Immunohistochemical and in situ hybridization studies on glycine transporter 1 after transient ischemia in the rat forebrain

Glycine is a critical factor in ischemia as reduced astrocytic and increased extracellular glycine levels aggravate the neurotoxic effect of glutamate and consequently, increase the extent of brain damage. Extracellular levels of glycine are primarily regulated by the plasma membrane glycine transporter 1. In the present study, we examined the effects of transient ischemia (1 h occlusion of the middle cerebral artery; followed by 0 h, 0.5 h, 1 h, 2 h, 4 h, 24 h or 48 h reperfusion) on immunoreactivity and mRNA expression of glycine transporter 1 in the rat forebrain. In control animals, glycine transporter 1-immunoreactivity was strong in diencephalic and certain telencephalic structures, moderate in the globus pallidus, and rather low in the cortex and striatum. In situ hybridization studies revealed a similar distribution pattern of glycine transporter 1 mRNA expression. One hour occlusion of the middle cerebral artery resulted in a significant decrease in ipsilateral glycine transporter 1-immunoreactivity and mRNA expression in a circumscribed region of the preoptic/hypothalamic area; both the immunoreactivity and mRNA exhibited further reductions with increasing reperfusion time. In contrast, the cerebral cortex and the globus pallidus showed an increase of glycine transporter 1-immunoreactivity after 0.5 h reperfusion; the elevation proved to be transient in the somatosensory cortex and remained sustained in the globus pallidus after longer reperfusion times. Western blot analysis of globus pallidus samples from the ipsilateral side confirmed higher glycine transporter 1 protein levels. These results suggest an elevated expression of the transporter protein facilitating the glial uptake of glycine from the extracellular space. However, glycine transporter 1 mRNA expression was not significantly different in the penumbra regions from the corresponding contralateral sites of the injury. Together, these findings indicate that post-translational mechanisms are of primary importance in elevating glycine transporter 1 protein levels following transient ischemia.