Is pharmacological neuroprotection dependent on reduced glutamate release?

BACKGROUND AND
PURPOSE: The aim of this study was to determinate the possible role of the ionotropic glutamate receptor in the expression of irreversible electrophysiological changes induced by in vitro ischemia and to test whether the neuroprotective action of various neurotransmitter agonists and drugs 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 deprivation).
RESULTS: In vitro ischemia lasting 10 minutes produced an irreversible loss of the field potential recorded from striatal slices after cortical stimulation. Preincubation of the slices with 3 micromol/L 6-cyano-7-nitroquinoxaline-2,3-dione (an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid [AMPA] receptor antagonist) allowed a significant recovery of the field potential amplitude (P<0.05, n=6), whereas incubation with 30 micromol/L aminophosphonovaleric acid (an N-methyl-D-aspartate receptor antagonist) did not produce a significant recovery after 10 minutes of ischemia (P>0.05, n=7). Bath application of 3 mmol/L glutamate for 5 minutes produced a complete but reversible inhibition of the field potential amplitude. When a similar application was coupled with a brief period of ischemia (5 minutes), which produced, per se, only a transient inhibition of the field potential, it caused an irreversible loss of this parameter. We also tested the possible neuroprotective effect of neurotransmitter agonists reducing the release of glutamate from corticostriatal terminals. Agonists acting on purinergic (adenosine), muscarinic (oxotremorine), and metabotropic glutamate receptors (L-serine o-phosphate [L-SOP]) significantly (P<0.001, n=8 for each agonist) reduced glutamatergic synaptic potentials, with each showing different potencies. The EC(50) was 26.4 micromol/L for adenosine, 0. 08 micromol/L for oxotremorine, and 0.89 micromol/L for L-SOP. Concentrations of these agonists producing the maximal inhibition of the synaptic potential were tested on the ischemia-induced irreversible loss of field potential. Adenosine (P<0.05, n=9) and oxotremorine (P<0.05, n=8) showed significant neuroprotective action, whereas 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 EC(50) values (phenytoin, 33.5 micromol/L; gabapentin, 96.8 micromol/L; lamotrigine, 26.7 micromol/L; riluzole, 6 micromol/L; and sipatrigine, 2 micromol/L). Concentration of these drugs producing maximal inhibition of the amplitude of corticostriatal potentials showed a differential neuroprotective action on the ischemic electrical damage. Phenytoin (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, whereas gabapentin (P>0.05, n=11) was ineffective. The neuroprotective action of transmitter agonists and clinical drugs was not related to their ability in decreasing glutamate release, as detected by changes in the paired-pulse facilitation protocol.
CONCLUSIONS: Ionotropic glutamate receptors, and particularly AMPA-like receptors, play a role in the irreversible loss of field potential amplitude induced by ischemia in the striatum. Drugs acting by reducing glutamatergic corticostriatal transmission may show a neuroprotective effect. However, their efficacy does not seem to be directly related to their capability to decrease glutamate release from corticostriatal terminals. We suggest that additional modulatory actions on voltage-dependent conductances and on ischemia-induced ion distribution at the postsynaptic site may also exert a crucial role.