Suppression of TRPV4 channels ameliorates anti-dipsogenic effects under hypoxia in the subfornical organ of rats

Central regulation of body fluid homeostasis

Extracellular fluids, including blood, lymphatic fluid, and cerebrospinal fluid, are collectively called body fluids. The Na⁺ concentration ([Na⁺]) in body fluids is maintained at 135-145 mM and is broadly conserved among terrestrial animals. Homeostatic osmoregulation by Na⁺ is vital for life because severe hyper- or hypotonicity elicits irreversible organ damage and lethal neurological trauma. To achieve "body fluid homeostasis" or "Na homeostasis", the brain continuously monitors [Na⁺] in body fluids and controls water/salt intake and water/salt excretion by the kidneys. These physiological functions are primarily regulated based on information on [Na⁺] and relevant circulating hormones, such as angiotensin II, aldosterone, and vasopressin. In this review, we discuss sensing mechanisms for [Na⁺] and hormones in the brain that control water/salt intake behaviors, together with the responsible sensors (receptors) and relevant neural pathways. We also describe mechanisms in the brain by which [Na⁺] increases in body fluids activate the sympathetic neural activity leading to hypertension.

Compound K induces neurogenesis of neural stem cells in thrombin induced nerve injury through LXRα signaling in mice

Intracerebral hemorrhage (ICH) causes neurological function deficit due to the loss of neurons surrounding the hematoma. Increased neurogenesis of endogenous neural stem cells (EnNSCs) is believed to increase cell proliferation and differentiation, thereby improving the neurological deficit. However, there are still limited drugs that are effective for treating neurological deficit. So, the effects of compound K (CK) in EnNSCs were measured after thrombin-induced mice models both in vivo and in vitro, and investigated the probable mechanisms of CK during pro-neurogenesis. The results revealed that 10 μM CK promotes neurogenesis, proliferation and reduces apoptosis of EnNSCs after induction by thrombin. After that, CK treatment increased the neurogenesis of EnNSCs through liver X receptor α (LXRα) signaling pathway using adeno-associated virus knockdown and knocked out mice of LXRα gene. Finally, intraperitoneal injection of 10 mg/kg CK improved the neurogenesis of subventricular zone (SVZ), myelin repair and behavioral deficit after stereotaxic injection of thrombin in the basal ganglia of mice, and this process involved LXRα. These observations provided evidence regarding the effect of CK in pro-neurogenesis via LXRα activation, and suggested further evaluation of it due to its potential role as an effective modulator in the treatment of ICH.

The interaction of SiO2 nanoparticles with the neuronal cell membrane: activation of ionic channels and calcium influx

AIM

To clarify the mechanisms of interaction between SiO₂ nanoparticles (NPs) and the plasma membrane of GT1-7 neuroendocrine cells, with focus on the activation of calcium-permeable channels, responsible for the long lasting calcium influx and modulation of the electrical activity in these cells.

MATERIALS & METHODS

Nontoxic doses of SiO₂ NPs were administered to the cells. Calcium imaging and patch clamp techniques were combined with a pharmacological approach.

RESULTS

TRPV4, Cx and Panx-like channels are the major components of the NP-induced inward currents. Preincubation with the antioxidant N-acetyl-L-cysteine strongly reduced the [Ca²⁺]_i increase.

CONCLUSION

These findings suggest that SiO₂ NPs directly activate a complex set of calcium-permeable channels, possibly by catalyzing free radical production.

Effects of altitude changes on mild-to-moderate closed-head injury in rats following acute high-altitude exposure

Mild-to-moderate closed-head injury (mmCHI) is an acute disease induced by high-altitudes. It is general practice to transfer patients to lower altitudes for treatment, but the pathophysiological changes at different altitudes following mmCHI remain unknown. The present study simulated acute high-altitude exposure (6,000 m above sea level) in rats to establish a model of mmCHI and recorded their vital signs. The rats were then randomly assigned into different altitude exposure groups (6,000, 4,500 and 3,000 m) and neurological severity score (NSS), body weight (BW), brain magnetic resonance imaging (MRI), brain water content (BWC) and the ratio of BW/BWC at 6, 12 and 24 h following mmCHI, and the glial fibrillary acidic protein levels were analysed in all groups. The results revealed that within the first 24 h following acute high-altitude exposure, mmCHI induced dehydration, brain oedema and neuronal damage. Brain injury in rats was significantly reversed following descent to 4,500 m compared with the results from 6,000 or 3,000 m. The results indicated that subjects should be transported as early as possible. Furthermore, avoiding large-span descent altitude was beneficial to reduce neurological impairment. The examination of brain-specific biomarkers and MRI may further be useful in determining the prognosis of high-altitude mmCHI. These results may provide guidance for rescuing high altitude injuries.

