Cocaine- and amphetamine-regulated transcript (CART) and food intake: Behavior in search of anatomy

MCH and feeding behavior-interaction with peptidic network

Numerous works associate the MCH peptide, and the hypothalamic neurons that produce it, to the feeding behavior and energy homeostasis. It is commonly admitted that MCH is an orexigenic peptide, and MCH neurons could be under the control of arcuate NPY and POMC neurons. However, the literature data is not always concordant. In particular questions about the intrahypothalamic circuit involving other neuropeptides and about the mechanisms through which MCH could act are not yet clearly answered. For example, which receptors mediate a MCH response to NPY or alpha-MSH, does MCH act alone, is there any local anatomical organization within the tuberal LHA? A review of the current literature is then needed to help focus attention on these unresolved and often neglected issues.

Synergistic effect of CART (cocaine- and amphetamine-regulated transcript) peptide and cholecystokinin on food intake regulation in lean mice

Background

CART (cocaine- and amphetamine-regulated transcript) peptide and cholecystokinin (CCK) are neuromodulators involved in feeding behavior. This study is based on previously found synergistic effect of leptin and CCK on food intake and our hypothesis on a co-operation of the CART peptide and CCK in food intake regulation and Fos activation in their common targets, the nucleus tractus solitarii of the brainstem (NTS), the paraventricular nucleus (PVN), and the dorsomedial nucleus (DMH) of the hypothalamus.

Results

In fasted C57BL/6 mice, the anorexigenic effect of CART(61-102) in the doses of 0.1 or 0.5 μg/mouse was significantly enhanced by low doses of CCK-8 of 0.4 or 4 μg/kg, while 1 mg/kg dose of CCK-A receptor antagonist devazepide blocked the effect of CART(61-102) on food intake. After simultaneous administration of 0.1 μg/mouse CART(61-102) and of 4 μg/kg of CCK-8, the number of Fos-positive neurons in NTS, PVN, and DMH was significantly higher than after administration of each particular peptide. Besides, CART(61-102) and CCK-8 showed an additive effect on inhibition of the locomotor activity of mice in an open field test.

Conclusion

The synergistic and long-lasting effect of the CART peptide and CCK on food intake and their additive effect on Fos immunoreactivity in their common targets suggest a co-operative action of CART peptide and CCK which could be related to synergistic effect of leptin on CCK satiety.

Distribution and neuropeptide coexistence of nucleobindin-2 mRNA/nesfatin-like immunoreactivity in the rat CNS

The protein fragment nesfatin-1 was recently implicated in the control of food intake. Central administration of this fragment results in anorexia and reduced body weight gain, whereas antisense or immunological nesfatin-1 antagonism causes increased food intake and overweight. Nesfatin-1 is derived from the precursor nucleobindin-2 (NUCB2). To identify the neurocircuitry underpinning the catabolic effects of NUCB2/nesfatin-1, we have used in situ hybridization and immunohistochemistry to map the distribution of this protein and its mRNA in the rat CNS and performed double-labeling experiments to localize its expression to functionally defined neuronal populations. These experiments confirm previous observations but also present several novel NUCB2 cell populations. Both NUCB2 mRNA and nesfatin-like immunoreactivity was most concentrated in the hypothalamus, in the supraoptic, paraventricular, periventricular and arcuate nuclei and the lateral hypothalamic area/perifornical region. Additionally, outside of the hypothalamus, labeling was observed in the thalamic parafascicular nucleus, the Edinger-Westphal nucleus, locus coeruleus, ventral raphe system, nucleus of solitary tract and in the preganglionic sympathetic intermediolateral cell column of the spinal cord, and the pituitary anterior and intermediate lobes. In neurons, immunoreactivity was almost exclusively confined to perikarya and primary dendrites with virtually no labeling of axonal terminals. Double-labeling immunohistochemistry revealed colocalization of nesfatin with vasopressin and oxytocin in magnocellular neuroendocrine neurons, thyrotropin-releasing hormone, corticotropin-releasing hormone, somatostatin, neurotensin, and growth-hormone-releasing hormone in parvocellular neuroendocrine neurons, pro-opiomelanocortin (but not neuropeptide Y) in the arcuate nucleus and melanin-concentrating hormone (but not hypocretin) in the lateral hypothalamus. Furthermore, nesfatin was extensively colocalized with cocaine- and amphetamine-regulated transcript in almost all NUCB2-expressing brain regions. These data reveal a wider distribution of NUCB2/nesfatin-1 than previously known, suggesting that the metabolic actions of this protein may involve not only feeding behavior but also endocrine and autonomic effects on energy expenditure. In addition, the subcellular distribution of nesfatin-like immunoreactivity indicates that this protein may not be processed like a conventional secreted neuromodulator.

