Regional differences in the morphology of the rat subfornical organ

Anatomical Organization of the Rat Subfornical Organ

The subfornical organ (SFO) is a sensory circumventricular organ located along the anterodorsal wall of the third ventricle. SFO lacks a complete blood-brain barrier (BBB), and thus peripherally-circulating factors can penetrate the SFO parenchyma. These signals are detected by local neurons providing the brain with information from the periphery to mediate central responses to humoral signals and physiological stressors. Circumventricular organs are characterized by the presence of unique populations of non-neuronal cells, such as tanycytes and fenestrated endothelium. However, how these populations are organized within the SFO is not well understood. In this study, we used histological techniques to analyze the anatomical organization of the rat SFO and examined the distribution of neurons, fenestrated and non-fenestrated vasculature, tanycytes, ependymocytes, glia cells, and pericytes within its confines. Our data show that the shell of SFO contains non-fenestrated vasculature, while fenestrated capillaries are restricted to the medial-posterior core region of the SFO and associated with a higher BBB permeability. In contrast to non-fenestrated vessels, fenestrated capillaries are encased in a scaffold created by pericytes and embedded in a network of tanycytic processes. Analysis of c-Fos expression following systemic injections of angiotensin II or hypertonic NaCl reveals distinct neuronal populations responding to these stimuli. Hypertonic NaCl activates ∼13% of SFO neurons located in the shell. Angiotensin II-sensitive neurons represent ∼35% of SFO neurons and their location varies between sexes. Our study provides a comprehensive description of the organization of diverse cellular elements within the SFO, facilitating future investigations in this important brain area.

Circumventricular organs of human brain visualized on post-contrast 3D fluid-attenuated inversion recovery imaging

PURPOSE

Although contrast-enhanced three-dimensional T2 fluid-attenuated inversion recovery (3D T2-FLAIR) images are useful for assessing various neuronal diseases, physiological enhancement of the circumventricular organs on the images have not been investigated. We aimed to assess the physiological appearance of the circumventricular organs on contrast-enhanced 3D T2-FLAIR images.

METHODS

We studied 3-T MR images of the brain of 30 individuals with no apparent brain abnormalities. In ten areas of the brain, the degree of contrast enhancement on 3D T2-FLAIR and magnetization-prepared rapid gradient-echo (MPRAGE) images was evaluated using a 4-point grading system. The pre- and post-contrast mean contrast ratios (CRs) of the anterior pituitary gland, median eminence, and pineal gland were compared.

RESULTS

On post-contrast 3D T2-FLAIR images, marked enhancement was most frequently scored in the median eminence, followed by the choroid plexus, posterior pituitary gland, and pineal gland. In 10 of the 30 cases, the vascular organ of the lamina terminalis and the area postrema were enhanced but the subcommissural organ was not. The difference in the mean pre- and post-contrast CRs of the median eminence and pineal gland was statistically significant, while that of the anterior pituitary gland was not.

CONCLUSION

On contrast-enhanced 3D T2-FLAIR images, the circumventricular organs show variable enhancement. Our findings help to recognize physiological and abnormal enhancement of brain structures on contrast-enhanced 3D T2-FLAIR images.

Mechanisms of brain renin angiotensin system-induced drinking and blood pressure: importance of the subfornical organ

It is critical for cells to maintain a homeostatic balance of water and electrolytes because disturbances can disrupt cellular function, which can lead to profound effects on the physiology of an organism. Dehydration can be classified as either intra- or extracellular, and different mechanisms have developed to restore homeostasis in response to each. Whereas the renin-angiotensin system (RAS) is important for restoring homeostasis after dehydration, the pathways mediating the responses to intra- and extracellular dehydration may differ. Thirst responses mediated through the angiotensin type 1 receptor (AT1R) and angiotensin type 2 receptors (AT2R) respond to extracellular dehydration and intracellular dehydration, respectively. Intracellular signaling factors, such as protein kinase C (PKC), reactive oxygen species (ROS), and the mitogen-activated protein (MAP) kinase pathway, mediate the effects of central angiotensin II (ANG II). Experimental evidence also demonstrates the importance of the subfornical organ (SFO) in mediating some of the fluid intake effects of central ANG II. The purpose of this review is to highlight the importance of the SFO in mediating fluid intake responses to dehydration and ANG II.

