Gut vagal afferents are not necessary for the eating-stimulatory effect of intraperitoneally injected ghrelin in the rat - PubMed (original) (raw)
Comparative Study
Gut vagal afferents are not necessary for the eating-stimulatory effect of intraperitoneally injected ghrelin in the rat
Myrtha Arnold et al. J Neurosci. 2006.
Abstract
Ghrelin is unique among gut peptides in that its plasma level increases during fasts and its administration stimulates eating. Although ghrelin physiology has been intensively studied, whether its eating-stimulatory effect arises from endocrine-neural signal transduction at peripheral or central sites remains unresolved. To address this issue, we tested the effects of subdiaphragmatic vagal deafferentation (SDA), the most complete and selective vagal deafferentation method available, on ghrelin-induced eating. SDA was verified with a cholecystokinin satiation test, retrograde labeling of vagal motor neurons in the dorsal motor nucleus of the vagus with fluorogold, and anterograde labeling of vagal afferents in the nucleus tractus solitarius with wheat germ agglutinin-horseradish peroxidase. Intraperitoneal injections of 10-40 microg/kg ghrelin stimulated eating as robustly in rats with verified complete SDA as in sham-operated controls. Ghrelin also stimulated eating in rats with total subdiaphragmatic vagotomies. We also recorded the electrophysiological responses of gastric load-sensitive vagal afferent neurons to intravenous ghrelin. Ghrelin (10 nmol) phasically (0-30 s) increased activity in two of seven gastric load-sensitive fibers in the absence of gastric loads and tonically (5-30 min) increased activity in only one fiber. Ghrelin did not affect any of the eight fibers tested in the presence of 1-3 ml gastric loads. We conclude that although phasic increases in plasma ghrelin may affect the activity of a fraction of gastric load-sensitive vagal afferents, the acute eating-stimulatory effect of intraperitoneal ghrelin does not require vagal afferent signaling.
Figures
Figure 1.
Inverted dark-field photomicrographs of representative sections of the subpostremal dorsal vagal complex from rats with SHsda, intentionally incomplete SDA, and complete SDA. All rats showed retrograde labeling of vagal motor neurons in the left DMX. Anterograde labeling of vagal afferents in the overlying left NTS was absent after SDA. Sol, Solitary tract. Wheat germ agglutinin–HRP (2%) in saline (1.5 μl) was pressure injected into the left nodose ganglion using a micromanipulator. See Materials and Methods for additional details.
Figure 2.
In series 1, 20 and 40 μg/kg BW ghrelin increased food intake in rats with verified complete SDA and SHsda at each time point measured. BW ghrelin (10 μg/kg) failed to reliably stimulate eating in either group. Post hoc (Bonferroni–Holm): for 20 μg/kg ghrelin, ghrelin − PBS difference, p < 0.05 for each time and group; for 40 μg/kg ghrelin, p < 0.01 for each time and group; differences in eating-stimulatory effect of 20 or 40 μg/kg BW ghrelin between SDA and SHsda rats; not significant. Error bars represent means ± SE of 12 SHsda and nine SDA rats. See Results for additional details.
Figure 3.
In series 2, 40 μg/kg BW ghrelin again increased food intake similarly in rats with verified complete SDA and SHsda at each time point measured. Post hoc (Bonferroni–Holm): 30 min, p < 0.001 for SDA and SHsda; 60 min, p < 0.05 for SDA and SHsda; 120 min, p < 0.02 for SDA and p < 0.01 for SHsda; differences in eating-stimulatory effect of ghrelin between SDA and SHsda rats, not significant. Error bars represent means ± SE of 17 SHsda and 15 SDA rats. See Results for additional details.
Figure 4.
Inverted fluorescence photomicrographs of representative sections of the subpostremal dorsal vagal complex from rats with sham vagotomy (SHtvx) or complete subdiaphragmatic vagotomy (TVX). SHtvx rats showed retrograde labeling of vagal motor neurons in the DMX; TVX rats did not. Each rat was intraperitoneally injected with 2 mg of fluorogold (Fluorochrome) in 1 ml of saline. See Materials and Methods for additional details.
Figure 5.
BW ghrelin (40 μg/kg) increased food intake similarly in rats with verified complete subdiaphragmatic vagotomy (TVX) and sham vagotomy (SHtvx) at each time point measured. Post hoc comparisons (Bonferroni–Holm): ghrelin − PBS difference at 30 min, p < 0.02 for both TVX and SHtvx; at 60 and 120 min, p < 0.05 for both TVX and SHtvx; differences in eating-stimulatory effect of ghrelin between TVX and SHtvx rats, not significant. Error bars represent means ± SE of nine SHtvx and eight TVX rats. See Results for additional details.
Figure 6.
Intracarotid infusion of ghrelin (10 nmol in 250 μl saline at 1 ml/min) did not reliably affect the activity of gastric load-sensitive vagal afferents tested without gastric loads. Lines represent results from each of seven fibers recorded in seven different rats (a–g). Neural activity was recorded for the 30 s periods immediately before infusion and starting at infusion onset (preinfusion and postinfusion onset) and for 60 s periods once each 5 min for 40 min after infusions. See Results for additional details.
Figure 7.
A, Gastric loads volume-dependently increased single-unit vagal afferent activity and intracarotid infusion of ghrelin (10 nmol in 250 μl of saline at 1 ml/min) did not reliably affect this response (left). In contrast, CCK-8 (100 pmol) infusion significantly increased the response to gastric load (right). Post hoc comparisons (Tukey's HSD): effects of each gastric load alone (except 1 ml in left panel), p < 0.05; all ghrelin-control differences are not significant; CCK-control differences after one to three gastric loads, *p < 0.05. B, Activity of a representative gastric load-sensitive vagal afferent fiber in response to gastric loads of 1, 2, and 3 ml of saline (infusion at arrows). C, Activity of a representative gastric load-sensitive vagal afferent fiber in response to gastric loads of 1, 2, and 3 ml of saline in combination with intracarotid (ica) infusions of 10 nmol ghrelin (both infusions at arrows).
References
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