At least two different types of sigma σ (σ1 and σ2) receptors have been identified. A structural feature common to high-affinity (Ki < 10 nM) σ1 ligands is: C-N(R)-X-Ph; both C and Ph are associated with regions of bulk tolerance. Numerous other ligands bind, but typically do so with lower affinity.
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R. A. GLENNON (2005) studied this question.