Why can't rodents vomit? A comparative behavioral, anatomical, and physiological study

Abstract

The vomiting (emetic) reflex is documented in numerous mammalian species, including primates and carnivores, yet laboratory rats and mice appear to lack this response. It is unclear whether these rodents do not vomit because of anatomical constraints (e.g., a relatively long abdominal esophagus) or lack of key neural circuits. Moreover, it is unknown whether laboratory rodents are representative of Rodentia with regards to this reflex. Here we conducted behavioral testing of members of all three major groups of Rodentia; mouse-related (rat, mouse, vole, beaver), Ctenohystrica (guinea pig, nutria), and squirrel-related (mountain beaver) species. Prototypical emetic agents, apomorphine (sc), veratrine (sc), and copper sulfate (ig), failed to produce either retching or vomiting in these species (although other behavioral effects, e.g., locomotion, were noted). These rodents also had anatomical constraints, which could limit the efficiency of vomiting should it be attempted, including reduced muscularity of the diaphragm and stomach geometry that is not well structured for moving contents towards the esophagus compared to species that can vomit (cat, ferret, and musk shrew). Lastly, an in situ brainstem preparation was used to make sensitive measures of mouth, esophagus, and shoulder muscular movements, and phrenic nerve activity-key features of emetic episodes. Laboratory mice and rats failed to display any of the common coordinated actions of these indices after typical emetic stimulation (resiniferatoxin and vagal afferent stimulation) compared to musk shrews. Overall the results suggest that the inability to vomit is a general property of Rodentia and that an absent brainstem neurological component is the most likely cause. The implications of these findings for the utility of rodents as models in the area of emesis research are discussed.

Figure 1. Mammalian phylogenetic tree , , .

Specific species listed in the tree branches are examples and may not include all those contained in each class; species included in the current study are marked with a yellow highlight. A “+” sign notes a species with a well established emetic response (demonstrated in laboratory studies) (e.g., , , , , , –[79]).

Figure 2. Behavioral test chambers.

A) Floor surface areas for chambers used for behavioral testing in different rodent species. Dashed lines indicate the locations of quadrants used to score locomotion during video playback. B) Larger chamber used to test nutria and beaver. All test chambers had a clear glass floor and video recordings of the ventral surface of animals were collected by reflection in a mirror (45° angle). This design is based on taste reactivity testing, which is focused on the recording of mouth movements in laboratory rodents .

Figure 3. Anatomical measures of the esophagus, diaphragm, and stomach.

Esophagus length measures were total (from gastroesophageal border to caudal extremity of larynx) and abdominal (below the diaphragm) components. Esophagus circumference was measured directly above the gastroesophageal border. The diaphragm was measured for muscular and non-muscular regions. Stomach shape was measured by placing a horizontal line on the gastroesophageal and gastroduodenal borders and creating a vertical division to determine left and right stomach surface areas. A measure of gastric shape included statistical analysis using 100 points, with 4 restricted landmark points (points 1, 75, 76, and 100) placed on the anatomical borders with the esophagus and duodenum (only a few of these points are shown in blue; starting at point 1 on the gastroesophageal border and moving clockwise).

Figure 4. The in situ brainstem preparation for musk shrews, mice, and rats.

Animals were deeply anesthetized, decerebrated, and perfused with artificial blood. Recordings included the phrenic nerve activity, esophagus and mouth contraction force, and shoulder displacement. Electrocardiogram (ECG) was recorded from pins placed in the lateral edges of the preparation. Perfusion pressure was measured with a pressure tranducer located close to tip of the aorta perfusion catheter. The location of vagus nerve electrical stimulation is also shown. This preparation is adapted from Paton and colleagues –, .

Figure 5. Effects of emetic agents on locomotion of rodent species.

