Conversion of dynorphin-(1-9) to [Leu5]enkephalin by the mouse vas deferens in vitro

Long-term pharmacological kindling increases in vitro release of IR-Met and IR-Leu-enkephalin from amygdala

Met-enkephalin release is increased from amygdala and striatum 1 and 15 days after pharmacological kindling with pentylenetetrazol, following potassium-induced depolarization in vitro via a Ca2+ dependent mechanism. Leu-enkephalin release was only enhanced in amygdala and striatum 1 day after the last seizure. IR-Met-enkephalin amygdala tissue content enhanced 1 and 15 days after seizure. In striatum, we found an IR-Met-enkephalin decrease 35 days after the last stimulus. IR-Leu-enkephalin amygdala tissue content enhanced 1 day after the last seizure, and no significant increases were found in striatum 1, 15 and 35 days after the last seizure. In this paper, we show that opioid peptides release is differentially enhanced in rat brain for several days after the last seizure, thus suggesting that opioid peptides may have a protective action against seizure activity.

Evidence that the agonist action of dynorphin A(1-8) in the guinea-pig myenteric-plexus may be mediated partly through conversion to [Leu5]enkephalin

1. The agonist action of the opioid peptide dynorphin A(1-8) on the myenteric plexus-longitudinal muscle of the guinea-pig ileum has been characterized. 2. The endogenous opioid peptide dynorphin A(1-8) was rapidly degraded by slices of myenteric plexus-longitudinal muscle of the guinea-pig ileum. 3. A product of the degradation was the delta-receptor preferring [Leu5]enkephalin. Levels of [Leu5]enkephalin were markedly increased in the presence of the peptidase inhibitors bestatin, thiorphan and captopril. 4. In the myenteric plexus dynorphin A(1-8) acted as a kappa-receptor agonist. In the presence of bestatin, thiorphan and captopril a mu-receptor agonist effect was observed. This mu-agonist action was lost in the presence of N-[1-(RS)-carboxy-2-phenylethyl]Ala-Ala-Phe-p-aminobenzoate, an inhibitor of the endopeptidase enzyme EC 3.4.24.15. 5. The results suggest that formation of [Leu5]enkephalin from dynorphin A(1-8) may be an important conversion process. The enzyme responsible may be the Zn2(+)-metalloendopeptidase, EC 3.4.24.15.

High-performance liquid chromatographic system for the separation of dynorphin A (1-17) fragments and its application in enzymolysis studies with rat nerve terminal membranes

A high-performance liquid chromatographic method capable of separating a large number of C- and N-terminal degradation fragments of dynorphin A (1-17) (dyn 1-17) in 1 h has been developed. The system has been applied to study the metabolism profiles of various dyn 1-17-derived peptides following in vitro incubation with rat striatum and spinal cord nerve terminal membranes. In addition to the removal of the N-terminal amino acid Tyr, major sites of cleavage between the following amino acids could be established: Leu5-Arg6 in dyn 1-7 (formation of dyn 1-5); Arg6-Arg7 and Leu5-Arg6 in dyn 1-8 (formation of dyn 1-6 and dyn 1-5, respectively); Arg7-Ile8 in dyn 1-9 (formation of dyn 1-7) and Arg9-Pro10 in dyn 1-10 (formation of dyn 1-9). Studies with inhibitors of the enzymes involved show that dyn 1-5 is formed directly from dyn 1-8 via an endopeptidase insensitive to the angiotensin-converting enzyme inhibitor MK 422 acting on the scissile Leu5-Arg6 bond in dyn 1-8. The method circumvents the use of [3H]Tyr-labelled dynorphins, which have the inherent drawback that fragments lacking the N-terminal Tyr cannot be detected. Owing to the high resolution, also for the larger dynorphins dyn 1-14, dyn 1-15 and dyn 1-16, the chromatographic system should prove especially useful in the elucidation of the enzymolysis pattern of dyn 1-17. Furthermore, the method offers a way to evaluate simultaneously the selectivity of new enzyme inhibitors for several cleavage sites in the same assay.

Formation of [Leu5]enkephalin from dynorphin A(1-8) by rat central nervous tissue in vitro

[3H]Dynorphin A(1-8) is readily metabolised by rat lumbosacral spinal cord tissue in vitro, affording a variety of products including a significant amount (20% recovered activity) of [3H][Leu5]enkephalin. In the presence of the peptidase inhibitors bestatin, captopril, thiorphan, and leucyl-leucine, [3H][Leu5]enkephalin was the major metabolic product, accounting for 60% of recovered activity. Production of [3H][Leu5]enkephalin was seen across all gross brain regions. The enzyme responsible for the cleavage has an optimal substrate length of 8-13 amino acids and is inhibited by N-[1-(RS)-carboxy-2-phenylethyl]-Ala-Ala-Phe-p-aminobenzoate, a site-directed inhibitor of the metalloendopeptidase EC 3.4.24.15. However the enzymic breakdown also has properties in common with involvement of endo-oligopeptidase A. Possible consequences of the formation of [Leu5]-enkephalin from the smaller dynorphins are discussed.

