Emerging concepts from the recent literature
Activating 5-HT2AR Enhances Morphine-Mediated MOR Internalization
Morphine is a powerful analgesic whose beneficial effects become blunted as tolerance develops. Mechanistically, morphine induces very little internalization of its receptor, the μ-opioid receptor (MOR) [a G protein coupled receptor (GPCR)], whereas the MOR agonists [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) and etorphine produce less tolerance, promote enhanced desensitization and internalization of MOR (where internalization correlates with resistance to tolerance), and downregulate MOR expression. GPCRs often cross-regulate each other’s signaling pathways, and because the serotonin 2A receptor (5-HT2AR), also a GPCR, is frequently expressed at the same brain sites as the MOR, Lopez-Gimenez and colleagues sought to determine whether 5-HT2AR could alter morphine’s weak ability to promote internalization of MOR. Induced expression of 5-HT2AR led to increased MOR expression, and this was blocked by mianserin (an inverse agonist of 5-HT2AR) and by YM254890 (an inhibitor of Gq, which is associated with 5-HT2AR). On the other hand, addition of 5-HT and morphine to this heterologous expression system led to sensitization, internalization, and downregulation of the MOR. Intriguing observations made elsewhere reinforce the claim that 5-HT treatment might be useful in retarding morphine-induced tolerance. Indeed, administration of SSRIs, which keep 5-HT concentrations elevated, have been proposed to suppress tolerance and dependence on morphine. [Lopez-Gimenez, J.F., Vilaro, M.T., and Milligan, G. Morphine desensitization, internalization and down-regulation of the mu opioid receptor is facilitated by serotonin 5-HT2A receptor co-activation.Mol. Pharmacol. Epub ahead of print; doi: 10.1124/mol.108.048272 (2008).]
—JW Nelson, ASPET
Inhibiting Inhibition: Targeting MicroRNAs with Small Molecules
MicroRNAs (miRs) participate as gene regulators, binding to the 3′ untranslated region of target messenger RNAs (mRNAs) and promoting mRNA degradation. Up to 30% of all human genes are thought regulated by miRs. Deregulation of miRs has been linked to several disease processes and human malignancies, and numerous miRs are placed on the list of potential oncogenes and tumor suppressors. Until recently, the inhibition of miR actions had only been achieved with antisense nucleic acids. Gumireddy et al. have developed a mechanistically unbiased phenotypic screen for small molecule inhibitors of a miR-dependent pathway and have identified two potential inhibitors. Lentiviral reporter constructs that measure miR activity, containing recognition sequences downstream of a luciferase reporter gene, served as sensors of the presence of mature microRNA. In HeLa cells, in the presence of miR-21, luciferase expression was substantially suppressed. Screening over 1,000 compounds led to the identification of a diazobenzene compound that suppressed the miR-21-induced inhibition of luciferase signal. A secondary screen of structurally related moieties containing the same azobenzene core resulted in the identification of a second compound that was highly specific for miR-21 [and five times more active (EC50=2 μM)] and not cytotoxic. These findings pave the way for the development of specific pharmacological probes to identify the roles of microRNAs in numerous disease processes as well as a potential new class of therapeutics. [Gumireddy, K., Young, D.D., Xiong, X., Hogenesch, J.B., Huang, Q., and Deiters, A. Small molecule inhibitors of MicroRNA miR21 function. Angew. Chem. Int. Ed. Engl. Epub ahead of print; doi: 10.1002/anie.200801555 (2008).]
—PE Queiroz-Oliveira, U Pittsburgh
General Anesthetics Enhance Post-Operative Pain via TRPA1 and TRPV1
Inhalational anesthetics, while extremely effective at depressing the CNS of patients undergoing surgery, also seem to exacerbate post-operative inflammation. Reports of increased surgery site specific pain have also been noted. Ahern and colleagues recently found that general anesthetics, such as isoflurane (ISO), activate Transient Receptor Potential (TRP) A1, a pain-sensing channel expressed in ~25% of sensory neurons. The vanilloid type 1 receptor (TRPV1) is, in addition to being more widely expressed than TRPA1, nearly always found in the same tissues and cells as TRPA1. Ahern and colleagues now find that volatile general anesthetics (VGAs) sensitize TRPV1 to capsaicin and protons. Thus, the half-maximal concentration required for TRPV1 activation is significantly reduced in the presence of ISO. Additionally, thermal and voltage sensitivities of TRPV1 were enhanced in the presence of ISO, and ISO increased the current evoked by TRPV1 in HEK293 cells. Clinically relevant concentrations of the more “pungent” anesthetics (i.e., desflurane and enflurane) resulted in greater proton-evoked currents from TRPV1 than did the less pungent ISO or sevoflurane. Protein Kinase C (PKC) and bradykinin, a signaling molecule involved in pain sensation, enhanced the VGA-mediated activation of TRPV1. Removal of PKC sites of phosphorylation on TRPV1 resulted in no activation, indicating that PKC participates in TRPV1 activation. Importantly, bradykinin enhanced ISO-mediated TRPV1 activity, supporting the hypothesis that tissue injury can amplify the excitatory activities of VGAs on sensory neurons. [Cornett, P., Matta, J., and Ahern, G. General anesthetics sensitize the capsaicin receptor TRPV1. Mol. Pharmacol. Epub ahead of print; doi: 10.1124/mol.108.049684 (2008).]
—JW Nelson, ASPET
- © American Society for Pharmacology and Experimental Theraputics 2008
