Exposure toN-methyl- d-aspartate increases release of arachidonic acid in primary cultures of rat hippocampal neurons and not in astrocytes

Abstract

The release of [ 3H]arachidonic acid (ARA) was investigated from prelabelled primary cultures of hippocampal neurons and astroglial cells. The activation ofN-methyl- d-aspartate (NMDA) subtype of glutamate receptors resulted in a dose-dependent stimulation of [ 3H]ARA release. The half maximal effect was obtained at about 15 μM NMDA, whereas the maximum concentration (50 μM NMDA) produced about a 2-fold increase in 7-day-old cultures. This elevation in [ 3H]ARA release was blocked in a dose-related manner by the NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV), and by Mg 2+ which blocks NMDA receptor-linked Ca 2+ ion channels. The removal of external Ca 2+ inhibited NMDA-induced release, whereas treatment with calcimycin (A 23187, a Ca 2+ ionophore) greatly increased the [ 3H]ARA release. The inhibitors of phospholipase A 2, nordihydroguaiaretic acid and mepacrine, decreased the NMDA-dependent [ 3H]ARA release in a dose-related manner, maximum inhibition reaching to about 90% at high doses. Entry of Ca 2+ brought about by opening the voltage-sensitive channels by high K + had no effect on the release of [ 3H]ARA, indicating that NMDA gated channels are situated in a part of the neuron where Ca 2+ entry through this route is more efficiently coupled to the activation of phospholipase A 2. Treatment with NMDA had no significant effect on [ 3H]ARA release in hippocampal astroglial cells as opposed to neurons. This was not due to inability of astrocytes to release ARA, for ATP still evoked [ 3H]ARA release, and this was markedly inhibited by mepacrine. It is suggested that ARA act as both intracellular and intercellular messengers in the functioning of NMDA receptors in synaptic transmission and plasticity in the hippocampus.