Context-Dependent Pharmacology Exhibited by Negative Allosteric Modulators of Metabotropic Glutamate Receptor 7

Phenotypic studies of mice lacking metabotropic glutamate receptor subtype 7 (mGluR7) suggest that antagonists of this receptor may be promising for the treatment of central nervous system disorders such as anxiety and depression. Suzuki et al. (J Pharmacol Exp Ther 323: 147-156, 2007) recently reported the in vitro characterization of a novel mGluR7 antagonist called 6-(4-methoxyphenyl)-5-methyl-3-(4-pyridinyl)-isoxazolo[4,5-c]pyridin-4(5H)-one (MMPIP), which noncompetitively inhibited the activity of orthosteric and allosteric agonists at mGluR7. We describe that MMPIP acts as a noncompetitive antagonist in calcium mobilization assays in cells coexpressing mGluR7 and the promiscuous G protein G alpha(15). Assessment of the activity of a small library of MMPIP-derived compounds using this assay reveals that, despite similar potencies, compounds exhibit differences in negative co-operativity for agonist-mediated calcium mobilization. Examination of the inhibitory activity of MMPIP and analogs using endogenous G(i/o)-coupled assay readouts indicates that the pharmacology of these ligands seems to be context-dependent, and MMPIP exhibits differences in negative cooperativity in certain cellular backgrounds. Electrophysiological studies reveal that, in contrast to the orthosteric antagonist (2S)-2-amino-2-[(1S,2S)-2-carboxyclycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495), MMPIP is unable to block agonist-mediated responses at the Schaffer collateral-CA1 synapse, a location at which neurotransmission has been shown to be modulated by mGluR7 activity. Thus, MMPIP and related compounds differentially inhibit coupling of mGluR7 in different cellular backgrounds and may not antagonize the coupling of this receptor to native G(i/o) signaling pathways in all cellular contexts. The pharmacology of this compound represents a striking example of the potential for context-dependent blockade of receptor responses by negative allosteric modulators.

Intravenous tolerance effectively overcomes enhanced pro-inflammatory responses and EAE severity in the absence of IL-12 receptor signaling

Intravenous (i.v.) administration of autoantigen effectively induces Ag-specific tolerance against experimental autoimmune encephalomyelitis (EAE). We and others have shown enhanced EAE severity in mice lacking IL-12 or its receptor, strongly suggesting an immunoregulatory effect of IL-12 signaling. To examine the role of IL-12 responsiveness in autoantigen-induced tolerance in EAE, we administered autoantigen i.v. in two distinct treatment regimes to wildtype and IL-12Rβ2(-/-) mice, immunized to develop EAE. Administration at the induction phase suppressed EAE in wildtype and IL-12Rβ2(-/-) mice however the effect was somewhat less potent in the absence of IL-12Rβ2. Expression of pro-inflammatory cytokines such as IFN-γ, IL-17 and IL-2, was inhibited in wild-type tolerized mice but less so in IL-12Rβ2(-/-) mice. I.v. antigen was also effective in suppressing disease in both genotypes when given during the clinical phase of disease with similar CNS inflammation, demyelination and peripheral inflammatory cytokine profiles observed in both genotypes. There was however a mild impact of a lack of IL-12 signaling on Treg induction during tolerance induction compared to WT mice in this treatment regime. These findings show that the enhanced severity of EAE that occurs in the absence of IL-12 signaling can be effectively overcome by i.v. autoantigen, indicating that this therapeutic effect is not primarily mediated by IL-12 and that i.v. tolerance could be a powerful approach in suppressing severe and aggressive MS.

ROLE OF INTRACELLULAR AND EXTRACELLULAR CALCIUM IN HISTAMINE-RELEASE FROM RAT PERITONEAL MAST-CELLS

The present study provides evidence for a number of calcium pools important in histamine secretion from the mast cell. Firstly, calcium loosely bound to the cell membrane, and in rapid equilibrium with the extracellular environment, may be utilized for histamine release induced by most secretagogues. Secondly, all inducers are able to mobilize deeply buried or internal stores of calcium to initiate exocytosis. Finally, calcium bound to regulatory sites in the membrane may modulate the secretory process, Removal of calcium from the latter sites by brief treatment with chelating agents markedly enhances the secretory response in the absence of extracellular calcium, probably by facilitating the mobilization of bound stores of the ion, Saturation of these sites in the presence of excess calcium inhibits the release process and may restrict influx of the cation.