Effects of temperature on the in vitro binding of 3H-raclopride to rat striatal dopamine-D2 receptors.

The influence of temperature on the in vitro binding of 3H-raclopride to rat striatal dopamine-D2 receptors was investigated. The KD-values obtained in Scatchard plots were approximately 1.6 nM at temperatures between 15 degrees and 30 degrees. At 37 degrees the KD-value was found to be 3.0 nM, indicating a lower affinity without affecting the number of the receptors (Bmax). The rate of association of 3H-raclopride to the receptors was decreased with decreasing temperatures, and at 6 degrees more than 150 min. incubation was needed to reach the steady state level. From the association constants, the activation energy of the binding reaction was calculated to be 80 kJ/mol. The driving forces of the binding reaction was suggested to be a change in entropy at temperatures up to 30 degrees but a change in enthalpy at 37 degrees.