Peptide E: opiate receptor binding profile in the rat brain membrane assay. Comparison with beta-endorphin

Enkephalin affects ion transport via the enteric nervous system in guinea-pig ileum

The endogenous opioid enkephalin drives ion transport towards absorption. To determine the site and mechanism of this effect, fractionated stripping of guinea-pig ileum was carried out. The muscularis propria, including myenteric plexus, was removed by partial stripping. The submucosa, including the submucosal plexus, plus the muscularis mucosae were removed by total stripping. For binding studies, epithelial cells were removed by the method of Weiser leaving the lamina propria mucosae with the mucosal plexus. Radio-receptor-assay with (3H)2-D-ala-5-D-leu-enkephalin revealed enkephalin binding sites in the submucosa plus muscularis mucosae (KD = 3.6 nmol l-1; Vmax = 7.3 fmol mg-1) and in the lamina propria mucosae (KD = 4.2 nmol l-1; Vmax = 5.1 fmol mg-1. The binding was stereospecific in both layers. No binding was detected on epithelial cells. In the Ussing chamber, partially stripped ileum exhibited spontaneous ISC which was abolished by addition of tetrodotoxin (TTX) or by total stripping indicating that this ISC was neuronally stimulated by the submucosal plexus. Electrogenic chloride secretion was identified as contributing to this ISC, since the TTX-sensitive part of ISC in the partially stripped ileum was lacking in Cl- and HCO3-free medium, reappeared after addition of Cl consistent with Michaelis-Menten kinetics (Km = 19 nmol l-1) and was reversed by serosal addition of bumetanide. In addition, enkephalin increased electroneutral NaCl-absorption as obtained by Na- and Cl-flux measurements. Enkephalin decreased this spontaneous neuronally stimulated electrogenic Cl-secretion in the partially stripped ileum, but had no effect in totally stripped ileum if ISC was stimulated at the cellular level by theophylline or PGE1. We conclude that ganglia located in the submucosal plexus regulate intestinal ion transport. Enkephalin acts by presynaptic inhibition via receptors on these neurons in the submucosa and/or via receptors on their neurites in the lamina propria mucosae.

Opioid receptors in rat neostriatum: radioautographic distribution at the electron microscopic level

The distribution of mu-opioid receptors, selectively labeled in vitro with a monoiodinated Met-enkephalin analog [( 125I]FK 33-824), was analyzed by light and electron microscopic radioautography in sections from the neostriatum of the rat. In the light microscope, patches of high receptor densities were detected amidst a moderately labeled matrix. The number of silver grains, as counted in 1-micron thick plastic-embedded sections, was 3 times greater inside the patches than in the intervening matrix. In both compartments, the proportion of labeled binding sites associated with the neuropil was significantly higher (greater than 70%) than that associated with nerve cell bodies or myelinated fascicles. Quantitative analyses of electron microscopic radioautographs revealed that the majority of silver grains corresponding to specifically bound [125I]FK molecules originated from radioactive sources associated with apposed neuronal membranes. Of the total number of specific binding sites, 53% was associated with axodendritic, 18% with axoaxonic and 3% with axosomatic interfaces. The occurrence of multiple labeled foci along the plasma membrane of certain perikarya and dendrites suggested that some of the binding sites might be associated with somato/dendritic elements. The high incidence of labeling along axoaxonic interfaces indicated that others were linked to the membrane of axons and/or axon terminals. A major finding of the present study was that only a small proportion of specific FK binding sites (7% of total) was associated with synaptic junctions. Labeled synapses were primarily of the asymmetric type and were found predominantly on dendritic branches and spines. A few were observed on nerve cell bodies. Labeled symmetric synapses were rare and encountered exclusively on dendritic branches. The high frequency with which specifically labeled binding sites were found to be associated with neuronal interfaces involving axonal processes strongly suggests that even if non-junctional these binding sites correspond to functional receptors. Whether these receptors are activated by endogenous ligand molecules released by the labeled terminals themselves or from terminals located at a distance from the labeled interfaces remains to be determined.

Reptilian enkephalins: implications for the evolution of proenkephalin

To shed light on the evolution of the opioid peptide precursor proenkephalin, we have investigated the profile of immunoreactive enkephalins in reptilian brain using six antisera directed toward different portions of the proenkephalin molecule. Three different orders of reptiles--turtles, alligators, and lizards--were studied; these orders represent lineages diverging more than 250 million years ago. Reptilian brain was found to contain large quantities of Met5-enkephalin (Met5-enk), Leu5-enkephalin (Leu5-enk), and Met5-enk-Arg6-Phe7 (MERF); gel filtration and reverse-phase chromatography of acid extracts prepared from turtle brain indicated that nearly all of the immunoreactivity corresponding to these peptides could be ascribed to the authentic penta- to heptapeptides. Immunoreactive metorphamide, but not amidorphin, was present in all three species. Unlike mammalian brain, reptilian brain does not contain immunoreactive Met5-enk-Arg6-Gly7-Leu8 (MERGL). These results indicate that reptilian proenkephalin, while similar to mammalian enkephalin in containing Met5-enk, Leu5-enk, MERF, and metorphamide, nevertheless differs from mammalian proenkephalin. Reptilian proenkephalin either may contain the MERGL sequence in a form not recognized by the MERGL antibody, or may lack the MERGL sequence entirely.

Protease inhibition increases the analgesic potency of peptide E

Peptide E (PE), an opioid peptide from adrenal medulla, is known to possess very low analgesic potency when injected intracerebroventricularly in mice. The present study showed that PE analgesia was increased by co-injection with protease inhibitors (bacitracin and bestatin).