The Effects of Dextromethorphan Against the Toxicity of Photoactivated Caged Glutamate In Vitro

Abstract

Abstract: : Purpose: One of the goals of our current work is to design a neurotransmitter-based stimulator for the purpose of artificial vision. This requires the quantal release of neurotransmitter, constrained in space and time to small synaptic domains of a single neuron or small group of neurons. Control of neurotransmitter release can be controlled via high-speed iontophoresis after photoactivation of a caged pro-drug. Glutamate is the primary stimulatory molecule in the central nervous system. If large quantities are released, however, excitotoxic cell death may occur. Some of our previous work has shown that while the ‘caged’ form of glutamate is less excitotoxic than glutamate, some uncaged forms may be more toxic. The purpose of this study was to examine the effects of the weak NMDA receptor antagonist, dextromethorphan (Dex), against the toxicity associated with both glutamate and photoactivated caged glutamate. Methods: Visual cortical neurons were cultured from embryonic Sprague-Dawley rat pups incubated for 8 days before exposure to either 50 uM L-glutamate, 50 uM p-hydroxyphenacyl (hPA) glutamate, 50 uM p-hydroxy-m-methoxyphenacyl (mPA) glutamate, m,m'-dimethoxy-p-hydroxyphenacyl (dPA) glutamate, with and without 75 uM Dex. Cellular viability was assessed 24 hours after the initial exposure using a trypan blue dye exclusion assay. Results: Cultures treated with both glutamate and Dex, or uncaged hPA-glutamate and Dex demonstrated a significantly higher rate of neuronal viability than those treated with glutamate or uncaged hPA-glutamate alone (p<.001, p<.05, respectively). Cultures treated with both uncaged mPA-glutamate and Dex, and uncaged dPA-glutamate and Dex did not show a difference in neuronal viability from uncaged mPA or dPA glutamate alone. Conclusions: Dex significantly reduced the toxicity of uncaged hPA-glutamate. Dex did not fully protect against the toxicity of the photoactivated mPA and dPA. Thus, the toxic mechanisms underlying these caged glutamates may not be exclusively NDMA-receptor mediated.

Keywords: neurotransmitters/neurotransmitter systems • excitatory neurotransmitters • cell death/apoptosis