Thermodynamics of the binding of BTCP (GK 13) and related derivatives on the dopamine neuronal carrier

We have studied the thermodynamic properties of the binding of a coherent series of uptake inhibitors derived from BTCP (GK 13 = N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine) to the dopamine neuronal carrier labelled with [ 3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]4-[3-phenyl-2-propenyl]-piperazine). GK 13 (30 nM) and its 2-naphthyl derivative GK 189 (15 nM) competitively inhibited the specific binding of [ 3H]GBR 12783 to sites present in rat striatal membranes. Hill numbers calculated for the inhibition of the specific binding of [ 3H]GBR 12783 by BTCP derivatives were close to 1 (range 0.79–1.18). Increasing the temperature from 0° to 30°C induced a decrease in the affinity of [ 3H]GBR 12783 and GK derivatives which was generally less pronounced than that obtained when temperature was raised from 30°C to 37°C. Increasing the incubation temperature led to a decrease in both enthalpy (ΔH°) and entropy (°S°). We observed at 37° a large negative enthalpy change (range −48, −79 kJ/mol) and a negative, binding unfavorable, change in entropy. This indicates that the GK derivatives binding is enthalpy-driven. Furthermore, data obtained in the present study show that changes in thermodynamic parameters are not a function of the inhibitor's affinity for the dopamine neuronal carrier and this suggests that bonds involved in the inhibitor-carrier interaction are more likely related to the carrier configuration than to the chemical structure of the inhibitor.