Stimulation of gastric secretion and enhanced gastric mucosal damage following central administration of pancreatic polypeptide (PP) in rats

The area postrema lesions alter the inhibitory effects of peripherally infused pancreatic polypeptide on pancreatic secretion

Circulating PP binds to specific receptors in the DVC through the AP, but the mechanism through which these brain receptors affect pancreatic secretion is not clear. We hypothesize that the removal of the AP (APX) will alter the effects of PP on pancreatic secretion. APX or sham procedures were performed in anesthetized male Wistar rats. After a 1-month recovery, one group of rats were infused with either PP (30 or 100 pmol/kg per h) or vehicle under basal or 2-DG-stimulated (75 mg/kg, i.v. bolus) conditions for studying pancreatic exocrine secretion. A second parallel group was sacrificed for examination of PP receptor binding in the brain stem. A third group received an intraperitoneal injection of PP at the dose of 4.15x10(4) pmol/kg (200 microg/kg) and c-fos expression in the brain stem was examined. APX eliminated PP binding sites in the DVC as assessed by autoradiography. PP infusion caused a dose-dependent decrease in basal protein secretion. APX partially reversed PP inhibition of basal protein secretion when infused at 30 pmol/kg per h, and at 100 pmol/kg per h stimulated pancreatic fluid secretion and reversed the inhibition of protein secretion. During 2-DG stimulation the effects of PP and 2-DG on pancreatic fluid and protein secretion were parallel. PP dose-dependently inhibited 2-DG-stimulated secretion in sham rats. APX reduced the pancreatic fluid (54%) and protein (46%) secretory response to 2-DG. However, PP at 30 pmol/kg per h remained a potent inhibitor of 2-DG-stimulated pancreatic secretion in APX rats. This effect was blunted with PP at 100 pmol/kg per h in APX rats, possibly related to the stimulatory effect of high-dose PP in APX rats without 2-DG. Furthermore, i.p. PP induced significantly greater c-fos activation of NTS neurons in APX rats than sham rats, despite the apparent absence of PP binding sites in the DVC. We conclude that in awake rats, PP inhibits basal secretion, in part, through the AP. Furthermore, and unlike PYY, PP inhibits 2-DG-stimulated pancreatic secretion, and it does so through an AP-independent mechanism. The possibility that the mechanism may involve the DVC cannot be excluded since i.p. injection of PP activates c-fos expression in DVC neurons. Thus, PP and PYY may regulate different components of the pancreatic secretory control system through unique pathways.

Distribution of pancreatic polypeptide receptors in the rat brain

Pancreatic polypeptide (PP) is a regulatory peptide that modulates gastrointestinal function. Previously we demonstrated PP receptors in the brainstem and interpeduncular nucleus, and the PP receptors in the brainstem appear to modulate gastric motility and pancreatic exocrine secretion. The purpose of this study is to extend our understanding of the distribution of PP receptors in the rat brain in order to determine the systems that are potentially modulated by PP. Rat brains were studied using 125I-PP receptor autoradiography on cryostat sections of the entire brain cut in three planes (horizontal, sagittal, and coronal). Brain regions exhibiting PP binding sites were confirmed when identified in all three planes of section. Saturable PP binding was identified in the hypothalamus (arcuate and paraventricular n), the rostral forebrain (medial preoptic area, anterior olfactory nucleus, islands of Calleja, the dorsal endopiriform n, piriform cortex, and the bed n of the stria terminalis), medial amygdaloid n; the thalamus (anteromedial thal. n; reuniens thal. n; and paraventricular thal n), the interpeduncular red nucleus, substantia nigra, parabrachial n; locus coeruleus, mesencephalic trigeminal n, dorsal motor n of the vagus, the n solitary tract, and the area postrema. We conclude that PP receptors are distributed widely throughout the rat brain. The distribution of many of these PP binding sites corresponds to brain regions regulating digestion and autonomic function. We speculate, based on the patterns of binding in the olfactory and limbic systems, that PP receptors might be involved in positive reinforcement of ingestion behavioral as well as modulation of gastrointestinal function.

Apolipoprotein A-IV acts centrally in the brain to reduce the severity of gastric ulceration in the rat

We have recently reported that apolipoprotein A-IV (apo A-IV), a protein associated with lipoproteins, acts in the brain to inhibit gastric acid secretion. In the present study, we determined whether or not apo A-IV has an anti-ulcer action via the central nervous system using Sprague-Dawley rats. Intracisternal injection of apo A-IV dose-dependently (2.0-4.0 micrograms/rat) reduced the severity of gastric mucosal lesions induced by intravenous injection of 2-deoxy-D-glucose or subcutaneous administration of indomethacine. A higher dose (15 micrograms) of apo A-IV injected intraperitoneally failed to inhibit the development of gastric mucosal damage evoked by these ulcerogenic agents. These results suggest for the first time that apo A-IV has an anti-ulcer action through a central mechanism.

Intracisternal injection of pancreatic polypeptide stimulates gastric emptying in rats

In the present study, we evaluated the effects of central administration of pancreatic polypeptide (PP) on gastric emptying of a liquid meal in conscious rats using a phenol red method. Intracisternal injection of PP (0.5-2.0 micrograms) speeds gastric emptying of a test meal in a dose-dependent manner. In contrast, i.p. injection of PP at the same doses inhibited gastric emptying in a dose-related fashion. Bilateral gastric branch vagotomy abolished the increase in gastric emptying evoked by intracisternal PP. These results demonstrate that PP speeds gastric emptying through the vagal system when given centrally but slows emptying when given peripherally.