Effect of leptin on nitrergic neurons in the lateral hypothalamic area and the supraoptic nucleus of rats.

The adipocyte-derived hormone, leptin, plays a key role in the maintenance of energy homeostasis. Leptin binds to the long form of its receptor, which is predominantly expressed in various hypothalamic regions, including the lateral hypothalamic area (LH) and supraoptic nucleus (SO). Several studies have suggested that leptin directly activates neuronal nitric oxide synthase, leading to increased nitric oxide production. We used histochemistry for nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) as a marker for nitric oxide synthase activity and assessed the effect of leptin on nitrergic neurons in the LH and SO of rats. We found that intraperitoneal administration of leptin led to a significant increase in the number of NADPH-d-positive neurons in the LH and SO. In addition, the intensity (optical density) of NADPH-d staining in LH and SO neurons was significantly elevated in rats that received leptin compared with saline-treated rats. These findings suggest that nitrergic neurons in the LH and SO may be implicated in mediating the central effects of leptin.

Therapeutic potential of orally applied KB-R7943 in streptozotocin-induced neuropathy in rats.

Both peripheral neuropathy and depression can be viewed as neurodegeneration's consequences of diabetes, at least in part coexisting with or resulting from sodium-calcium dysbalance. This study aims to assess the therapeutic potential of the orally applied reverse-mode inhibitor of the sodium-calcium exchanger (NCX) KB-R7943 in the streptozotocin (STZ) diabetes model in rats. A pilot pharmacokinetic (PK) study with high-performance liquid chromatography with high-resolution tandem mass spectrometric detection revealed higher drug exposure (AUC), lower volume of distribution (Vd) and clearance (Cl), and faster decline of the plasma concentration (ƛ) in rats with diabetes vs. controls. Brain and heart accumulation and urinary excretion of the unmetabolized KB-R7943 at least 24 h were also demonstrated in all rats. However, heart and hippocampus KB-R7943 penetration (AUCtissue/AUCplasma) was higher in controls vs. diabetic rats. The development of thermal, mechanical, and chemical-induced allodynia was assessed with the Cold plate test (CPT), Randall-Stiletto (R-S) test, and 0.5% formalin test (FT). Amitriptyline 10 mg/kg, KB-R7943 5 mg/kg, or 10 mg/kg p.o once daily was applied from the 28th to the 49th day. The body weight, coat status, CPT, R-S, and FT were evaluated on days (-5), 0, and 42. On day 41, a forced swim test and 24-h spontaneous physical activities were assessed. The chronic treatment effects were calculated as % of the maximum. A dose-depended amelioration of neuropathic and depression-like effects was demonstrated. The oral application of KB-R7943 for potentially treating neurodegenerative consequences of diabetes merits further studies. The brain, heart, and kidneys are essential contributors to the PKs of this drug, and their safety involvement needs to be further characterized.

Alterations of the glutamatergic system in diabetes mellitus.

Diabetes mellitus (DM) is a chronic disease characterized by elevated blood glucose levels caused by a lack of insulin production (type 1 diabetes) or insulin resistance (type 2 diabetes). It is well known that DM is associated with cognitive deficits and metabolic and neurophysiological changes in the brain. Glutamate is the main excitatory neurotransmitter in the central nervous system that plays a key role in synaptic plasticity, learning, and memory processes. An increasing number of studies have suggested that abnormal activity of the glutamatergic system is implicated in the pathophysiology of DM. Dysfunction of glutamatergic neurotransmission in the central nervous system can provide an important neurobiological substrate for many disorders. Magnetic resonance spectroscopy (MRS) is a non-invasive technique that allows a better understanding of the central nervous system factors by measuring in vivo the concentrations of brain metabolites within the area of interest. Here, we briefly review the MRS studies that have examined glutamate levels in the brain of patients with DM. The present article also summarizes the available data on abnormalities in glutamatergic neurotransmission observed in different animal models of DM. In addition, the role of gut microbiota in the development of glutamatergic alterations in DM is addressed. We speculate that therapeutic strategies targeting the glutamatergic system may be beneficial in the treatment of central nervous system-related changes in diabetic patients.

Inhibition of nitric oxide synthase or cystathionine gamma-lyase abolishes leptin-induced fever in male rats.

