Pharmacological differences between immunoisolated native brain and heterologously expressed rat α4β2 nicotinic receptors

Native brain and heterologously expressed rat α4β2 nicotinic receptors (in Xenopus oocytes and CV-1 cells) were immunoisolated with the anti-α4 antibody mAb 299 and their pharmacological properties were compared using [³H](±)epibatidine, the novel N-alkylnicotinium analog N-n-octylnicotinium iodide (NONI), and the ganglionic antagonist trimethaphan (TRM). The equilibrium dissociation constant (K_d) for [³H](±)epibatidine binding to the native and heterologously expressed receptors ranged from 13 to 21 pM. The Hill coefficients for [³H](±)epibatidine binding to the native and expressed receptors ranged from 0.8 to 1.1 and were consistent with a single high-affinity site. NONI inhibited 30 pM [³H](±)epibatidine binding to the native and expressed receptors with similar potency (IC₅₀ values of 6-7 μM). However, [³H](±)epibatidine dissociated 2-3 times more slowly from the native, than from the expressed receptors and TRM inhibited 30 pM [³H](±)epibatidine binding to the native receptors (IC₅₀ value of 330 μM) less potently than it did to the receptors expressed in oocytes (IC₅₀ value of 16 μM) or CV-1 cells (IC₅₀ value of 55 μM). The differences between the native and expressed [³H](±)epibatidine dissociation rate constants and IC₅₀ values for TRM were significant for both host cell types, although the values for the CV-1-expressed receptors were closer to the native ones than were those for the oocyte-expressed receptors. Thus, the epibatidine and trimethaphan binding sites in native and expressed α4β2 receptors appear to have significantly different structural or chemical properties.