Electron microscopic study of neuropeptide Y-containing nerve elements of the guinea pig small intestine

Ultrastructural co-localization of neuropeptide Y and vasoactive intestinal polypeptide in neurosecretory vesicles of submucous neurons in the rat jejunum

The localization of vasoactive intestinal polypeptide and neuropeptide Y in submucous nerves of rat jejunum was studied using both single-label pre-embedding immunocytochemistry and post-embedding double-label immunogold techniques. Vasoactive intestinal polypeptide-immunoreactive fibres and cell bodies were regularly observed in submucous plexus and a similar distribution was seen for neuropeptide Y. Varicose fibres were observed in single-label studies and when areas of specific interest were subjected to double-label immunogold protocols these immunoreactive profiles exhibited vesicles clearly stained for both vasoactive intestinal polypeptide and neuropeptide Y. Synaptic vesicles in immunopositive fibres observed close to the mucosa (and elsewhere in the submucosa) were dense-cored with an average diameter of 80 nm. Nerves associated with vascular elements only stained for neuropeptide Y, not for vasoactive intestinal polypeptide. These findings suggest that these two unrelated enteric peptides are co-released in the vicinity of the mucosal lining and the likely implications of such co-release are discussed.

Human pancreatic polypeptide, neuropeptide Y and peptide YY reduce the contractile motility by depressing the release of acetylcholine from the myenteric plexus of the guinea pig ileum

Transmural stimulation (TS; 15 V, 0.5 msec, 1-5 Hz, for 30 sec) caused a contraction in a frequency-dependent manner of the longitudinal muscle with myenteric plexus of the guinea pig ileum. Two-min premedication with human pancreatic polypeptide (HPP, 10(-8) M-10(-6) M), neuropeptide Y (NPY, 10(-8) M-10(-6) M), and peptide YY (PYY, 10(-8) M-10(-6) M) partially, by less than 35%, reduced TS (15 V, 0.5 msec, 10 Hz)-evoked contraction in a dose-dependent manner without affecting the resting tensions. Hexamethonium, phentolamine, prazocine, yohimbine, propranolol, naloxone and theophylline had no effects on the inhibitory actions of these peptides on TS-evoked contractions. TS (15 V, 0.5 msec, 10 Hz)-evoked 3H-ACh release was significantly reduced by 2-min premedication of HPP (10(-6) M), NPY (10(-6) M) and PYY (10(-6) M). The order of potency of these depressant effects on TS-evoked contraction and 3H-ACh release was PYY greater than NPY greater than HPP. It is suggested that the members of PP family have inhibitory effects on the contractile motility by depressing ACh release from myenteric plexus of the guinea pig ileum.

Cholinergic stimulation of neuropeptide Y secretion from the isolated perfused rat stomach

Neuropeptide Y (NPY) is present in both extrinsic sympathetic adrenergic nerve terminals and intrinsic nerves of the gastrointestinal (GI) tract. Based on this localization a number of functions have been attributed to GI NPY including regulation of blood flow, intestinal fluid and electrolyte transport, and motility. There is nothing currently known, however, about the regulation of its secretion from GI nerves. The effect of cholinergic agonists and antagonists on secretion of NPY immunoreactivity (NPY-IR) from the isolated perfused rat stomach was investigated in the present study. Perfusate samples were extracted and concentrated on SepPak cartridges. Basal levels of NPY-IR varied between 98 and 147 fmol/min. Release was stimulated by high potassium concentrations (50 mM) and acetylcholine (ACh; 1 microM). ACh-induced secretion was unaffected by atropine, but inhibited by hexamethonium. Further evidence for a nicotinic component in the regulation of NPY-IR secretion was obtained by the finding of hexamethonium-induced reduction in basal secretion and stimulation of secretion by 1,1-dimethyl-4-phenyl-piperazinium (DMPP). In conclusion, cholinergic agonists and antagonists can modulate gastric NPY-IR secretion, and the cholinergic stimulatory effects are probably mediated via nicotinic receptor stimulation at the level of the intrinsic ganglia.

