Is pharmacological neuroprotection dependent on reduced glutamate release?

Background and Purpose-The aim of this study was to determinate the possibl e role of the ionotropic glutamate receptor in the expression of irreversib le electrophysiological changes induced by in vitro ischemia and to test wh ether the neuroprotective action of various neurotransmitter agonists and d rugs of clinical interest is related to a presynaptic inhibitory action at glutamatergic synapses. Methods-Intracellular and extracellular recordings have been performed in a rat corticostriatal slice preparation, Different pharmacological compounds have been tested on corticostriatal glutamatergic transmission in control conditions and in an in vitro model of ischemia (oxygen and glucose depriva tion). Results-In vitro ischemia lasting 10 minutes produced an irreversible loss of the field potential recorded from striatal slices after cortical stimula tion. Preincubation of the slices with 3 mu mol/L 6-cyano-7-nitroquinoxalin e-2,3-dione (an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid [A MPA] receptor antagonist) allowed a significant recovery of the field poten tial amplitude (P<0.05, n=6), whereas incubation with 30 mu mol/L aminophos phonovaleric acid (an N-methyl-D-aspartate receptor antagonist) did not pro duce a significant recovery after 10 minutes of ischemia (P>0.05, n=7). Bat h application of 3 mmol/L glutamate for 5 minutes produced a complete but r eversible inhibition of the field potential amplitude. When a similar appli cation was coupled with a brief period of ischemia (5 minutes), which produ ced, per se, only a transient inhibition of the field potential, it caused an irreversible loss of this parameter. We also tested the possible neuropr otective effect of neurotransmitter agonists reducing the release of glutam ate from corticostriatal terminals. Agonists acting on purinergic (adenosin e), muscarinic (oxotremorine), and metabotropic glutamate receptors (L-seri ne o-phosphate [L-SOP]) significantly (P<0.001, n=8 for each agonist) reduc ed glutamatergic synaptic potentials, with each showing different potencies . The EC50 was 26.4 mu mol/L for adenosine, 0.08 mu mol/L for oxotremorine, and 0.89 mu mol/L for L-SOP, Concentrations of these agonists producing th e maximal inhibition of the synaptic potential were tested on the ischemia- induced irreversible loss of field potential. Adenosine (P<0.05, n=9) and o xotremorine (P<0.05, n=8) showed significant neuroprotective action, wherea s L-SOP was ineffective (P>0.05, n=10). Similarly, putative neuroprotective drugs significantly (P<0.001, n=10 for each drug) reduced the amplitude of corticostriatal potential, with different EC50 values (phenytoin, 33.5 mu mol/L; gabapentin, 96.8 mu mol/L; lamotrigine, 26.7 mu mol/L; riluzole, 6 m u mol/L; and sipatrigine, 2 mu mol/L). Concentration of these drugs produci ng maximal inhibition of the amplitude of corticostriatal potentials showed a differential neuroprotective action on the ischemic electrical damage. P henytoin (P<0.05, n=10), lamotrigine (P<0.05, n=10), riluzole (P<0.05, n=9) , and sipatrigine (P<0.001, n=10) produced a significant neuroprotection, w hereas gabapentin (P>0.05, n=11) was ineffective. The neuroprotective actio n of transmitter agonists and clinical drugs was not related to their abili ty in decreasing glutamate release, as detected by changes in the paired-pu lse facilitation protocol. Conclusions-Ionotropic glutamate receptors, and particularly AMPA-like rece ptors, play a role in the irreversible loss of field potential amplitude in duced by ischemia in the striatum. Drugs acting by reducing glutamatergic c orticostriatal transmission may show a neuroprotective effect. However, the ir efficacy does not seem to be directly related to their capability to dec rease glutamate release from corticostriatal terminals. We suggest that add itional modulatory actions on voltage-dependent conductances and on ischemi a-induced ion distribution at the postsynaptic site may also exert a crucia l role.