Altered mechanosensitive properties of vagal afferent fibers innervating the stomach following gastric surgery in rats

Abstract

Background and aims: Several types of gastric surgeries have been associated with early satiety, dyspepsia and food intolerances. We aimed to examine alterations in gastric vagal afferents following gastric surgery-fundus ligation.

Methods: Six week old, male Sprague-Dawley rats underwent chronic ligation (CL) of the fundus. Sham rats underwent abdominal surgery, but without ligation. Another group of rats underwent acute ligation (AL) of the fundus immediately prior to experiments. CL and sham rats were allowed to grow to age 3-4 months. Food intake and weights were recorded post-operatively. Gastric compliance and gastric wall thickness was measured at baseline and during gastric distension (GD). Extracellular recordings were made to examine response characteristics of vagal afferent fibers to GD and to map the stomach receptive field (RF). The morphological structures of afferent terminals in the stomach were examined with retrograde tracings from the nodose ganglion.

Results: The CL group consumed significantly less food and weighed less than sham control. The mean compliance of the CL group was significantly less than control, but higher than the AL group. The spontaneous firing and responses to GD of afferent fibers from the CL rats were significantly higher than AL rats. There was a marked expansion of the gastric RF in the CL rats with significant reorganization and regeneration of intramuscular array (IMA) terminals. There was no difference in total wall or muscle thickness among the groups.

Conclusion: CL results in aberrant remodeling of IMAs with expansion of the gastric RF and alters the mechanotransduction properties of vagal afferent fibers. These changes could contribute to altered sensitivity following gastric surgery.

Figure 1

Schematic diagram of fundus ligation

Figure 2

Food consumption measured weekly for 1 hour (A) and during 3 hours (B) following overnight fast. Consumptions was greater in the sham animals compared to CL throughout the first 5 weeks measured (p<0.05)

Figure 3

Mean total body weight of sham rats and chronic fundus ligated rats. CL animals had lower weights compare to naïve at all weeks measured following surgery (p<0.05).

Figure 4

Pressure-volume curves obtained after instillation of increasing volumes of saline in AL, CL and sham rats. Values are expressed as mean and SEM. *p<0.05 vs. chronic ligated, ^#p<0.05 vs sham.

Figure 5

Example of typical response of mechanosensitive afferents near the corpus and response to GD (10–40mmHg). In each panel, the top tracing represents the frequency histogram (1s binwidth), the middle tracing is the nerve action potential and the lower tracing is the distension pressure. The spontaneous firing and response to GD was significantly higher in CL animals compared to AL.

Figure 6

Summary data of stimulus response function of vagal afferents to GD in CL AL and sham (non-ligated) rats. The spontaneous firing of neurons in the CL animals was significantly higher than AL. Also, the response to mechanical distension was significantly higher in CL animals compared to AL at pressures ≥ 20mmHg. There was no difference in the spontaneous firing of neurons or response to distension between AL and sham group. Values are expressed as mean and SEM. *p<0.05.

Figure 7

Vagal sensory terminal remodeling associated with fundic ligation. Afferent terminals, labeled with Dextran-Biotin, illustrating the patterns observed in the proximal corpus near the fundus in the dorsal wall of the stomach of sham (A) and fundic ligated rats (B, C). A: An intramuscular array (IMA). The IMA terminal runs diagonally from the upper right to the lower left of the panel. This example is a characteristic IMA, in that it consists of multiple telodendria running parallel to each other (and smooth muscle fibers) in a single muscle layer. B: A disorganized, apparently hyperplastic IMA near the site of the fundic ligation surgery. Originating from a single axon, the terminal is morphologically similar to an IMA, yet the telodendria lack the uniform/parallel characteristics of an IMA. C: Another type of dystrophic afferent neurite terminating in a putative growth cone profile near the site of the fundic ligation. Scale bars = 50 μm.

Figure 8

Ultrasound images of the gastric wall in CL rats. A. The gastric image is from the corpus at a time when there was no volume distension and demonstrates the measure of wall thickness. B. Total wall thickness was calculated by measuring the distance from the inner mucosal surface to the outer serosal surface using a line that bisected the center of the stomach lumen.