Seasonal Responses Of Gastrointestinal Tract Phenotypes And Functional Regulating Factors In Spermophilus Dauricus

Changes in ambient temperature,food availability and photoperiod trigger seasonal acclimatization in digestive phenotypic flexibility of wild animals.The most studies to date are focused on a few morphological features of gastrointestinal tract to assess phenotypic flexibility in rodents.However,the studies on hibernator and the neurochemical responses in seasonal flexibility are lacking.The Daurian ground squirrel(Spermophilus dauricus)is a typical fat-storing hibernator.The aims of this work are to address the gastrointestinal digestive strategies at different levels and potential self-regulating mechanisms when the Daurian ground squirrel adapts to the seasonal variation and hibernation.Here,the morphological and histological parameters were assessed,while the key functional factors involved in regulating gastrointestinal activity and appetite,i.e.neuronal nitric oxide synthase(nNOS,an indicator of nitric oxide production),neuropeptide Y(NPY),vasoactive intestinal peptide(VIP),cholecystokinin(CCK),ghrelin(GHRL)and melatonine(MT)were also examined.The results of morphological parameters were as follows: The Daurian ground squirrel weighed more in summer and autumn than did in spring and winter;the total wet and fresh weights of gastrointestinal tract were the heaviest in autumn and the lightest in winter across seasons(P < 0.05),although there was no significant seasonal changes in the total length and total dry weight(P > 0.05);the stomach of the ground squirrels kept relative stable except that the dry weight in autumn was higher than in summer(P < 0.05);the length of small intestine was shorter in autumn than in summer(P < 0.05),and the fresh weight,net fresh weight and dry weight of small intestine in winter were lighter than in autumn(P < 0.05);the length of caecum and fresh weight in winter was lower than in summer(P < 0.05),and the dry weight in winter was lighter than in spring(P <0.05).The length of large intestine in winter was the longer than in summer and autumn(P< 0.05),and the dry weight in winter was lighter than in spring(P < 0.05).The results of histological parameters were as follows: The thickness of stomach mucosa in autumn was the thickest over seasons(P < 0.05),whereas thickness of muscular layer in summer was the thickest(P < 0.05).The thickness of stomach submucosa was thicker in autumn and winter than in spring and summer(P < 0.05);the thickness of mucosa of small intestine was the thinnest in winter(P < 0.05).The villus height in autumn and winter was shorter than in spring and summer(P < 0.05).The thickness of muscular layer in summer and autumn than in spring and winter(P < 0.05).The diameter of small intestine in spring was the biggest(P < 0.05).However,the submucosa thickness of small intestine showed nonsignificant seasonal differences(P >0.05);the thickness of mucosa and muscular layer in caecum is the highest in autumn(P <0.05),in contrast,the thickness of submucosa showed nonsignificant seasonal differences(P > 0.05);the thickness of muscular layer of large intestine was the thinnest in winter,and there were not significant seasonal differences in other parameters(P > 0.05).The results of gene expression via RT-PCR assessing were as follows: The nNOS expression of the stomach in autumn and winter were lower than in spring and summer(P< 0.05),and the expression of NPY and VIP in autumn and winter were lower than in spring(P < 0.05);the CCK of the stomach showed very few or no expression.The nNOS expression of small intestine in autumn and winter was higher than in spring and summer(P < 0.05),and the NPY expression in autumn and winter was higher than in spring(P <0.05).The VIP expression in winter was the highest over seasons(P < 0.05),and the CCK expression in winterand summerwas higher than in spring(P < 0.05).The results of Elisa assessing showed that the concentrations of serum CCK-8 in winter was the lowest and the highest in autumn over seasons(P < 0.05);the concentrations of serum GHRL in winter was lower than in summer and autumn(P < 0.05);the concentrations of serum MT in spring was the lower than in summer and autumn(P < 0.05).The above results indicated that the Daurian ground squirrel exhibits seasonal plasticity in gastrointestinal tract,and which were especially affected by autumn(fattening period)and winter(hibernating period).The different digestive organs showed different varying degrees of flexibility at length,fresh weight,dry weight and histological parameters in seasonal variation.Small intestine showed the most dramatically changes to seasonal climatization,although the significant seasonal changes in the total length and total weight were not found,indicting the adjustment of digestive plasticity is probablly local adjustment.The expression of nNOS,NPY,VIP and CCK effectively corresponded with seasonal changes,and they were differentially expressed in stomach and small intestine.which may be different in modulating function properties of gastrointestinal seasonal acclimatization.Serum CCK-8,GHRL and MT were also responsed to seasonal variation,and may modulate food intake by endocrinic way.Thus,gastrointestinal tract of the Daurian ground squirrel may self-regulate at different levels(morphology,histology,gene,and hormone)to cope the seasonal changes and variable energy demands.