Leptin activates hypothalamic CART neurons projecting to the spinal cord

Effects of leptin on spinally projecting neurons in the PVN of the hypothalamus

The study examined whether intravenous (i.v.) leptin increased Fos-production in spinally projecting neurons in the hypothalamic paraventricular nucleus (PVN). We combined (i) rhodamine tagged microspheres injected into the upper thoracic spinal cord and (ii) Fos (marker of neuronal activation) immunohistochemistry. Effects of recombinant murine leptin were compared to vehicle (containing lipopolysaccharide a contaminant present in the leptin solution). Following leptin, 10% of the spinally projecting neurons contained Fos. However, vehicle elicited similar effects and there was no significant difference between the groups.

Two important systems in energy homeostasis: melanocortins and melanin-concentrating hormone

Our understanding of the regulation of appetite and energy balance has advanced significantly over the past decade as several peptides, centrally or peripherally expressed, have been characterized and shown to profoundly influence food intake and energy expenditure. (1)The growing number of putative appetite-regulating neuropeptides includes peptides that are orexigenic (appetite-stimulating) signals and anorectic peptides. Neuropeptide Y (NPY), melanin concentrating hormone (MCH), orexins A and B, galanin, and agouti -related peptide (AgRP) all act to stimulate feeding while alpha-melanocyte stimulating hormone (alphaMSH), corticotropin releasing hormone (CRH), cholecystokinin (CCK), cocaine and amphetamine regulated transcript (CART), neurotensin, glucagon-like peptide 1 (GLP 1), and bombesin have anorectic actions.(1) Leptin, expressed in the periphery in white adipose tissue, acts in the CNS to modulate the expression of several of these hypothalamic peptides.(1) This creates a functional link between the adipose tissue and the brain that translates the information on body fat provided by leptin to input into energy balance regulating processes. In the current review we examine the significant role of the melanocortin system (alphaMSH, agouti and AgRP peptides, and their receptors and mahogany protein) and melanin concentrating hormone in the regulation of energy balance.

Interconnections between the neuroendocrine hypothalamus and the central autonomic system. Geoffrey Harris Memorial Lecture, Kitakyushu, Japan, October 1998

Tract-tracing techniques in combination with immunohistochemistry and in situ hybridization were used in intact and operated rats (hypothalamic lesions, transections of neuronal pathways) to localize and characterize neuronal connections between the hypothalamus and autonomic centers. Viscerosensory and somatosensory signals which relay in the spinal cord and the medulla oblongata reach the hypothalamus through various catecholaminergic and noncatecholaminergic neuronal pathways. Vice versa, the hypothalamus influences autonomic activities through humoral and neurohumoral pathways. Descending hypothalamic efferents carry feedback signals to viscerosensory and brainstem catecholaminergic neurons and regulatory inputs to parasympathetic (dorsal vagal nucleus) and sympathetic (thoracolumbar intermediolateral cell column) preganglionic neurons. These fibers arise mainly from neurons of the paraventricular, arcuate, perifornical, and dorsomedial nuclei and the lateral hypothalamus. The major neuroanatomical observations are the following: (1) pathways between the hypothalamus and autonomic centers are bidirectional: the ascending and descending fibers may use the same avenues; (2) the descending axons are mainly peptidergic (CRF, vasopressin, oxytocin, somatostatin, enkephalin, POMC, and cANP), while the ascending fibers are both peptidergic (enkephalin, NPY, neurotensin, dynorphins) and catecholaminergic; (3) descending hypothalamic axons terminate directly on the sensory, preganglionic, and catecholaminergic neurons in the medulla and the spinal cord; (4) hypothalamic projections to the autonomic centers are always bilateral; (5) while medullary autonomic and catecholaminergic fibers innervate hypothalamic neurons directly, spinohypothalamic axons are relayed on neurons in the lateral hypothalamus.

CART peptide analysis by Western blotting

CART peptides have been implicated in leptin-regulated feeding, reward and reinforcement, neurotropism, and other processes. In this Western blotting study, at least six different CART peptides varying from 4 to 14 kD were found in rat brain, pituitary, gut, and adrenal gland. The peptides may be processed differently in different tissues and one species found in the rat was not found in human hypothalamus. The higher molecular weight species are likely to include preproCART and proCART, while lower molecular weight peptides may be CART 55-102 and 62-102, physiologically active fragments.

