The appearance of Fos protein-like immunoreactivity in the hypothalamus of developing rats in response to cold ambient temperatures

This study examined cellular activity in the hypothalamus of developing rats in response to cold environmental temperatures. The appearance of the nuclear protein, Fos, in response to cold ambient temperatures in rats three to 35 days of age was used as a marker of neuronal activation. Fos-positive nuclei were first seen in response to cold ambient temperatures in the ventromedial nucleus at three days of age, the paraventricular nucleus at eight days of age, the preoptic-anterior hypothalamus at 10 days of age and the anterior hypothalamic nucleus at 21 days of age. The rectal temperature of the 10-day-old pups dropped by less than half that measured in the nine-day-old pups after 1 h in the cold. It is possible that the activation of neurons in the preoptic-anterior hypothalamus in the 10-day-old animal may contribute to the decreased hypothermia observed in the 10-day-old after 1 h in the cold. The high density of Fos-like immunoreactive-labeled nuclei in the preoptic-anterior hypothalamus in the 10-day-old rats that were exposed to the cold environment indicates increased neuronal activity at this site. The results suggest that, at least in the preoptic-anterior hypothalamus, the appearance of the cellular response to cold ambient temperatures is coincidental with a thermogenic response.