Characterization of CB1 cannabinoid receptors using receptor peptide fragments and site-directed antibodies

The cannabinoid system and cytokine network

Many advances have been made in the last few years concerning our understanding of the receptors and ligands composing the cannabinoid system. Likewise, the science surrounding cytokine biology has advanced enabling us to measure these proteins more precisely as well as understand and interpret the meaning of changes in their levels. Scientists wishing to study the health consequences of smoking marijuana as well as understand the possible role of endogenous cannabimimetic ligands in immune regulation have continued to study the influence of these substances on the regulation and development of the cytokine network. Research has shown that two major cannabinoid receptor subtypes exist and that subtype 1 (CB1) is expressed primarily in the brain whereas subtype 2 (CB2) is expressed primarily in the periphery. A variety of ligands for these receptors based on the cannabinoid structure have been synthesized and studied as well as low affinity compounds, noncannabinoid ligands, and endogenous ligands derived from fatty acid eicosanoids. Highly selective receptor antagonists have also been introduced and studied. Synthetic, low affinity ligands such as (+)-HU-211 and DMH-11C have been shown to cause anti-inflammatory effects possibly through inhibiting the production and action of TNF-alpha and other acute phase cytokines. In addition, suppression of TNF and other cytokines such as GM-CSF, IL-6, IFNgamma, and IL-12 has also been seen following exposure to high affinity and psychoactive ligands such as marijuana and THC. However, some of these ligands have also been shown to increase rather than decrease interleukins such as IL-1, IL-4, IL-10, and IL-6, cytokines such as TNF-alpha, and chemokines such as IL-8, MIP-1, and RANTES. The endogenous ligand, anandamide, has been shown in culture to either suppress the proliferation response to prolactin or enhance the response to cytokines such as IL-3 and IL-6. This eicosanoid has also been shown to increase the production of interleukins and other cytokines. Cannabinoid receptors have been shown to be involved in some but not all of these effects. It is clear that psychoactive and nonpsychoactive compounds have demonstrated effects in vivo and in vitro on the production and function of a variety of cytokines. Depending upon the model system, these effects are often conflicting, and the involvement of cannabinoid receptors is unclear. However, enough evidence exists to suggest that the cannabinoid system significantly impacts the functioning of the cytokine network, and this association may provide clues to the mechanisms of certain immune diseases and form the basis for new immunotherapies.

Azido- and isothiocyanato-substituted aryl pyrazoles bind covalently to the CB1 cannabinoid receptor and impair signal transduction

3-Azidophenyl- and 3-isothiocyanatophenyl-and 2-(5'-azidopentyl)- and 2-(5'-isothiocyanatopentyl)pyrazoles were synthesized to determine whether these compounds could behave as covalently binding ligands for the CB1 cannabinoid receptor in rat brain membranes. Heterologous displacement of [3H]CP55940 indicated that the apparent affinity of these compounds for the CB1 receptor was similar to that of the parent compound, SR141716A, with the exception of the 3-isothiocyanato derivatives, which showed a 10-fold loss of affinity. The 3-azidophenyl and 3-isothiocyanatophenyl compounds behaved as antagonists against the cannabinoid agonist desacetyllevonantradol in activation of G proteins [guanosine 5'-O-(y-[35S]thio)triphosphate ([35S]GTPgammaS) binding] and regulation of adenylyl cyclase. The 2-(5'-azidopentyl)- and 2-(5'-isothiocyanatopentyl)pyrazoles were poor antagonists for [35S]GTPgammaS binding, and both compounds failed to antagonize the cannabinoid regulation of adenylyl cyclase. After incubation with the isothiocyanato analogues or UV irradiation of the azido analogues, the 3-substituted aryl pyrazoles formed covalent bonds with the CB1 receptor as evidenced by the loss of specific binding of [3H]CP55940. In the case of the isothiocyanato analogues, the log concentration-response curve for cannabinoid-stimulated [35S]GTPgammaS binding was shifted to the right, indicating that loss of receptors compromised signal transduction capability. These irreversibly binding antagonists might be useful tools for the investigation of tolerance and receptor down-regulation in both in vitro and in vivo studies.

