Leptin receptors, NPY, and tyrosine hydroxylase in autonomic neurons supplying fat depots in a pig

Novel insight into perirenal adipose tissue: A neglected adipose depot linking cardiovascular and chronic kidney disease

Obesity is associated with adverse metabolic diseases including cardiovascular disease (CVD) and chronic kidney disease (CKD). These obesity-related diseases are highly associated with excess fat accumulation in adipose tissue. However, emerging evidence indicates that visceral adiposity associates more with metabolic and cardiovascular risk factors. Perirenal adipose tissue, surrounding the kidney, is originally thought to provides only mechanical support for kidney. However, more studies demonstrated perirenal adipose tissue have a closer association with renal disease than other visceral fat deposits in obesity. Additionally, perirenal adipose tissue is also an independent risk factor for CKD and even associated more with CVD. Thus, perirenal adipose tissue may be a connection of CVD with CKD. Here, we will provide an overview of the perirenal adipose tissue, a neglected visceral adipose tissue, and the roles of perirenal adipose tissue linking with CVD and CKD and highlight the perirenal adipose tissue as a potential strategy for future therapeutics against obesity-related disease.

Perirenal Fat: A Unique Fat Pad and Potential Target for Cardiovascular Disease

Although visceral obesity is recognized as a risk factor for cardiovascular diseases (CVDs), the efficacy of omental fat removal in CVD treatment is still controversial. There is a need to identify other visceral fat depots for CVD management. This review aims to provide a summary on perirenal fat as an important risk factor for CVD. Studies on epidemiology, anatomy, and function of perirenal fat were reviewed. Observational studies in humans suggest that excessive perirenal fat increases the risk of hypertension and coronary heart disease. Anatomy studies prove that perirenal fat is unique compared to other connective tissues in that it is well vascularized, innervated, and drains into the lymphatic system. Other special morphological features include a complete fascia border, sympathetic-independent development of architecture, and proximity to the kidneys. Based on these anatomical features, perirenal fat regulates the cardiovascular system presumably via neural reflex, adipokine secretion, and fat-kidney interaction. These new insights suggest that perirenal fat may constitute a promising target for CVD management.

Alterations of neurochemical expression of the coeliac-superior mesenteric ganglion complex (CSMG) neurons supplying the prepyloric region of the porcine stomach following partial stomach resection

The purpose of the present study was to determine the response of the porcine coeliac-superior mesenteric ganglion complex (CSMG) neurons projecting to the prepyloric area of the porcine stomach to peripheral neuronal damage following partial stomach resection. To identify the sympathetic neurons innervating the studied area of stomach, the neuronal retrograde tracer Fast Blue (FB) was applied to control and partial stomach resection (RES) groups. On the 22nd day after FB injection, following laparotomy, the partial resection of the previously FB-injected stomach prepyloric area was performed in animals of RES group. On the 28th day, all animals were re-anaesthetized and euthanized. The CSMG complex was then collected and processed for double-labeling immunofluorescence. In control animals, retrograde-labelled perikarya were immunoreactive to tyrosine hydroxylase (TH), dopamine β-hydroxylase (DβH), neuropeptide Y (NPY) and galanin (GAL). Partial stomach resection decreased the numbers of FB-positive neurons immunopositive for TH and DβH. However, the strong increase of NPY and GAL expression, as well as de novo-synthesis of neuronal nitric oxide synthase (nNOS) and leu5-Enkephalin (LENK) was noted in studied neurons. Furthermore, FB-positive neurons in all pigs were surrounded by a network of cocaine- and amphetamine-regulated transcript peptide (CART)-, calcitonin gene-related peptide (CGRP)-, and substance P (SP)-, vasoactive intestinal peptide (VIP)-, LENK- and nNOS- immunoreactive nerve fibers. This may suggest neuroprotective contribution of these neurotransmitters in traumatic responses of sympathetic neurons to peripheral axonal damage.

