Food-induced expression of orexin receptors in rat duodenal mucosa regulates the bicarbonate secretory response to orexin-A

Abstract

Presence of appetite-regulating peptides orexin-A and orexin-B in mucosal endocrine cells suggests a role in physiological control of the intestine. Our aim was to characterize orexin-induced stimulation of duodenal bicarbonate secretion and modulation of secretory responses and mucosal orexin receptors by overnight food deprivation. Lewis x Dark Agouti rats were anesthetized and proximal duodenum cannulated in situ. Mucosal bicarbonate secretion (pH stat) and mean arterial blood pressure were continuously recorded. Orexin-A was administered intra-arterially close to the duodenum, intraluminally, or into the brain ventricles. Total RNA was extracted from mucosal specimens, reverse transcribed to cDNA and expression of orexin receptors 1 and 2 (OX1 and OX2) measured by quantitative real-time PCR. OX1 protein was measured by Western blot. Intra-arterial orexin-A (60-600 nmol.h(-1).kg(-1)) increased (P < 0.01) the duodenal secretion in fed but not in fasted animals. The OX1 receptor antagonist SB-334867, which was also found to have a partial agonist action, abolished the orexin-induced secretory response but did not affect secretion induced by the muscarinic agonist bethanechol. Atropine, in contrast, inhibited bethanechol but not orexin-induced secretion. Orexin-A infused into the brain ventricles (2-20 nmol.kg(-1).h(-1)) or added to luminal perfusate (1.0-100 nM) did not affect secretion, indicating that orexin-A acts peripherally and at basolateral receptors. Overnight fasting decreased mucosal OX1 and OX2 mRNA expression (P < 0.01) as well as OX1 protein expression (P < 0.05). We conclude that stimulation of secretion by orexin-A may involve both receptor types and is independent of cholinergic pathways. Intestinal OX receptors and secretory responses are markedly related to food intake.

An Orexinergic Projection from Perifornical Hypothalamus to Raphe Pallidus Increases Rat Brown Adipose Tissue Thermogenesis

“…Subsequent injection of SB334867 in rRPa had no effect on the amplitudes or time courses of the increases in BAT SNA and BAT thermogenesis evoked by either Orx-A injection in rRPa or NMDA injection in PeF-LH. Consistent with our observation, SB334867 was reported to have partial agonist activity (Bengtsson et al, 2007) and chronic intracerebroventricular infusion of SB334867 acted similarly to orexin to increase BAT temperature in free-behaving rats (Verty et al, 2010). Although these observations call into question the efficacy of this drug as a selective orexin receptor antagonist, complexities of the microcircuitry in rRPa or the localization of orexin receptors to specific neuronal populations could explain the ability of SB334867 to activate BAT thermogenesis.…”

OX1 Orexin/Hypocretin Receptor Signaling through Arachidonic Acid and Endocannabinoid Release

“…SB-334867 alone weakly inhibited the forskolin response (Fig. 7A), which suggests that SB-334867 shows some weak partial agonist activity at OX 1 receptors, in line with some previous observations (Bengtsson et al, 2007) (J. Putula and J. P. Kukkonen, unpublished observations). AM-251 at 10 M strongly but incompletely inhibited the response to 1 M HU-210 (data not shown; see also below).…”

International Union of Basic and Clinical Pharmacology. LXXXVI. Orexin Receptor Function, Nomenclature and Pharmacology

“…In contrast to gastric secretion, however, bicarbonate secretion in the duodenum is peripherally rather than centrally mediated and has been shown in rats to be affected by nutritional status (Bengtsson et al, 2007). At the cellular level, fasting was shown to reduce the expression of OX 1 receptor mRNA in duodenal enterocytes and to inhibit OX-A induced calcium mobilization (Bengtsson et al, 2009).…”