Circumventricular organs: Neuroendocrine interfaces between the brain and the hemal milieu

The circumventricular organs (CVOs) of the mammalian brain are the subfornical organ, organum vasculosum of the lamina terminalis (OVLT), median eminence/neurohypophysis, subcommisural organ, pineal gland, area postrema, and choroid plexus. While these CVOs have individual characteristics, they do share a number of common properties that distinguish them from other parts of the central nervous system. Lack of a blood-brain barrier (except for the subcommisural organ) due to the presence of fenestrated capillary endothelium, and extensive perivascular spaces are two such properties and provide some of the morphological basis for neuroendocrine roles of the CVOs. Centrally synthesized hormones may be secreted at CVOs (e.g., hypophysiotropic hormones at the median eminence, LHRH at the OVLT, and melatonin at the pineal gland). On the other hand, circulating hormones have access to the interstitium of CVOs and exert influences on CNS function via specific receptors in CVOs such as the subfornical organ, OVLT, and area postrema. These are the only CVOs that contain neuronal perikarya. Neurons in them have efferent links to other brain regions, which influence homeostatic mechanisms including regulation of body fluids and cardiovascular function. In this regard, there is evidence suggesting that the OVLT and subfornical organ may be the site of osmoreceptors subserving thirst and vasopressin secretion. The octapeptide angiotensin II also exerts actions on several of the CVOs to induce water drinking, vasopressin secretion, and increased blood pressure. Considerable research has been carried out to clarify the role of particular CVOs in these central actions of angiotensin II, and it seems likely that the subfornical organ is of prime importance in the rat, although this is not necessarily the case in all species. Both the subfornical organ and OVLT are rich in angiotensin II binding sites in all species investigated, whereas the receptors are present in the area postrema and median eminence to a variable degree across species. Their relative importance in the central actions of angiotensin II may therefore differ between species.