by chronic treatment with levodopa. Image analysis and statistical assessment were performed using dedicated software. We have previously demonstrated the formation of heteromers between GPR55 and CB₁ receptor (CB₁-GPR55_Hets), which is highly expressed in the central nervous system (CNS), but also with the CB₂ receptor (CB₂-GPR55_Hets). Compared to the baseline expression of CB₁-GPR55_Hets in control animals, our results showed increased expression levels in basal ganglia input nuclei of MPTP-treated animals. These observed increases in CB₁-GPR55_Hets returned back to baseline levels upon chronic treatment with levodopa in dyskinetic animals. Obtained data regarding CB₂-GPR55_Hets were quite similar, with somehow equivalent amounts in control and dyskinetic animals, and with increased expression levels in MPTP animals. Taken together, the detected increased expression of GPR55-endocannabinoid heteromers appoints these GPCR complexes as potential non-dopaminergic targets for PD therapy.