THERMODYNAMICS OF THE BINDING OF BTCP-(GK-13) AND RELATED DERIVATIVESON THE DOPAMINE NEURONAL CARRIER

We have studied the thermodynamic properties of the binding of a coher ent series of uptake inhibitors derived from BTCP (GK 13 = N-[1-(2-ben zo(b)thiophenyl)cyclohexyl]piperidine) to the dopamine neuronal carrie r labelled with [H-3]GBR 12783 methoxy)ethyl]4-{3-phenyl-2-propenyl}-p iperazine). GK 13 (30 nM) and its 2-naphthyl derivative GK 189 (15 nM) competitively inhibited the specific binding of [H-3]GBR 12783 to sit es present in rat striatal membranes. Hill numbers calculated for the inhibition of the specific binding of [H-3]GBR 12783 by BTCP derivativ es were close to 1 (range 0.79-1.18). Increasing the temperature from 0 degrees to 30 degrees C induced a decrease in the affinity of [3H]GB R 12783 and GK derivatives which was generally less pronounced than th at obtained when temperature was raised from 30 degrees C to 37 degree s C. Increasing the incubation temperature led to a decrease in both e nthalpy (Delta H degrees) and entropy (Delta S degrees). We observed a t 37 degrees C a large negative enthalpy change (range -48, -79 kJ/mol ) and a negative, binding unfavorable, change in entropy. This indicat es that the GK derivatives binding is enthalpy-driven. Furthermore, da ta obtained in the present study show that changes in thermodynamic pa rameters are not a function of the inhibitor's affinity for the dopami ne neuronal carrier and this suggests that bonds involved in the inhib itor-carrier interaction are more likely related to the carrier config uration than to the chemical structure of the inhibitor.