A dynorphin-like opioid in the central nervous system of an amphibian.

Abstract

Evidence is provided for the existence of a biologically active opioid in toad (Bufo marinus) brain that is immunoreactive with antiserum raised against dynorphin-(1-13). Compared with porcine dynorphin, this opioid is similar in apparent MW on the basis of gel permeation chromatography and is more hydrophobic on the basis of high-performance liquid chromatography. After purification, its opioid biological activity was demonstrated on the guinea pig ileum myenteric plexus-longitudinal muscle preparation. It was less potent, and had a similar sensitivity to antagonism by naloxone, in comparison with porcine dynorphin. Because it is immunoreactive with antiserum specific for porcine dynorphin, it probably has considerable sequence homology. Generally, the tissue distribution of immunoreactive dynorphin in the toad is similar to that in the rat, with highest concentrations in the neurointermediate lobe of the pituitary. The anterior lobe of the toad pituitary contains considerably lower concentrations than are found in the rat anterior lobe. There appear to be 3 size classes of immunoreactive dynorphin in toad neural tissue, each with apparent MW below 12,000, similar to the size classes of immunoreactive dynorphin found in pig and rat. In toad spinal cord (and possibly in brain) there is immunoreactive dynorphin of greater apparent MW, which has not been reported in mammalian tissue. The contribution of each molecular size to the total immunoreactivity varies from tissue to tissue and is different from that observed in rat.