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.

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.

Effect of long-term caffeine administration on depressive-like behavior in rats exposed to chronic unpredictable stress.

Chronic unpredictable stress (CUS) was used to study the effects of a long-term treatment with either caffeine (8 mg/kg, orally) or desipramine (DMI) (10 mg/kg, intraperitoneally) in Wistar rats. The CUS procedure was applied for 6 weeks. Animals underwent a 2-week drug-free CUS procedure. Drugs were administered for 4 weeks alongside the stress and both drug and stress were continued throughout the behavioral testing period. CUS-exposed rats showed depressive-like behavior with reduced weight gain, reduced consumption of sucrose solution, increased immobility in the forced swimming test, and hypolocomotion in an open field. For the open field and elevated plus maze, calculation of an anxiety index confirmed that CUS increased anxiety, which was accompanied by an increase in the core temperature. DMI counteracted these physical and behavioral changes. Caffeine caused similar effects to DMI on weight gain, motor activity, anxiety level, and core temperature. In CUS-exposed rats, caffeine showed antidepressant and anxiolytic activity, accompanied by increased hippocampal dopamine and serotonin levels. However, no significant change in weight gain or core temperature was observed after caffeine treatment in CUS-exposed rats. These results suggest that, similar to the antidepressant DMI, long-term caffeine exposure exerts an antidepressant and anxiolytic effect in the CUS model. The involvement of the dopaminergic and serotonergic systems is discussed.

Effects of GABA-transaminase inhibitor Vigabatrin on thermoregulation in rats.

Vigabatrin is a GABA derivative (gamma-vinyl GABA) which inhibits irreversibly the enzyme activity of GABA transaminase and thus increased indirectly brain GABA concentrations. We have used body temperature assay to examine the effects of Vigabatrin on thermoregulation in intact rats. In order to understand the mechanism of thermoregulatory action of Vigabatrin at cellular level, we have investigated its effect on individual warm-sensitive preoptic area/anterior hypothalamus (PO/AH) neurons in rat brain slice preparations. The results of the present study suggest that Vigabatrin produced dose-dependent hypothermia in rats and also increased temperature sensitivity of warm-sensitive PO/AH neurons.

Effect of GABAergic substances on firing rate and thermal coefficient of hypothalamic neurons in the juvenile chicken.

The goal of the study is to investigate the GABAergic action on firing rate (FR) and temperature coefficient (TC) on hypothalamic neurons in the juvenile chicken. Extracellular recordings were obtained from 37 warm-sensitive, 32 cold-sensitive and 56 temperature-insensitive neurons in brain slices to determine the effect of GABA(A)-receptor agonist muscimol, GABA(A)-receptor antagonist bicuculline, GABA(B)-receptor agonist baclofen and GABA(B)-receptor antagonist CGP 35348. Muscimol and baclofen in equimolar concentrations (1 microM) significantly inhibited FR of the neurons, regardless of their type of thermosensitivity. In contrast, bicuculline, as well as CGP 35348 (10 microM) increased FR of the majority of the neurons. The TC of most chick hypothalamic neurons could not be estimated during muscimol application because FR was completely inhibited. GABA(B)-receptor agonist specifically increased TC. This effect was restricted to cold-sensitive neurons, which were determined in a high number. The TC was significantly increased (p<0.05) by baclofen and significantly decreased (p<0.05) by CGP 35348. The effects of muscimol and baclofen on FR and TC were prevented by co-perfusion of the appropriate antagonists bicuculline and CGP 35348. The results suggest that the fundamental mechanisms of GABAergic influence on temperature sensitive and insensitive neurons in the chicken PO/AH are conserved during evolution of amniotes.

Effects of GABAB-agonist on rat hypothalamic neurons: functional antagonism with mu-receptor agonist.

Extracellular and whole-cell patch clamp recordings were made from neurons in slices of the preoptic area/anterior hypothalamus (PO/AH) of rats, to investigate the effects of the GABA(B)-receptor agonist baclofen on neuronal response characteristics, as well as its interactions with mu-opioid receptor agonist PL-017 on the level of central temperature controller. Baclofen decreased tonic activity (firing rate) in all types of neurons, but increased temperature sensitivity (temperature coefficient, TC) in warm-sensitive neurons. The decrease in firing rate during baclofen application was accompanied with significant membrane hyperpolarization and decrease of input resistances. The tonic activity (in all type of PO/AH neurons), as well as the temperature sensitivity (in warm-sensitive neurons), were inhibited by mu-opioid receptor agonist PL-017. Remarkably, the effect on temperature sensitivity was abolished and absence of synergism in regard to firing rate decrease occurred, when baclofen and PL-017 were applied simultaneously. Our results are step of understanding the complicated mechanisms of action of neurotransmitters and their interactions on the level of central temperature controller-the neurons of the PO/AH.