Functional magnetic resonance imaging and immunohistochemical study of hypothalamic function following oral glucose ingestion in rats

Background The hypothalamus plays a central role in the regulation of metabolism by sensing metabolic demands andreleasing regulatory neurotransmitters.This study investigated the response of the hypothalamus to glucose ingestion inrats by blood oxygen level-dependent functional magnetic resonance imaging(BOLD-fMRI)and immunohistochemicaltechniques to determine the role of the hypothalamus in glycoregulation during disturbances in carbohydrate metabolism.Methods The signal intensity of the hypothalamus was monitored by fMRI for 60 minutes after oral glucose intake in 48healthy rats(age 14 months),which included 24 normal weight rats(weighing(365±76.5)g)and 24 overweight rats(weighing(714±83.5)g).Then,12 rats(6 normal,6 overweight)underwent a repeat fMRI scan after consuming anequivalent amount of water without glucose on a separate day.The procedure for fMRI with water intake was the sameas for glucose ingestion,fMRI data was processed using time cluster analysis and intensity averaging method.After fMRIthe expression of neuropeptide Y(NPY)and 5-hydroxytryptamine(5-HT)in the hypothalamus of all rats was determinedby immunohistochemistry.Positive cells for NPY or 5-HT were counted.Results There was a transient,but significant,decrease in fMRI signal intensity in all rats(mean(3.12±0.78)%)in thehypothalamus within 19.5-25.5 minutes of oral glucose ingestion.In overweight rats,the decrease in signal intensity inresponse to the glucose ingestion was more markedly attenuated than that observed in normal weight rats((2.2±1.5)%vs(4.2±0.7)% inhibition,t=2.12,P<0.05).There was no significant response in the hypothalamus after oral wateringestion.The percentage of NPY positive cells in obese rats were slightly lower than those in control group(21% vs 23%t=0.71,P>0.05);but there was no significant difference between the two groups;the percentage of 5-HT positive cells inobese rats were significantly lower than those in the control group(22% vs 31%,t=3.25,P<0.01).Conclusions There is a transient,but significant,decrease in BOLD signal intensity in the hypothalamus followingglucose ingestion,which is similar to that observed in humans.The response of the hypothalamus to glucose ingestionwas different in overweight and normal weight rats.The percentage of NPY positive cells in obese rats were lower thanthose in the control group,although this difference was not statistically significant.The percentage of 5-HT positive cellsin obese rats was significantly lower than those in the control group.