Dynorphin-A (1-8) is contained within perikarya, nerve fibres and nerve terminals of rat duodenum

Slowing intestinal transit by PYY depends on serotonergic and opioid pathways

Slowing of intestinal transit by fat is abolished by immunoneutralization of peptide YY (PYY), demonstrating a key role for this gut peptide. How PYY slows intestinal transit is not known. We tested the hypothesis that the slowing of intestinal transit by PYY may depend on an ondansetron-sensitive serotonergic pathway and a naloxone-sensitive opioid pathway. In a fistulated dog model, occluding Foley catheters were used to compartmentalize the small intestine into proximal (between fistulas) and distal (beyond midgut fistula) half of gut. Buffer (pH 7.0) was perfused into both proximal and distal gut, and PYY was delivered intravenously. Ondansetron or naloxone was mixed with buffer and delivered into either the proximal or distal half of gut. Intestinal transit was measured across the proximal half of the gut. The slowing of intestinal transit by PYY was abolished when either ondansetron or naloxone was delivered into the proximal, but not the distal gut, to localize the two pathways to the efferent limb of the slowing response. In addition, 5-HT slows intestinal transit with marker recovery decreased from 76.2 +/- 3.6% (control) to 33.5 +/- 2.4% (5-HT) (P < 0.0001) but was reversed by naloxone delivered into the proximal gut with marker recovery increased to 79.9 +/- 7.2% (P < 0.0005). We conclude that the slowing of intestinal transit by PYY depends on serotonergic neurotransmission via an opioid pathway.

Glucose stimulation of pancreatic beta-cell lines induces expression and secretion of dynorphin

To investigate adaptive responses of pancreatic beta-cells to hyperglycemia, genes induced by glucose stimulation were identified by subtraction cloning. Among 53 clones representing differentially expressed genes, 20 encoded the endogenous opioid precursor, prodynorphin. The amino acid sequence of murine prodynorphin is identical to the rat protein in sequences comprising the opioid peptides and 86% identical in the remainder of the molecule. Stimulation of MIN6 cells increased prodynorphin RNA levels to more than 20-fold in proportion to physiological glucose concentrations. Similar induction levels were observed in murine betaTC3 and rat Rinm5F beta-cell lines. Prodynorphin RNA expression increased within 1 h of glucose stimulation, achieved maximal levels by 4 h, and remained elevated for at least 24 h. By using RIA, MIN6 cells were shown to contain and secrete increased amounts of dynorphin-A following glucose stimulation. Treatment of MIN6 cells with KCl, forskolin, or isobutyl-methyl-xanthine strongly induced prodynorphin RNA expression, suggesting that induction may be related to secretion-coupled signaling pathways. The induction of prodynorphin in several beta-cell lines is consistent with previous demonstrations of beta-cell synthesis of other endogenous opioids, including beta-endorphin, and suggests that opioids may have a potentially significant role in regulating beta-cell secretion.

Measurement and chromatographic characterization of prodynorphin-derived peptides in the guinea-pig ileum

Guinea-pig ileum was dissected and the mucosa, submucosa and external musculature extracted with aqueous acetic acid for measurement of four prodynorphin-derived peptides, namely dynorphin A 1-8, dynorphin A 1-17, dynorphin B, and alpha-neoendorphin. The peptide-like immunoreactive material extracted from the external musculature was characterized by multi-dimensional chromatographic analysis and compared to synthetic porcine standards. The chromatographic methods utilized were: reversed-phase high performance liquid chromatography (RP-HPLC), using two different eluants; cation exchange high performance liquid chromatography (CE-HPLC) and gel filtration chromatography. The dynorphin A 1-8-like immunoreactive material was homogeneous and coeluted with the standard in all chromatographic modes. The dynorphin A 1-17-like and dynorphin B-like immunoreactive material was heterogeneous but showed a peak that coeluted with synthetic standard in all chromatographic modes. The alpha-neoendorphin-like immunoreactive material also appeared to be heterogeneous with the major component on CE-HPLC coeluting with the synthetic peptide standard while the major component on RP-HPLC eluted differently. It was concluded that the guinea-pig ileum contains immunoreactivity for peptides derived from all coding regions of the prodynorphin gene and that these peptides may be present in multiple immunoreactive forms.

