Subtype selective antagonism of substantia nigra pars compacta Group I metabotropic glutamate receptors protects the nigrostriatal system against 6-hydroxydopamine toxicity in vivo

Evidence suggests that increased glutamatergic input to the substantia nigra pars compacta as a result of hyperactivity of subthalalmic nucleus output pathways may contribute to the progressive degeneration of nigral dopaminergic neurones in Parkinson's disease (PD), a debilitating neurodegenerative disorder which affects similar to 1% of people aged over 65. Substantial electrophysiological evidence suggests that the excitation of nigral dopaminergic neurones is regulated by the activation of Group I metabotropic glutamate receptors (mGluR), comprising mGluR1 and mGluR5 subtypes. As activation of these receptors by endogenous glutamate may promote multiple cascades leading to excitotoxic neuronal death, it may be hypothesised that functional antagonism of Group I mGluR should be neuroprotective and could form the basis of a novel neuroprotective treatment for PD. To investigate this hypothesis, the neuroprotective potential of the selective competitive mGlu1 antagonist (+)-2-methyl-4-carboxyphenylglycine ((S)-(+)-alpha-amino-4-carboxy-2-methlybenzeneacetic acid; LY367385) and the selective allosteric mGlu5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) was tested in a rodent 6-hydroxydopamine (6-OHDA) model of PD in vivo. Both acute and subchronic intranigral administration of either LY367385 or MPEP resulted in significant neuroprotection of nigral tyrosine hydroxylase immunoreactive cell bodies, which correlated closely with prevention of striatal monoamine depletion following 6-OHDA lesioning. This neuroprotective action of LY367385 and MPEP displayed a clear concentration-dependent effect, suggesting a receptor-mediated mechanism of action. LY367385 produced robust neuroprotection at all concentrations tested (40, 200 and 1000 nmol in 4 mu L), whilst MPEP displayed a bell-shaped neuroprotective profile with significant neuroprotection at low concentrations (2 and 10 nmol in 4 mu L) but not at higher concentrations (50 nmol). Importantly, subchronic intranigral administration of MPEP and LY367385 appeared to slow the degeneration of remaining nigral dopaminergic neurones and prevented further striatal dopamine depletion in animals with established 6-OHDA induced nigrostriatal lesions, suggesting that these compounds may significantly influence disease progression in this model.