Leptin stimulates sympathetic axon outgrowth

The neurohormone leptin regulates energy homeostasis. Circulating levels of leptin secreted by adipose tissue act on hypothalamic neurons in the brain leading to decreased appetite and increased energy expenditure. Although leptin signaling in the central nervous system (CNS) is fundamental to its ability to regulate the body's metabolic balance, leptin also has a variety of effects in many peripheral tissues including the heart, the liver, and the sympathetic nervous system. Leptin stimulation of the hypothalamus can stimulate glucose uptake via the sympathetic nervous system in heart, muscle, and brown adipose tissue. Leptin receptors (Ob-Rb) are also expressed by peripheral sympathetic neurons, but their functional role is not clear. In this study, we found that leptin stimulates axonal growth of both adult and neonatal sympathetic neurons in vitro. Leptin stimulates acute activation of the transcription factor STAT3 via phosphorylation of tyrosine 705. STAT3 phosphorylation is required for leptin-stimulated sympathetic axon outgrowth. Thus, circulating levels of leptin may enhance sympathetic nerve innervation of peripheral tissues.

Robust internal elastic lamina fenestration in skeletal muscle arteries

Holes within the internal elastic lamina (IEL) of blood vessels are sites of fenestration allowing for passage of diffusible vasoactive substances and interface of endothelial cell membrane projections with underlying vascular smooth muscle. Endothelial projections are sites of dynamic Ca²⁺ events leading to endothelium dependent hyperpolarization (EDH)-mediated relaxations and the activity of these events increase as vessel diameter decreases. We tested the hypothesis that IEL fenestration is greater in distal vs. proximal arteries in skeletal muscle, and is unlike other vascular beds (mesentery). We also determined ion channel protein composition within the endothelium of intramuscular and non-intramuscular skeletal muscle arteries. Popliteal arteries, subsequent gastrocnemius feed arteries, and first and second order intramuscular arterioles from rat hindlimb were isolated, cut longitudinally, fixed, and imaged using confocal microscopy. Quantitative analysis revealed a significantly larger total fenestration area in second and first order arterioles vs. feed and popliteal arteries (58% and 16% vs. 5% and 3%; N = 10 images/artery), due to a noticeably greater average size of holes (9.5 and 3.9 µm² vs 1.5 and 1.9 µm²). Next, we investigated via immunolabeling procedures whether proteins involved in EDH often embedded in endothelial cell projections were disparate between arterial segments. Specific proteins involved in EDH, such as inositol trisphosphate receptors, small and intermediate conductance Ca²⁺-activated K⁺ channels, and the canonical (C) transient receptor potential (TRP) channel TRPC3 were present in both popliteal and first order intramuscular arterioles. However due to larger IEL fenestration in first order arterioles, a larger spanning area of EDH proteins is observed proximal to the smooth muscle cell plasma membrane. These observations highlight the robust area of fenestration within intramuscular arterioles and indicate that the anatomical architecture and endothelial cell hyperpolarizing apparatus for distinct vasodilatory signaling is potentially present.

Diet-induced obesity severely impairs myelinated aortic baroreceptor reflex responses

Diet-induced obesity (DIO) attenuates the arterial cardiac baroreceptor reflex, but the mechanisms and sites of action are unknown. This study tested the hypothesis that DIO impairs central aortic baroreceptor pathways. Normal chow control (CON) and high-fat-chow obesity-resistant (OR) and obesity-prone (OP) rats were anesthetized (inactin, 120 mg/kg) and underwent sinoaortic denervation. The central end of the aortic depressor nerve (ADN) was electrically stimulated to generate frequency-dependent baroreflex curves (5-100 Hz) during selective activation of myelinated (A-fiber) or combined (A- and C-fiber) ADN baroreceptors. A mild stimulus (1 V) that activates only A-fiber ADN baroreceptors induced robust, frequency-dependent depressor and bradycardic responses in CON and OR rats, but these responses were completely abolished in OP rats. Maximal activation of A fibers (3 V) elicited frequency-dependent reflexes in all groups, but a dramatic deficit was still present in OP rats. Activation of all ADN baroreceptors (20 V) evoked even larger reflex responses. Depressor responses were nearly identical among groups, but OP rats still exhibited attenuated bradycardia. In separate groups of rats, the reduced heart rate (HR) response to maximal activation of ADN A fibers (3 V) persisted in OP rats following pharmacological blockade of β(1)-adrenergic or muscarinic receptors, suggesting deficits in both parasympathetic nervous system (PNS) and sympathetic nervous system (SNS) reflex pathways. However, the bradycardic responses to direct efferent vagal stimulation were similar among groups. Taken together, our data suggest that DIO severely impairs the central processing of myelinated aortic baroreceptor control of HR, including both PNS and SNS components.

