Negative feedforward control of body fluid homeostasis by hepatorenal reflex. Hypertens Res. 2011;34:895-905. [PMID: 21716295 DOI: 10.1038/hr.2011.88]

Neuroimmune modulation in liver pathophysiology

The liver, the largest organ in the human body, plays a multifaceted role in digestion, coagulation, synthesis, metabolism, detoxification, and immune defense. Changes in liver function often coincide with disruptions in both the central and peripheral nervous systems. The intricate interplay between the nervous and immune systems is vital for maintaining tissue balance and combating diseases. Signaling molecules and pathways, including cytokines, inflammatory mediators, neuropeptides, neurotransmitters, chemoreceptors, and neural pathways, facilitate this complex communication. They establish feedback loops among diverse immune cell populations and the central, peripheral, sympathetic, parasympathetic, and enteric nervous systems within the liver. In this concise review, we provide an overview of the structural and compositional aspects of the hepatic neural and immune systems. We further explore the molecular mechanisms and pathways that govern neuroimmune communication, highlighting their significance in liver pathology. Finally, we summarize the current clinical implications of therapeutic approaches targeting neuroimmune interactions and present prospects for future research in this area.

Physiological Mechanisms of Dietary Salt Sensing in the Brain, Kidney, and Gastrointestinal Tract

Excess dietary salt (NaCl) intake is strongly correlated with cardiovascular disease and is a major contributing factor to the pathogenesis of hypertension. NaCl-sensitive hypertension is a multisystem disorder that involves renal dysfunction, vascular abnormalities, and neurogenically-mediated increases in peripheral resistance. Despite a major research focus on organ systems and these effector mechanisms causing NaCl-induced increases in arterial blood pressure, relatively less research has been directed at elucidating how NaCl is sensed by various tissues to elicit these downstream effects. The purpose of this review is to discuss how the brain, kidney, and gastrointestinal tract sense NaCl including key cell types, the role of NaCl versus osmolality, and the underlying molecular and electrochemical mechanisms.

Two parabrachial Cck neurons involved in the feedback control of thirst or salt appetite

Thirst and salt appetite are temporarily suppressed after water and salt ingestion, respectively, before absorption; however, the underlying neural mechanisms remain unclear. The parabrachial nucleus (PBN) is the relay center of ingestion signals from the digestive organs. We herein identify two distinct neuronal populations expressing cholecystokinin (Cck) mRNA in the lateral PBN that are activated in response to water and salt intake, respectively. The two Cck neurons in the dorsal-lateral compartment of the PBN project to the median preoptic nucleus and ventral part of the bed nucleus of the stria terminalis, respectively. The optogenetic stimulation of respective Cck neurons suppresses thirst or salt appetite under water- or salt-depleted conditions. The combination of optogenetics and in vivo Ca2+ imaging during ingestion reveals that both Cck neurons control GABAergic neurons in their target nuclei. These findings provide the feedback mechanisms for the suppression of thirst and salt appetite after ingestion.

Pharmacodynamics Research on Danggui-Shaoyao-San through Body Fluid Indexes of Spleen Deficiency-water Dampness Rats using Bio-impedance Technology

BACKGROUND

Spleen deficiency-water dampness symptom is closely related to body fluid-mediated organism metabolism and circulation. However, previous clinical evaluation of spleen deficiency-water dampness model was based only on body weight, D-xylose excretion rate, serum gastrin content, etc. Therefore, we established a large sample of normal rats and model rats experiment to verify the scientific nature of bio-impedance measuring body fluid indexes for evaluation of the modeling state. Pharmacodynamics research on Danggui-Shaoyao- San (DSS) was conducted through body fluid index changes of rats using bio-impedance technology.

METHODS

A spleen deficiency-water dampness symptom rat model was established through an inappropriate diet combined with excess fatigue. Experimental rats were divided into a normal control group, a model control group, a positive drug control group (hydrochlorothiazide), a blood-activating group, a water-disinhibiting group, and a DSS group. Total Body Water/Body Weight (TBW%), extracellular fluid/total body water content (ECF%), intracellular fluid/total body water content (ICF%), extracellular fluid/intracellular fluid (ECF/ICF), fat mass/body weight (FM%), fat-free mass/body weight (FFM%), and fat mass/fat-free mass (FM/FFM) of 150 rats were detected by a Bio-Imp Vet Body analyzer.

RESULTS

The TBW% of the model control group increased significantly, and the FM/FFM was significantly reduced compared with the normal group (P < 0.05) (P < 0.01), showing symptoms of spleen deficiency and diarrhea; the TBW% of the blood-activating group, and the waterdisinhibiting group decreased significantly, and the FM/FFM increased significantly (P < 0.05) (P < 0.01). The TBW% and FM/FFM in the water-disinhibiting group had returned to nearnormal values compared with the model control group. The blood-activating and waterdisinhibiting split prescriptions in DSS are both effective in treating spleen deficiency-water dampness rats. Comparatively, the fluid-regulating effect of split prescriptions in DSS was even stronger than that of DSS as shown in the present study.

CONCLUSIONS

These findings suggest that using bio-impedance technology to measure body fluid indexes can pave a road for further exploring the molecular mechanism of the reason why the blood-activating and disinhibit-water split prescriptions in DSS are both effective in treating spleen deficiency-water dampness rats.

