Role of corticotropin-releasing factor, somatostatin and leptin in vagal nerve function and control of gastric emptying

Abstract

The overall aim of this thesis was to study the effects of centrally acting CRF and leptin, and peripherally acting somatostatin on gastric emptying of glucose. Freely moving rats bearing chronic intragastric fistulas received intragastric infusions (1.0 ml/min) of glucose (12.5 % or 25 %) during 6-18 min. The stomachs were evacuated immediately, or 6-36 min after intragastric infusion offset, and solute emptied was determined. Drugs were delivered into the lateral (i.c.v.) or fourth (4th i.c.v.) cerebral ventricle, or injected subcutaneously. In some experiments, animals were sacrificed by decapitation and trunk blood was collected for subsequent RIA analysis of gastrointestinal hormones. Presence of leptin receptor-like immunoreactivity in nuclei of the caudal brainstem, and the possible co-localization with choline-acetyl transferase, was studied with immunohistochemistry.

CRF injected i.c.v. (60 pmol-1.28 nmol) increased plasma levels of somatostatin in freely fed rats, but did not affect plasma levels of CCK or gastrin. Plasma levels of somatostatin were unaffected by CRF in food deprived rats, as well as in food deprived rats pretreated with 10 µg CCK, i.p. Peripheral (i.p.) injection did not affect plasma levels of somatostatin, CCK or gastrin.

To evaluate the effect of peripheral somatostatin receptor activation on gastric emptying, rats received injections of the agonist octreotide (0.0013-1.39 µg/kg, s.c.). OctreodOctreotidede dose dependently suppressed gastric emptying during gastric fill. The effect was reversed by pretreatment with CPP-1, but not by subdiaphragmatic vagotomy. A new finding was that the octreotide-induced emptying suppression during fill was followed by a rapid increase in postprandial emptying, and the transition was locked to the offset of the intragastric infusion, regardless of its duration.

Administration of CRF (10-1000 pmol) 4th i.c.v., suppressed gastric emptying of glucose during gastric fill, indicating that CRF acts in the caudal brainstem to suppress emptying. The CRF-induced emptying suppression was blocked by pretreatment with the somatostatin antagonist CPP-1 (40 µg/kg. s.c.), which by itself was without effect, suggesting that the brainstern CRF-elicited suppression of gastric emptying was mediated by peripheral somatostatin. The data suggest a novel, CRF-somatostatin linkage mechanism in the brainstem control of gastric emptying.

Immunohistochemistry using an antibody that recognizes all leptin receptor (LR) isoforms equally, showed many strongly immunoreactive (IR) neurones in the dorsal motor nucleus of the vagus nerve (DMX) of rats. There were only weakly LR-IR neurones present in the nucleus of the solitary tract (NTS) and in the hypoglossal nucleus. In the DMX, LR-IR was co-localized with choline-acetyltransferase. The data suggest that LR are present in cholinergic neurones of the DMX Injection of leptin (0.013-3.9 µg) 4th i.c.v. suppressed gastric emptying of glucose during fill, but did not affect postprandial emptying. The suppression of emptying during fill was abolished by vagotomy, but not by pretreatment with a CRF antagonist. The data show that leptin acts in the caudal brainstem, independently of CRF receptor activation, to suppress gastric emptying through a vagal route. The findings are consistent with the notion of functionally significant leptin receptors in the DMX of rats.

Subdiaphragmatic vagotomy, as well as gastric branch vagotomy, increased gastric emptying of glucose during gastric fill. Gastric branch vagotomy was without effect on postprandial emptying, whereas subdiaphragmatic: vagotomy slightly decreased emptying in the post-fill period. These findings show that the gastric branch of the vagus contributes to inhibition of emptying during gastric fill, whereas other vagal fibers appear to contribute to control of emptying after fill. In addition, the findings support the notion of separable control mechanisms for glucose gastric emptying during versus after gastric fill.