Immunocytochemical localization of neuropeptides in the adrenal medulla

The distribution of neuropeptides exhibits pronounced interspecies heterogeneity. Neuropeptides may function as hormones secreted from chromaffin cells or as neurotransmitters/neuromodulators released from nerve terminals. However, other possible functions such as trophic or intracellular effects should also be considered. Thus, to understand the role of neuropeptides, it is important to explore their localization in different species. The distribution of enkephalins, neurotensin, neuropeptide Y, calcitonin gene-related peptide, and galanin in the adrenal medulla of rat, cat, hamster, and mouse is presented in detail.

Release of endogenous opioid peptides displaces [3H]diprenorphine binding in rat hippocampal slices

Pharmacological depolarization by KCl or veratrine reduced [3H]diprenorphine binding to opioid receptors in the hippocampal slice in a transient, calcium-dependent, and peptide-sensitive manner. These results suggest that endogenous opioid peptides were released from synaptic terminals and competitively displaced [3H]diprenorphine binding to opioid receptors. [3H]diprenorphine binding was significantly reduced by calcium-dependent depolarization throughout the hippocampus as determined by subsequent receptor autoradiography and quantitative densitometry. Displacement of binding was evident at sites in the CA1 and CA3 regions, the dentate gyrus, and the subiculum. The most dramatic reduction was evident in stratum lacunosum moleculare of CA3. Correlating the sites of maximal [3H]diprenorphine displacement with the previously described distribution of the opioid peptides suggests that the perforant path fibers release enkephalins in stratum lacunosum moleculare of CA3 and stratum moleculare of the dentate gyrus, and that mossy fibers may release both dynorphins and enkephalins near stratum pyramidale of CA3 and stratum granulosum. The lack of complete overlap between the distribution of opioid terminals and the sites of displacement indicates that these peptides may diffuse a moderate distance to their sites of action. Radioligand displacement defines the sites of endogenous opioid binding, suggests the likely sources of peptide release, and thus predicts the sites of endogenous opioid action within the hippocampus.

Opioid receptors inhibit the adenylate cyclase in guinea pig cochleas

The effects of mu- and delta-preferring agonists on adenylate cyclase activity have been investigated in vitro in homogenates of guinea pig cochleas. Morphine, Leu-enkephalin, D-Ala2, N-methyl-Phe4, Gly-ol5-enkephalin (DAGO) and D-Ser2-Leu-enkephalin-Thr (DSLET) each inhibited the synthesis of cyclic AMP. This effect was reversed by naloxone which had a greater affinity in blocking the effect of the mu-preferring agonists (morphine, DAGO) than in blocking the effect of the delta-preferring agonists (Leu-enkephalin, DSLET). Finally, no additive effects were observed when various combinations of two agonists were used. These results indicate that opioid receptors exist in the guinea pig cochlea and that they are negatively linked to adenylate cyclase. The different affinities shown by naloxone to reverse the inhibition induced by the mu- and delta-preferring agonists suggest that morphine and DAGO act through mu-receptors, whereas Leu-enkephalin and DSLET act through delta-receptors. Since no additive effects have been found when combining two different agonists, it can be hypothesized that the mu- and delta-receptors are coupled to the same pool of adenylate cyclase. It may be proposed from these findings that in vivo enkephalins inhibit the synthesis of cyclic AMP via mu- and delta-receptors. However, whether this effect occurs at a presynaptic level (within opioid-containing olivocochlear varicosities) or at the postsynaptic level (within dendrites of the primary auditory neurons) remains to be determined.

Pharmacological properties of a proenkephalin A-derived opioid peptide: BAM 18

BAM 18 is a derivative of the opioid precursor proenkephalin A. Although it exists in rat and guinea-pig brain in relatively high concentrations, its physiological function is presently unknown. In the present study we have determined the opioid receptor selectivity of this peptide using radioligand binding and peripheral tissue bioassay. When selective binding conditions were used, BAM 18 bound to the mu opioid receptor with an affinity three times that of the kappa opioid receptor and over 10 times that of the delta opioid receptors (Ki = 0.29, 0.75, and 3.2 nM respectively). BAM 18 also displayed mixed receptor selectivity in in vitro bioassay. Ke values for naloxone antagonism of BAM 18 agonist activity in the electrically stimulated guinea-pig ileum and the mouse vas deferens were 4.3 and 9.9 nM, respectively. These data indicate that BAM 18 binds to all three opioid receptor subtypes with a selectivity profile of mu greater than kappa greater than delta.