The subfornical organ: a novel site of action of cholecystokinin

The subfornical organ (SFO) is an important sensory circumventricular organ implicated in the regulation of fluid homeostasis and energy balance. We investigated whether the SFO is activated by the hormone cholecystokinin (CCK). CCK₁ and CCK₂ receptors were identified in the SFO by RT-PCR. Dissociated SFO neurons that responded to CCK (40/77), were mostly depolarized (9.2 ± 0.9 mV, 30/77), but some were hyperpolarized (-7.3 ± 1.1 mV, 10/77). We next examined the responses of SFO neurons in vivo to CCK (16 μg/kg ip), in the presence and absence of CCK₁ or CCK₂ receptor antagonists (devazepide; 600 μg/kg and L-365,260; 100 μg/kg, respectively), using the functional activation markers c-Fos and phosphorylated extracellular signal-related kinase (p-ERK). The nucleus of the solitary tract (NTS) served as a control for CCK-induced activity. There was a significant increase in c-Fos expression in the NTS (259.2 ± 20.8 neurons) compared with vehicle (47.5 ± 2.5). Similarly, in the SFO, c-Fos was expressed in 40.5 ± 10.6 neurons in CCK-treated compared with 6.6 ± 2.7 in vehicle-treated rats (P < 0.01). Devazepide significantly reduced the effects of CCK in the NTS but not in SFO. L-365,260 blocked the effects of CCK in both brain regions. CCK increased the number of p-ERK neurons in NTS (27.0 ± 4.0) as well as SFO (18.0 ± 4.0), compared with vehicle (8.0 ± 2.6 and 4.3 ± 0.6, respectively; P < 0.05). Both devazepide and L-365,260 reduced CCK-induced p-ERK in NTS, but only L-365,260 reduced it in the SFO. In conclusion, the SFO represents a novel brain region at which circulating CCK may act via CCK₂ receptors to influence central autonomic control.

Effects of acute heat exposure on prosencephalic c-Fos expression in normohydrated, water-deprived and salt-loaded rats

In the present study, the distribution pattern of c-Fos protein immunoreactivity (Fos-IR) in prosencephalic areas of the brain involved in thermoregulatory and osmoregulatory responses was investigated, in rats exposed or not exposed to a hyperthermic environment, under three different conditions: normohydration, dehydration induced by water deprivation and hyperosmolarity induced by an acute intragastric salt load. Normohydrated, water-deprived or salt-loaded male Wistar rats (270+/-30 g) were submitted or not to acute heat exposure (33 degrees C for 45 min). A separate group of animals was submitted to the same experimental protocol and had blood samples collected before and after the heating period to measure serum osmolarity and sodium. The brains were processed for c-Fos immunohistochemistry using the avidin-biotin peroxidase method. After analyzing Fos-IR in the brains of animals in the present study, three different types of prosencephalic areas were identified: (1) those that respond to hydrational and to heat conditions, with an interaction between these two factors (PaMP and SON); (2) those that respond to hydrational and to heat conditions, but with no interaction between these factors (MnPO, LSV and OVLT); and (3) those that respond only to hydrational status (SFO and PaLM).

Structure of the subfornical organ: a review

In this review, the light microscopic and fine structural characteristics of neurons, axons, dendrites, glial cells, and capillaries and their topography within the subfornical organ are summarized, with an emphasis on recent findings. Structure-function relationships are discussed whenever possible and put into perspective in a concluding section.

