Three-dimensional distribution of Fos-positive neurons in the supramammillary nucleus of the rat exposed to novel environment

The supramammillary nucleus (SuM) in the hypothalamus is proposed to regulate the function of the hippocampus through distinct fiber connection. Several investigations suggest that the SuM is relevant to anxiety and defensive behavior. Function of the SuM, however, is not known exactly. In order to demonstrate the spatial activation of the SuM in physiologically behaving rats, we investigated Fos induction in the SuM by exposure to novel environment. To correct uneven background in microscopic preparations, we applied a convolution filter, resulting in reliable automatic counting of Fos-positive neurons and analyzed the distribution of Fos-positive neurons in the whole region of SuM. A large number of Fos-positive neurons were observed throughout the entire SuM after rats exposed to a novel open field. A three-dimensional density map revealed that density of the Fos-positive neurons was highest in the medial SuM, especially in its core regions. Based on these results we suggest that the medial SuM modulates defensive behavior and that the lateral SuM modulates emotional and memory functions of the hippocampus.