Distribution of hypothalamic, medullary and lamina terminalis neurons expressing Fos after hemorrhage in conscious rats.

The immunohistochemical detection of the protein, Fos, has been used as an anatomical marker of activated neurons. Three conscious rats were hemorrhaged (4 ml, 20-25% of blood volume) and the distribution of Fos-stained neurons was compared to that in 4 rats which did not have blood removed. In hemorrhaged rats, a higher concentration of Fos-stained neurons was present in the lamina terminalis, particularly the subfornical organ and organum vasculosum of the lamina terminalis, and in the supraoptic and paraventricular nuclei of the hypothalamus. In the medulla, Fos-stained neurons were restricted to the nucleus of the tractus solitarius, area postrema and the ventrolateral medulla. We hypothesize that those neurons are involved in mediating the physiological responses to hemorrhage.