Muscarinic sites in the amphibian central nervous system: Characterization and temperature studies

Associations of the muscarinic cholinergic antagonist [13H]quinuclidinyl benzilate (³H-QNB) with neural membranes from the frog (Rana pipiens pipiens) are described. This ligand interacts with a single set of identical independent sites on these membranes. The set of sites has characteristics suggesting that it represents a population of muscarinic receptor structures. The first thermodynamic association constant of ³H-QNB with the set of sites is 1.7 ± 0.53 x 10⁹ M^-1. The pseudo-first rate constant of association is 2.74 ± 0.42 x 10⁶ M^-1 sec ^-1. Temperatures studies demonstrate several transitions in both the kinetic and thermodynamic properties of association. Finally when membrane composition is modified by temperature acclimation of the animals, significant changes are observed in both antagonist and agonist interactions with the receptor structure.