Distribution and projections of cholecystokinin‐immunoreactive neurons in the hypothalamic paraventricular nucleus of rat

Abstract Analysis of coronal sections from colchicine‐treated rat brains reveals that CCK‐immunoreactivity (CCK‐ir) is present in two distinguishable neuronal systems in the paraventricular nucleus (PVN). More than 60% of these cells were found to be typical parvicellular neurons; the remainder were magnocellular neurons. The magnocellular CCK‐ir neurons were concentrated in the medial magnocellular subdivision, while more caudally they formed a ring around a zone of unstained magnocellular neurons. Immunostained parvicellular neurons predominate in medial and periventricular parvicellular subdivisions. The efferent projections of CCK‐ir neurons were investigated by looking for retrograde accumulation of CCK‐ir in cell bodies after selective knife cuts. A parasagittal cut of the lateral retrochiasmatic area as well as transection of the rostral median eminence resulted in an accumulation of CCK‐ir material in a large number of both parvi‐ and magnocellular neurons. After pituitary stalk lesions, however, increased staining was only seen in magnocellular neurons. It is inferred that the magnocellular (presumed oxytocin‐CCK) cells send their axons to the pituitary, whereas axons of CCK‐ir parvicellular neurons appear to terminate in the median eminence. After transection of the medial forebrain bundle (MFB), immunostaining increased in a small number of scattered transected fibers proximal to the knife cut and in a few perikarya in the PVN, indicating that very few CCK cells may send descending fibers to the lower brainstem.