Comparison of binding parameters of σ1 and σ2 binding sites in rat and guinea pig brain membranes: novel subtype-selective trishomocubanes

Abstract

Comparisons of binding parameters of [ 3H](+)-pentazocine and [ 3H]1,3-di- o-tolylguanidine (DTG) at σ binding sites in guinea pig and rat brain membranes demonstrated that [ 3H](+)-pentazocine binds to a single high-affinity site, whereas [ 3H]DTG binds to two high-affinity sites in both species. The K d values of the radioligands were similar in both types of membranes. However, the density of σ 1 sites in guinea pig was significantly higher than that of rat. Novel trishomocubanes were tested for their affinities at σ 1 and σ 2 binding sites in guinea pig brain membranes using [ 3H](+)-pentazocine and [ 3H]DTG as the radioligands. N-(4-Phenylbutyl)-3-hydroxy-4-azahexacyclo[5.4.1.0 2,6.0 3,10.0 5,9.0 8,11]dodecane (ANSTO-14) showed the highest affinity for the σ 1 site ( K i = 9.4 nM) and 19-fold σ 1 σ 2 selectivity, as a result of increasing the alkyl chain between the cubane moiety and the aromatic ring. N-(3′-Fluorophenyl)methyl-3-hydroxy-4-azahexacyclo[5.4.1.0 2,6.0 3,10.0 5,9.0 8,11]dodecane (ANSTO-19), displayed the highest affinity for σ 2 sites ( K i = 19.6 nM) and 8-fold σ 2 σ 1 selectivity due to a fluoro substitution in the meta position of the aromatic ring. These represent structurally novel lead compounds, especially for the development of selective σ 2 receptor ligands.