The human eye expresses high levels of CB1 cannabinoid receptor mRNA and protein

We used reverse transcriptase polymerase chain reaction to detect the expression of the central and peripheral cannabinoid receptors (CB1 and CB2, respectively) mRNA, and Western blotting to show the presence of the CB1 protein in subregions of the human eye. CB2 mRNA transcripts were undetectable, while levels of CB1 mRNA were significantly expressed in the human retina (25.8 +/- 2.46%), ciliary body (210 +/- 11.55%) and iris (62.7 +/- 5.94%) when compared with those of the normalizing reference gene beta2 microglobulin. The CB1 gene encodes a functional protein which is detected in its glycosylated (63 kDa) and unglycosylated (54 kDa) form in the same areas by a specific purified antibody raised against the amino terminus (residues 1-77) of the CB1 receptor. These results further support the proposed role of the CB1 receptor in controlling intraocular pressure, helping to explain the antiglaucoma properties of marijuana.

Immunocytochemical demonstration of CB1 cannabinoid receptors in the anterior lobe of the pituitary gland

Both exogenous and endogenous cannabinoids can influence hormone secretion from the anterior pituitary gland. A large body of information proves that the primary target of these effects is the neuroendocrine hypothalamus. However, recent studies using cannabinoid (CB) receptor autoradiography, messenger RNA in-situ hybridization and in-vitro analysis, indicate direct effects of cannabinoids at the level of the anterior pituitary gland itself. In the present paper, the immunocytochemical distribution of CB in the adult rat anterior pituitary was studied using specific polyclonal antibodies against CB1 (central) and CB2 (peripheral) receptors. Due to its resolution, this method allowed identification of individual anterior pituitary cells possessing cannabinoid receptors. The specific hormone immunoreactive cells with receptor-like immunoreactivity were compared on adjacent sections. CB1-like immunoreactivity (CB1ir) was found in the lactotroph cells as well as in luteinizing hormone (LH) secreting gonadotrophs. The CB1ir positive material present in the cytoplasm of these cells was less homogeneous than the hormone immunoreactive material, and it was also seen at the periphery of the cells, presumably on the cell membrane. No CB1ir was found in growth hormone (GH) secreting cells and it was hardly seen in the corticotrophs. No CB1ir was detected in the posterior pituitary. CB2ir was not observed in any part of the pituitary gland. The results support the view that the site of action of cannabinoids on neuroendocrine regulatory mechanisms may be both at pituitary and hypothalamic levels. We suggest that at least the direct effect of cannabinoids on the regulation of LH and prolactin secretion is mediated via CB1 cannabinoid receptors in the anterior pituitary.

Effects of delta9-THC on VIP-induced prolactin secretion in anterior pituitary cultures: evidence for the presence of functional cannabinoid CB1 receptors in pituitary cells

Peripheral administration of cannabinoid CB1 receptor agonists to laboratory rats induce a brief rise in plasma prolactin (PRL) levels followed by a prolonged decrease in PRL secretion from the pituitary. While the inhibitory component of this biphasic response depends on the cannabinoid-induced activation of dopamine release from hypothalamic terminals located in the median eminence, the neurobiological mechanisms underlying the activation phase of PRL release remains to be explained. In the present study the possible direct effect of the cannabinoid receptor agonist delta9-Tetrahydrocannabinol (THC) on prolactin secretion and cAMP accumulation was examined in anterior pituitary cultures. THC (0.1 and 1 microM) increased cAMP levels, and induced PRL release (1 and 10 mu). THC did not affect vasoactive intestinal peptide (VIP, 0.5 microM) induced cAMP accumulation in pituitary cultures, showing additive effects at THC 1 microM concentration. However, THC did prevent VIP-dependent increases in prolactin secretion. These results indicate that THC, through a direct pituitary action, activates both the synthesis of cAMP and PRL release and interferes with intracellular mechanisms involved in PRL secretion by VIP. These actions could be mediated through cannabinoid CB1 receptors which were found to be present in anterior pituitary cells, including lactotrophs, as revealed by immunocytochemistry with a specific polyclonal antibody raised against the CB1 receptor protein.

