Infant cold exposure changes Fos expression to acute cold stimulation in adult hypothalamic brain regions.

It is known that cold exposure as an infant results in a permanent cold tolerance which is accompanied by a reduced rate of colonic temperature decline and increased metabolic heat production to cold stimulation. The present study was aimed to elucidate the central mechanism of cold tolerance of adult rats with cold exposure as infants. Newborn infants were exposed to cold ambient (4 degrees C) daily for 2 weeks, and when they were 15-weeks old, Fos expression to acute cold stimulation (10 degrees C) in whole brain regions was investigated using quantitative immunohistochemistry. Acute cold stimulation induced a significant increase of Fos-positive neurons in many nuclei of whole brain areas in adult rats both with and without cold exposure as infants. However, the number of Fos-positive neurons was significantly less in the rats with cold exposure than without cold exposure as infants, particularly in the hypothalamic nuclei such as the lateral septal nucleus (LS), preoptic area (POA), parvocellular paraventricular nucleus (pPVN0, ventromedial hypothalamic nucleus (VMH) and supramammillary nucleus (SuM). However, there was no significant difference in the number of Fos-positive neurons in the extra-hypothalamic regions between the rats with and without cold exposure as infants. These results suggest that the central mechanism of cold tolerance with cold exposure as infants is due to metabolic changes of the hypothalamic brain regions.