Identification of the tridecapeptide dynorphin B (rimorphin) within perikarya of rat duodenum

Using an immunofluorescence microscopic staining technique, the opioid peptide dynorphin B (rimorphin) was revealed within neuronal cell bodies of the rat duodenum. Dynorphin B immunoreactive perikarya were revealed in the myenteric and submucousal plexus as well as in the longitudinal muscle layer. They were large in diameter and round in shape and they contained a large round nucleus. Because no dynorphin B immunofluorescent nerve fibre and terminal could be noted it might be that dynorphin B is further cleaved by proteases into the bioactive opioid pentapeptide Leu-enkephalin and dynorphin B(6-13). These findings might also indicate that dynorphin B is processed within duodenal perikarya and that it has important physiological roles in the rat duodenum.

Alpha-neo-endorphin coexists with dynorphin-A(1-8) within intramurally lying perikarya of rat duodenum

By the use of the immunofluorescent microscopic staining technique, adjacent serial sections through the rat duodenum were alternately stained with specific antisera directed to the opioid peptides alpha-neo-endorphin and dynorphin-A(1-8). alpha-Neo-endorphin immunoreactivity has been revealed exclusively within perikarya lying intramurally in the longitudinal muscle layer. These alpha-neo-endorphin and dynorphin-A(1-8) immunoreactive perikarya were large in diameter, round in shape, contained a large and round nucleus, and were recognized only occasionally there. alpha-Neo-endorphin immunoreactivity was coexistent with dynorphin-A(1-8)-positive material within these perikarya. Since no alpha-neo-endorphin material was detected within duodenal nerve fibres and terminals, it might be concluded that this peptide is further enzymatically cleaved to the opioid pentapeptide Leu-enkephalin during its axonal transport from intramural perikarya to nerve terminals and during its storage there.

Dynorphin-A(1-8) and gamma-melanotropin exist within different myenteric plexus neurons of rat duodenum

Using the adjacent serial section staining technique and the double staining elution method, it was demonstrated that the opioid peptide dynorphin-A(1-8), originating from the prodynorphin precursor, and gamma 3-melanotropin (gamma 3-MSH), originating from the pro-opiomelanocortin (POMC) precursor, did not co-exist within myenteric plexus perikarya of the rat duodenum. This finding resembles that of the rat brain. Whether gamma 3 -MSH and dynorphin-A(1-8) act synergistically or antagonize each other in some physiological functions or have no interaction at all in the rat duodenum is as yet unknown.

Colocalization of dynorphin-A(1-17) and dynorphin-A(1-8) within some perikarya of rat duodenum: immunohistochemical evidence for the presence of two separate dynorphinergic systems

Adjacent serial sections through the rat duodenum were alternately stained for immunofluorescence microscopic studies with specific anti-sera directed to the opioid peptides dynorphin-A(1-17) and dynorphin-A(1-8), respectively. This resulted in the evidence that two separate dynorphinergic neuron populations are present there: intramural neurons, revealing a colocalization of dynorphin-A(1-17) and dynorphin-A(1-8), were round, contained a large and round nucleus and were lying sporadically in the longitudinal muscle layer as well as bulb-shaped neurons expressing only a dynorphin-A(1-8) immunoreactivity. The latter were recognized abundantly in the myenteric plexus. Myenteric plexus nerve fibres and terminals were immunoreactive for dynorphin-A(1-8), but not for dynorphin-A(1-17). Dynorphin-A(1-8) immunostained nerve terminals formed close contacts with large non-dynorphinergic myenteric plexus perikarya. These findings might indicate that dynorphin-A(1-8) is processed directly from its prodynorphin ('preproenkephalin B') precursor within myenteric plexus perikarya and indirectly via dynorphin-A(1-17) within intramural perikarya, indicating the presence of two separate dynorphinergic systems in the rat duodenum.