Cinnabarinic acid, an endogenous metabolite of the kynurenine pathway, activates type 4 metabotropic glutamate receptors.

Cinnabarinic acid is an endogenous metabolite of the kynurenine pathway that meets the structural requirements to interact with glutamate receptors. We found that cinnabarinic acid acts as a partial agonist of type 4 metabotropic glutamate (mGlu4) receptors, with no activity at other mGlu receptor subtypes. We also tested the activity of cinnabarinic acid on native mGlu4 receptors by examining 1) the inhibition of cAMP formation in cultured cerebellar granule cells; 2) protection against excitotoxic neuronal death in mixed cultures of cortical cells; and 3) protection against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity in mice after local infusion into the external globus pallidus. In all these models, cinnabarinic acid behaved similarly to conventional mGlu4 receptor agonists, and, at least in cultured neurons, the action of low concentrations of cinnabarinic acid was largely attenuated by genetic deletion of mGlu4 receptors. However, high concentrations of cinnabarinic acid were still active in the absence of mGlu4 receptors, suggesting that the compound may have off-target effects. Mutagenesis and molecular modeling experiments showed that cinnabarinic acid acts as an orthosteric agonist interacting with residues of the glutamate binding pocket of mGlu4. Accordingly, cinnabarinic acid did not activate truncated mGlu4 receptors lacking the N-terminal Venus-flytrap domain, as opposed to the mGlu4 receptor enhancer, N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC). Finally, we could detect endogenous cinnabarinic acid in brain tissue and peripheral organs by high-performance liquid chromatography-tandem mass spectrometry analysis. Levels increased substantially during inflammation induced by lipopolysaccharide. We conclude that cinnabarinic acid is a novel endogenous orthosteric agonist of mGlu4 receptors endowed with neuroprotective activity.

Switch in the expression of mGlu1 and mGlu5 metabotropic glutamate receptors in the cerebellum of mice developing experimental autoimmune encephalomyelitis and in autoptic cerebellar samples from patients with multiple sclerosis.

Recent evidence suggests that changes in the expression of membrane receptors/ion channels in cerebellar Purkinje cells contribute to the onset of cerebellar motor symptoms in patients with multiple sclerosis (MS). We examined the expression of group-I metabotropic glutamate receptors (mGlu1 and mGlu5 receptors) in the cerebellum of mice developing experimental autoimmune encephalomyelitis (EAE) and in autoptic cerebellar samples of MS patients. EAE was induced in mice by immunization with the 35-55 fragment of MOG (myelin oligodendrocyte glycoprotein). EAE mice showed a progressive loss of mGlu1a receptors in the cerebellum, associated with an increased expression of mGlu5 receptors. These changes were restricted to Purkinje cells and their dendritic arborization, as shown by immunohistochemistry. A reduced expression of mGlu1a receptors in cerebellar Purkinje cells was also found in 7 of 9 MS patients. In addition, a light/moderate to very strong mGlu5 receptor immunoreactivity was detected in Purkinje cells of 8 MS patients, but was always absent in non-MS control patients. In EAE mice, an acute treatment with the mGlu1 receptor enhancer, 9H-xanthene-9-carboxylic acid (4-trifluoromethyl-oxazol-2-yl)-amide (RO0711401), significantly improved motor coordination, whereas treatment with the mGlu5 receptor antagonists, 2-methyl-6-(phenylethynyl)-pyridine (MPEP) and 6-methyl-2-(phenylazo)-3-pyridinol (SIB-1757), had no effect. We conclude that mGlu1 receptor enhancers improve motor symptoms associated with EAE and might be helpful as symptomatic drugs in patients with MS.