Administration of a 5HT3 receptor antagonist increases the intake of diets containing Eucalyptus secondary metabolites by marsupials
Lawler, IR, Foley, WJ, Pass, GJ & Eschler, BM 1998, Journal of Comparative Physiology B, vol. 168, no. 1, pp. 611-618.
Jensenone, an acylphloroglucinol derivative that is a secondary metabolite in Eucalyptus, was found to cause common ringtail and brushtail possums to regulate their food intake to consume only a defined maximum amount. Injecting jensenone into possums’ stomachs reduced their food consumption so as not to exceed a maximum dosage, although individuals responded variably. After injecting ondansetron, an antagonist of serotonin 5HT3 receptors, the maximum amount of food able to be consumed increased significantly, suggesting that jensenone’s negative effects occur by acting on 5HT3 serotonin receptors, thereby causing ‘nausea’ and food aversion.
When jensenone was added to the possums’ diets, they were able reduce their food consumption to limit their dosage of jensenone, regardless of concentration. Brushtail possums consumed a maximum amount of 70 mg kg-0.75 d-1 whereas ringtail possums were able to consume much higher levels, averaging 140 mg kg-0.75 d-1. Injections of ondansetron increased the amount of jensenone brushtail possums consumed, but no increases in food consumption were seen in brushtails with no jensenone in their diet or in ringtail possums. This effect was also seen with ondansetron being added to the food, rather than being injected. The stomach wash with jensenone suggested that the maximum dosage among the possums remained the same irrespective of ingestion method, but did not yield significant results due to some of the test animals vomiting during the experiment.
By using ondansetron, a selective 5HT3 antagonist, this study honed in on serotonin at this specific receptor site as the cause of the feeding regulation, rather than much more general drugs or combinations of drugs used in previous experiments that could not be traced to specific physiological pathways. How 5HT3 influences feeding regulation is still unknown as nausea is a subjective sensation and it unclear whether animals perceive it similarly to humans. It is also unknown whether jensenone actually has negative physiological effects other than making the animals feel ‘nauseous’.
By determining the mechanism through which high jensenone levels influence possum food intake, we can acquire a more comprehensive understanding of food selection and diet variation in marsupials.
Summarised by Laura Wait
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