Vertical bars indicate median quadrants moved in the test chambers for each species group (see Fig. 2). Animals were injected with saline (sc or ig) or the emetic agents apomorphine (sc), veratrine (sc), or CuSO₄ (ig) and observed for 40 min. Dark circles indicate raw movement scores for each animal and vertical lines represent the range of scores. * = p<0.05, Mann-Whitney U, comparison to saline control groups.

Figure 6. Behaviors scored after animals were injected with saline (sc or ig) or the emetic agents apomorphine (sc), veratrine (sc), or CuSO₄ (ig).

Data represent the percentage of animals of each species that showed specific behaviors for the 40 min test. No emetic responses were detected in any of these rodent species.

Figure 7. Representative anatomical images of the diaphragm and stomach in test species of Rodentia and emetic species. Bar = 1 cm.

Figure 8. Diaphragm density and area measures.

A) Density of the diaphragm (g/cm²). B) Percentage of diaphragm area composed of muscle compared to ligament. The SEM for musk shrews is small and hidden by the vertical bar. See Figure 3 for a diagram showing the location of these measures. * = p<0.05, planned contrast, a rodent species compared to all emetic species. Data represent mean ± SEM.

Figure 9. Esophagus and stomach area measures.

A) Abdominal esophagus circumference/total esophagus length (cm). B) Abdominal esophagus length/total esophagus length. C) Percentage of stomach area to the left of vertical division. See Figure 3 for a diagram showing location of these measures. * = p<0.05, planned contrast, a rodent species compared to all emetic species. Data represent mean ± SEM.

Figure 10. Stomach shape analysis.

A) Initial X and Y coordinates of ventral stomach shapes for the Rodentia group (n = 32) and emetic group (n = 25). B) All stomach shapes were aligned using a Procrustes analysis (including translation, rotation, and scaling). C) Average group values for the Procrustes transformed stomach shapes: Rodentia (in black) and emetic group (in red). Hotelling T² statistic = 0.59, p<0.001, Rodentia compared to emetic group. X and Y coordinates in each figure represent arbitrary units in image graphics.

Figure 11. Representative recordings of the mouth movement, esophagus movement, and phrenic nerve activity from the mouse (C57BL6), rat (Sprague-Dawley), and musk shrew in the in situ brainstem preparation.

Vertical dashed lines indicate the start of the contraction of the esophagus after resiniferatoxin (RTX) was perfused through the brainstem (Fig. 4). Plots show 15 s pre-event versus 15 s post-event (see Fig. 12 for group averages). Mouth and esophageal recordings indicate force (g), with positive deflections showing opening of the mouth and shortening of the esophagus. Lines and event marks above each trace indicate events detected by computer software (DataView; http://www.st-andrews.ac.uk/~wjh/dataview/).

Figure 12. Average effects of resiniferatoxin (RTX; 40 nM) treatment on mouth, esophagus, and phrenic nerve responses from the brainstems of mouse (C57BL6), rat (Sprague-Dawley), and musk shrew (Fig. 4).

A) Mouth, esophagus, phrenic nerve events during the 15 s before and after (pre- and post-event) alignment to the first large esophageal movement (an esophageal movement that was greater than baseline movements). B) Effects when data are aligned to the first large mouth movement. * = p<0.05, Wilcoxon Signed Rank test, number of pre-events versus number of post-events. δ = p<0.05, Kruskal-Wallis one-way ANOVA, species effect for difference between pre-and post-event values. Data represent mean ± SEM.

Figure 13. Average effects of vagal afferent electrical stimulation (2.5, 5, 10, and 20 V applied for 30 s) on mouth, esophagus, and phrenic nerve responses from the brainstems of mouse (C57BL6), rat (Sprague-Dawley), and musk shrew (Fig. 4). Top row:

The effects of stimulation on mouth, esophagus, and shoulder movements. Bottom row: The effects of stimulation on tonic esophageal force (measured in the first 5 s after the start of stimulation). * = p<0.05, Dunnett’s test, versus 2.5 V condition. Data represent mean ± SEM.