Aquaporin-1 in blood vessels of rat circumventricular organs

Abstract

Although the water channel protein aquaporin-1 (AQP1) is widely observed outside the rat brain in continuous, but not fenestrated, vascular endothelia, it has not previously been observed in any endothelia within the normal rat brain and only to a limited extent in the human brain. In this immunohistochemical study of rat brain, AQP1 has also been found in microvessel endothelia, probably of the fenestrated type, in all circumventricular organs (except the subcommissural organ and the vascular organ of the lamina terminalis): in the median eminence, pineal, subfornical organ, area postrema and choroid plexus. The majority of microvessels in the median eminence, pineal and choroid plexus, known to be exclusively fenestrated, are shown to be AQP1-immunoreactive. In the subfornical organ and area postrema in which many, but not all, microvessels are fenestrated, not all microvessels are AQP1-immunoreactive. In the AQP1-immunoreactive microvessels, the AQP1 probably facilitates water movement between blood and interstitium as one component of the normal fluxes that occur in these specialised sensory and secretory areas. AQP1-immunoreactive endothelia have also been seen in a small population of blood vessels in the cerebral parenchyma outside the circumventricular organs, similar to other observations in human brain. The proposed development of AQP1 modulators to treat various brain pathologies in which AQP1 plays a deleterious role will necessitate further work to determine the effect of such modulators on the normal function of the circumventricular organs.

Aquaporins in the Spinal Cord

“…These observations are in agreement with sporadical labeling of blood vessels in parenchyma of rat and human brains [20,22]. However, these observations seem to be exceptional since in general AQP1 is not expressed in the capillaries of neurovascular units and BBB [16,19,57].…”

“…Therefore, this channel is involved in the secretion of cerebrospinal fluid (CSF) [18]. The cerebrovascular endothelium in the CNS lacks AQP1 [16,19] with the exception of the blood vessels in circumventricular organs [20] and small capillaries occasionally found in the brain and spinal cord parenchyma [20,21,22]. Outside of the brain, AQP1 expression was found in the dorsal horns of the spinal cord, in the dorsal root ganglia and its projecting nerves, particularly in the sciatic nerve (Table 1) [14,21,23].…”

Aquaporin water channels in the nervous system

“…AQP1 is also expressed in vascular endothelial cells throughout the body but is notably absent in the cerebrovascular endothelium 41 , except in circumventricular organs 43 . Cerebrovascular endothelial cells express AQP1 in culture 44 and when the BBB is disrupted in malignant brain tumours 45 .…”

The origins of the circumventricular organs

“…; Wilson et al. ), and the pathological expression of AQP4 autoantibodies has been identified as a cause of neuromyelitis optica, an inflammatory autoimmune disease that also affects CVOs, resulting in intractable nausea and vomiting (Lennon et al. ; Matiello et al.…”