Demonstration of a putative receptor site for cholecystokinin in rat brain

High affinity (apparent K_d {reversed tilde equals} 0.65 nM) and saturable (B_max {reversed tilde equals} 6.6 fmoles/mg protein) binding sites for cholecystokinin (CCK) have been demonstrated in crude synaptosomal membranes from rat cerebral cortex. Scatchard analysis of the equilibrium binding data indicates a single population of non-interacting sites. The specific binding of ¹²⁵I-CCK₃₃ to brain membranes is reversible (t _{1 2} {reversed tilde equals} 6 minutes), and can be inhibited by structurally related CCK analogues but not by unrelated neuropeptides or drugs. Specific CCK binding reaches equilibrium at a temperature-dependent rate, and is abolished when membranes are pre-treated with heat. Kinetic and competition studies suggest that these high affinity binding sites represent physiologically-relevant receptors for CCK in brain.