Seasonal and thermal effects on the emetic responses of ranid frogs

We examined the relationship between time of the year and sensitivity to emetics in the frogs Rana rugosa and Rana nigromaculata. In response to apomorphine hydrochloride at a dosage of 50 μg/g body mass (wet mass), both species vomited from the late autumn through the winter, when they naturally hibernate, but not during the spring or summer months, when they are normally active. Exposure to low temperature (7 °C) for 29 days made R. rugosa sensitive to apomorphine even in the summer. Exposure to high temperature (20.0–23.0 °C) for 7–10 days in the winter resulted in loss of the frogs' sensitivity to apomorphine. Based on these results, we conclude that seasonal fluctuation in sensitivity to apomorphine is caused by changes in temperature. Sensitivity to copper sulfate, administered orally at a dosage of 0.4 mg/g, also changed with the time of year. However, in contrast to emesis induced with apomorphine, there was an increase in the latency to emesis induced with copper sulfate in the winter compared with the summer for both species. Thus, the emetic responsiveness of ranid frogs depends on both seasonal changes in temperature and on the agent used to provoke emesis.