Chemistry of the gastrointestinal hormones and hormone-like peptides and a sketch of their physiology and pharmacology

Gastrointestinal hormones and the dialogue between gut and brain

The landmark discovery by Bayliss and Starling in 1902 of the first hormone, secretin, emerged from earlier observations that a response (pancreatic secretion) following a stimulus (intestinal acidification) occurred after section of the relevant afferent nerve pathway. Nearly 80 years elapsed before it became clear that visceral afferent neurons could themselves also be targets for gut and other hormones. The action of gut hormones on vagal afferent neurons is now recognised to be an early step in controlling nutrient delivery to the intestine by regulating food intake and gastric emptying. Interest in these mechanisms has grown rapidly in view of the alarming global increase in obesity. Several of the gut hormones (cholecystokinin (CCK); peptide YY3-36 (PYY3-36); glucagon-like peptide-1 (GLP-1)) excite vagal afferent neurons to activate an ascending pathway leading to inhibition of food intake. Conversely others, e.g. ghrelin, that are released in the inter-digestive period, inhibit vagal afferent neurons leading to increased food intake. Nutrient status determines the neurochemical phenotype of vagal afferent neurons by regulating a switch between states that promote orexigenic or anorexigenic signalling through mechanisms mediated, at least partly, by CCK. Gut-brain signalling is also influenced by leptin, by gut inflammation and by shifts in the gut microbiota including those that occur in obesity. Moreover, there is emerging evidence that diet-induced obesity locks the phenotype of vagal afferent neurons in a state similar to that normally occurring during fasting. Vagal afferent neurons are therefore early integrators of peripheral signals underling homeostatic mechanisms controlling nutrient intake. They may also provide new targets in developing treatments for obesity and feeding disorders.

Three-dimensional structure and position of porcine motilin in sodium dodecyl sulfate micelles determined by 1H NMR

The solution structure of the porcine gastrointestinal peptide hormone motilin was determined in the presence of sodium dodecyl sulfate (SDS) micelles at 28 degrees C using 1H nuclear magnetic resonance, full relaxation matrix analysis, and structure calculations based on restrained molecular dynamics. The structure of motilin in SDS micelles is described by a reverse gamma-turn and a beta-turn of type II in the N terminal end, an alpha-helical region in the middle of the molecule, and an extended structure at the C terminus. The position of the motilin molecule relative to the SDS micelles was probed by adding spin-labeled stearic acids, containing 12-doxyl or 5-doxyl spin-labels. We observed selective broadening of the proton resonances of residues 3-5 and concluded that they must be located in the interior of the micelle. These experiments suggest a structural model in which the hydrophobic N terminus consists of two well-defined turns buried in the interior of the micelle, whereas the amphiphilic alpha-helical part is located at the surface of the micelle. Spectral density mapping using a 13C label on the alphaC of Leu10 gave overall rotational correlation times taum of 6.6 and 4.5 ns at 35 and 45 degrees C, respectively. The long correlation time in combination with a high order parameter (S = 0.92) indicates that motilin has a rigid structure in the complex with the SDS micelle.

Effect of chronically administered cholecystokinin on the nude mouse

The nude mouse has been used to evaluate the effect of cholecystokinin (CCK) on xenografted tissues, but little is known about long-term actions of cholecystokinin on native organs in this animal. We investigated the impact of chronically administered synthetic cholecystokinin octapeptide on the nude mouse. Six groups of eight animals each received intraperitoneal injections twice daily for 14 days with diluent or a 4-log range of cholecystokinin. Overall health, behavior, and body weight were unaffected by this treatment. Among the seven organs examined at necropsy, pancreas alone showed a dose-related increase in weight. Pancreatic DNA content decreased with increasing dosages of CCK-8, while RNA content exhibited a biphasic response to CCK-8. The only histological abnormality occurred in the pancreas and was confined to the higher doses. These data indicate for the first time the action of CCK on the non-tumor-bearing nude mouse. Unlike other animal models, the nude mouse responds to cholecystokinin administration with pancreatic hypoplasia and hypertrophy, which is accompanied by pancreatitis at higher doses.

Phorbol esters stimulate the potassium-induced release of cholecystokinin from slices of cerebral cortex, caudato-putamen and hippocampus incubated in vitro

Incubation of slices of caudato-putamen, cerebral cortex and hippocampus for 5 to 15 minutes with phorbol 12,13-dibutyrate (PDB) or phorbol 12-myristate 13-acetate (PMA) increased potassium evoked cholecystokinin (CCK) release from 139% to 296% of control. The inactive 4 alpha phorbol and 4 alpha PDB did not alter CCK release. None of the active or inactive phorbols tested altered basal CCK release. These results suggest that there may be similarities in the regulation of CCK release in different brain regions. Although the physiological factors which regulate CCK release may differ in these tissues, it is possible that their common action is mediated by the products of inositol phospholipid turnover.

