Subfornical organ

The subfornical organ (SFO) is one of the circumventricular organs of the brain. Its name comes from its location on the ventral surface of the fornix near the interventricular foramina (foramina of Monro), which interconnect the lateral ventricles and the third ventricle. Like all circumventricular organs, the subfornical organ is well-vascularized, and like all circumventricular organs except the subcommissural organ, some SFO capillaries have fenestrae, which increase capillary permeability. The SFO is considered a sensory circumventricular organ because it is responsive to a wide variety of hormones and neurotransmitters, as opposed to secretory circumventricular organs, which are specialized in the release of certain substances. The subfornical organ (SFO) is one of the circumventricular organs of the brain. Its name comes from its location on the ventral surface of the fornix near the interventricular foramina (foramina of Monro), which interconnect the lateral ventricles and the third ventricle. Like all circumventricular organs, the subfornical organ is well-vascularized, and like all circumventricular organs except the subcommissural organ, some SFO capillaries have fenestrae, which increase capillary permeability. The SFO is considered a sensory circumventricular organ because it is responsive to a wide variety of hormones and neurotransmitters, as opposed to secretory circumventricular organs, which are specialized in the release of certain substances. As noted above, capillaries in some subregions within the SFO are fenestrated, and thus lack a blood-brain barrier. All circumventricular organs except the subcommissural organ contain fenestrated capillaries, a feature that distinguishes them from most other parts of the brain. The SFO can be divided into six anatomical zones based on its capillary topography: two zones in the coronal plane and four zones in the sagittal plane. The central zone is composed of the glial cells, neuronal cell bodies and a high density of fenestrated capillaries. Conversely, the rostral and caudal areas have a lower density of capillaries and are mostly made of nerve fibers, with fewer neurons and glial cells seen in this area. Functionally, however, the SFO may be viewed in two portions, the dorsolateral peripheral division, and the ventromedial core segment. The subfornical organ contains endothelin receptors mediating vasoconstriction and high rates of glucose metabolism mediated by calcium channels. The subfornical organ is active in many bodily processes, including, but not limited to, osmoregulation, cardiovascular regulation, and energy homeostasis. Most of these processes involve fluid balance through the control of the release of certain hormones, particularly angiotensin or vasopressin. The subfornical organ's impact on the cardiovascular system is mostly mediated through its influence on fluid balance. The SFO plays a role in vasopressin regulation. Vasopressin is a hormone that, when bound to receptors in the kidneys, increases water retention in the cardiovascular system by decreasing the amount of fluid transferred from the blood to the urine by the kidneys. This regulation of blood volume affects other aspects of the cardiovascular system. Increased or decreased blood volume influences blood pressure, which is regulated by baroreceptors, and can in turn affect the strength of ventricular contraction in the heart, although heart rate is generally not affected by blood volume. Additional research has demonstrated that the subfornical organ may be an important intermediary through which leptin acts to maintain blood pressure within normal physiological limits via descending autonomic pathways associated with cardiovascular control. SFO neurons have also been experimentally shown to send efferent projections to regions involved in cardiovascular regulation including the lateral hypothalamus, with fibers terminating in the supraoptic (SON) and paraventricular (PVN) nuclei, and the anteroventral 3rd ventricle (AV3V) with fibers terminating in the OVLT and the median preoptic area. The subfornical organ has also been shown to have a significant impact on appetite. These mechanisms are not as clear as the neural mechanisms by which the SFO regulates fluid balance; however the most prevalent theory links the SFO's role in appetite control to its influence on energy, particularly glucose consumption. Recent study has focused on the SFO as an area particularly important in the regulation of energy. The observation that subfornical neurons are perceptive of a wide range of circulating energy balance signals and that electrical stimulation of the SFO in rats resulted in food intake supports the SFO’s importance in energy homeostasis. Additionally, it is assumed that the SFO is the lone forebrain structure capable of constant monitoring of circulating concentrations of glucose, due to its lack of a blood-brain barrier. This responsiveness to glucose again serves to solidify the SFO’s integral role as a regulator of energy homeostasis. Other circumventricular organs are the area postrema in the brainstem and the OVLT. The OVLT and SFO are both interconnected with the nucleus medianus, and together these three structures comprise the so-called 'AV3V' region - the region anterior and ventral to the third ventricle. The AV3V region is important in the regulation of fluid and electrolyte balance, by controlling thirst, sodium excretion, blood volume regulation, and vasopressin secretion.