Expression of cannabinoid CB₁R–GPR55 heteromers in neuronal subtypes of the Macaca fascicularis striatum

Abstract

The cannabinoid CB1 receptor (CB1R) is the most abundant G protein–coupled receptor in the central nervous system, consistent with the important role of endocannabinoids as neuromodulators. Cannabinoids also modulate the function of G protein–coupled receptor 55 (GPR55), which forms heteroreceptor complexes with the CB1R in the striatum. The aim was to characterize cannabinoid CB1R–GPR55 heteromers (CB1R/GPR55Hets) in the basal ganglia input nuclei of nonhuman primates, Macaca fascicularis, both in projection neurons and interneurons, by the in situ proximity ligation assay. Striatal projecting neurons were identified by the retrograde neuroanatomical tracer, biotinylated dextran amine (BDA), injected into external or internal subdivisions of the globus pallidus. Triple immunofluorescent stains were carried out to visualize (1) BDA‐labeled neurons, (2) CB1R/GPR55Hets, and (3) striatal interneurons positive for choline acetyltransferase, parvalbumin, calretinin, or nitric oxide synthase. CB1R/GPR55Hets were identified within both types of projection neurons as well as all interneurons except those that are cholinergic. Moreover, CB1R/GPR55Hets were found specifically in the neuronal cell surface, and also in intracellular membranes. Further research efforts will be needed to confirm the intracellular occurrence of heteromers and their potential as therapeutic targets in diseases related to motor control imbalances, particularly within a parkinsonian context (with or without levodopa‐induced dyskinesia).

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

“…The above-commented study of identification of neurons expressing CB 1 -GPR55_Hets in the striatum of naïve animals also concluded that the heteromers are not only expressed on cell surface but in intracellular structures (Martínez-Pinilla et al 2020). In this regard, it has been reported that the CB 1 R may be present in mitochondria and may mediate regulation of mitochondrial function by cannabinoids (Bénard et al 2012;Hebert-Chatelain et al 2014;Melser et al 2017;Gutiérrez-Rodríguez et al 2018).…”

“…Triple immunofluorescent stains identified heteromers in parvalbumin, calretinin and nitric oxide positive interneurons. In contrast, cholinergic interneurons lacked heteromer expression (Martínez-Pinilla et al 2020).…”

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

“…The CB 1 receptor (CB 1 R) is the most abundant G-protein-coupled receptor (GPCR) superfamily member in mammalian CNS, it is particularly abundant in the olfactory bulb, hippocampus, basal ganglia, and cerebellum [22][23][24]. Moderate/low levels of this receptor have been found in the cerebral cortex, amygdala, hypothalamus, some areas of the brainstem, and the spinal cord [22,25]. Regarding CB 2 receptor (CB 2 R) expression in the CNS, it is primarily identified in microglia but also in some neurons in diverse brain areas including the cerebral cortex, hippocampus, and globus pallidus [23,[26][27][28][29][30][31].…”

“…Regarding CB 2 receptor (CB 2 R) expression in the CNS, it is primarily identified in microglia but also in some neurons in diverse brain areas including the cerebral cortex, hippocampus, and globus pallidus [23,[26][27][28][29][30][31]. At the neuronal level, these two receptors are preferentially located in the plasma membrane of pre-and post-synaptic terminals, but they can appear in the soma and dendrites of glutamatergic and serotonergic neurons, as well as in GABAergic interneurons [25,29,32,33]. Cannabinoids, endogenous, natural, or synthetic, may limit excitotoxic damage and enhance synaptic plasticity via CB 1 R or may exert anti-inflammatory and immunomodulatory actions via CB 2 R [17,34].…”

“…Although early studies proved the binding of CP55940, a synthetic cannabinoid, to GPR55 and activation of this receptor by anandamide [35], it is now assumed that the binding of cannabinoids to the receptor occurs at an allosteric site [36][37][38]. GPR55 is abundant in various regions within the CNS, both in neurons and glial cells; it has been described in the hippocampus, thalamic nuclei, and basal ganglia of rodent, primate, and human brains [25,30,39,40]. While CB 1 R and CB 2 R couple primarily to the adenylyl cyclase-inhibiting heterotrimeric G i/o protein [41][42][43], GPR55 likely couples to G q/G11 and G 12 /G 13 , showing a pleiotropic pharmacological profile that includes the activation of different Rho and ERK signal transduction pathways [41,44,45].…”