Electron spin resonance study of the synaptosome opiate receptor: kinetics of stereospecific binding of spin labeled morphine.

Morphine, spin labeled on the 3- or 6-position has been used as the opiate ligand in a study of the time course of stereospecific opiate binding to intact synaptosomes isolated from non-cerebellar rat brain. The broadening of electron spin resonance lines induced by immobilization of the ligand on binding has been used to determine the concentration of bound opiate. The stereospecificity of the reaction was measured by comparing ligand binding in the presence of thousand-fold molar excesses of dextrorphan or levorphanol. Using both static and flow techniques, the binding process has been continuously monitored at times greater than 4.8 s after mixing spin labeled morphine with synaptosomes. It is shown that for this ligand and receptor preparation, binding takes place primarily during a delayed, abrupt process whose rate and time of onset are temperature dependent and reflect the presence of added opiate agonist or antagonist.