Light and electron microscopy of neuropeptide Y-containing nerves in human liver, gallbladder, and pancreas

Neuropeptide Y-containing nerve fibers were identified by light and electron microscopic immunocytochemistry in the human liver, gallbladder, and pancreas. In the liver, neuropeptide Y-containing nerve fibers were distributed richly in Glisson's sheath and were prominent around the walls of the interlobular vein, interlobular hepatic artery, and hepatic bile duct. The fibers also formed a dense network surrounding the hepatocytes. The nerve terminals were found close to the endothelial cells of blood vessels, as well as being distributed in Disse's space, where they appeared to terminate. Occasionally these terminals contacted directly the membrane of a hepatocyte. In the gallbladder, neuropeptide Y fibers were found in each layer, with an especially dense network in the lamina propria. The fibers also ran close to the epithelium and parallel to the muscle bundles. Blood vessels throughout the gallbladder were well supplied with such nerve fibers. In the pancreas, neuropeptide Y fibers were found mainly near blood vessels and partly in gaps between exocrine glands, seeming to terminate on certain endocrine cells. Nerve terminals were located in the vascular walls and adjacent to the surface of exocrine acinar cells. These studies provide a basis for correlating the neuropeptide Y distribution with pharmacological and physiological studies in humans.

Effect of neuropeptide Y on jejunal water and ion transport in humans

Neuropeptide Y is a neurotransmitter in enteric and postganglionic sympathetic neurons. In animal models of intestinal water and ion transport, neuropeptide Y decreases stimulated secretion but has no consistent effect on basal transport. In the present study, the effect of neuropeptide Y on jejunal water and electrolyte transport in healthy volunteers was investigated under basal conditions and during intestinal secretion induced by intraluminal administration of prostaglandin E2. The triple-lumen tube technique was used for perfusion of the small intestine with a plasmalike electrolyte solution containing polyethylene glycol as a nonabsorbable volume marker. After an initial control period (saline IV) neuropeptide Y was administered IV at a dose of 400 pmol.kg-1.h-1. Neuropeptide Y significantly increased net absorption of water, sodium, potassium, and chloride under basal conditions. The peptide significantly reduced the secretion of these electrolytes induced by an intraluminal prostaglandin E2 concentration of 5 mumol/L and reduced net water secretion by 36%. The results of the current study suggest that neuropeptide Y can change intestinal water and ion transport from secretion toward absorption.

The effects of selective amino acid substitution upon neuropeptide Y antisecretory potency in rat jejunum mucosa

The antisecretory potency of NPY and a series of truncated and structural analogues of NPY have been tested upon mucosal preparations of rat small intestine. Single amino acid substitutions, i.e., [Ile34]NPY, [Pro34]NPY, resulted in severe attenuation and loss of biological activity, respectively, and neither peptide affected NPY responses. An agonist order of potency: NPY greater than or equal to [Glu16,Ser18,Ala22,Leu28,31]NPY (ESALL-NPY) greater than [Cys2,Aoc5-24,DCys27]NPY (C2-NPY) greater than [Aoc5-24]NPY greater than [Des-Ser3,Des- Lys4]C2-NPY much greater than [Cys5,Aoc7-20,DCys24]NPY (C5-NPY) greater than equal to [DCys7,Aoc8-17, Cys20]NPY (C7-NPY) greater than [Aoc8-17]NPY greater than or equal to [Ile34]C7-NPY much greater than [Aoc2-27]NPY much greater than [Pro34]C2-NPY was obtained. The use of analogues based upon the tertiary structural model of NPY with varying amounts of N- and C-terminal helical regions removed and replaced with a single 8-aminooctanoic acid residue (Aoc) has allowed us to assess the structural requirements for activation of the regions in close apposition to each other. The polyproline helix, beta-turn and majority of the amphipathic alpha-helix serve a structural role bringing N- and C-terminal residues together for optimal receptor recognition and activation.

