The cannabinoid type-1 receptor carboxyl-terminus, more than just a tail.

The cannabinoid type-1 (CB(1)) receptor is a G protein-coupled receptor that binds the main active ingredient of marijuana, Δ(9)-tetrahydrocannabinol, and has been implicated in several disease states, including drug addiction, anxiety, depression, obesity, and chronic pain. In the two decades since the discovery of CB(1), studies at the molecular level have centered on the transmembrane core. This interest has now expanded as we discover that other regions of CB(1), including the CB(1) carboxyl-terminus, have critical structures that are important for CB(1) activity and regulation. Following the recent description of the three dimensional structure of the full-length CB(1) carboxyl-terminal tail [Biopolymers (2009) vol. 91, pp. 565-573], several residues and structural motifs including two α-helices (termed H8 and H9) have been postulated to interact with common G protein-coupled receptor accessory proteins, such as G-proteins and ß-arrestins. This discourse will focus on the CB(1) carboxyl-terminus; our current understanding of the structural features of this region, evidence for its interaction with proteins, and the impact of structure on the binding and regulatory function of CB(1) accessory proteins. The involvement of the carboxyl-terminus in the receptor life cycle including activation, desensitization, and internalization will be highlighted.

Inhibition of brain [(3)H]cimetidine binding by improgan-like antinociceptive drugs.

[(3)H]cimetidine, a radiolabeled histamine H(2) receptor antagonist, binds with high affinity to an unknown hemoprotein in the brain which is not the histamine H(2) receptor. Improgan, a close chemical congener of cimetidine, is a highly effective pain-relieving drug following CNS administration, yet its mechanism of action remains unknown. To test the hypothesis that the [(3)H]cimetidine-binding site is the improgan antinociceptive target, improgan, cimetidine, and 8 other chemical congeners were studied as potential inhibitors of [(3)H]cimetidine binding in membrane fractions from the rat brain. All compounds produced a concentration-dependent inhibition of [(3)H]cimetidine binding over a 500-fold range of potencies (K(i) values were 14.5 to >8000nM). However, antinociceptive potencies in rats did not significantly correlate with [(3)H]cimetidine-binding affinities (r=0.018, p=0.97, n=10). These results suggest that the [(3)H]cimetidine-binding site is not the analgesic target for improgan-like drugs.

High-affinity binding of [3H]cimetidine to a heme-containing protein in rat brain.

[(3)H]Cimetidine (3HCIM) specifically binds to an unidentified site in the rat brain. Because recently described ligands for this site have pharmacological activity, 3HCIM binding was characterized. 3HCIM binding was saturable, heat-labile, and distinct from the histamine H(2) receptor. To test the hypothesis that 3HCIM binds to a cytochrome P450 (P450), the effects of nonselective and isoform-selective P450 inhibitors were studied. The heme inhibitor KCN and the nonselective P450 inhibitor metyrapone both produced complete, concentration-dependent inhibition of 3HCIM binding (K(i) = 1.3 mM and 11.9 muM, respectively). Binding was largely unaffected by inhibitors of CYP1A2, 2B6, 2C8, 2C9, 2D6, 2E1, and 19A1 but was eliminated by inhibitors of CYP2C19 (tranylcypromine) and CYP3A4 (ketoconazole). Synthesis and testing of CC11 [4(5)-(benzylthiomethyl)-1H-imidazole] and CC12 [4(5)-((4-iodobenzyl)-thiomethyl)-1H-imidazole] confirmed both drugs to be high-affinity inhibitors of 3HCIM binding. On recombinant human P450s, CC12 was a potent inhibitor of CYP2B6 (IC(50) = 11.7 nM), CYP2C19 (51.4 nM), and CYP19A1 (140.7 nM) and had a range of activities (100-494 nM) on nine other isoforms. Although the 3HCIM binding site pharmacologically resembles some P450s, eight recombinant human P450s and three recombinant rat P450s did not exhibit 3HCIM binding. Inhibition by KCN and metyrapone suggests that 3HCIM binds to a heme-containing brain protein (possibly a P450). However, results with selective P450 inhibitors, recombinant P450 isoforms, and a P450 antibody did not identify a 3HCIM-binding P450 isoform. Finally, CC12 is a new, potent inhibitor of CYP2B6 and CYP2C19 that may be a valuable tool for P450 research.

CC12, a high-affinity ligand for [3H]cimetidine binding, is an improgan antagonist.

