I imagine its the same receptor, but that one stereoisomer changes the conformation of the receptor protein in a different way to the other isomer.
This happens very frequently, biochemically speaking, thus D-methorphan (dextromethorphan, DXM), is a cough suppressant, acting on the NMDA receptor (sigma receptor perhaps, like PCP? I don't know that one for sure), but the other isomer, laevo-methorphan acts as a fairly potent mu opioid agonist.