This project focuses on the receptor superfamily of ligand-gated ion channels, including the N-methyl-D-aspartate receptor (NMDA-R), the nicotinic cholinergic receptor (nAChR), and the GABAA receptor (GABA-R). NMDA-R is of interest because phencyclidine binds to sites inside the open state of the ionic channel formed by the receptor. This receptor has been linked to neurodegeneration in various conditions, including ischemia, epilepsy, and Huntington's disease. Our studies show that NMDA-R is regulated by tissue redox phenomena, as some reducing agents (e.g., ascorbic acid, hydroquinone) produce inactivation, whereas others (dithiothreitol) facilitate receptor activation. Furthermore, we found that ascorbic acid protects cultured cortical cells from the NMDA- and glutamate-induced toxicity. Using [3H]spermidine, we have identified and characterized polyamine receptors, which also appear to modulate activity of NMDA-R. Our assay system may provide a template for the design of therapeutic agents to alter the function of the NMDA-R. [3H]Mecamylamine was used as a new radioligand probe for the cationic channels of nAChRs. We discovered that the binding of [3H]mecamylamine and [3H]chlorpromazine is markedly increased by some purinergic nucleotides (e.g., ATP), suggesting that they may be important modulators of nAChR. Preliminary studies have shown that polyamines negatively modulate the binding of [3H]mecamylamine but not [3H]methylcarbamylcholine (a ligand for the acetylcholine recognition site of nAChRs) in brain. Since both receptors are ligand-gated cationic channels, it seems possible that polyamines generally are regulators of such receptors. Pregnenolone sulfate (PS) and dehydroepiandrosterone sulfate (DHEAS) are modulators of GABA-R. We examined binding of [3H]PS in the rat brain, noting that this steroid seems to interact at the interface of the receptor with associated phospholipids. We identified and characterized specific binding sites for [3H]DHEAS, and demonstrated, by biochemical and electrophysiological methods, that DHEAS is a noncompetitive negative modulator of the GABAA receptor complex.