Peptides in rat brain immunoreactive for the amino terminus of cholecystokinin 33: distribution and chromatography

Antisera directed against the amino-terminus of porcine CCK 33 detects related immunoreactivity in rat brain extracts, the distribution of which follows that of CCK 8. Sephadex chromatography indicates that several immunoreactive peptides are present with a molecular weight range of 2600-3500. These peptides are likely to be CCK 39 or CCK 33 and the amino terminal segments of CCK 39/33 without the CCK 8 sequence. The presence of CCK 39/33 and its amino-terminal fragments without CCK 22 and its amino-terminal fragments confirms the absence of CCK 22 in the rat brain. This cleavage at CCK 22 is one of the major differences between the processing of CCK in rat brain and gut and may reflect differences in their physiological roles.

Aspects of measurement and analysis of regulatory peptides

Although almost all methods of mass measurement of regulatory peptides still depend on the high affinity antibody, the traditional Yalow and Berson radioimmunoassay technique is becoming outdated. Pure monoclonal antibodies allow excess antibody two site assay techniques with a variety of different labels (preferentially non-radioactive) of great sensitivity and speed. The large amounts of particular monoclonal antibodies available allow several different laboratories to use the same reagents and have increased comparability. Unfortunately many regulatory peptides exist in multiple molecular forms and attention must be paid to antibody region specificity. Improved methods of extraction of regulatory peptides from plasma tissue allow more accurate quantitation. New techniques for rapid high resolution chromatography make distinction of different molecular forms much easier than hitherto. Better education in techniques and/or attention to inter-assay standards are necessary to improve the comparability of regulatory peptide measurement in the future.

Lithium preincubation stimulates the potassium-induced release of cholecystokinin from slices of cerebral cortex and caudate-putamen incubated in vitro

Potassium-evoked cholecystokinin (CCK) release from slices of caudate-putamen and cerebral cortex, but not hippocampus incubated in vitro was increased by 152-175% by preincubation for 40 min with 10 mM lithium. These results and previous studies suggest that although different physiological agents regulate CCK release in these brain regions, these agents may share a common intracellular mediator which may be a product of inositol phospholipid turnover.

The regulation of cholecystokinin release from rat caudatoputamen in vitro

Cholecystokinin (CCK) is one of the most abundant neuropeptides in the rat caudatoputamen (cp). CCK perikarya innervating cp are thought to originate in neurons in the claustrum/piriform cortex area of the amygdala. Previous studies on the release of CCK from cp have focused on the influence of dopamine agonists. The present study has examined the influence of other neuroactive substances on the release of CCK. The release of CCK from rat cp slices in vitro stimulated by potassium was quantitated with a specific CCK radioimmunoassay. This potassium-stimulated release of CCK was Ca2+-dependent. Maximal stimulation of CCK release was observed at 55 mM potassium. Several lines of evidence indicate that the release of CCK from cp is inhibited by some other substance (or substances) released by a Ca2+-dependent mechanism from cp along with CCK. Release media from cerebral cortex or cp (called 'conditioned media' or CM) inhibits the release of CCK from fresh slices of cp but not from cerebral cortex. The release of dopamine from cp is unaffected by CM from cortex or cp. The identity of the substance in CM which inhibits CCK release from cp is still under investigation, though it appears not to be CCK, dopamine, acetylcholine, somatostatin, leucine enkephalin or gamma-aminobutyric acid.

Exogenous cholecystokinin (CCK) reduces neonatal rat brain opioid receptor density and CCK levels

Newborn rats were given saline or cholecystokinin8 (CCK8) (5 micrograms/kg, twice daily) i.p. for 3 weeks. On day 21, effects on brain development were assessed. CCK-like immunoreactivity was measured in 7 brain regions; a small (12-18%) but significant decrease in endogenous levels of this peptide was detected in cerebral cortex, medulla and pons of the CCK-treated rats. Morphometric measurements revealed a slight reduction in thickness of most cerebral cortical sections within the CCK-treated group. The area of a midsagittal section of the cerebellum was unchanged except for the Purkinje/granule cell layer, which was smaller in CCK-treated animals. Levels of mu-, delta- and kappa-opioid receptors were estimated by homologous displacement binding assays using selective radioligands. The CCK treatment resulted in a significant decrease in levels of mu- (11%) and delta- (13%)-sites in the cerebral cortex. Neither binding affinities nor kappa-receptor densities were altered. Other animals received the same treatment regimens for 21 days and were maintained for an additional 29 days without treatment; these rats had reductions only in cortical mu-sites (15%). Chronic intraventricular administration of CCK (0.1 microgram/h) to adult rats did not elicit a similar down-regulation of cortical mu or delta receptors, suggesting that the effects observed in neonates reflected developmental processes.