Comparing the hypothalamic and extrahypothalamic actions of endogenous hyperleptinemia

To determine whether the depletion of body fat caused by adenovirus-induced hyperleptinemia is mediated via the hypothalamus, we used as a "bioassay" for hypothalamic leptin activity the hypothalamic expression of a leptin-regulated peptide, cocaine- and amphetamine-regulated transcript (CART). The validation of this strategy was supported by the demonstration that CART mRNA was profoundly reduced in obese rats with impaired leptin action, whether because of ablation of the ventromedial hypothalamus (VMH) or a loss-of-function mutation in the leptin receptor, as in Zucker diabetic fatty rats. We compared leptin activity in normal rats made hyperleptinemic by adenovirus-leptin treatment (43 +/- 9 ng/ml, cerebrospinal fluid leptin 100 pg/ml) with normal rats made hyperleptinemic by a 60% fat intake (19 +/- 4 ng/ml, cerebrospinal fluid leptin 69 +/- 22 pg/ml). CART was increased 5-fold in the former and 2-fold in the latter, yet in adenovirus-induced hyperleptinemia, body fat had disappeared, whereas in high-fat-fed rats, body fat was abundant. Treatment of the high-fat-fed rats with adenovirus-leptin further increased their hyperleptinemia to 56 +/- 6 ng/ml without changing CART mRNA or food intake, indicating that leptin action on hypothalamus had not been increased. Nevertheless, their body fat declined 36%, suggesting that an extrahypothalamic mechanism was responsible. We conclude that in diet-induced obesity body-fat depletion by leptin requires supraphysiologic plasma concentrations that exceed the leptin-transport capacity across the blood-brain barrier.

Narcolepsy in orexin knockout mice: molecular genetics of sleep regulation

Neurons containing the neuropeptide orexin (hypocretin) are located exclusively in the lateral hypothalamus and send axons to numerous regions throughout the central nervous system, including the major nuclei implicated in sleep regulation. Here, we report that, by behavioral and electroencephalographic criteria, orexin knockout mice exhibit a phenotype strikingly similar to human narcolepsy patients, as well as canarc-1 mutant dogs, the only known monogenic model of narcolepsy. Moreover, modafinil, an anti-narcoleptic drug with ill-defined mechanisms of action, activates orexin-containing neurons. We propose that orexin regulates sleep/wakefulness states, and that orexin knockout mice are a model of human narcolepsy, a disorder characterized primarily by rapid eye movement (REM) sleep dysregulation.

Distinct patterns of neuropeptide gene expression in the lateral hypothalamic area and arcuate nucleus are associated with dehydration-induced anorexia

We have investigated the hormonal and hypothalamic neuropeptidergic substrates of dehydration-associated anorexia. In situ hybridization and hormone analyses of anorexic and paired food-restricted rats revealed two distinct profiles. First, both groups had the characteristic gene expression and endocrine signatures usually associated with starvation: increased neuropeptide Y and decreased proopiomelanocortin and neurotensin mRNAs in the arcuate nucleus (ARH); increased circulating glucocorticoid but reduced leptin and insulin. Dehydrated animals are strongly anorexic despite these attributes, showing that the output of leptin- and insulin-sensitive ARH neurons that ordinarily stimulate eating must be inhibited. The second pattern occurred only in anorexic animals and had two components: (1) reduced corticotropin-releasing hormone (CRH) mRNA in the neuroendocrine paraventricular nucleus (PVH) and (2) increased CRH and neurotensin mRNAs in the lateral hypothalamic (LHA) and retrochiasmatic areas. However, neither corticosterone nor suppressed PVH CRH gene expression is required for anorexia after dehydration because PVH CRH mRNA in dehydrated adrenalectomized animals is unchanged from euhydrated adrenalectomized controls. We also showed that LHA CRH mRNA was strongly correlated with the intensity of anorexia, increased LHA CRH gene expression preceded the onset of anorexia, and dehydrated adrenalectomized animals (which also develop anorexia) had elevated LHA CRH gene expression with a distribution pattern similar to intact animals. Finally, we identified specific efferents from the CRH-containing region of the LHA to the PVH, thereby providing a neuroanatomical framework for the integration by the PVH of neuropeptidergic signals from the ARH and the LHA. Together, these observations suggest that CRH and neurotensin neurons in the LHA constitute a novel anatomical substrate for their well known anorexic effects.

