Emetic Responses and Neural Activity in Young Musk Shrews during the Breast-Feeding/Weaning Period: Comparison between the High and Low Emetic Response Strains Using a Shaking Stimulus

The high emetic response (HER) strain and low emetic response (LER) strain of musk shrews (Suncus murinus) markedly differ in the emetic reflex in adults. However, there have been no studies on young musk shrews. We gave a shaking stimulus to young musk shrews aged 10 days or more that were obtained by mating within each strain and observed emetic responses. In the HER strain, no animal aged 10 days vomited, but vomiting was observed in 1 of 5 animals each aged 12 and 14 days, 2 of 5 animals aged 16 days, and all animals aged 18 days or more. In the LER strain, no vomiting was observed until the age of 14 days, but at the age of 16 days or more, 1 or 2 of 5 animals at each age vomited. After stimulation, activated neurons of the dorsal vagal complex and the dorsal reticular formation of the nucleus ambiguus (Amb) were examined by Fos immunohistochemistry. This morphometric study demonstrated that the numbers of Fos-positive neurons in the nucleus of the solitary tract and the dorsal reticular formation of the Amb were significantly larger in the animals that vomited in the HER strain than animals that did not vomit in the LER strain. We suggest that neurons in these regions are involved in emetic responses, as is the case in adult animals.