Computer-assisted modeling of multiple dextromethorphan and sigma binding sites in guinea pig brain

Computer-assisted, simultaneous analysis of self- and cross-displacement experiments demonstrated the existence of several binding sites in guinea pig brain for dextromethorphan, (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP)- and 1,3-di-o-totyl guanidine (DTG). Dextromethorphan binds with high affinity to two sites (R₁ K_d 50–83 and R₂ K_d 8–19 nM) and with low affinity to two additional sites (R₃ and R₄). (+)-3-PPP binds to one high-affinity (R₁ K_d 24–36 nM), to one intermediate-affinity (R₃ K_d 210–320 nM), and to two (R₂ and R₄) low-affinity sites. DTG binds with almost identical high affinity to two different sites (R₁ K_d 22–24 and R₃ K_d 13–16 nM). These results confirm that dextromethorphan, (+)-3-PPP, and DTG bind to the common DM₁/σ₁ site (R₁). The binding of DTG to two different sites with identical affinities precludes the use of this compound as a specific marker for σ receptors. Besides, haloperidol displaces labeled ligands from both high-affinity DTG sites (R₁ and R₃) with high affinity. Thus, haloperidol sensitivity should not be used as the single criterion to identify a putative receptor. The resolution of these novel sites also may provide new insights into the multiple effects of antipsychotic drugs. In addition, this investigation has important implications regarding the methods that must be applied to characterize multiple binding sites and their relations with putative receptors.