CHANGES IN NEURONS AND GLIA IN RESPONSE TO GLUTAMATERGIC EXCITOTOXIC EVENTS

Abstract

Glutamate, aspartate, NMDA receptor (NMDARl and 2 subunits), and nitric oxide synthase (NOS) immunoreactive neurons were studied in the arcuate nucleus (Arc) of adult mice neonatally treated with monosodium glutamate which is known to cause extensive neuronal loss in this hypothalamic nucleus. It was found that intensely stained glutamate and aspartate immunoreactive neurons present in the Arc of control mice were completely absent in the MSG-lesioned Arc as well as the ventromedial nucleus lateral to the Arc. Similarly, NMDARl immunoreactive neurons were mostly absent in the MSG-lesioned Arc but remained intact in the ventromedial nucleus. There was also a substantial loss of NMDAR2A/B immunoreactivity within the Arc. In contrast, NOS immunoreactive neurons in the Arc survived the neonatal glutamate treatment, although they appeared to be less intensely stained. It may be speculated that NOS neurons may be selectively protected from damage partially due to its ability to generate NO+ which exerts neuroprotective action by nitrosylation of free sulfhydryl groups in the redox modulatory site of the NMDA receptor. Developmental studies showed that NOS-immunoreactivity was generally high in the developing brain compared to the adult brain. Over the period of the first 19 postnatal days, MSG-treatment (from day 2 - 10) did not cause any significant change in the NOS-immunostaining in the Arc. In contrast, microglial and astroglial response was observed. Staining of both microglia and astroglia reached their maximal levels at day 8 possibly indicating maximal neural damage at this timepoint. Thereafter, microglial activation declined rapidly while astroglial response persisted for another 11 days. These temporal profiles of microglial and astroglial response in relation to neuronal degeneration raised the possibility that microglial activation may be more involved with neural degeneration and astroglial response with neural regeneration. In another model of excitotoxic lesioning, KA was injected directly into the striatum to further investigate the potential involvement of NO. Both ChAT- and NOS-containing neurons were damaged within 1h postinjection. NOS-positive neurons reappeared in the lesioned striatum only 24h postinjection, along with an accumulation of small NOS-containing cells. Small glia-like ChAT-positive cells also reappeared at the same time. These cells do not have the morphology of normal neurons. The activation of astroglia and microglia increased within 1h after injection, peaking from 24h - 2 days after injection. These findings provided some evidence that an activation of NOS in astroglia/microglia, in response to KA injection, might be closely associated with the initiation and progression of neuronal injury in the striatum, consistent with the idea that glial cells play a significant role in excitotoxic events. Mice treated neonatally with MSG have been shown to suffer some learning and memory deficits presumably due to diminished hippocampal cholinergic functions. In studying ChAT and NMDAR immunostaining in the hippocampus of MSG-treated mice, no significant damage was found in both parameters suggesting that there were no structural changes in both systems. Although not conclusive, it also suggests that the impairment of the hippocampal cholinergic mechanisms in these mice is only functional.