In animals, sporadic injections of the mitochondrial toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) selectively damage dopaminergic neurons but do not fully reproduce the features of human Parkinson's disease. We have now developed a mouse Parkinson's disease model that is based on continuous MPTP administration with an osmotic minipump and mimics many features of the human disease. Although both sporadic and continuous MPTP administration led to severe striatal dopamine depletion and nigral cell loss, we find that only continuous administration of MPTP produced progressive behavioral changes and triggered formation of nigral inclusions immunoreactive for ubiquitin and α-synuclein. Moreover, only continuous MPTP infusions caused long-lasting activation of glucose uptake and inhibition of the ubiquitin-proteasome system. In mice lacking α-synuclein, continuous MPTP delivery still induced metabolic activation, but induction of behavioral symptoms and neuronal cell death were almost completely alleviated. Furthermore, the inhibition of the ubiquitinproteasome system and the production of inclusion bodies were reduced. These data suggest that continuous low-level exposure of mice to MPTP causes a Parkinson-like syndrome in an α-synuclein-dependent manner.

An abundant presynaptic protein, α-synuclein, is centrally involved in the pathogenesis of Parkinson's disease. However, conflicting data exist about the normal function of α-synuclein, possibly because α-synuclein is redundant with the very similar β-synuclein. To investigate the functions of synucleins systematically, we have now generated single- and double-knockout (KO) mice that lack α- and/or β-synuclein. We find that deletion of synucleins in mice does not impair basic brain functions or survival. We detected no significant changes in the ultrastructure of synuclein-deficient synapses, in short- or long-term synaptic plasticity, or in the pool size or replenishment of recycling synaptic vesicles. However, protein quantitations revealed that KO of synucleins caused selective changes in two small synaptic signaling proteins, complexins and 14-3-3 proteins. Moreover, we found that dopamine levels in the brains of double-KO but not single-KO mice were decreased by ≈20%. In contrast, serotonin levels were unchanged, and dopamine uptake and release from isolated nerve terminals were normal. These results show that synucleins are not essential components of the basic machinery for neurotransmitter release but may contribute to the long-term regulation and/or maintenance of presynaptic function.

Metabotropic glutamate (mGlu) receptors have been considered as potential targets for neuroprotective drugs, but the lack of specific drugs has limited the development of neuroprotective strategies in experimental models of acute or chronic central nervous system (CNS) disorders. The advent of potent and centrally available subtype-selective ligands has overcome this limitation, leading to an extensive investigation of the role of mGlu receptor subtypes in neurodegeneration during the last 2 years. Examples of these drugs are the noncompetitive mGlu1 receptor antagonists, CPCCOEt and BAY-36-7620; the noncompetitive mGlu5 receptor antagonists, 2-methyl-6-(phenylethynyl)pyridine, SIB-1893, and SIB-1757; and the potent mGlu2/3 receptor agonists, LY354740 and LY379268. Pharmacologic blockade of mGlu1 or mGlu5 receptors or pharmacologic activation of mGlu2/3 or mGlu4/7/8 receptors produces neuroprotection in a variety of in vitro or in vivo models. MGlu1 receptor antagonists are promising drugs for the treatment of brain ischemia or for the prophylaxis of neuronal damage induced by synaptic hyperactivity. MGlu5 receptor antagonists may limit neuronal damage induced by a hyperactivity of N-methyl-d-aspartate (NMDA) receptors, because mGlu5 and NMDA receptors are physically and functionally connected in neuronal membranes. A series of observations suggest a potential application of mGlu5 receptor antagonists in chronic neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer disease. MGlu2/3 receptor agonists inhibit glutamate release, but also promote the synthesis and release of neurotrophic factors in astrocytes. These drugs may therefore have a broad application as neuroprotective agents in a variety of CNS disorders. Finally, mGlu4/7/8 receptor agonists potently inhibit glutamate release and have a potential application in seizure disorders. The advantage of all these drugs with respect to NMDA or AMPA receptor agonists derives from the evidence that mGlu receptors do not “mediate,” but rather “modulate” excitatory synaptic transmission. Therefore, it can be expected that mGlu receptor ligands are devoid of the undesirable effects resulting from the inhibition of excitatory synaptic transmission, such as sedation or an impairment of learning and memory.

Epigenetic mechanisms are involved in the pathophysiology of depressive disorders and are unique potential targets for therapeutic intervention. The acetylating agent L -acetylcarnitine (LAC), a well-tolerated drug, behaves as an antidepressant by the epigenetic regulation of type 2 metabotropic glutamate (mGlu2) receptors. It caused a rapid and long-lasting antidepressant effect in Flinders Sensitive Line rats and in mice exposed to chronic unpredictable stress, which, respectively, model genetic and environmentally induced depression. In both models, LAC increased levels of acetylated H3K27 bound to the Grm2 promoter and also increased acetylation of NF-ĸB-p65 subunit, thereby enhancing the transcription of Grm2 gene encoding for the mGlu2 receptor in hippocampus and prefrontal cortex. Importantly, LAC reduced the immobility time in the forced swim test and increased sucrose preference as early as 3 d of treatment, whereas 14 d of treatment were needed for the antidepressant effect of chlorimipramine. Moreover, there was no tolerance to the action of LAC, and the antidepressant effect was still seen 2 wk after drug withdrawal. Conversely, NF-ĸB inhibition prevented the increase in mGlu2 expression induced by LAC, whereas the use of a histone deacetylase inhibitor supported the epigenetic control of mGlu2 expression. Finally, LAC had no effect on mGlu2 knockout mice exposed to chronic unpredictable stress, and a single injection of the mGlu2/3 receptor antagonist LY341495 partially blocked LAC action. The rapid and long-lasting antidepressant action of LAC strongly suggests a unique approach to examine the epigenetic hypothesis of depressive disorders in humans, paving the way for more efficient antidepressants with faster onset of action.

Dual metabotropic glutamate 2/3 (mGlu2/3) receptor agonists have been examined with success in the clinic with positive proof of efficacy in several tests of anxiety and schizophrenia. Moreover, a large body of evidence has accumulated that these drugs have significant neuroprotective potential. An important discussion in the field deals with dissecting effects on mGlu2 versus effects on mGlu3 receptors, which is relevant for the potential use of subtype-selective agonists or allosteric activators. We addressed this issue using mGlu2 and mGlu3 receptor knock-out mice. We used mixed cultures of cortical cells in which astrocytes and neurons were plated at different times and could therefore originate from different mice. Cultures were challenged with NMDA for the induction of excitotoxic neuronal death. The mGlu2/3 receptor agonist, (−)-2-oxa-4-aminocyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268), was equally neuroprotective in cultures containing neurons from wild-type,mGlu2^−/−, ormGlu3^−/−mice. Neuroprotection was instead abolished when astrocytes lacked mGlu3 receptors, unless neuronal mGlu2 receptors were also absent. The latter condition partially restored the protective activity of LY379268. Cultures in which neurons originated frommGlu2^−/−mice were also intrinsically resistant to NMDA toxicity. Inin vivoexperiments, systemic administration of LY379268 protected striatal neurons against NMDA toxicity in wild-type andmGlu2^−/−mice but not inmGlu3^−/−mice. In addition, LY379268 was protective against nigrostriatal degeneration induced by low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine only in mice lacking mGlu2 receptors. We conclude that neuroprotection by mGlu2/3 receptor agonists requires the activation of astrocytic mGlu3 receptors, whereas, unexpectedly, activation of mGlu2 receptors might be harmful to neurons exposed to toxic insults.