Effects on food and water intake of the alpha 1-adrenoceptor agonists amidephrine and SK&F-89748

Locus coeruleus anchors a trisynaptic circuit controlling fear-induced suppression of feeding

The circuit mechanisms underlying fear-induced suppression of feeding are poorly understood. To help fill this gap, mice were fear conditioned, and the resulting changes in synaptic connectivity among the locus coeruleus (LC), the parabrachial nucleus (PBN), and the central nucleus of amygdala (CeA)-all of which are implicated in fear and feeding-were studied. LC neurons co-released noradrenaline and glutamate to excite PBN neurons and suppress feeding. LC neurons also suppressed inhibitory input to PBN neurons by inducing heterosynaptic, endocannabinoid-dependent, long-term depression of CeA synapses. Blocking or knocking down endocannabinoid receptors in CeA neurons prevented fear-induced depression of CeA synaptic transmission and fear-induced suppression of feeding. Altogether, these studies demonstrate that LC neurons play a pivotal role in modulating the circuitry that underlies fear-induced suppression of feeding, pointing to new ways of alleviating stress-induced eating disorders.

Genetic and Neurobiological Analyses of the Noradrenergic-like System in Vulnerability to Sugar Overconsumption Using a Drosophila Model

Regular overconsumption of sugar is associated with obesity and type-2 diabetes, but how genetic factors contribute to variable sugar preferences and intake levels remains mostly unclear. Here we provide evidence for the usefulness of a Drosophila larva model to investigate genetic influence on vulnerability to sugar overconsumption. Using genetic and RNA interference approaches, we show that the activity of the Oamb gene, which encodes a receptor for octopamine (OA, the invertebrate homologue of norepinephrine), plays a major role in controlled sugar consumption. Furthermore, Oamb appears to suppress sugar food intake in fed larvae in an acute manner, and neurons expressing this Oamb receptor do not overlap with neurons expressing Octβ3R, another OA receptor previously implicated in hunger-driven exuberant sugar intake. Together, these results suggest that two separate sub-circuits, defined by Oamb and Octβ3R respectively, co-regulate sugar consumption according to changes in energy needs. We propose that the noradrenergic-like system defines an ancient regulatory mechanism for prevention of sugar overload.

Long-term α1B-adrenergic receptor activation shortens lifespan, while α1A-adrenergic receptor stimulation prolongs lifespan in association with decreased cancer incidence

The α1-adrenergic receptor (α1AR) subtypes, α1AAR and α1BAR, have differential effects in the heart and central nervous system. Long-term stimulation of the α1AAR subtype prolongs lifespan and provides cardio- and neuro-protective effects. We examined the lifespan of constitutively active mutant (CAM)-α1BAR mice and the incidence of cancer in mice expressing the CAM form of either the α1AAR (CAM-α1AAR mice) or α1BAR. CAM-α1BAR mice have a significantly shortened lifespan when compared with wild-type (WT) animals; however, the effect was sex dependent. Female CAM-α1BAR mice lived significantly shorter lives, while the median lifespan of male CAM-α1BAR mice was not different when compared with that of WT animals. There was no difference in the incidence of cancer in either sex of CAM-α1BAR mice. The incidence of cancer was significantly decreased in CAM-α1AAR mice when compared with that in WT, and no sex-dependent effects were observed. Further study is warranted on cancer incidence after activation of each α1AR subtype and the effect of sex on lifespan following activation of the α1BAR. The implications of a decrease in cancer incidence following long-term α1AAR stimulation could lead to improved treatments for cancer.

Octopamine-mediated circuit mechanism underlying controlled appetite for palatable food in Drosophila

The easy accessibility of energy-rich palatable food makes it difficult to resist food temptation. Drosophila larvae are surrounded by sugar-rich food most of their lives, raising the question of how these animals modulate food-seeking behaviors in tune with physiological needs. Here we describe a circuit mechanism defined by neurons expressing tdc2-Gal4 (a tyrosine decarboxylase 2 promoter-directed driver) that selectively drives a distinct foraging strategy in food-deprived larvae. Stimulation of this otherwise functionally latent circuit in tdc2-Gal4 neurons was sufficient to induce exuberant feeding of liquid food in fed animals, whereas targeted lesions in a small subset of tdc2-Gal4 neurons in the subesophageal ganglion blocked hunger-driven increases in the feeding response. Furthermore, regulation of feeding rate enhancement by tdc2-Gal4 neurons requires a novel signaling mechanism involving the VEGF2-like receptor, octopamine, and its receptor. Our findings provide fresh insight for the neurobiology and evolution of appetitive motivation.

New approaches to the pharmacological treatment of obesity: can they break through the efficacy barrier?