Enzyme histochemical studies of membrane proteases in rat subfornical organ

Localization of membrane proteases glutamyl aminopeptidase (EAP), microsomal alanyl aminopeptidase (mAAP), dipeptidyl peptidase IV (DPP IV) and gamma-glutamyl transpeptidase (gamma-GTP) were studied in vessels of the rat subfornical organ (SFO), ependyma which cover the surface of the SFO, and adjacent brain structures. Results of enzyme histochemical reactions showed strong activity for EAP, mAAP, and gamma-GTP, but absence of DPP IV in microvessels of SFO. The ependyma which cover the SFO was positive for gamma-GTP, but negative for other studied proteases. Our results showed that the spectrum of enzymes in the majority of the vessels of SFO is similar to that of the microvessels of the adjacent brain tissue which were positive for EAP, mAAP, and gamma-GTP, but negative for DPP IV. The relative intensity of the enzyme reactions in vessels varied from central to lateral locations in the SFO and the adjacent brain tissue. There was also a difference in the relative reaction intensity from one enzyme to the other. The presence and heterogeneous distribution of the enzymes are consistent with the hypothesis that membrane proteases of the microvascular endothelium constitute an enzyme-barrier between blood and parenchyma of the SFO and between blood and brain tissue, and may be involved in metabolism or modulation of various peptides when they contact the plasma membrane of the endothelial cells of the vessels.

Water deprivation and subfornical organ activity in the pigeon a [14C]2-deoxyglucose study

Following varying degrees of water deprivation (0, 24 and 72 h), functional activity in the subfornical organ (SFO) of pigeons was measured using the [14C]2-deoxyglucose method. Increasing levels of water deprivation produced a significant increase in glucose uptake in SFO. The magnitude of the effect was systematically correlated with morphologically defined SFO subdivisions.

Timetables of cytogenesis in the rat subfornical organ

Timetables of neurogenesis and ependymal cell production in the rat subfornical organ (SFO) were determined by examining the offspring of pregnant rats injected with [3H]thymidine on E13-E14, E14-E156, ... E21-E22, respectively. The proportion of postmitotic cells originating each embryonic day was determined by analyzing, in the adult offspring, the progressive reduction in the proportion of labeled precursors from the maximum amount seen in the E13-E14 group. Neurogenesis was found to occur over an extended period of time, beginning on E12 and continuing through E21. Ependymal cells were generated E15 through E21. Both neuron and ependymal cell production occurred in a triphasic pattern and followed an anterior (older) to posterior (younger) gradient. The anterior to posterior production gradient may be related to the morphological variation which exists along this plane. A production gradient intrinsic to a particular levels was found only in the posterior SFO, where peripheral neurons form earlier than core neurons. That neurogenetic gradient may be related to the core-periphery topographical patterns found in other studies, and suggests that the core neurons, since they are among the last to be formed, may be interneurons.

Focal metabolic effects of angiotensin and captopril on subregions of the rat subfornical organ

Angiotensin infusion increased glucose metabolism in 4 of 7 subdivisions of the rat subfornical organ, the effect being stronger in ventromedial compared to dorsolateral zones across the rostrocaudal axis. [Sar1-Leu8]Angiotensin II attenuated metabolic responses to intravenous angiotensin in all subfornical organ subregions. Brattleboro rats, having high circulating levels of angiotensin, displayed greater rates of glucose metabolism than Long-Evans rats in all subregions, differences that were eliminated by captopril, an inhibitor of angiotensin converting enzyme. The studies reveal focal subfornical organ zones where in vivo metabolic activity corresponds to cytoarchitectonic evidence for topographical processing within this angiotensin-sensitive structure.

Quantitative fine structure of capillaries in subregions of the rat subfornical organ

The differentiated cytology across subregions of the rat subfornical organ (SFO) prompted our hypothesis that ultrastructural features of capillary endothelial cells would vary topographically and quantitatively within this small nucleus. We used electron microscopic and computer-based morphometric methods to assess fine structural dimensions of the capillary endothelium in four distinct subregions of the SFO from Long-Evans and homozygous Brattleboro rats. Three types of capillary were present. Type III capillaries (resembling those of endocrine glands) had an average wall thickness of 0.17 microns, 54% thinner than those of Type I and II capillaries. Pericapillary spaces around Type III capillaries measured 56 microns2, 100% larger than for Type I vessels (resembling those of skeletal muscle). Only Type III capillaries contained fenestrations (9 per microns2 of endothelial cell) and were the predominant type of capillary in central and caudal subregions of the SFO. Type I capillaries, prevalent in the transitional subregion between the central and rostral parts of the SFO, had 10 cytoplasmic vesicles per micron2 of endothelial cell area, a number not different from that of Type III capillaries but 3x the frequency found in Type II vessels. Type II capillaries (those typical of "blood-brain barrier" endothelium) had low vesicular density (3 per microns2), no fenestrations, and no pericapillary spaces. Luminal diameters and the densities of mitochondria and intercellular junctions were not different among capillary types or subregions in the SFO. Furthermore, there were no morphometric differences for any capillary dimensions between Long-Evans and Brattleboro rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Topography and morphometry of capillaries in the rat subfornical organ