Activation of arcuate nucleus neurons by systemic administration of leptin and growth hormone-releasing peptide-6 in normal and fasted rats

Both leptin and growth hormone secretagogues are believed to have stimulatory effects on the hypothalamic growth hormone pulse generator, though whether these are achieved through the same pathway is unknown. Systemic administration of a normally maximal effective dose of the growth hormone secretagogue GHRP-6 to male rats causes the induction of c-Fos protein in the ventromedial aspect of the hypothalamic arcuate nucleus. The effect of the same dose of GHRP-6 is, however, much greater in animals that have been fasted for 48 h, suggesting that in the food-replete rat, arcuate neurons either show reduced sensitivity to endogenous growth hormone secretagogues or they are under the tonic inhibitory influences of other factors. The major populations of arcuate neurons activated by GHRP-6 have been shown to contain neuropeptide Y or growth hormone-releasing factor, while leptin is thought to be inhibitory to neuropeptide Y neurons. Leptin did not alter the response of the rats to GHRP-6. However, it was able by itself to induce c-Fos protein immunoreactivity in the ventral, including the ventrolateral, arcuate nucleus of fasted rats. This is a clear demonstration of the acute activation of arcuate neurons in the rat following systemic leptin injection and suggests that GHRP-6 and leptin act on the growth hormone axis via different pathways.

Leptin differentially regulates NPY and POMC neurons projecting to the lateral hypothalamic area

Recent studies have reinforced the view that the lateral hypothalamic area (LHA) regulates food intake and body weight. We identified leptin-sensitive neurons in the arcuate nucleus of the hypothalamus (Arc) that innervate the LHA using retrograde tracing with leptin administration. We found that retrogradely labeled cells in the Arc contained neuropeptide Y (NPY) mRNA or proopiomelanocortin (POMC) mRNA. Following leptin administration, NPY cells in the Arc did not express Fos but expressed suppressor of cytokine signaling-3 (SOCS-3) mRNA. In contrast, leptin induced both Fos and SOCS-3 expression in POMC neurons, many of which also innervated the LHA. These findings suggest that leptin directly and differentially engages NPY and POMC neurons that project to the LHA, linking circulating leptin and neurons that regulate feeding behavior and body weight homeostasis.

CART peptides: novel addiction- and feeding-related neuropeptides

CART peptides are novel, putative brain-gut neurotransmitters and co-transmitters that probably have a role in drug abuse, the control of feeding behavior, sensory processing, stress and development. They are abundant, processed and apparently released. Exogenously applied peptides cause inhibition of feeding and have neurotrophic properties. Although the precise sequences, relative abundance and efficacy of all CART peptides are currently being determined, small molecules that are active at putative CART receptors could have substantial therapeutic promise.

The role of the central nervous system in hypertension

Normally, the kidney plays the dominant role in setting long-term arterial pressure, and the nervous system acts primarily as a short-term regulator, adjusting arterial pressure to acute challenges (eg, standing, running, and stress). However, in several animal models and in subsets of hypertensive human patients, the nervous system seems to play a more significant role in the chronic elevation of arterial pressure. Many clinical studies suggest that the peripheral sympathetic nerves are intimately involved in hypertension, and researchers recently characterized abnormalities in the brain that seem to predispose animal models to sympathetic nervous system overactivity and hypertension. Together, the current data strongly suggest that the brain, via the sympathetic nervous system, directly contributes to some forms of hypertension and indirectly contributes to all of them. This review is not intended as an exhaustive examination of all studies on the role of the nervous system in hypertension but rather focuses on several intriguing experiments that provide provocative new insights on this topic.

Leptin binding in the arcuate nucleus is increased during fasting

The arcuate nucleus (ARC) mediates the anorexic effects of leptin and expresses the long form (Ob-Rb) of the leptin receptor. To determine whether ARC leptin binding increases when plasma leptin levels are low during fasting, [125I]-leptin specific binding to rat brain slices was measured by quantitative autoradiography. [125I]-leptin specific binding was dense in the ARC and increased 2-fold after a 48-h fast (P<0.001). These findings suggest that leptin receptor binding in the ARC is upregulated during fasting and that fasting changes the sensitivity of the ARC to leptin.