In this review we assess the range of centrally active anorectics that are either in human clinical trials, or are likely to be so in the near future. We describe their weight loss efficacy, mode of action at both pharmacological and behavioural levels, where understood, together with the range of side effects that might be expected in clinical use. We have however evaluated these compounds against the considerably more rigorous criteria that are now being used by the Federal Drugs Agency and European Medicines Agency to decide approvals and market withdrawals. Several trends are evident. Recent advances in the understanding of energy balance control have resulted in the exploitation of a number of new targets, some of which have yielded promising data in clinical trials for weight loss. A second major trend is derived from the hypothesis that improved weight loss efficacy over current therapy is most likely to emerge from treatments targeting multiple mechanisms of energy balance control. This reasoning has led to the development of a number of new treatments for obesity where multiple mechanisms are targeted, either by a single molecule, such as tesofensine, or through drug combinations such as qnexa, contrave, empatic, and pramlintide+metreleptin. Many of these approaches also utilise advances in formulation technology to widen safety margins. Finally, the practicality of peptide therapies for obesity has become better validated in recent studies and this may allow more rapid exploitation of novel targets, rather than awaiting the development of orally available small molecules. We conclude that novel, more efficacious and better tolerated treatments for obesity may become available in the near future.

Tesofensine, a novel triple monoamine reuptake inhibitor, induces appetite suppression by indirect stimulation of alpha1 adrenoceptor and dopamine D1 receptor pathways in the diet-induced obese rat

Tesofensine is a novel monoamine reuptake inhibitor that inhibits both norepinephrine, 5-HT, and dopamine (DA) reuptake function. Tesofensine is currently in clinical development for the treatment of obesity, however, the pharmacological basis for its strong effect in obesity management is not clarified. Using a rat model of diet-induced obesity (DIO), we characterized the pharmacological mechanisms underlying the appetite suppressive effect of tesofensine. DIO rats treated with tesofensine (2.0 mg/kg, s.c.) for 16 days showed significantly lower body weights than vehicle-treated DIO rats, being reflected by a marked hypophagic response. Using an automatized food intake monitoring system during a 12 h nocturnal test period, tesofensine-induced hypophagia was investigated further by studying the acute interaction of a variety of monoamine receptor antagonists with tesofensine-induced hypophagia in the DIO rat. Tesofensine (0.5-3.0 mg/kg, s.c.) induced a dose-dependent and marked decline in food intake with an ED(50) of 1.3 mg/kg. The hypophagic response of tesofensine (1.5 mg/kg, s.c.) was almost completely reversed by co-administration of prazosin (1.0 mg/kg, alpha(1) adrenoceptor antagonist) and partially antagonized by co-administration of SCH23390 (0.03 mg/kg, DA D(1) receptor antagonist). In contrast, tesofensine-induced hypophagia was not affected by RX821002 (0.3 mg/kg, alpha(2) adrenoceptor antagonist), haloperidol (0.03 mg/kg, D(2) receptor antagonist), NGB2904 (0.1 mg/kg, D(3) receptor antagonist), or ritanserin (0.03 mg/kg, 5-HT(2A/C) receptor antagonist). Hence, the mechanism underlying the suppression of feeding by tesofensine in the obese rat is dependent on the drug's ability to indirectly stimulate alpha(1) adrenoceptor and DA D(1) receptor function.

Neuropeptide Y, alpha-melanocyte-stimulating hormone, and monoamines in food intake regulation

Obesity is increasing in severity and prevalence in the United States and represents a major public health issue. No effective pharmacologic treatment leading to sustained weight loss currently exists. The growing interest in the regulation of food intake stems from the current drug treatments for obesity, almost all of which interfere with the monoamine system. Our knowledge of potential interactions between the orexigenic and anorexigenic pathways is limited and fragmented, making the development of targeted drug therapy for obesity difficult. The present review of the interaction of neuropeptides and monoamines emphasizes the complexity of the central mechanisms that regulate feeding behavior. Two main systems are implicated in food intake regulation: neuropeptide Y (NPY) and pro-opiomelanocortin. alpha-Melanocyte-stimulating hormone is a tridecapeptide cleaved from pro-opiomelanocortin that acts to inhibit food intake. The predominant NPY orexigenic receptors are NPY-Y1 and NPY-Y5, and the two anorexigenic melanocortin receptors involved in hypothalamic food intake control are MC3-R and MC4-R. Both neuropeptides interact with monoamines in the hypothalamus to control physiologic states such as hunger, satiation, and satiety. Serotonin suppresses food intake and body weight, acting mainly through the serotonin 1B receptor. Dopamine regulates hunger and satiety by acting in specific hypothalamic areas, through the D1 and D2 receptors. Noradrenaline activation of alpha1- and beta2-adrenoceptors decreases food intake, and stimulation of the alpha2-adrenoceptor increases food intake. A better understanding of the detailed mechanisms underlying the pathogenesis of hyperphagia and hypophagia is needed to develop new therapeutic approaches to obesity.