Novel autonomic neurotransmitters and intestinal function

A wide variety of substances, including amines and peptides, have been detected within the complex neuronal pathways of the enteric nervous system using immunohistochemical techniques. In this article we have discussed some of the more recent data on the effects of these substances on intestinal activity. We have also commented on the many difficulties associated with ascribing neurotransmitter status to individual compounds. The technique of immunoblockade of neurogenic functional responses has been used in an attempt to identify some of the putative neurotransmitter substances. The search for selective antagonists continues.

The effect of neuropeptide Y and peptide YY on electrogenic ion transport in rat intestinal epithelia

1. Neuropeptide Y (NPY), peptide YY (PYY) and, to a lesser extent, human pancreatic polypeptide (HPP) reduced short-circuit current (SCC) in a concentration-dependent manner in epithelial preparations of rat jejunum and descending colon. 2. From concentration-response curves in the jejunum EC50 values of 3 nM for PYY and 10 nM for NPY were obtained. HPP was much less potent, the threshold concentration being around 100 nM, and NPY 13-36 was inactive. 3. Repeated exposure of jejunal preparations to either NPY or PYY led to a rapid desensitization. Cross-desensitization to the actions of these two peptides was also observed. Neither tetrodotoxin (TTX) nor phentolamine affected responses to either NPY or PYY on the jejunum. 4. Responses to both peptides were inhibited by the presence of transport inhibitors, particularly diphenylamine-2-carboxylate (DPC, a chloride channel blocker) and piretanide (Na+-K+-2Cl- co-transport inhibitor). These results may indicate that the reduction in SCC caused by the neuropeptides is due to a net increase in chloride movement in the apical to basolateral direction. 5. 36Cl-flux studies identified an inhibition of chloride secretion as the predominant mechanism of action of NPY and PYY, together with a smaller stimulation of chloride absorption. No significant changes in the movement of 22Na were seen in either direction. 6. The cyclo-oxygenase inhibitors piroxicam (5 microM) and indomethacin (5 microM) significantly reduced the responses to both NPY and PYY in rat jejunum. From this and other evidence it was concluded that the peptides depended for their effect on the endogenous formation of eicosanoids, the prevention of which attenuated the SCC reduction due to the peptides.

The role of ultrastructural investigations in neurotoxicology

The ways in which ultrastructural approaches have been applied to the investigation of xenobiotic-induced toxicity of the nervous system have been briefly reviewed. These approaches have been grouped in 3 broad areas, viz. morphology, function and composition. Firstly, morphological approaches permit the visualisation of changes in intercellular relationships, the identification of the subcellular target(s) of a xenobiotic substance and the discrimination between what may appear ostensibly to be identical cellular responses to one or more chemically distinct toxins. Secondly, functional approaches using, e.g. cytochemistry, ion precipitation, immunocytochemistry and autoradiography provide indications of metabolic state, the identity or the intra- or extracellular location of the "reactive species". Thirdly, those approaches, viz. electronprobe X-ray microanalysis and electron energy loss spectroscopy which provide information of the elemental composition of cells and tissues permit an assessment of the subcellular distribution and compartmentalisation of endogenous substances and toxic or therapeutic xenobiotics. In concert, ultrastructural approaches possess the ability to contribute unique information on the effects of exposure of cells of the nervous system to toxic substances and so direct further investigation towards an understanding of the mechanism of action.

Neuropeptides and the microcircuitry of the enteric nervous system

The discovery of neuropeptides in enteric neurons has revolutionized the study of the microcircuitry of the enteric nervous system. From immunohistochemistry, it is now clear that some individual enteric neurons contain several different neuropeptides with or without other transmitter-specific markers and that these markers occur in various combinations. There is evidence from experiments in which nerve pathways are interrupted that populations of enteric neurons with different combinations of markers have different projection patterns, sending their processes to distinct targets using different routes. Correlations between the neurochemistry of enteric neurons and the types of synaptic inputs they receive are also beginning to emerge from electrophysiological studies. These findings imply that enteric neurons are chemically coded by the combinations of peptides and other transmitter-related substances they contain and that the coding of each population correlates with its role in the neuronal pathways that control gastrointestinal function.