Protective effect of metabotropic glutamate receptor inhibition on amyotrophic lateral sclerosis–cerebrospinal fluid toxicity in vitro

Conflicting results have been reported concerning the toxicity of cerebrospinal fluid from patients with amyotrophic lateral sclerosis (ALS-CSF) when added to neuronal cultures. The possible toxic factor(s) and the exact mode of action (e.g. requirement of glial cells) have not been identified so far. Glutamate is a potential candidate for this toxic effect, since antagonists of ionotropic glutamate receptors have been shown to attenuate ALS-CSF toxicity.We studied the effects of ALS-CSF on mixed and motoneuron-enriched chick embryonic spinal cord cultures. We found a toxic action of ALS-CSF in both culture types which could not be attenuated by 5 kDa-filtration or 15 min 90 °C heating.Nevertheless, the metabotropic glutamate receptor (mGluR) group I antagonist 1-aminoindan-1,5-dicarboxylic acid, but also the group I agonist (s)-3,5-dihydroxyphenylglycine (DHPG) exerted protective effects against ALS-CSF toxicity. In this experimental setting, DHPG may functionally act via a receptor blockade due to sustained activation. No protective effect was seen with the mGluR group III inhibitor (RS)-α-cyclopropyl-4-phosphonophenylglycine (CPPG). Addition of DHPG did not increase the protective action of the AMPA inhibitor 6-chloro-4-hydroxyquinoline-2-carboxylic acid (6-CKU). Addition of l-glutamate did not mimic these toxic ALS-CSF effects in motoneuron-enriched cultures.Our experiments demonstrate that ALS-CSF toxicity is mediated by a small heat-resistant molecule which may act directly on neurons. Since blockade of group I mGluRs exerts a protective effect, the possibility of targeting these mGluRs pharmacologically in motoneuron disease should be kept in mind.