Sigma Receptors
Sigma receptors were initially thought to be the phencyclidine binding site on the NMDA receptor complex or a type of opioid receptor. They have since been distinguished from these two receptors and two subtypes of sigma receptors are now known; σ1 and σ2. σ1 and σ2 are distinguished by their stereoselectivity for pentazocine, SK&F 10047 and cyclazocine and have different tissue distribution patterns and subcellular localizations.