Improgan, a chemical congener of cimetidine, is a highly effective non-opioid analgesic when injected into the CNS. Despite extensive characterization, neither the improgan receptor, nor a pharmacological antagonist of improgan has been previously described. Presently, the specific binding of [(3)H]cimetidine (3HCIM) in brain fractions was used to discover 4(5)-((4-iodobenzyl)thiomethyl)-1H-imidazole, which behaved in vivo as the first improgan antagonist. The synthesis and pharmacological properties of this drug (named CC12) are described herein. In rats, CC12 (50-500nmol, i.c.v.) produced dose-dependent inhibition of improgan (200-400nmol) antinociception on the tail flick and hot plate tests. When given alone to rats, CC12 had no effects on nociceptive latencies, or on other observable behavioral or motor functions. Maximal inhibitory effects of CC12 (500nmol) were fully surmounted with a large i.c.v. dose of improgan (800nmol), demonstrating competitive antagonism. In mice, CC12 (200-400nmol, i.c.v.) behaved as a partial agonist, producing incomplete improgan antagonism, but also limited antinociception when given alone. Radioligand binding, receptor autoradiography, and electrophysiology experiments showed that CC12's antagonist properties are not explained by activity at 25 sites relevant to analgesia, including known receptors for cannabinoids, opioids or histamine. The use of CC12 as an improgan antagonist will facilitate the characterization of improgan analgesia. Furthermore, because CC12 was also found presently to inhibit opioid and cannabinoid antinociception, it is suggested that this drug modifies a biochemical mechanism shared by several classes of analgesics. Elucidation of this mechanism will enhance understanding of the biochemistry of pain relief.

Redox response of the endogenous calcineurin inhibitor Adapt 78.

Adapt 78 (DSCR 1/calcipressin/MCIP 1) is a potent natural inhibitor of calcineurin, an important intracellular phosphatase that mediates many cellular responses to calcium. We previously reported two major cytosolic isoforms (1 and 4) of Adapt 78, and that isoform 4 is an oxidative and calcium stress-response protein. Using a higher cell culture density and new antibody, we again observed that both major isoforms localized to the cytosol, but a significant level of isoform 4 (but not isoform 1) was also detected in the nucleus where it was present in the non-soluble region and not associated with RNA. Exposure of cells to hydrogen peroxide led to the significant loss of isoform 4 from the nucleus with a moderate increase in cytosolic localization. The change in isoform 4 phosphorylation state in response to oxidative stress, characterized by a loss of the lesser (hypo) phosphorylated Adapt 78, was not due to accelerated degradation, although general Adapt 78 degradation was proteosome mediated. Finally, stimulation of Jurkat and primary T-lymphocyte signaling led to isoform 4 induction. This induction was BAPTA, diphenylene iodonium, and N-acetylcysteine inhibitable, and accompanied by induction of the classic immune response mediator and calcineurin-pathway-stimulated interleukin-2. These studies reveal new redox-related activities for Adapt 78 isoform 4, which may contribute to its known calcineurin-regulating and cytoprotective activities, and further suggest that Adapt 78 plays a role in basic T-cell response.

Activation of spinal histamine H3 receptors inhibits mechanical nociception.

Previous studies have suggested a possible pain-modulatory role for histamine H(3) receptors, but the localization of these receptors and nature of this modulation is not clear. In order to explore the role of spinal histamine H(3) receptors in the inhibition of nociception, the effects of systemically (subcutaneous, s.c.) and intrathecally (i.t.) administered histamine H(3) receptor agonists were studied in rats and mice. Immepip (5 mg/kg, s.c.) produced robust antinociception in rats on a mechanical (tail pinch) test but did not alter nociceptive responses on a thermal (tail flick) test. In contrast, this treatment in mice (immepip, 5 and 30 mg/kg, s.c.) did not change either mechanically or thermally evoked nociceptive responses. When administered directly into the spinal subarachnoid space, immepip (15-50 microg, i.t.) and R-alpha-methylhistamine (50 microg, i.t.) had no effect in rats on the tail flick and hot plate tests, but produced a dose- and time-dependent inhibition (90-100%) of nociceptive responses on the tail pinch test. This attenuation was blocked by administration of thioperamide (10 mg/kg, s.c.), a histamine H(3) receptor antagonist. Intrathecally administered thioperamide also reversed antinociceptive responses induced by systemically administered immepip, which demonstrates a spinal site of action for the histamine H(3) receptor agonist. In addition, intrathecally administered immepip (25 microg) produced maximal antinociception on the tail pinch test in wild type, but not in histamine H(3) receptor knockout (H(3)KO) mice. These findings demonstrate an antinociceptive role for spinal histamine H(3) receptors. Further studies are needed to confirm the existence of modality-specific (i.e. mechanical vs. thermal) inhibition of nociception by these receptors, and to assess the efficacy of spinally delivered histamine H(3) receptor agonists for the treatment for pain.