Ionic mechanisms underlying tonic and burst firing behavior in subfornical organ neurons: a combined experimental and modeling study

Subfornical organ (SFO) neurons exhibit heterogeneity in current expression and spiking behavior, where the two major spiking phenotypes appear as tonic and burst firing. Insight into the mechanisms behind this heterogeneity is critical for understanding how the SFO, a sensory circumventricular organ, integrates and selectively influences physiological function. To integrate efficient methods for studying this heterogeneity, we built a single-compartment, Hodgkin-Huxley-type model of an SFO neuron that is parameterized by SFO-specific in vitro patch-clamp data. The model accounts for the membrane potential distribution and spike train variability of both tonic and burst firing SFO neurons. Analysis of model dynamics confirms that a persistent Na⁺ and Ca²⁺ currents are required for burst initiation and maintenance and suggests that a slow-activating K⁺ current may be responsible for burst termination in SFO neurons. Additionally, the model suggests that heterogeneity in current expression and subsequent influence on spike afterpotential underlie the behavioral differences between tonic and burst firing SFO neurons. Future use of this model in coordination with single neuron patch-clamp electrophysiology provides a platform for explaining and predicting the response of SFO neurons to various combinations of circulating signals, thus elucidating the mechanisms underlying physiological signal integration within the SFO. NEW & NOTEWORTHY Our understanding of how the subfornical organ (SFO) selectively influences autonomic nervous system function remains incomplete but theoretically results from the electrical responses of SFO neurons to physiologically important signals. We have built a computational model of SFO neurons, derived from and supported by experimental data, which explains how SFO neurons produce different electrical patterns. The model provides an efficient system to theoretically and experimentally explore how changes in the essential features of SFO neurons affect their electrical activity.

The subfornical organ: A novel site for prolactin action

Prolactin (PRL) is a peptide hormone that performs over 300 biological functions, including those that require binding to prolactin receptor (PRL-R) in neurones within the central nervous system (CNS). To enter the CNS, circulating PRL must overcome the blood-brain barrier. Accordingly, areas of the brain that do not possess a blood-brain barrier, such as the subfornical organ (SFO), are optimally positioned to interact with systemic PRL. The SFO has been classically implicated in energy and fluid homeostasis but has the potential to influence oestrous cyclicity and gonadotrophin release, which are also functions of PRL. We aimed to confirm and characterise the expression of PRL-R in the SFO, as well as identify the effects of PRL application on membrane excitability of dissociated SFO neurones. Using a quantitative real-time polymerase chain reaction, we found that PRL-R mRNA in the SFO of male and female Sprague Dawley rats did not significantly differ between juvenile and sexually mature rats (P = .34), male and female rats (P = .97) or across the oestrous cycle (P = .54). Patch-clamp recordings were obtained in juvenile male rats to further investigate the actions of PRL at the SFO. Dissociated SFO neurones perfused with 1 μmol L^-1 PRL resulted in 2 responsive subpopulations of neurones; 40% depolarised (n = 15/43, 11.3 ± 1.7 mV) and 14% hyperpolarised (n = 6/43, -6.7 ± 1.4 mV) to PRL application. Within the range of 10 pmol L^-1 to 1 μmol L^-1 , the concentrations of PRL were not significantly different in either the magnitude (P = .53) or proportion (P = .19) of response. Furthermore, PRL application significantly reduced the transient K⁺ current in 67% of SFO neurones in voltage-clamp configuration (n = 6/9, P = .02). The stability in response to PRL and expression of PRL-R in the SFO suggests that PRL function is conserved across physiological states and circulating PRL concentrations, prompting further investigations aiming to clarify the nature of PRL function in the SFO.

Adropin Is a Key Mediator of Hypoxia Induced Anti-Dipsogenic Effects via TRPV4-CamKK-AMPK Signaling in the Circumventricular Organs of Rats

Water intake reduction (anti-dipsogenic effects) under hypoxia has been well established, but the underlying reason remains unknown. Our previous report indicated that activated TRPV4 neurons in SFO are associated with anti-dipsogenic effects under hypoxia. Although low partial pressure of blood oxygen directly activates TRPV4, humoral factors could also be involved. In the present study, we hypothesize that adropin, a new endogenous peptide hormone, was rapidly increased (serum and brain) concomitant with reduced water intake in early hypoxia. Also, the nuclear expression of c-Fos, a marker for neuronal activation, related to water-consumption (SFO and MnPO) was inhibited. These effects were mitigated by a scavenger, rat adropin neutralizing antibody, which effectively neutralized adropin under hypoxia. Interestingly, injection of recombinant adropin in the third ventricle of the rats also triggered anti-dipsogenic effects and reduced c-Fos positive cells in SFO, but these effects were absent when TRPV4 was knocked down by shRNA. Moreover, adropin-activated CamKK-AMPK signaling related to TRPV4 calcium channel in SFO in normoxia. These results revealed that dissociative adropin was elevated in acute hypoxia, which was responsible for anti-dipsogenic effects by altering TRPV4-CamKK-AMPK signaling in SFO.