Changes in pericytic expression of NG2 and PDGFRB and vascular permeability in the sensory circumventricular organs of adult mouse by osmotic stimulation.

The blood-brain barrier (BBB) is a barrier that prevents free access of blood-derived substances to the brain through the tight junctions and maintains a specialized brain environment. Circumventricular organs (CVOs) lack the typical BBB. The fenestrated vasculature of the sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO) and area postrema (AP), allows parenchyma cells to sense a variety of blood-derived information, including osmotic ones. In the present study, we utilized immunohistochemistry to examine changes in the expression of NG2 and platelet-derived growth factor receptor beta (PDGFRB) in the OVLT, SFO and AP of adult mice during chronic osmotic stimulation. The expression of NG2 and PDGFRB was remarkably prominent in pericytes, although these angiogenesis-associated proteins are highly expressed at pericytes of developing immature vasculature. The chronic salt loading prominently increased the expression of NG2 in the OVLT and SFO and that of PDGFRB in the OVLT, SFO and AP. The vascular permeability of low-molecular-mass tracer fluorescein isothiocyanate was increased significantly by chronic salt loading in the OVLT and SFO but not AP. In conclusion, the present study demonstrates changes in pericyte expression of NG2 and PDGFRB and vascular permeability in the sensory CVOs by chronic osmotic stimulation, indicating active participation of the vascular system in osmotic homeostasis.

ENaC-expressing neurons in the sensory circumventricular organs become c-Fos activated following systemic sodium changes.

The sensory circumventricular organs (CVOs) are specialized collections of neurons and glia that lie in the midline of the third and fourth ventricles of the brain, lack a blood-brain barrier, and function as chemosensors, sampling both the cerebrospinal fluid and plasma. These structures, which include the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), are sensitive to changes in sodium concentration but the cellular mechanisms involved remain unknown. Epithelial sodium channel (ENaC)-expressing neurons of the CVOs may be involved in this process. Here we demonstrate with immunohistochemical and in situ hybridization methods that ENaC-expressing neurons are densely concentrated in the sensory CVOs. These neurons become c-Fos activated, a marker for neuronal activity, after various manipulations of peripheral levels of sodium including systemic injections with hypertonic saline, dietary sodium deprivation, and sodium repletion after prolonged sodium deprivation. The increases seen c-Fos activity in the CVOs were correlated with parallel increases in plasma sodium levels. Since ENaCs play a central role in sodium reabsorption in kidney and other epithelia, we present a hypothesis here suggesting that these channels may also serve a related function in the CVOs. ENaCs could be a significant factor in modulating CVO neuronal activity by controlling the magnitude of sodium permeability in neurons. Hence, some of the same circulating hormones controlling ENaC expression in kidney, such as angiotensin II and atrial natriuretic peptide, may coordinate ENaC expression in sensory CVO neurons and could potentially orchestrate sodium appetite, osmoregulation, and vasomotor sympathetic drive.

The viral mimetic polyinosinic:polycytidylic acid (poly I:C) induces cellular responses in primary cultures from rat brain sites with an incomplete blood-brain barrier.

Primary microcultures of the organum vasculosum laminae terminalis (OVLT) and the area postrema (AP), brain sites with an incomplete blood-brain barrier, were established from topographically excised rat pup tissue, with cellular identification by marker protein-specific immunocytochemistry. Employing the ratio calcium imaging technique, we showed for the first time that polyinosinic:polycytidylic acid (poly I:C) can induce calcium signalling in single OVLT and AP cells. Poly I:C stimulation caused fast, transient rises in intracellular calcium in about 5% of neurons and astrocytes and some microglial cells. Frequently, the responses of astrocytes and microglial cells showed a shorter onset-latency compared to neurons. In addition, exposure to poly I:C led to a time dependent release of bioactive tumour necrosis factor (TNF) and interleukin-6 (IL-6) into the supernatants of OVLT and AP cultures. The demonstration of direct cellular responses of OVLT- and AP-intrinsic cells to stimulations with poly I:C is in agreement with the discovered existence of Toll-like receptor 3 (TLR3), the cognate receptor for poly I:C, in the brain.

The serotonergic system in fish.

Neurons using serotonin (5-HT) as neurotransmitter and/or modulator have been identified in the central nervous system in representatives from all vertebrate clades, including jawless, cartilaginous and ray-finned fishes. The aim of this review is to summarize our current knowledge about the anatomical organization of the central serotonergic system in fishes. Furthermore, selected key functions of 5-HT will be described. The main focus will be the adult brain of teleosts, in particular zebrafish, which is increasingly used as a model organism. It is used to answer not only genetic and developmental biology questions, but also issues concerning physiology, behavior and the underlying neuronal networks. The many evolutionary conserved features of zebrafish combined with the ever increasing number of genetic tools and its practical advantages promise great possibilities to increase our understanding of the serotonergic system. Further, comparative studies including several vertebrate species will provide us with interesting insights into the evolution of this important neurotransmitter system.

Sensory circumventricular organs in health and disease.

