The neuroprotective properties of bis(7)-tacrine, a novel dimeric acetylcholinesterase (AChE) inhibitor, on glutamate-induced excitotoxicity were investigated in primary cultured cerebellar granule neurons (CGNs). Exposure of CGNs to 75 M glutamate resulted in neuronal apoptosis as demonstrated by Hoechst staining, TUNEL, and DNA fragmentation assays. The bis(7)-tacrine treatment (0.01-1 M) on CGNs markedly reduced glutamate-induced apoptosis in dose-and time-dependent manners. However, donepezil and other AChE inhibitors, even at concentrations of inhibiting AChE to the similar extents as 1 M bis(7)-tacrine, failed to prevent glutamate-induced excitotoxicity in CGNs; moreover, both atropine and dihydro-␤-erythroidine, the cholinoreceptor antagonists, did not affect the anti-apoptotic properties of bis(7)-tacrine, suggesting that the neuroprotection of bis(7)-tacrine appears to be independent of inhibiting AChE and cholinergic transmission. In addition, ERK1/2 and p38 pathways, downstream signals of N-methyl-D-aspartate (NMDA) receptors, were rapidly activated after the exposure of glutamate to CGNs. Bis(7)-tacrine inhibited the apoptosis and the activation of these two signals with the same efficacy as the coapplication of PD98059 and SB203580. Furthermore, using fluorescence Ca 2؉ imaging, patch clamp, and receptor-ligand binding techniques, bis(7)-tacrine was found effectively to buffer the intracellular Ca 2؉ increase triggered by glutamate, to reduce NMDA-activated currents and to compete with [ 3 H]MK-801 with an IC 50 value of 0.763 M in rat cerebellar cortex membranes. These findings strongly suggest that bis(7)-tacrine prevents glutamate-induced neuronal apoptosis through directly blocking NMDA receptors at the MK-801-binding site, which offers a new and clinically significant modality as to how the agent exerts neuroprotective effects.

Minocycline has been shown to have remarkably neuroprotective qualities, but underlying mechanisms remain elusive. We reported here the robust neuroprotection by minocycline against glutamate-induced apoptosis through regulations of p38 and Akt pathways. Pre-treatment of cerebellar granule neurons (CGNs) with minocycline (10-100 lM) elicited a dose-dependent reduction of glutamate excitotoxicity and blocked glutamate-induced nuclear condensation and DNA fragmentations. Using patch-clamping and fluorescence Ca 2+ imaging techniques, it was found that minocycline neither blocked NMDA receptors, nor reduced glutamatecaused rises in intracellular Ca 2+. Instead, confirmed by immunoblots, minocycline in vivo and in vitro was shown to directly inhibit the activation of p38 caused by glutamate. A p38-specific inhibitor, SB203580, also attenuated glutamate excitotoxicity. Furthermore, the neuroprotective effects of minocycline were blocked by phosphatidylinositol 3-kinase (PI3-K) inhibitors LY294002 and wortmannin, while pharmacologic inhibition of glycogen synthase kinase 3b (GSK3b) attenuated glutamate-induced apoptosis. In addition, immunoblots revealed that minocycline reversed the suppression of phosphorylated Akt and GSK3b caused by glutamate, as were abolished by PI3-K inhibitors. These results demonstrate that minocycline prevents glutamate-induced apoptosis in CGNs by directly inhibiting p38 activity and maintaining the activation of PI3-K/Akt pathway, which offers a novel modality as to how the drug exerts protective effects.

The excessive activation of the N-methyl-D-aspartate receptor (NMDAR)/nitric oxide (NO) pathway has been proposed to be involved in the neuropathology of various neurodegenerative disorders. In this study, NO was found to mediate glutamateinduced excitotoxicity in primary cultured neurons. Compared with the NO synthase (NOS) inhibitor, N G -monomethyl-L-arginine (L-NMMA), and the NMDAR antagonist memantine, bis(7)-tacrine was found to be more potent in reducing NO-mediated excitotoxicity and the release of NO caused by glutamate. Moreover, like L-NMMA but not like 5H-dibenzo [a,d]cyclohepten-5,10-imine (MK-801) and memantine, bis(7)-tacrine showed greater neuroprotection and inhibition on NO release when neurons were pretreated for a prolonged time between 0 and 24 h and remained quite potent even when neurons were post-treated 1 h after the glutamate challenge. Bis(7)-tacrine was additionally found to be as moderately potent as memantine in competing with [ 3 H]MK-801, inhibiting NMDA-evoked currents and reducing glutamate-triggered calcium influx, which eventually reduced neuronal NOS activity. More importantly, at neuroprotective concentrations, bis(7)-tacrine substantially reversed the overactivation of neuronal NOS caused by glutamate without interfering with the basal activity of NOS. Furthermore, in vitro pattern analysis demonstrated that bis(7)-tacrine competitively inhibited both purified neuronal and inducible NOS with IC 50 values at 2.9 and 9.3 M but not endothelial NOS. This result was further supported by molecular docking simulations that showed hydrophobic interactions between bis(7)-tacrine and three NOS isozymes. Taken together, these results strongly suggest that the substantial neuroprotection against glutamate by bis(7)-tacrine might be mediated synergistically through the moderate blockade of NMDAR and selective inhibition of neuronal NOS.The precise mechanisms leading to the pathogenesis of chronic and acute neurodegenerative disorders have not yet been fully elucidated. However, increasing evidence has shown that these diseases may share a final common pathway to neuronal injury as a result of the excitotoxicity caused by the overstimulation of glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype (Yuan and Yankner, 2000;

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Clinical trials have demonstrated positive proof of efficacy of dual metabotropic glutamate receptor 2/3 (mGluR2/3) agonists in both anxiety and schizophrenia. Importantly, evidence suggests that these drugs may also be neuroprotective against glutamate excitotoxicity, implicated in the pathogenesis of Parkinson's disease (PD). However, whether this neuroprotection also translates into functional recovery is unclear. In the current study, we examined the neuroprotective efficacy of the dual mGluR2/3 agonist, 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC), and whether this is accompanied by behavioral recovery in a rodent 6-hydroxydopamine (6-OHDA) model of PD. We now report that delayed post lesion treatment with 2R,4R-APDC (10 nmol), results in robust neuroprotection of the nigrostriatal system, which translated into functional recovery as measured by improved forelimb use asymmetry and reduced (+)-amphetamine-induced rotation compared to vehicle treated animals. Interestingly, these beneficial effects were associated with a decrease in microglial markers in the SNc, which may suggest an antiinflammatory action of this drug.