Structure-activity relationship of CART (cocaine- and amphetamine-regulated transcript) peptide fragments

CART (cocaine- and amphetamine-regulated transcript) peptides are neuropeptides abundant in the central nervous system and periphery found to be involved in the regulation of food intake behavior and other physiological processes. Recently, we reported specific binding of (125)I-CART(61-102) to the rat adrenal pheochromocytoma cell line PC12, both intact cells and cell membranes. In this study, several fragments of CART(61-102) corresponding to its structural loops were synthesized and tested for their potency in binding experiments using PC12 intact cells and cell membranes and in feeding test with fasted mice. From all shorter peptides tested, only CART(74-86) and CART(62-86) containing disulfide bridges kept partial binding potency of the original molecule with K(i) in 10(-5) and 10(-4)M range. However, these fragments were not able to inhibit food intake after their central administration up to a dose of 4 nmol/mouse. The results showed that a compact structure containing three disulfide bridges is necessary for preservation of full biological activity of CART peptides.

Cocaine- and amphetamine-regulated transcript (CART) peptide specific binding in pheochromocytoma cells PC12

CART (cocaine- and amphetamine-regulated transcript) peptides have been studied for ten years. We report specific binding of 125I-CART(61-102) to the rat adrenal pheochromocytoma PC12 cell line, both intact cells and cell membranes. Saturation binding to intact plated cells resulted in Kd of 0.48+/-0.16 nM and Bmax of 2228+/-529 binding sites/cell. 125I-CART(61-102) was also bound to PC12 cells differentiated using nerve growth factor to the neuronal phenotype with non-specific binding below 20%, and Kd of 1.90+/-0.27 nM and Bmax of 11,194+/-261 binding sites/cell. In competitive binding experiments, CART(61-102), CART(55-102) and di-iodinated CART(61-102) were bound to PC12 cell membranes with Ki in low nM range; their affinity to intact non-differentiated and differentiated cells was in low 10(-8) M range. In order to prove that iodination did not eliminate the pharmacological properties of CART, we tested the biological activity of di-iodinated CART(61-102). It decreased food intake in in vivo feeding experiment on fasted mice in a dose of 1 microg/mouse to the same extent as CART(61-102) in a dose of 0.5 microg/mouse.

Regulation of rat pituitary cocaine- and amphetamine-regulated transcript (CART) by CRH and glucocorticoids

Cocaine- and amphetamine-regulated transcript (CART) was originally isolated from rat brain, but CART is also synthesized and stored in the anterior pituitary. The localization of pituitary CART and factors regulating its synthesis are largely unknown. The regulation of pituitary CART synthesis and release in response to CRH and glucocorticoids was examined in vitro and in vivo. CART immunoreactivity (CART-IR) was released from anterior pituitary segments. This release was increased 15-fold in response to corticotropin-releasing hormone (CRH). Intraperitoneal administration of CRH to rats significantly increased plasma CART-IR. Furthermore, CART-IR content and plasma CART-IR were significantly increased in adrenalectomized rats, and anterior pituitary CART mRNA expression, CART-IR content, and plasma CART-IR were significantly decreased in corticosterone-treated rats. Plasma CART-IR showed a pattern of diurnal variation similar to that of ACTH and corticosterone, and plasma CART-IR was positively correlated with corticosterone. CART-IR was detectable in the medium of the corticotroph cell line AtT-20. Dual in situ hybridization for prepro-CART (ppCART) mRNA expression and immunocytochemistry for ACTH showed localization of ppCART mRNA to a subpopulation of ACTH-immunoreactive cells. These findings demonstrate that pituitary CART expression and release are regulated by CRH and the glucocorticoid environment and that pituitary CART is partly localized to corticotrophs.

