Organization of the hamster paraventricular hypothalamic nucleus.

The hamster periventricular hypothalamic area has been the focus of functional research concerning photoperiodic time measurement. These studies have relied upon the extensive analysis of rat paraventricular nucleus because there has been a general absence of anatomical description in the hamster. The present work sought to remedy this problem by investigating the structure of the hamster paraventricular nucleus with respect to the localization of cells immunoreactive to vasopressin, oxytocin, 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 parvicellular. The magnocellular divisions are characterized by many large neurons immunoreactive to oxytocin and vasopressin, which are generally absent from the parvicellular divisions. In contrast, corticotrophin-releasing hormone-immunoreactive cells are plentiful in most of the parvicellular areas. Spinally projecting cells are found in two rostral areas, one dorsally and a second, more ventral area. More caudally, the two regions merge within the posterior magnocellular division. Cells of the ventral group are frequently immunoreactive for one of the three peptides. Cells identified by peripheral injection of retrograde label are found in the rostral magnocellular divisions but not in the caudal posterior magnocellular division. Areas in which these cells also contain peptide are also described. The features of the hamster paraventricular nucleus are compared to those in the rat and apparent species differences are discussed.