Hepatic Innervations and Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disorder. Increased sympathetic (noradrenergic) nerve tone has a complex role in the etiopathomechanism of NAFLD, affecting the development/progression of steatosis, inflammation, fibrosis, and liver hemodynamical alterations. Also, lipid sensing by vagal afferent fibers is an important player in the development of hepatic steatosis. Moreover, disorganization and progressive degeneration of liver sympathetic nerves were recently described in human and experimental NAFLD. These structural alterations likely come along with impaired liver sympathetic nerve functionality and lack of adequate hepatic noradrenergic signaling. Here, we first overview the anatomy and physiology of liver nerves. Then, we discuss the nerve impairments in NAFLD and their pathophysiological consequences in hepatic metabolism, inflammation, fibrosis, and hemodynamics. We conclude that further studies considering the spatial-temporal dynamics of structural and functional changes in the hepatic nervous system may lead to more targeted pharmacotherapeutic advances in NAFLD.

The Medullary Targets of Neurally Conveyed Sensory Information from the Rat Hepatic Portal and Superior Mesenteric Veins

Vagal and spinal sensory endings in the wall of the hepatic portal and superior mesenteric veins (PMV) provide the brain with chemosensory information important for energy balance and other functions. To determine their medullary neuronal targets, we injected the transsynaptic anterograde viral tracer HSV-1 H129-772 (H129) into the PMV wall or left nodose ganglion (LNG) of male rats, followed by immunohistochemistry (IHC) and high-resolution imaging. We also determined the chemical phenotype of H129-infected neurons, and potential vagal and spinal axon terminal appositions in the dorsal motor nucleus of the vagus (DMX) and the nucleus of the solitary tract (NTS). PMV wall injections generated H129-infected neurons in both nodose ganglia and in thoracic dorsal root ganglia (DRGs). In the medulla, cholinergic preganglionic parasympathetic neurons in the DMX were virtually the only targets of chemosensory information from the PMV wall. H129-infected terminal appositions were identified on H129-infected somata and dendrites in the DMX, and on H129-infected DMX dendrites that extend into the NTS. Sensory transmission via vagal and possibly spinal routes from the PMV wall therefore reaches DMX neurons via axo-somatic appositions in the DMX and axo-dendritic appositions in the NTS. However, the dearth of H129-infected NTS neurons indicates that sensory information from the PMV wall terminates on DMX neurons without engaging NTS neurons. These previously underappreciated direct sensory routes into the DMX enable a vago-vagal and possibly spino-vagal reflexes that can directly influence visceral function.

Autonomic Nervous System and the Liver

The liver is innervated by both the sympathetic and the parasympathetic nerve systems. These nerves are derived from the splanchnic and vagal nerves that surround the portal vein, hepatic artery, and bile duct. The afferent fiber delivers information regarding osmolality, glucose level, and lipid level in the portal vein to the central nervous system (CNS). In contrast, the efferent fiber is crucial in the regulation of metabolism, blood flow, and bile secretion. Furthermore, liver innervation has been associated with hepatic fibrosis, regeneration, and circadian rhythm. Knowledge of these mechanisms can be applied for potential liver disease treatment.

Extrahepatic complications to cirrhosis and portal hypertension: Haemodynamic and homeostatic aspects

In addition to complications relating to the liver, patients with cirrhosis and portal hypertension develop extrahepatic functional disturbances of multiple organ systems. This can be considered a multiple organ failure that involves the heart, lungs, kidneys, the immune systems, and other organ systems. Progressive fibrosis of the liver and subsequent metabolic impairment leads to a systemic and splanchnic arteriolar vasodilatation. This affects both the haemodynamic and functional homeostasis of many organs and largely determines the course of the disease. With the progression of the disease, the circulation becomes hyperdynamic with cardiac, pulmonary as well as renal consequences for dysfunction and reduced survival. Infections and a changed cardiac function known as cirrhotic cardiomyopathy may be involved in further aggravation of other complications such as renal failure precipitating the hepatorenal syndrome. Patients with end-stage liver disease and related complications as for example the hepatopulmonary syndrome can only radically be treated by liver transplantation. As a bridge to this treatment, knowledge on the mechanisms of the pathophysiology of complications is essential for the choice of vasoactive drugs, antibiotics, drugs with specific effects on fibrogenesis and inflammation, and drugs that target specific receptors.

Hepatic nervous system and neurobiology of the liver

The liver has a nervous system containing both afferent and efferent neurons that are involved in a number of processes. The afferent arm includes the sensation of lipids, glucose, and metabolites (after eating and drinking) and triggers the nervous system to make appropriate physiological changes. The efferent arm is essential for metabolic regulation, modulation of fibrosis and biliary function and the control of a number of other processes. Experimental models have helped us to establish how: (i) the liver is innervated by the autonomic nervous system; and (ii) the cell types that are involved in these processes. Thus, the liver acts as both a sensor and effector that is influenced by neurological signals and ablation. Understanding these processes hold significant implications in disease processes such as diabetes and obesity, which are influenced by appetite and hormonal signals.

The absence of intrarenal ACE protects against hypertension

Activation of the intrarenal renin-angiotensin system (RAS) can elicit hypertension independently from the systemic RAS. However, the precise mechanisms by which intrarenal Ang II increases blood pressure have never been identified. To this end, we studied the responses of mice specifically lacking kidney angiotensin-converting enzyme (ACE) to experimental hypertension. Here, we show that the absence of kidney ACE substantially blunts the hypertension induced by Ang II infusion (a model of high serum Ang II) or by nitric oxide synthesis inhibition (a model of low serum Ang II). Moreover, the renal responses to high serum Ang II observed in wild-type mice, including intrarenal Ang II accumulation, sodium and water retention, and activation of ion transporters in the loop of Henle (NKCC2) and distal nephron (NCC, ENaC, and pendrin) as well as the transporter activating kinases SPAK and OSR1, were effectively prevented in mice that lack kidney ACE. These findings demonstrate that ACE metabolism plays a fundamental role in the responses of the kidney to hypertensive stimuli. In particular, renal ACE activity is required to increase local Ang II, to stimulate sodium transport in loop of Henle and the distal nephron, and to induce hypertension.