A comprehensive stereological analysis was performed to define capillary dimensions in individual subregions of the subfornical organ in Long-Evans, homozygous Brattleboro, and Sprague-Dawley rats. Capillary density, volume fraction, length, surface area, and diameter were assessed in four regions in the sagittal plane (rostral, "transitional," central, and caudal) and two zones in the coronal plane (dorsal and ventromedial). The ventromedial zones in the central and caudal regions correspond to areas of dense perikarya and neuropil containing neural afferent inputs to the subfornical organ (e.g., putative fiber terminals for angiotensin II), whereas the dorsal zones of these regions are apparently the predominant sites of perikarya having efferent projections directed outside of the organ. The morphometric analysis revealed heterogeneous capillary density across subregions of the subfornical organ (range of 132 to 931 capillaries/mm2 in the three rat groups). Capillaries in the ventromedial zones of the central and caudal regions had significantly greater density, volume fraction, and surface area, but smaller diameters, than those in the adjacent dorsal zones and more rostral regions. Across all subregions within the dorsal zone, there was generally a consistent morphometric pattern in the three rat groups. No differences in capillary dimensions in any part of the subfornical organ were found between the Long-Evans and Brattleboro rats. A qualitative electron microscopic investigation of endothelial cells in each subregion of the subfornical organ in Long-Evans rats revealed at least three types of capillary oriented according to region: in the rostral region were capillaries having no endothelial fenestrations or pericapillary spaces, and few vesicles, in the "transitional" region between the rostral and central regions, capillaries having no endothelial fenestrations, substantial numbers of vesicles, and narrow but perceptible pericapillary spaces were found, and in the central and caudal regions, capillaries having abundant endothelial fenestrations and vesicles, expansive pericapillary labyrinths, and relatively thin walls were present. These findings from light microscopic morphometry and electron microscopy in rats indicate a heterogeneity of capillary organization that shows topographical correspondence to the cytology and putative functions of the subfornical organ.

Angiotensin II sensitive neurons in the supraoptic nucleus, subfornical organ and anteroventral third ventricle of rats in vitro

The angiotensin II (AII) sensitivity of neurons in the supraoptic nucleus (SON), subfornical organ (SFO) and the region near the anteroventral part of the third ventricle (AV3V) was investigated using extracellular recording in the rat brain slice preparation by adding AII (10(-10)-10(-6) M) to the perfusion medium. Forty seven (44%) of 106 SON neurons, 62 (66%) of 94 SFO neurons and 28 (33%) of 86 AV3V neurons were excited by AII. One cell was inhibited by AII in the SON and one in the SFO. The threshold concentration to evoke responses in the SON neurons was approximately 10(-9) M, but neurons in the SFO and AV3V showed clear excitatory responses to AII at 10(-10) M. In the SON, 18 (40%) of 45 phasic firing neurons (putative vasopressin neurons) and 29 (48%) of 61 nonphasic firing neurons (including putative oxytocin neurons) were excited by AII. The excitatory effect of AII was reversibly antagonized by a specific antagonist saralasin and persisted after synaptic blockade in medium with low [Ca2+] and high [Mg2+]. We conclude that AII can stimulate both vasopressin and oxytocin release, acting directly upon SON neurons and also that both the SFO and AV3V are important receptive sites for AII (although the SFO is relatively more sensitive) which contributes SON input and modulates release of these hormones.