Integration and sensory experience-dependent survival of newly-generated neurons in the accessory olfactory bulb of female mice

Newborn neurons generated by proliferative progenitors in the adult subventricular zone (SVZ) integrate into the olfactory bulb circuitry of mammals. Survival of these newly-formed cells is regulated by the olfactory input. The presence of new neurons in the accessory olfactory bulb (AOB) has already been demonstrated in some mammalian species, albeit their neurochemical profile and functional integration into AOB circuits are still to be investigated. To unravel whether the mouse AOB represents a site of adult constitutive neurogenesis and whether this process can be modulated by extrinsic factors, we have used multiple in vivo approaches. These included fate mapping of bromodeoxyuridine-labelled cells, lineage tracing of SVZ-derived enhanced green fluorescent protein-positive engrafted cells and neurogenesis quantification in the AOB, in both sexes, as well as in females alone after exposure to male-soiled bedding or its derived volatiles. Here, we show that a subpopulation of SVZ-derived neuroblasts acquires proper neurochemical profiles of mature AOB interneurons. Moreover, 3D reconstruction of long-term survived engrafted neuroblasts in the AOB confirms these cells show features of fully integrated neurons. Finally, exposure to male-soiled bedding, but not to its volatile compounds, significantly increases the number of new neurons in the AOB, but not in the main olfactory bulb of female mice. These data show SVZ-derived neuroblasts differentiate into new functionally integrated neurons in the AOB of young and adult mice. Survival of these cells seems to be regulated by an experience-specific mechanism mediated by pheromones.

Abundance of triacylglycerols in ganglia and their depletion in diabetic mice: implications for the role of altered triacylglycerols in diabetic neuropathy

Herein, we report the first study on the mass distribution and molecular species composition of abundant triacylglycerols (TAG) in ganglia. This study demonstrates five novel findings. First, unanticipated high levels of TAG were present in all examined ganglia from multiple species (e.g. mouse, rat and rabbit). Second, ganglial TAG mass content is location-dependent. Third, the TAG mass levels in ganglia are species-specific. Fourth, dorsal root ganglial TAG mass levels in streptozotocin-induced diabetic mice are dramatically depleted relative to those found in untreated control mice. Fifth, mouse ganglial TAG mass levels decrease with age although molecular species composition is not changed. Collectively, these results indicate that TAG is an important component of ganglia and may potentially contribute to pathological alterations in peripheral neuronal function in diabetic neuropathy.

Are hypothalamic neurons transsynaptically connected to porcine adipose tissue?

Specific anatomical sites and pathways responsible for mediating metabolic and neuroendocrine effects of leptin are still poorly understood. Therefore, we examined distribution of leptin receptor-containing neurons transsynaptically connected with the porcine fat tissue by means of combined viral transneuronal tracing and immunohistochemical staining method. Pseudorabies virus (PRV) was injected into the perirenal fat tissue in pigs, and after survival periods of 3, 5, 7, 9, and 11 days, hypothalami were processed immunohistochemically with primary antisera against PRV and leptin receptor (OBR). PRV labeled neurons were found in paraventricular nucleus (PVN), ventromedial nucleus (VMN), anterior hypothalamic area (AHA), preoptic area (PA), arcuate nucleus (ARC), and supraoptic nucleus (SON) by nine days after injection of the virus. Double-labeling immunofluorescence demonstrated that OBR were co-localized in nearly all virus-infected neurons. The present results provide the first morphological data demonstrating a multisynaptic circuit of neurons of CNS origin which innervates porcine fat tissue.