Signal transduction of eicosanoid CB1 receptor ligands

The eicosanoid ligand, arachidonylethanolamide (anandamide), interacts with the CB1 cannabinoid receptor in the brain to signal its response. Pharmacophoric points of interaction between this agonist and the receptor have been proposed based upon structure-activity relationship studies of ligand binding to the receptor. Three dimensional quantitative structure-activity relationship (3D-QSAR) models have been constructed based upon the corresponding pharmacophoric points predicted for cannabinoid ligands delta9-tetrahydrocannabinol and 9-nor-9beta-hydroxyhexa-hydrocannabinol. A novel data set has been used to test the statistical validity of these models. Once the ligand interacts with the CB1 receptor, signal transduction occurs via G-proteins of the Gi/o family which are shown to be associated with the receptor. Evidence suggests that the juxtamembrane region of the C-terminal of the CB1 receptor is critical for activation of these G-proteins.

Dual intracellular signaling pathways mediated by the human cannabinoid CB1 receptor

It has long been established that the cannabinoid CB1 receptor transduces signals through a pertussis toxin-sensitive Gi/Go inhibitory pathway. Although there have been reports that the cannabinoid CB1 receptor can also mediate an increase in cyclic AMP levels, in most cases the presence of an adenylyl cyclase costimulant or the use of very high amounts of agonist was necessary. Here, we present evidence for dual coupling of the cannabinoid CB receptor to the classical pathway and to a pertussis toxin-insensitive adenylyl cyclase stimulatory pathway initiated with low quantities of agonist in the absence of any costimulant. Treatment of Chinese hamster ovary (CHO) cells expressing the cannabinoid CB1 receptor with the cannabinoid CP 55,940, {(-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hyd roxypropyl) cyclohexan-1-ol} resulted in cyclic AMP accumulation in a dose-response manner, an accumulation blocked by the cannabinoid CB1 receptor-specific antagonist SR 141716A, {N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-me thyl-1H-pyrazole-3-carboxamide hydrochloride}. In CHO cells coexpressing the cannabinoid CB1 receptor and a cyclic AMP response element (CRE)-luciferase reporter gene system, CP 55,940 induced luciferase expression by a pathway blocked by the protein kinase A inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide hydrochloride (H-89). Under the same conditions the peripheral cannabinoid CB2 receptor proved to be incapable of inducing cAMP accumulation or luciferase activity. This incapacity allowed us to study the luciferase activation mediated by CB /CB2 chimeric constructs, from which we determined that the first and second internal loop regions of the cannabinoid CB1 receptor were involved in transducing the pathway leading to luciferase gene expression.

The CB1 cannabinoid receptor in the brain

The CB1 cannabinoid receptor in brain is a G-protein-coupled receptor that exists as a protein possessing seven transmembrane helices that span the membrane. The intracellular surface is able to interact with f1p4oteins of the Gi/o family to regulate effector proteins, including adenylate cyclase, Ca2+ channels, and K+ channels, and to stimulate the mitogen-activated protein kinase pathway. The CB1 cannabinoid receptor recognizes three classes of agonist ligands: cannabinoid, eicosanoid, and aminoalkylindole. These agonist subtypes may interact with the CB1 cannabinoid receptor by some common points of association, yet may have subtle differences in the way that they interact with the receptor protein. This may be evident in the allosteric regulation by monovalent cations and individual agonists. The juxtamembrane region of the C-terminal is able to activate G-proteins. It is proposed that conformational changes in the receptor induced by agonist ligands may alter the conformation or exposure of the juxtamembrane C-terminal region extending from helix VII.

Cannabinoid receptor CB1 mRNA is highly expressed in the rat ciliary body: implications for the antiglaucoma properties of marihuana

We used RT-PCR to measure relative differences in cannabinoid receptor (CB) mRNAs in the rat eye, comparing CB1 or CB2 transcripts to that of the normalizing reference gene beta2 microglobulin (beta2m). Significantly higher levels of CB1 mRNA levels were found in the ciliary body (0.84+/-0.05% of beta2m) than in the iris, (0.34+/-0.04% of beta2m), retina (0.07+/-0.005% of beta2m) and choroid (0.06+/-0.005% of beta2m). CB2 mRNA was undetectable. This expression pattern supports a specific role for the CB1 receptor in controlling intraocular pressure, helping to explain the antiglaucoma property of cannabinoids.