Circumventricular organs (CVOs) are specialized brain structures located around the third and fourth ventricles. They differ from the rest of the brain parenchyma in that they are highly vascularised areas that lack a blood-brain barrier. These neurohaemal organs are classified as "sensory", when they contain neurons that can receive chemical inputs from the bloodstream. This review focuses on the sensory CVOs to describe their unique structure, and their functional roles in the maintenance of body fluid homeostasis and cardiovascular regulation, and in the generation of central acute immune and febrile responses. In doing so, the main neural connections to visceral regulatory centres such as the hypothalamus, the medulla oblongata and the endocrine hypothalamic-pituitary axis, as well as some of the relevant chemical substances involved, are described. The CVOs are vulnerable to circulating pathogens and can be portals for their entry in the brain. This review highlights recent investigations that show that the CVOs and related structures are involved in pathological conditions such as sepsis, stress, trypanosomiasis, autoimmune encephalitis, systemic amyloidosis and prion infections, while detailed information on their role in other neurodegenerative diseases such as Alzheimer's disease or multiple sclerosis is lacking. It is concluded that studies of the CVOs and related structures may help in the early diagnosis and treatment of such disorders.

Induction of Fos immunoreactivity labeling in rat forebrain metabolic loci by caudal fourth ventricular infusion of the monocarboxylate transporter inhibitor, alpha-cyano-4-hydroxycinnamic acid.

Caudal fourth ventricular (CV4) infusion of the monocarboxylate transporter inhibitor, alpha-cyano-4-hydroxycinnamic acid (4CIN), causes hyperglycemia coincident with Fos expression in the hindbrain nucleus tractus solitarius, a rare central source of metabolic deficit signaling. The present studies examined the hypothesis that hindbrain lactoprivic signaling activates central autonomic pathways that regulate systemic glucostasis by examining the effects of this drug treatment paradigm on patterns of Fos expression in forebrain structures that integrate sensory input from metabolic sensors and coordinate motor responses to energy shortages. Two hours after CV4 infusion of graded doses of 4CIN or vehicle alone, adult female rats were sacrificed by transcardial perfusion and sections through the telencephalic and diencephalic metabolic loci were processed for Fos immunoreactivity (-ir). Fos labeling of the hypothalamic paraventricular (PVH), dorsomedial (DMH), and ventromedial (VMH) nuclei was significantly elevated, relative to the vehicle-treated controls, in response to the lowest dose of 4CIN, e.g. 10 microg/animal. Treatment with higher doses of 4CIN (25 or 50 microg) further augmented numbers of Fos-ir-positive neurons in these structures, and also elicited staining of the bed nuclei of the stria terminalis (BST), medial preoptic (MPN), arcuate (ARH), supraoptic (SO), and anterior hypothalamic nuclei (AHN), and lateral hypothalamic area (LHA). Mean numbers of Fos-immunolabeled neurons in the ARH, DMH, LHA, AHN, MPN, and SO were not different between animals infused with 25 versus 50 microg 4CIN, whereas neuronal labeling in the VMH, BST, and PVH was significantly greater in the high- versus the middle-dose groups. The present data show that pharmacological inhibition of lactate uptake within the caudal hindbrain results in dose-dependent neuronal Fos immunoexpression within characterized forebrain components of the central metabolic circuitry, and that these patterns of neuronal transcriptional activation parallel observed drug effects on blood glucose levels. These results suggest that lactoprivic signaling by metabolic 'sensing' neurons in the caudal hindbrain initiates central neural mechanisms that control systemic energy availability, and that local lactate-'sensitive' neurons are connected neuroanatomically with principal higher-order autonomic metabolic loci that regulate glucostasis.

CART in the dorsal vagal complex: sources of immunoreactivity and effects on Fos expression and food intake.

CART-peptide (CARTp) has been shown to suppress food intake, particularly when injected into the 4th ventricle of rats, and the presence of CART in nodose ganglia suggested a role in satiation. Based on retrograde tracing from the DVC combined with CART immunohistochemistry and supranodose vagotomy, we found that CART immunoreactivity in varicose fibers of the dorsal vagal complex originates from vagal afferents, sparse projections from the medullary reticular formation and the arcuate/retrochiasmatic nucleus of the hypothalamus, and most likely also from local CART neurons in the area postrema and NTS. In the nodose ganglia, 17% of neurons with projections to the stomach and 41% to the duodenum express CART-IR. CART-IR vagal afferents significantly contribute to the rich fiber plexus in mainly the commissural NTS and the adjacent area postrema. Injections of CARTp into the 4th ventricle strongly suppressed sucrose drinking and stimulated expression of c-Fos in the NTS. Injections of CARTp directly into various subnuclei of the NTS were less effective in suppressing food intake. The findings suggest that the critical site for CART's suppression of food intake is not in the termination zone of CART-containing vagal afferents in the commissural NTS, and that CART release from vagal afferent terminals plays a minor role in satiation. The functional role of CART in vagal afferents and the site of food intake suppression by 4th ventricular CARTp remain to be determined.