Endogenous CART peptide regulates mu opioid and serotonin 5-HT(2A) receptors

Previous experiments conducted in this laboratory showed that intracerebroventricular (i.c.v.) administration of IgG antibodies directed against selected neuropeptides changed the density of CNS receptors, suggesting that neuropeptides in the cerebrospinal fluid can perform a regulatory role. To further test this hypothesis, we administered anti-CART peptide (the peptide product of cocaine amphetamine related transcript) IgG to rats via the i.c.v. route, and measured the density of opioid mu and delta receptors, beta-adrenergic and alpha(2)-adrenergic receptors and serotonin 5-HT(2A) receptors using ligand binding methods. We also used Western blots to determine the expression level of the mu, delta and 5-HT(2A) receptors. The results demonstrated that anti-CART peptide IgG up-regulates mu and 5-HT(2A) receptor in the hippocampus and caudate We conclude that CART peptides in the cerebrospinal fluid may exert regulatory effects in the brain.

The neuroanatomical axis for control of energy balance

The hypothalamic feeding-center model, articulated in the 1950s, held that the hypothalamus contains the interoceptors sensitive to blood-borne correlates of available or stored fuels as well as the integrative substrates that process metabolic and visceral afferent signals and issue commands to brainstem mechanisms for the production of ingestive behavior. A number of findings reviewed here, however, indicate that sensory and integrative functions are distributed across a central control axis that includes critical substrates in the basal forebrain as well as in the caudal brainstem. First, the interoceptors relevant to energy balance are distributed more widely than had been previously thought, with a prominent brainstem complement of leptin and insulin receptors, glucose-sensing mechanisms, and neuropeptide mediators. The physiological relevance of this multiple representation is suggested by the demonstration that similar behavioral effects can be obtained independently by stimulation of respective forebrain and brainstem subpopulations of the same receptor types (e.g., leptin, CRH, and melanocortin). The classical hypothalamic model is also challenged by the integrative achievements of the chronically maintained, supracollicular decerebrate rat. Decerebrate and neurologically intact rats show similar discriminative responses to taste stimuli and are similarly sensitive to intake-inhibitory feedback from the gut. Thus, the caudal brainstem, in neural isolation from forebrain influence, is sufficient to mediate ingestive responses to a range of visceral afferent signals. The decerebrate rat, however, does not show a hyperphagic response to food deprivation, suggesting that interactions between forebrain and brainstem are necessary for the behavioral response to systemic/ metabolic correlates of deprivation in the neurologically intact rat. At the same time, however, there is evidence suggesting that hypothalamic-neuroendocrine responses to fasting depend on pathways ascending from brainstem. Results reviewed are consistent with a distributionist (as opposed to hierarchical) model for the control of energy balance that emphasizes: (i) control mechanisms endemic to hypothalamus and brainstem that drive their unique effector systems on the basis of local interoceptive, and in the brainstem case, visceral, afferent inputs and (ii) a set of uni- and bidirectional interactions that coordinate adaptive neuroendocrine, autonomic, and behavioral responses to changes in metabolic status.

Hypothalamic and vagal neuropeptide circuitries regulating food intake

It has been recognized for some time that a number of different neuropeptides exert powerful effects on food intake. During the last few years, the neurocircuitry within which these peptides operate has also begun to be elucidated. Peptidergic feeding-regulatory neurones are found both in the hypothalamus and the brainstem, where they act as input stations for hormonal and gastrointestinal information, respectively. These cell populations both project to several other brain regions and interconnect extensively. The present review summarizes the neuroanatomy and connectivity of some prominent peptides involved in food intake control, including neuropeptide Y, melanocortin peptides, agouti gene-related protein, cocaine- and amphetamine-regulated transcript, orexin/hypocretin, melanin-concentrating hormone and cholecystokinin. Disturbances in the hypothalamic neuropeptide systems have been implicated in the phenotype of a genetic model of fatal hypophagia, the mouse anorexia (anx) mutation, which is also discussed.

