[³H]Thienylcyclohexylpiperidine Binding Activity in Brain Synaptic Membranes Treated with Triton X‐100

Abstract: Binding activity of [³H]thienylcyclohexylpiperidine was examined using rat brain synaptic membranes treated with Triton X‐100. This compound is proposed to be a noncompetitive antagonist for the N‐methyl‐d‐aspartate (NMDA)‐sensitive subclass of brain excitatory amino acid receptors. The activity decreased in proportion to increasing concentrations of the detergent up to 0.08%. In vitro addition of l‐glutamate (Glu) partially restored the decreased activity caused by this Triton treatment, whereas further addition of glycine (Gly) entirely reversed the loss of activity to the level found in membranes extensively washed but not treated with a detergent. These stimulatory effects were found to be due to the acceleration of the association of ligand. The rank order of potentiation of the activity coincided well with that of the affinity for the NMDA‐sensitive subclass among numerous Glu analogs. The potentiation by Gly as well as Glu was invariably prevented by competitive NMDA antagonists, such as dl‐2‐amino‐5‐phosphonovalerate and (×)‐3‐(2‐carboxypiperazin‐4‐yl)propyl‐1‐phosphonate, but not by strychnine. No significant difference was observed between pharmacological profiles of the activities in synaptic membranes treated and not treated with Triton X‐100, except haloperidol. The potency of this σ‐ligand to inhibit the activity was greatly reduced by the Triton treatment in the presence of both Glu and Gly. These results suggest that the regulatory properties of Triton‐treated synaptic membranes remain unchanged in terms of the interaction within the NMDA receptor complex.