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The vomiting reflex takes place in many mammals, including primates and carnivores, but mice and rats in the laboratory appear to lack this response. A team of neurobiologists at the University of Pittsburgh decided to study why this is.

The researchers -- lead by associate professor of Medicine and Anesthesiology Charles Horn -- hypothesised that it could be for anatomical constraints (they have a relatively long oesophagus) or a lack of neural circuits. Furthermore they weren't clear whether it was simply lab rodents or all rodents.


The three major groups of rodents were put to the test: rodentia, or mouse-related (rat, mouse, vole, beaver); ctenohystrica (guinea pigs and nutrias), and squirrel related species. To be precise, there were eight beavers, eight mountain beavers, four guinea pigs, 17 mice and 26 rats. They were given emetic agents (substances that provoke the vomiting reflex), including apomorphine, veratine and copper sulphate. These substances were chosen because they are known to cause vomiting in a variety of species.

The behaviour of the animals was documented in observation chambers of varying sizes, depending on species. The smaller animals were injected with small quantities of the emetic agents, while larger animals were briefly anaesthetised before given a feeding tube with a solution of the emetics. Each animal received no more than two tests of the vomiting agents -- at least four hours apart. Their behaviour was filmed for at least 40 minutes after taking the emetic.

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The video was assessed by two trained staff who didn't know which animals had been given saline and which the emetics. They were looking for specific behaviours -- such as vomiting, retching, coughing, exhaling, chewing, falling over, licking, heaving, defecating, gnawing, salivating shaking -- as well as overall movement.

At the same time, the researchers analysed the animals' oesophagus, diaphragm and stomach to see whether there were anatomical reasons that might constrain the animals' ability to vomit. A small subsection of the animals were dissected and their stomach and oesophagus analysed. The results were then compared to species with a vomiting reflex: eight cats, 22 musk shrews and eight ferrets.

Finally, the team recorded mouth and oesophageal movements using sensitive force transducers in the brainstems of rats, mice and musk shrews to see if the rodents had subtle emetic-like responses that couldn't be observed. This involved a rather unpleasant procedure called the "working heart brainstem preparation", where animals are deeply anaesthetised, their brain stem is severed, and they are supplied with artificial blood. Recordings were made of their nerve activity and subtle movements, while heart activity and blood pressure to the brain were also measured.


The results revealed that none of the rodents showed retching or vomiting and that they had less muscular diaphragms and a longer oesophagus which might make it difficult for them to efficiently vomit. Some of the animals coughed or heaved slightly, but weren't sick and didn't salivate heavily. One of the emetics -- apomorphine -- was found to cause animals to become restless.

Beyond this, several metrics of emesis -- mouth, shoulder and phrenic nerve responses -- which are usually present in animals that can vomit, did not display in mice and rats. This suggests that rodents may lack the brainstem circuitry to generate patterned emetic responses. However, it does seem that these animals can experience "nausea", indicated by the fact that with repeated doses of emetic stimuli they show taste aversion to certain substances. "Understanding the lack of emesis in rodents has implications for the suitability of typical laboratory species, such as rats and mice, for the study of nausea and vomiting," concluded the authors.

You can read the full study here.