Modulation of the glutamate-evoked release of arachidonic acid from mouse cortical neurons: Involvement of a pH-sensitive membrane phospholipase A₂

Excitatory synaptic transmission is associated with changes in both extracellular and intracellular pH. Using mouse cortical neurons in primary cultures, we studied the sensitivity of glutamate-evoked release of ³H- arachidonic acid (³H-AA) to changes in extracellular pH (pH(o)) and related intracellular pH (pH(i)). As pH(o) was shifted from 7.2 to 7.8, the glutamate-evoked release of ³H-AA was enhanced by ~ threefold. The effect of alkaline pH(o) on the glutamate response was rapid, becoming significant within 2 min. ³H-AA release, evoked by both NMDA and kainate, was also enhanced by pH(o) alkalinization. NMDA- and kainate-induced increase in free intracellular Ca²⁺ was unaffected by changing pH(o) from 7.2 to 7.8, indicating that the receptor-induced Ca²⁺ influx is not responsible for the pH(o) sensitivity of the glutamate-evoked release of ³H-AA. Alkalinization of pH(i) obtained by incubating neurons in the presence of HCO₃ or NH₄ enhanced the glutamate-evoked release of ³H-AA, while ph(i) acidification obtained by blockade of Na⁺/H⁺ and Cl^-/HCO₃ exchangers decreased the glutamate response. Membrane-bound phospholipase A₂ (mPLA₂) activity was stimulated by Ca²⁺ in a pH-dependent manner, increasing its activity as pH was shifted from 7.2 to 7.8. This pH profile corresponds to the pH profile of the glutamate-, NMDA- and kainate-evoked release of ³H-AA. Taken together, these results indicate that the glutamate-evoked release of ³H-AA may be mediated by the pH-sensitive mPLA₂. Since excitatory neurotransmission mediated by glutamate results in both pH(o) and pH(i) changes and since AA enhances glutamatergic neurotransmission at both pre- and postsynaptic levels, the data reported here reveals a possible molecular mechanism whereby glutamate can modulate its own signalling efficacy in a pH-dependent manner by regulating the release of AA.