Immunohistochemical distribution of the secretogranin II-derived peptide EM66 in the rat hypothalamus: a comparative study with jerboa.

EM66 is a 66-amino acid peptide derived from secretogranin II, a member of granin acidic secretory protein family, by proteolytic processing. EM66 has been previously characterized in the jerboa (Jaculus orientalis) hypothalamus and its potential implication in the neuroendocrine regulation of feeding behaviour has been demonstrated. In the present study, an immunohistochemical analysis of the localization of EM66 within hypothalamic structures of rat was performed and compared to the distribution of EM66 in the jerboa hypothalamus. In the rat hypothalamus, as in the jerboa, EM66 immunostaining was detected in the parvocellular paraventricular, preoptic and arcuate nuclei, as well as the lateral hypothalamus which displayed an important density of EM66-producing neurones. However, unlike jerboa, the suprachiasmatic and supraoptic nuclei of the rat hypothalamus were devoid of cellular EM66-immunolabeling. Thus, the novel peptide EM66 may exert common neuroendocrine activities in rat and jerboa, e.g. control of food intake, and species-specific roles in jerboa such as the regulation of biological rhythms and hydromineral homeostasis. These results suggest the existence of differences between jerboas and rats in neuroendocrine regulatory mechanisms involving EM66.