Noradrenergic modulation of ephedrine-induced hypophagia

The hypophagic action of the sympathomimetic amine ephedrine (EPH) in the rat may reflect actions on central dopaminergic (DA) and noradrenergic (NE) systems. EPH indirectly facilitates DA and NE activity and acts as a partial agonist at alpha(1)-adrenergic receptors. Two approaches were used to assess the possible contribution of NE and DA pathways to EPH-induced hypophagia. In the first, regression analyses of published archival data were computed to characterize the relation between the hypophagic potency values of (-)-(EPH) and related sympathomimetic drugs, including (+)-amphetamine, aminorex, mazindol, and phentermine (data derived from Blosser JC et al., 1987) and the most potent action of these drugs on facilitating NE activity or DA activity in rat brain (data derived from Rothman RB et al., 2001). In the NE analyses, the ED(50) values for these drugs for the inhibition of eating in rats were significantly related (r = 0.91, P = 0.03) to the potency of each drug in facilitating NE activity (either release or inhibition of [(3)H]NE reuptake), whereas in the DA analyses the correlation between ED(50) values and DA activity for these drugs was also significant (r = 0.98, P = 0.003). The regression analyses are thus supportive of a role for NE or DA in the hypophagic capacity of EPH. Although an earlier study noted that administration of the putative DA antagonist pimozide in rats attenuated EPH hypophagia, pimozide exerts similar potency in antagonizing DA receptors and alpha(1)-adrenergic receptors. To clarify the role of alpha(1)-adrenoceptors in EPH-induced hypophagia, adult male rats were pretreated with the alpha(1)-adrenergic receptor antagonist prazosin (0.0.5 and 2 mg/kg) prior to the administration of (-)-EPH (0, 5, 10, or 20 mg/kg, IP). Prazosin pretreatment at 2.0 mg/kg significantly attenuated the hypophagia, but not the hypodipsia, induced by administration of 10 mg/kg and by 20 mg/kg (-)-EPH. Collectively, these results confirm a critical contribution of of alpha(1)-adrenoceptors to the hypophagic action of (-)-EPH in rats.

Norepinephrine and the control of food intake

The focus of the present review is to reconsider the role of endogenous norepinephrine (NE) in brain, specifically within the hypothalamic paraventricular nucleus (PVN), with regard to its potential role in eliciting eating or satiety. The PVN is innervated by NE fibers and is a site at which infusion of exogenous NE elicits eating at low doses. Two subtypes of alpha-adrenergic receptors within the PVN exert antagonistic actions on eating in the rat: activation of PVN alpha(2)-adrenoceptors increases eating, whereas activation of PVN alpha(1)-adrenoceptors suppresses eating. Pharmacologic manipulations that elevate NE can increase or decrease food intake, depending on the site and type of NE manipulation. Certain antiobesity drugs may act to reduce eating via release of NE and subsequent activation of alpha(1)-adrenoceptors. The PVN exhibits a reliable rhythm in the secretion of endogenous NE over the dark-and-light cycle, and this rhythm may interact with changes in numbers of PVN alpha(1)- and alpha(2)-adrenoceptors to modulate eating during the dark-and-light cycle. Push-and-pull and microdialysis studies indicate that NE secretion is strongly associated with eating, particularly at the start of the dark phase. The present review considers potential interactions of NE with substances such as leptin and neuropeptide Y that alter eating.

In vivo IL-1beta-induced modulation of G-protein alphaO subunit subclass in the hypothalamic ventromedial nucleus: implications to IL-1beta-associated anorexia

The intracerebroventricular (i.c.v.) administration of interleukin-1beta (IL-1beta) induces anorexia in rats at doses that yield estimated pathophysiological concentrations in the cerebrospinal fluid. IL-1beta also induces anorexia when administered into the hypothalamic ventromedial nucleus (VMN), an important brain site for the control of feeding. A variety of guanine nucleotide binding protein (G-protein) coupled receptors (e. g., for neurotransmitters and neuropeptides) participate in the integrative regulation of feeding. Our previous studies reported that the VMN G-protein alphaO common subunit subclass is involved in the control of normal feeding, and that IL-1beta modulates calcium channel currents via a pertussis toxin (PTX)-sensitive G-protein (GalphaO/Galphai). Here, we examined the profile of GalphaO protein expression in the hypothalamic VMN during IL-1beta-induced anorexia. Intracerebroventricular microinfusion of IL-1beta (0.5 to 8.0 ng/24 h for 72 h) into the third cerebral ventricle dose-dependently induced anorexia (p<0.001) and decreased the VMN GalphaO common protein levels (p<0.001). Heat-inactivated IL-1beta and IL-1beta plus IL-1 receptor antagonist (a competitive inhibitor of IL-1beta action) had no effect on food intake or on VMN GalphaO common protein content. RT-PCR analysis of VMN RNA from IL-1beta-treated rats generated an expression profile for GalphaO common subunit; however, no modulation at the mRNA level was observed. The results suggest that anorexia induced by the central administration of IL-1beta involves modification of G-protein alphaO common subunit profile in the central nervous system.

