Relation of neuropeptides to mammalian circumventricular organs.

In summary, highly vascularized CVOs of the mammalian brain are the site of increased vascular permeability for peptides and other molecules which generally do not cross the blood-brain barrier. In the CVOs the blood-brain barrier is shifted from the level of the capillaries to the tight junctions of the oligociliated ependymal cells. The neurohypophysis is the well known target of various peptidergic neuroendocrine neurons. In the neural lobe, peptide hormones from magnocellular neurons are stored and released into the general circulation in the median eminence, releasing and inhibiting hormones enter the hypothalamo-adenohypophyseal portal circulation. The OVLT appears to be an additional vascular outlet for LHRH and somatostatin. In the pineal, no pinealocytes stain positively for arginine-vasotocin; however, occasionally a single neurophysin (vasopressin or oxytocin) fiber has been observed. In the subfornical organ and area postrema which do not appear to have a primary neuroendocrine function, hemo-neural interactions may be important for effects of circulating peptides and other molecules on specific receptors. In the subcommissural organ, which does not have a special vascular permeability, ependymal cells secrete Reissner's fiber, a mucopolysaccharide, whose function in unclear.