Pharmacological studies of somatostatin and somatostatin-analogues: therapeutic advances and perspectives

This article is aimed at reviewing and analyzing studies that are related to the possible therapeutic use of a potent and ubiquitously-distributed hormone--somato-statin (SS-14), and its analogues. Administration of these substances has provided beneficial effects in treating acromegaly, gastro-intestinal hemorrhagic and hypersecretory disorders, acute pancreatitis, diabetes mellitus, and certain types of cancer. Further studies with SS-14-analogues might provide new therapies for treating certain life-threatening disorders of man.

Structure and expression of the gene encoding the vasoactive intestinal peptide precursor

The gene encoding the human vasoactive intestinal peptide (VIP) and the histidine-methionine amide (PHM-27) peptide hormone was isolated from lambda phage libraries. The human gene was found to be composed of seven exons spanning approximately 9 kilobase pairs. The first exon codes for an untranslated leader sequence, and the second exon codes for a putative signal peptide. DNA sequences coding for the VIP and PHM-27 hormones are located in two different exons. Southern blot analysis with genomic DNA suggested that a single copy of the VIP/PHM-27 gene is present in the human haploid genome. The expression of VIP/PHM-27 precursor mRNA in various tissues in the rat was analyzed by RNA gel blot hybridization. In the organs examined, expression was only detected in the brain and duodenum. RNA isolated from various regions of the rat brain--including the cortex, hypothalamus, and hippocampus--hybridized to both VIP- and PHM-27-specific probes. The same pattern of hybridization was found when VIP- and PHM-27-specific probes were used, suggesting that possible differences in the localization of VIP and PHM-27 peptides between different brain regions cannot be accounted for by differential RNA processing.

Vasoactive intestinal polypeptide (VIP) inhibits potassium-induced release of cholecystokinin (CCK) from rat caudato-putamen but not from cerebral cortex

CCK release elicited by 40 mM potassium from slices of rat caudato-putamen (cp) was inhibited by VIP. The effect of VIP was maximal at 10(-7) M. VIP does not inhibit CCK release from cerebral cortex at either 10(-7) or 10(-6) M. VIP is known to elevate levels of cAMP in rat brain. VIP inhibition of CCK release appears to be independent of activation of adenylate cyclase because treatment of cp slices with forskolin (2 X 10(-6) to 10(-4) M) does not mimic the inhibitory action of VIP.

Current contributions of peptide synthesis to studies on brain-gut-skin triangle peptides

The usefulness of a strong acid, such as MSA or TFMSA/TFA, as a deprotecting reagent in peptide synthesis was examined. By synthesizing several structurally related brain-gut-skin triangle peptides, a number of advantageous features of the thioanisole-mediated deprotecting procedure were demonstrated. New amino acid derivatives, Arg(Mts), Trp(Mts) and Asp(OChp), were introduced to improve the synthetic methodology of complex peptides and the superior properties of Cys(Ad) were evaluated.

Gastrin/cholecystokinin-like immunoreactive peptides in the Dungeness crab, Cancer magister (Dana): immunochemical and biological characterization

The purpose of this investigation was to characterize a gastrin/cholecystokinin-like immunoreactant (G/CCK-LI) extractable from the crab, Cancer magister. G/CCK-LI was extracted best in boiling water and was found mainly in the stomach, hemolymph and carapace. A relatively large immunoreactive peptide in the stomach and apparently smaller forms in the hemolymph and carapace were separated by Sephadex G-50 fractionation. Anion-exchange chromatography further fractionated the stomach form into three major peaks. The crab material cross-reacted with three antisera specific for the common C-terminus of gastrin/CCK, but cross-reacted much less with three antisera directed against other portions of the gastrin molecule. Partially purified crab stomach G/CCK-LI inhibited the binding of labeled CCK to mouse brain G/CCK receptors but not to rat pancreatic CCK receptors. The crab peptide did not stimulate rat gastric acid or rat pancreatic amylase secretion. These results indicate that the crab peptides are structurally similar to, but distinguishable from, the bioactive C-terminal amino acid sequence common to gastrins and CCKs.