Opioid receptors of bovine posterior pituitary neurosecretosomes are exclusively kappa.

Intact neurosecretosomes (NSS) from bovine posterior pituitary were prepared and characterized. Ligand binding studies were performed, using 3H-[D-Ala2-D-Leu5]enkephalin (DADL), 3H-etorphine and 3H-ethylketocyclazocine (EKC). The absence of specific binding of 3H-DADL and the inability of DADL to displace 3H-etorphine, implies the absence of mu, delta, or DADL-suppressible benzomorphan ("kappa-2") sites. Self- and cross displacement studies for etorphine and EKC suggested receptor heterogeneity. EKC fails to displace - 20% of specific binding of etorphine. Mathematical modelling indicates the presence of three classes of sites. The present findings imply that bovine posterior pituitary opioid receptors are exclusively of the kappa type.