Cocaine and amphetamine-regulated transcript peptide (CART(62-76))-induced changes in regional monoamine levels in ratbrain

Cocaine- and amphetamine-regulated transcript (CART) is a novel neuropeptide with neurotransmitter-like effects. In the present study we examined the influence of CART peptide fragment, CART(62-76), on the levels of catecholamines (dopamine and norepinephrine), serotonin and their metabolites in five regions of the rat brain. CART(62-76) was administered at 0.5 or 5.0 microg dose intracerebroventricularly. A high-pressure liquid chromatograph coupled to electrochemical array detector was used to analyse the tissue homogenates. In the frontal cortex, CART(62-76) increased the levels of 5-hydroxyindoleacetic acid (5-HIAA), a metabolite of serotonin (5-HT). In the striatum, CART(62-76) decreased the levels of noradrenaline and 5-HT. In the hypothalamus, CART(62-76) increased the levels of 5-HIAA. CART(62-76) had no significant effect in the hippocampus and cerebellum. Our data suggest that CART(62-76) peptide has no major effect on dopaminergic pathways, but it modulates the activity of striatal noradrenergic and corticostriatal and hypothalamic serotoninergic system in the rat brain. These regionally selective neurochemical changes may explain the effects of CART peptides on appetitive, emotional and locomotor behaviour.

Effect of LPS administration on the expression of POMC, NPY, galanin, CART and MCH mRNAs in the rat hypothalamus

Anorexia and weight loss are manifestations of inflammation seen both in patients and in experimental animal models such as the lipopolysaccharide (LPS)-treated rat. Using in situ hybridization, the levels of mRNAs encoding proopiomelanocortin (POMC), neuropeptide Y (NPY), galanin, melanin-concentrating hormone (MCH) and cocaine- and amphetamine-regulated transcript (CART) were investigated in the rat hypothalamus after a single intraperitoneal dose (125 microg/kg) of LPS. Four hours after LPS injection the food intake was significantly decreased. POMC and CART mRNA levels were increased in the arcuate nucleus, and MCH, CART and galanin mRNAs were all decreased in the lateral hypothalamic area in LPS-treated rats. Levels of mRNAs for NPY and galanin in the arcuate nucleus, and for MCH and CART in the zona incerta did not change significantly after LPS treatment. These findings support the hypothesis that LPS-induced factors mediate signalling to the POMC/CART neurons in the arcuate nucleus which could lead to reduced food intake by decreasing MCH, CART and galanin synthesis in target lateral hypothalamic neurons.

Hypothalamic CART and serum leptin levels are reduced in the anorectic (anx/anx) mouse

Cocaine- and amphetamine-regulated transcript (CART) is expressed in the hypothalamus, and putative peptides encoded by CART potently inhibit feeding when administered centrally. CART is strongly down-regulated in the lateral hypothalamic area and the arcuate nucleus in animal models of obesity with disrupted leptin signaling. Here we have used in situ hybridization and immunohistochemistry to study CART expression in mice homozygous for the anorexia (anx) mutation which are characterized by a much reduced food intake and premature death. anx/anx mice had significantly decreased levels of CART mRNA label and peptide-immunoreactive cell bodies and fibers in the arcuate nucleus and a lower number of detectable CART-expressing cells in the dorsomedial hypothalamic nucleus/lateral hypothalamic area. Moreover, serum leptin levels were significantly lower in anx/anx mice compared to normal littermates, most likely due to the prominent depletion of body fat in these animals. The decrease in the anorexigenic agents leptin and CART, may reflect a compensatory down-regulation in response to the energy-deprived state of anx/anx mice. Alternatively, the reduced arcuate CART expression may be a consequence of a molecular defect in the arcuate nucleus of these animals.