Interference of endogenous B endorphin with opiate binding in the anterior pituitary

Opiates have been shown to affect prolactin secretion directly at the pituitary level. However, opiate binding sites have not been characterized on anterior pituitary tissue. In the present work we show that a soluble factor present in anterior pituitary membrane preparation inhibit opiate binding on striatal membranes. The high concentrations of B-endorphin like immunoreactivity recovered from the supernatant of pituitary membrane preparations seem to account for this effect on striatal binding and to be responsible for a partial masking of pituitary opiate receptors. We thus attempted to improve membrane fractionation procedure in order to decrease B-endorphin contamination and thus to increase binding of opiate ligands on pituitary membranes. 3-H Etorphine binds to pituitary membranes with a constant of association (kon) of 20.8 10(6) M-1 min-1 and a constant of dissociation (hoff) of 33 min-1. Saturation experiments with 3-H Etorphine and 3-H ethylketocyclazocine indicate the presence of a single population of specific binding sites (KD) value of 12.2 +/- 3.9 nM a maximal binding capacity of 55.9 +/- 10.7 fm/mg protein for 3-H Etorphine binding). These results demonstrate the presence of opiate binding sites in the anterior pituitary. However the difficulty of the membrane preparation and the low density of binding sites did not allow a complete pharmacological characterization of these sites.