Subfornical organ

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Subfornical organ
Medial aspect of a brain sectioned in the median sagittal plane. (Subfornical organ not labeled, but fornix and foramen of Monro are both labeled near the center.)
Details
Identifiers
Latinorganum subfornicale
MeSHD013356
NeuroLex IDnlx_anat_100314
TA98A14.1.08.412
A14.1.09.449
TA25782
FMA75260
Anatomical terms of neuroanatomy

The subfornical organ (SFO) is one of the

neurotransmitters, as opposed to secretory circumventricular organs, which are specialized in the release of certain substances.[1][4][5]

Anatomy

photomicrograph, the subfornical organ (arrow) is located on the undersurface of the fornix in the upper part of the third ventricle. The cells in this coronal section of the brain were colored with a bluish dye ("Nissl stain"). The thalamus
is at the bottom of the photo. The bar at the lower right represents a distance of 200 μm (0.2mm).

As noted above, capillaries in some subregions within the SFO are fenestrated,

glial cells, neuronal cell bodies and a high density of fenestrated capillaries.[8] Conversely, the rostral and caudal areas have a lower density of capillaries[8] 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.[9]

The subfornical organ contains endothelin receptors mediating vasoconstriction and high rates of glucose metabolism mediated by calcium channels.[10]

General function

The subfornical organ is active in many bodily processes,[1][5] including osmoregulation,[9] cardiovascular regulation,[9] and energy homeostasis.[1][5] Most of these processes involve fluid balance through the control of the release of certain hormones, particularly angiotensin or vasopressin.[5]

Cardiovascular regulation

The impact of the SFO on the

baroreceptors, and can in turn affect the strength of ventricular contraction in the heart. 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.[1]

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.[5]

Relationship with other circumventricular organs

Other circumventricular organs participating in systemic regulatory processes are the

blood volume regulation, and vasopressin secretion.[1][5] The SFO, area postrema, and OVLT have capillaries permeable to circulating hormonal signals, enabling these three circumventricular organs to have integrative roles in cardiovascular, electrolyte, and fluid regulation.[1][5][8]

Hormones and receptors

nucleus medianus which is involved in controlling thirst. Thus, the subfornical organ is involved in fluid balance.[citation needed
]

Other important hormones have been shown to excite the SFO, specifically

neurotransmitters however seem to have an effect on deeper areas of the SFO than angiotensin, and antagonists of these hormones have been shown to also primarily effect the non-superficial regions of the SFO (other than atropine antagonists, which showed little effects). In this context, the superficial region is considered to be 15-55μm deep into the SFO, and the "deep" region anything below that.[citation needed
]

From these reactions to certain hormones and other molecules, a model of the neuronal organization of the SFO is suggested in which angiotensin-sensitive neurons lying superficially are excited by substances borne by blood or

inhibitory circuit is suggested on the output path.[5]

Genetics

The expression of various

mRNA that codes for angiotensin II receptors, allowing for a lower angiotensin concentration in the blood that produce the "thirst" response. It also has been observed to be a site of thyroid transcription factor 1 (TTF1) production, a protein generally produced in the hypothalamus.[11]

Pathology

Hypertension

Hypertension, or high blood pressure, is highly affected by the concentration of angiotensin. Injection of angiontensin has actually been long used to induce hypertension in animal test models to study the effects of various therapies and medications. In such experiments, it has been observed that an intact and functioning subfornical organ limits the increase in mean arterial pressure due to the increased angiotensin.[12]

Dehydration

As stated above, angiotensin receptors (AT1) have been shown to be upregulated due to water deprivation. These AT1 receptors have also shown an increased bonding with circulating angiotensin after water deprivation. These findings could indicate some sort of morphological change in the AT1 receptor, likely due to some signal protein modification of the AT1 receptor at a non-bonding site, leading to an increased affinity of the AT1 receptor for angiotensin bonding.[13]

Research

Feeding

Although generally viewed primarily as having roles in

peptides indicating satiety) and then stimulating hunger. It has been shown to induce drinking in rats as well as eating.[5]

References

  1. ^
    PMID 2891718
    .
  2. ^
    ISBN 978-0-12-547635-5
    .
  3. ^
    S2CID 26102264
    .
  4. ^
    PMID 26578857
    .
  5. ^
    S2CID 58947441
    .
  6. S2CID 38665940
    .
  7. ^
    PMID 1410407
    .
  8. ^
    PMID 1954559
    .
  9. ^
    S2CID 29552630
    .
  10. S2CID 12713010
    .
  11. PMID 12730191
    .
  12. PMID 3971518
    .
  13. PMID 9252492
    .


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