Protective Role of Nerve Growth Factor against Excitatory Amino Acid Injury during Neostriatal Cholinergic Neurons Postnatal Development

Abstract

The interaction between excitatory amino acids (EAAs) and nerve growth factor (NGF) levels were studied on neostriatal cholinergic neurons during postnatal development. Striatal choline acetyltransferase (ChAT) activity and NGF levels were determined 7 days following EAA injection in 7-, 15-, 21-, 30-, and 50-day-old rats. ChAT activity was decreased 7 days after kainate (KA), quinolinate (QUIN), or quisqualate (QUIS) lesion. The reduction was most pronounced in 30-day-old rats. KA injection produced the greatest decrease in ChAT activity. Conversely, KA did not change NGF levels. QUIN and QUIS increased NGF protein and these effects were maximal with lesions in 21-day-old rats. In order to further characterize the effect of EAAs on NGF levels and ChAT activity, the time-course of the lesion was studied. We used 30-day-old rats as the maximal sensitivity of cholinergic neurons to EAAs was observed at this age. ChAT activity decreased 2 days following QUIN or QUIS injection and 1 day after KA. The EAA agonists also changed NGF levels. QUIN induced an increase in NGF levels 1 day after lesion. This effect was maintained to the last time point examined. In contrast, KA and QUIS induced transient increases in NGF levels that were only detected 2 and 4 days after injection, respectively. To study whether NGF is able to regulate EAA excitotoxicity on striatal cholinergic neurons, we studied ChAT activity 7 days after simultaneous injection of NGF plus QUIN, KA, or QUIS. Intrastriatal injection of exogenous NGF was able to block the decrease in ChAT activity observed following EAA injection alone. In conclusion, our results show that striatal cholinergic neurons have different vulnerabilities to excitotoxicity induced by EAAs during development. ChAT activity was decreased and NGF was increased by EAAs. However, those EAAs (QUIN and QUIS) that increased NGF had less effect on ChAT activity than KA which had little effect on NGF levels, suggesting that an increase in endogenous NGF levels by these agents may decrease their toxicity. This was confirmed by our finding that exogenous NGF protects cholinergic neurons against excitotoxic lesion. The combined results suggest that sensitivity to EAAs and the regulation of NGF may be crucial to the development of striatal cholinergic neurons.