N‐Methyl‐D‐Aspartate‐Mediated Injury Enhances Quisqualic Acid‐Stimulated Phosphoinositide Turnover in Perinatal Rats

Abstract: Previous work in our laboratory demonstrated that ischemic‐hypoxic brain injury in postnatal day 7 rats causes a substantial increase in phosphoinositide (PPI) turnover stimulated by the glutamate analogue quisqualic acid (QUIS) in the hippocampus and striatum. To examine this phenomenon in more detail, we performed similar experiments after producing injury by unilateral intracerebral injections of the glutamate analogue N‐methyl‐D‐aspartate (NMDA). The 7‐day‐old rodent brain is hypersensitive to NMDA neurotoxicity and NMDA injection causes histopathology that closely resembles that produced by ischemia‐hypoxia. NMDA, 17 nmol in 0.5 μl, was injected into the right posterior striatum of 7‐day‐old rat pups and they were killed 3 days later. Hippocampal or striatal tissue slices were prepared from ipsilateral and contralateral hemispheres from vehicle‐injected control and from noninjected control rat pups. Slices were then incubated with myo‐[³H]inositol plus glutamate agonists or antagonists in the presence of lithium ions and [³H]inositol monophosphate ([³H]IP₁) accumulation was measured. The glutamate agonists, QUIS, L‐glutamic acid, and (RS)‐α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid, stimulated greater [³H]IP₁ release in tissue ipsilateral to the NMDA injection compared with that in the contralateral side and in control pups. The glutamate antagonists, D,L‐2‐amino‐7‐phosphonoheptanoic acid, 3‐[(+)‐2‐carboxypiperazin‐4‐yl]‐propyl‐l‐phosphoric acid, kynurenic acid, and 6,7‐dinitroquinoxaline‐2,3‐dione did not inhibit QUIS‐stimulated [³H]IP₁ release. The enhanced PPI turnover in the lesioned tissue was specific to glutamate receptors because carbachol (CARB) failed to elicit preferential enhanced stimulation. To investigate the possibility that alterations in the release of endogenous neurotransmitters had a role in potentiating QUIS‐stimulated PPI turnover after NMDA injection, we examined the effect of tetrodotoxin. Tetrodotoxin (0.5 μM) did not alter QUIS‐or CARB‐stimulated PPI hydrolysis in the lesioned or unlesioned tissue. The influence of extracellular calcium concentration on QUIS‐stimulated [³H]IP₁ formation was also examined after the NMDA lesion. Moderate reduction of calcium in the buffer (1 μM) enhanced the lesion effect. Low calcium buffer enhanced QUIS‐stimulated PPI turnover in the lesioned hippocampal slices, but reduced QUIS stimulation in contralateral slices and controls. In contrast, CARB‐stimulated PPI turnover was not enhanced in low Ca²⁺ buffer. A similar pattern of Ca²⁺ dependency was observed in striatal slices. Calcium‐free (<10 nM) buffer suppressed PPI turnover in all groups. These studies demonstrate that NMDA‐induced excitotoxic injury in neonatal rats causes a selective enhancement of QUIS‐stimulated PPI turnover that resembles the effects of ischemia‐hypoxia. In addition, we found that agonist‐stimulated PPI turnover is sensitive to the in vitro Ca²⁺ concentration. These changes could reflect altered coupling of non‐NMDA receptors to phospholipase C activity.