Non-opioid dynorphin binding site on secretory vesicles of a pituitary-derived cell line

Accumulating evidence indicates that the endogenous opioid peptides dynorphinA-(1-17) and dynorphinA-(1-13) interact not only with opioid but also with yet poorly characterized non-opioid receptors. The latter have been implicated in a number of the effects of dynorphins including induction of ACTH release in sheep and in AtT 20 cells, a pituitary-derived mouse cell line. AtT 20 cells do not express opioid receptors and therefore are particularly suitable for search of non-opioid dynorphin receptors. We report here that 3H-dynorphinA-(1-13)-NH2 associates specifically with AtT 20 cells, apparently through an uptake process and a binding site. Within the cell, it binds preferentially to fractions containing secretory vesicles, with a Kd of about 100 nM. DynorphinA-(1-17), and several non-opioid fragments of dynorphin, including A-(2-17), A-(2-16) and A-(2-13), compete with 3H-dynorphinA-(1-13)-NH2 for that site with IC50s ranging from 200 nM to 2 microM. ACTH(1-39) also competes with 3H-dynorphinA-(1-13)-NH2 for the site with an IC50 of about 300 nM. DynorphinA-(2-17) at microM concentrations stimulates release of ACTH from the isolated vesicles. The results indicate the presence of a non-opioid dynorphin binding site on the secretory vesicle fractions of AtT20 cells that might be involved in ACTH release. The ability of ACTH itself to compete for the binding sites associated with the vesicles suggest that those sites may be involved in an autocrine loop.