Apoptosis and necrosis in the circumventricular
organs after experimental subarachnoid hemorrhage as detected with annexin V and caspase 3 immunostaining.
It may originate from (1) periphery and then is transported by brain barriers to reach the brain parenchyma or to enter the brain through the circumventricular
organs [32-34], (2) de novo synthesis in the brain microvessel endothelium and CP [20, 35], and (3) local synthesis in microglial and dendritic cells, astrocytes, and even neurons .
The presence of abnormal prion protein accumulation in the pituitary gland and other circumventricular
organs before deposition of [PrP.sup.res] in surrounding brain has been reported in TSE-infected sheep (38).
We cannot however rule out upstream circumventricular
organs (CVO) such as the subfornical organ (SFO), which have an incomplete BBB and are sensitive to changes in osmolality and circulating peripheral hormones such as Ang II [11, 27, 40, 41].
We previously proposed that these agents slowly diffuse in the brain through the fenestrated vessels of circumventricular
organs [66, 67].
New studies have now shown that TLR4 is also expressed on CNS structures such as choroid plexus, circumventricular
organs, and leptomeninges .
It is still not clear if the expression of ABCB1 is only restricted to those brain blood vessels that are part of the BBB, or if it is equally expressed in fenestrated capillaries of the circumventricular
organs (area postrema in the brainstem, the subfornical organ, the median eminence, the pineal body, the vascular organ of the lamina terminalis, choroid plexus, and neurohypophysis) [55, 56].
These reports support the present results that increased water intake in the second hour of the 2 h feeding period in large-type goats fed on dry forage for 2 h twice daily is the result of increases in plasma osmolality, caused by the ruminal absorption of salts from the consumed feed, that in turn bypassed the blood-brain barrier and activated the osmosenstive neurons in the circumventricular
(4) The immune system could be regarded as a "diffuse sensory organ" that signals the presence of pathogens to the brain through different pathways, such as the vagus nerve, endothelial activation/dysfunction which leads to release or passive diffusions of cytokines and neurotoxic mediators, and the circumventricular
organs, especially the neurohypophysis.
Ang II can increase AP due to centrally mediated effects (26), acting in circumventricular
organs or in the NTS, either by activating the central sympathetic nervous system or by elevating plasma vasopressin, noradrenaline, and adrenaline levels (6,27).
Past experiments in which researchers had elecrically stimulated various circumventricular
organs in the brain of mice, including the SFO, had yielded inconsistent results.
Additionally, circulating factors that are able to cross the blood-brain barrier via circumventricular
organs can also influence central sympathetic outflow.