MODULATION OF THE GLUTAMATE-EVOKED RELEASE OF ARACHIDONIC-ACID FROM MOUSE CORTICAL-NEURONS - INVOLVEMENT OF A PH-SENSITIVE MEMBRANE PHOSPHOLIPASE A(2)

Excitatory synaptic transmission is associated with changes in both ex tracellular and intracellular pH. Using mouse cortical neurons in prim ary cultures, we studied the sensitivity of glutamate-evoked release o f H-3-arachidonic acid (H-3-AA) to changes in extracellular pH (pH(o)) and related intracellular pH (pH(i)). As pH(o) was shifted from 7.2 t d 7.8, the glutamate-evoked release of H-3-AA was enhanced by threefol d, The effect of alkaline pH(o) on the glutamate response was rapid, b ecoming significant within 2 min. H-3-AA release, evoked by both NMDA and kainate, was also enhanced by pH(o) alkalinization, NMDA- and kain ate-induced increase in free intracellular Ca2+ was unaffected by chan ging pH(o) from 7.2 to 7.8, indicating that the receptor-induced Ca2influx is not responsible for the pH(o) sensitivity of the glutamate-e voked release of H-3-AA. Alkalinization of pH(i) obtained by incubatin g neurons in the presence of HCO3- or NH4 enhanced the glutamate-evoke d release of H-3-AA, while pH(i) acidification obtained by blockade of Na+/H+ and Cl-/HCO3- exchangers decreased the glutamate response. Mem brane-bound phospholipase A(2) (mPLA(2)) activity was stimulated by Ca 2+ 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 H-3-AA, Taken togethe r, these results indicate that the glutamate-evoked release of H-3-AA may be mediated by the pH-sensitive mPLA(2). Since excitatory neurotra nsmission mediated by glutamate results in both pH(o) and pH(i) change s and since AA enhances glutamatergic neurotransmission at both pre- a nd postsynaptic levels, the data reported here reveals a possible mole cular mechanism whereby glutamate can modulate its own signalling effi cacy in a pH-dependent manner by regulating the release of AA.