Dynorphin-(1-13): the effect of in vivo treatment on opiate bindings in vitro

Mice were injected intracerebroventricularly with different doses of dynorphin-(1-13) in vivo and decapitated after different intervals of time; crude P2 fractions were then prepared and used for in vitro binding with [3H]dihydromorphine (DHM) and [3H][D-Ala2,D-Leu5]enkephalin. Dynorphin pretreatment was found to decrease DHM binding in vitro in a both time-dependent and dose-dependent manner. Its maximal effect lasted from 30 min to 3 h and recovery took place in 6-12 h. Analysis of the Scatchard plots showed that dynorphin decreased the number of high affinity-binding site for DHM, while KD of this site was essentially unaltered. Neither Bmax nor KD of low affinity site were much affected. A similar decrease in high affinity Bmax for DADLE was also obtained; however, the recovery was even more rapid. Extra washes of tissue did not significantly increase the binding, but preincubation of tissue in the presence of morphine reversed the dynorphin effect and increased DHM binding significantly to almost control levels. The probable mechanism of dynorphin in decreasing opiate receptor binding is discussed.