Leptin is an adipokine that regulates energy balance and immune function. Peripheral leptin administration elicits prostaglandin E2-dependent fever in rats. The gasotransmitters nitric oxide (NO) and hydrogen sulfide (H2S) are also involved in lipopolysaccharide (LPS)-induced fever response. However, there is no data in the literature indicating if these gasotransmitters have a role in leptin-induced fever response. Here, we investigate the inhibition of NO and H2S enzymes neuronal nitric oxide synthase (nNOS), inducible nitric oxide synthase (iNOS), and cystathionine γ-lyase (CSE) in leptin-induced fever response, respectively. Selective nNOS inhibitor 7-nitroindazole (7-NI), selective iNOS inhibitor aminoguanidine (AG), and CSE inhibitor dl-propargylglycine (PAG) were administered intraperitoneally (ip). Body temperature (Tb), food intake, and body mass were recorded in fasted male rats. Leptin (0.5 mg/kg ip) induced a significant increase in Tb, whereas AG (50 mg/kg ip), 7-NI (10 mg/kg ip), or PAG (50 mg/kg ip) caused no changes in Tb. AG, 7-NI, or PAG abolished leptin increase in Tb. Our results highlight the potential involvement of iNOS, nNOS, and CSE in leptin-induced febrile response without affecting anorexic response to leptin in fasted male rats 24 h after leptin injection. Interestingly, all the inhibitors alone had the same anorexic effect induced by leptin. These findings have implications for understanding the role of NO and H2S in leptin-induced febrile response.

Cafeteria diet-induced obesity reduces leptin-stimulated NADPH-diaphorase reactivity in the hypothalamic arcuate nucleus of rats.

Leptin is an adipokine that plays an important role in the regulation of energy homeostasis. The failure of endogenous and exogenous leptin to mediate its effects (for example, at suppressing appetite and decreasing body weight) has been termed leptin resistance. Hyperleptinemia and leptin resistance can be well demonstrated in animals in which obesity is induced by consumption of a palatable, high-calorie diet (e.g., cafeteria diet-induced obesity). Since leptin receptor signaling is known to be impaired in the hypothalamic arcuate nucleus (ARC) of obese rodents, we investigated the effect of leptin on nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) reactivity in the ARC of male Wistar rats with cafeteria diet-induced obesity. Our results have shown that after intraperitoneal administration of leptin, the number of NADPH-d positive neurons in the ARC was significantly lower in obese rats compared with that observed in normal weight rats. Additionally, we have found that leptin-induced NADPH-d staining in ARC neurons and the adjacent ependyma was decreased in obese rats. The results presented here suggest that the ability of leptin to activate nitric oxide synthase in neurons within the ARC as well as tanycytes and ependymal cells of the third ventricle is reduced in rats made obese by a cafeteria diet. We speculate that impairment in leptin-induced NO production presents a potential mechanism, involved in the pathogenesis of obesity and obesity-related disease states.

Increased NADPH-diaphorase reactivity in the hypothalamic paraventricular nucleus and tanycytes following systemic administration of leptin in rats.

Leptin, a hormone mainly produced by adipocytes in proportion to fat mass, is a key component in the regulation of energy homeostasis and reproductive, neuroendocrine, immune, and metabolic functions. Leptin binds to the leptin receptor, which is expressed throughout the central nervous system but particularly in neurons of several nuclei of the hypothalamus, such as the arcuate nucleus (ARC) and paraventricular nucleus (PVN). It has been found that nitric oxide (NO) plays an important role in mediating effects of leptin. Since PVN and ARC neurons are known to express leptin receptors, we investigated the effects of leptin on nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) reactivity in the PVN and ARC of male Wistar rats. Our results have shown that systemic administration of leptin resulted in increased NADPH-d positive cell number in the PVN and ARC, suggesting that both the PVN and ARC may be important centers in the hypothalamus for the leptin action, mediated by increased NO production. In addition, we have also observed that hypothalamic tanycytes in the ventral portion of the third ventricle were NADPH-d positive. We speculate that leptin may affect the release of neurohormones and hypothalamic neurogenesis by activating nitric oxide synthase in hypothalamic tanycytes.

Central, but not systemic, thermoregulatory effects of leptin are impaired in rats with obesity: interactions with GABAB agonist and antagonist.

Leptin is an adipokine that regulates body weight by decreasing appetite and increasing energy expenditure. Besides the effects on food intake, leptin can regulate energy expenditure at least in part by modulating thermogenesis. Many of the effects of leptin are attributable to action in the central nervous system, particularly in the hypothalamus. Common forms of obesity are associated with increased leptin levels and a failure to suppress feeding and mediate weight loss in response to exogenous leptin. This apparent leptin ineffectiveness defines a state of so-called leptin resistance. We examined the effect of leptin on core body temperature in rats with normal weight and diet-induced obesity (DIO), as well as thermoregulatory interactions between leptin and GABAB-agonist and an antagonist. We found that leptin retains the ability to induce hyperthermic effect in rats with DIO. Additionally, temperature responses produced by GABAB agonist and antagonist are altered in a state of obesity and by administration of leptin. We evaluated whether the medial preoptic area of the anterior hypothalamus (MPA) still remains sensitive to leptin action during DIO. Using extracellular recordings of neurons and phospho-signal transducer and the activator of transcription 3 (pSTAT3) immunohistochemistry, we have provided strong evidence that leptin signaling in the MPA is impaired in obese rats. We believe that leptin resistance in the MPA may play a role in the pathogenesis of obesity and obesity-related disease states.