ORGANIZATION OF THE HAMSTER PARAVENTRICULAR HYPOTHALAMIC NUCLEUS

The hamster periventricular hypothalamic area has been the focus of fu nctional research concerning photoperiodic time measurement. These stu dies have relied upon the extensive analysis of rat paraventricular nu cleus because there has been a general absence of anatomical descripti on in the hamster. The present work sought to remedy this problem by i nvestigating the structure of the hamster paraventricular nucleus with respect to the localization of cells immunoreactive to vasopressin, o xytocin, or corticotropin-releasing factor and of cells projecting to the spinal cord or to vascular sites outside the blood-brain barrier. The hamster paraventricular nucleus includes the medial, lateral, and posterior magnocellular divisions, the main parvicellular division, as well as the periventricular area and dorsal cap, which are also parvi cellular. The magnocellular divisions are characterized by many large neurons immunoreactive to oxytocin and vasopressin, which are generall y absent from the parvicellular divisions. In contrast, corticotrophin -releasing hormone-immunoreactive cells are plentiful in most of the p arvicellular areas. Spinally projecting cells are found in two rostral areas, one dorsally and a second, more ventral area. More caudally, t he two regions merge within the posterior magnocellular division. Cell s of the ventral group are frequently immunoreactive for one of the th ree peptides. Cells identified by peripheral injection of retrograde l abel are found in the rostral magnocellular divisions but not in the c audal posterior magnocellular division. Areas in which these cells als o contain peptide are also described. The features of the hamster para ventricular nucleus are compared to those in the rat and apparent spec ies differences are discussed.