No abstract

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 – 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. Non-radioactive 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.

Cocaine functions, in part, through agonist actions at sigma-1 (σ1 ) receptors, while roles played by sigma-2 (σ2 ) receptors are less established. Attempts to discriminate σ2 receptor-mediated effects of cocaine in locomotor hyperactivity assays have been hampered by the lack of potent and selective antagonists. Certain tetrahydroisoquinolinyl benzamides display high σ2 receptor affinity, and excellent selectivity for binding to σ2 over σ1 receptors. The behavioral properties of this structural class of σ ligands have not yet been investigated. The present study evaluated 5-bromo-N-[4-(6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-yl)-butyl)]-2,3-dimethoxy-benzamide, 1, a ligand shown by others to bind preferentially to σ2 over σ1 receptors, as well as dopamine D2 and D3 sites. First, we determined binding to monoamine transporters and opioid receptors, and noted 57-fold selectivity for σ2 receptors over the serotonin transporter, and >800-fold selectivity for σ2 receptors over the other sites tested. We then examined 1 in locomotor activity studies using male CD-1® mice, and saw no alteration of basal activity at doses up to 31.6 µmol/kg. Cocaine produced a fivefold increase in locomotor activity, which was attenuated by 66% upon pretreatment of mice with 1 at 31.6 µmol/kg. In vivo radioligand binding studies also were performed, and showed no occupancy of σ1 receptors or the dopamine transporter by 1, or its possible metabolites, at the 31.6 µmol/kg dose. Thus, ligand 1 profiles behaviorally as a σ2 receptor-selective antagonist that is able to counteract cocaine's motor stimulatory effects.