Cocaine occupancy of sigma₁ receptors and dopamine transporters in mice

Activation of sigma₁ (σ₁) receptors contributes to the behavioral and toxic effects of (-)-cocaine. We studied a key step, the ability of (-)-cocaine to occupy σ₁ receptors in vivo, using CD-1^® mice and the novel radioligand [¹²⁵I]E-N-1-(3'-iodoallyl)-N'-4-(3″,4″-dimethoxyphenethyl)-piperazine ([¹²⁵I]E-IA-DM-PE-PIPZE). (-)-Cocaine displayed an ED₅₀ of 68 μmol/kg for inhibition of specific radioligand binding in whole brain, with values between 73 and 80 μmol/kg for heart, lung, and spleen. For comparison, an ED₅₀ of 26 μmol/kg for (-)-cocaine occupancy of striatal dopamine transporters (DAT) was determined by inhibition of [¹²⁵I]3β-(4-iodophenyl)tropan-2β-carboxylic acid isopropyl ester ([¹²⁵I]RTI-121) binding. A chief finding is the relatively small potency difference between (-)-cocaine occupancy of σ₁ receptors and the DAT, although the DAT occupancy is likely underestimated. Interactions of (-)-cocaine with σ₁ receptors were assessed further using [¹²⁵I]E-IA-DM-PE-PIPZE for regional cerebral biodistribution studies and quantitative ex vivo autoradiography of brain sections. (-)-Cocaine binding to cerebral σ₁ receptors proved directly proportional to the relative site densities known for the brain regions. Nonradioactive E-IA-DM-PE-PIPZE gave an ED₅₀ of 0.23 μmol/kg for occupancy of cerebral σ₁ receptors, and a 3.16 μmol/kg (i.p.) dose attenuated (-)-cocaine-induced locomotor hyperactivity by 30%. This effect did not reach statistical significance, but suggests that E-IA-DM-PE-PIPZE is a probable σ₁ receptor antagonist. As groundwork for the in vivo studies, we used standard techniques in vitro to determine ligand affinities, site densities, and pharmacological profiles for the σ₁ and σ₂ receptors expressed in CD-1^® mouse brain.