Axoglial contacts in the area postrema of the cat: an ultrastructural study

Axoglial contacts were observed in an ultrastructural study of the area postrema of the cat. According to the disposition of the electron-dense projections attached to the adjoining membranes these contacts were classified as symmetrical or asymmetrical. The axon profiles contained aggregations of clear vesicles randomly distributed or grouped in cluster adjacent to the electron-dense projections. Dense core vesicles were occasionally seen. The neuroglial profiles were either astrocytic or ependymoglial in nature. The astrocytes showed a clear cytoplasm, polymorphous vesicles, mitochondria, glycogen granules, and bundles of filaments. The ependymal cells, in contrast, had a more electron-dense and granular appearance, tubular structures, irregular vesicular formations, profiles of smooth reticuloendoplasm, and filaments grouped in bundles or isolated in the cytoplasm. The possibility that these contacts might play a role in the chemical transfer from neurons to glial cells is discussed on the basis of existing biochemical data.

Fine structural organization of the subfornical organ. A concise review

This review of the subfornical organ, with special emphasis on the rat, summarizes the fine structural characteristics of the capillaries, the access route for blood-borne substances, the ependyma through which cerebrospinal fluid-borne substances penetrate the organ, neuronal perikarya, and types of synapses and axons, together with a brief discussion of the principal as yet unresolved problems.

Vasopressin release to central and peripheral angiotensin II in rats with lesions of the subfornical organ

Angiotensin II (Ang II), peripherally or centrally administered, increases plasma vasopressin concentrations in the rat. Peripherally injected Ang II was unable to effect the release of vasopressin in rats with subfornical organ (SFO) lesions. In contrast, a normal increase of plasma vasopressin levels was induced by centrally injected Ang II. These results suggest that peripherally administered Ang II elicits antidiuretic hormone (ADH) release by stimulating receptors in the SFO, whereas centrally administered Ang II acts at receptors outside the SFO.

Surface morphology of the subfornical organ: effects of low and high sodium chloride diet

Ependymal cells found in the subfornical organ of the rat were counted. Cells covered by small microvilli, small protrusions and smooth cells were frequently found. Also present were cells with long or short cilia, cels with large protrusions and supraependymal cells. High and low sodium diets reduced the number of cells with large protrusions. Microvilli-covered cells increased after a low sodium diet.

Circumventricular organs: receptors and mediators of direct peptide hormone action on brain

The concept of the brain as an endocrine target organ is not new, nor is it novel to consider the circumventricular organs as receptive regions of the brain for circulating substances. However, in this review we have emphasized the relatively novel concept that CVOs mediate exclusively the direct feedback actions of circulating peptide hormones on brain function. In addition we have presented speculations concerning the neural mechanisms by which signals arising from peptide hormone-receptor interaction might be relayed into the CNS, and indicated the possible involvement of CVO receptors in endocrine disorders. We hope that this analysis provides a conceptual framework for evaluating the functional relationship of circulating peptides to brain and inspires interest in this fascinating area of neuroendocrinology.

The efferent projections of the subfornical organ of the rat: a circumventricular organ within a neural network subserving water balance

The efferent projections of the subfornical organ (SFO) of rats were traced using the autoradiographic method of following anterograde transport of labelled proteins through axons. The efferents of the SFO go to two different areas. The first is the anteroventral third ventricular area of the preoptic region and the second is the hypothalamus particularly the neurosecretory, magnocellular nuclei. Specifically, the apparent terminal fields in the first area are in the nucleus medianus of the medial preoptic area (NM), the organum vasculosum of the lamina terminalis (OVLT), and the anterior periventricular area (PeV). Many efferent fibers to this area emerge from the rostral SFO, pass anteriorly over the anterior commissure in the midline and either descend along the anterior border of the NM or enter the PeV dorsally just beneath the anterior commissure. The apparent terminal fields within the hypothalamus are in the anterior and tuberal supraoptic nuclei (SONa and SONt), the paraventricular nucleus (PVN) including its rostral accessory cluster, the nucleus circularis (NC), the dorsal perifornical area (PFd), and in both the lateral preoptic area and lateral hypothalamus adjacent to the SON. Many efferent fibers to the hypothalamus emerge from the rostral SFO and enter the columns of the fornix, diverge with the ventral stria medullari to disperse medially and laterally over the columns of the fornix and along their dorsal border at the anterior dorsal level of the columns trajectory through the hypothalamus. These findings are discussed in terms of the SFO's role within a neural network mediating water balance behaviorally and physiologically.