Alpha1-adrenergic and dopaminergic receptors are involved in the anoretic effect of corticotropin-releasing factor in goldfish

This study investigates the noradrenergic and/or dopaminergic receptors subtypes involved in the anoretic action of CRF in goldfish. Agonists and antagonists of alpha1- and alpha2-adrenoceptors, and D1- and D2-dopaminergic receptors were intracerebroventricularly (i.c.v.) administered alone or in combination with CRF in the case of antagonists. Food intake and hypothalamic content of catecholamines and their metabolites were measured at 2 h postinjection. On one hand, alpha1-adrenergic receptor antagonist, but not alpha2, blocked the anoretic effect of CRF. Moreover, we found a blockade of CRF-induced anoretic action by pretreatment with specific D1- and D2-dopaminergic antagonists. On the other hand, the i.c.v. administration of CRF reduced hypothalamic norepinephrine (NE) and dopamine (DA) content, without modifications in their metabolism. Thus, our results suggest that the anoretic effect of CRF appears to be mediated by alpha1-adrenergic and dopaminergic receptors in fish. Second, the reduction in hypothalamic NE and DA synthesis could be due to a direct effect of CRF treatment and/or a secondary effect of food intake reduction.

Investigation of the mechanisms underlying the hypophagic effects of the 5-HT and noradrenaline reuptake inhibitor, sibutramine, in the rat

1. Sibutramine is a novel 5-hydroxytryptamine (5-HT) and noradrenaline reuptake inhibitor (serotonin-noradrenaline reuptake inhibitor, SNRI) which is currently being developed as a treatment for obesity. Sibutramine has been shown to decrease food intake in the rat. In this study we have used a variety of monoamine receptor antagonists to examine the pharmacological mechanisms underlying sibutramine-induced hypophagia. 2. Individually-housed male Sprague-Dawley rats were maintained on reversed phase lighting with free access to food and water. Drugs were administered at 09 h 00 min and food intake was monitored over the following 8 h dark period. 3. Sibutramine (10 mg kg-1, p.o.) produced a significant decrease in food intake during the 8 h following drug administration. This hypophagic response was fully antagonized by the alpha 1-adrenoceptor antagonist, prazosin (0.3 and 1 mg kg-1, i.p.), and partially antagonized by the beta 1-adrenoceptor antagonist, metoprolol (3 and 10 mg kg-1, i.p.) and the 5-HT receptor antagonists, metergoline (non-selective; 0.3 mg kg-1, i.p.); ritanserin (5-HT2A/2C; 0.1 and 0.5 mg kg-1, i.p.) and SB200646 (5-HT2B/2C; 20 and 40 mg kg-1, p.o.). 4. By contrast, the alpha 2-adrenoceptor antagonist, RX821002 (0.3 and 1 mg kg-1, i.p.) and the beta 2-adrenoceptor antagonist, ICI 118,551 (3 and 10 mg kg-1, i.p.) did not reduce the decrease in food intake induced by sibutramine. 5. These results demonstrate that beta 1-adrenoceptors, 5-HT2A/2C-receptors and particularly alpha 1-adrenoceptors, are involved in the effects of sibutramine on food intake and are consistent with the hypothesis that sibutramine-induced hypophagia is related to its ability to inhibit the reuptake of both noradrenaline and 5-HT, with the subsequent activation of a variety of noradrenaline and 5-HT receptor systems.

Effects of the alpha 1a-adrenoceptor antagonist RS-17053 on phenylpropanolamine-induced anorexia in rats

Activation of alpha 1-Adrenergic receptors via systemic administration of drugs such as phenylpropanolamine (PPA) and cirazoline results in the suppression of feeding in rats. Whether PPA acts via activation of the three currently identified alpha 1-Adrenoceptor subtypes is unknown. The intent of the present study was thus to examine the effects of systemic administration of the novel alpha 1a-Adrenoceptor antagonist RS-17053 on PPA-induced anorexia. Adult male rats (n = 6 to 8 per group) were pretreated (IP) with either 0, 0.1, 0.5, 2.5, or 10.0 mg/kg RS-17053 or with 2.0 mg/kg of the prototypical alpha 1-Adrenoceptor antagonist prazosin. Five minutes later, each rat was treated (IP) with either 0, 5, 10 or 15 mg/kg PPA. Food and water intakes were recorded for a 30 min period starting 10 min after the the treatment injection. Rats pretreated with vehicle and then treated with PPA exhibited a dose-dependent suppression of feeding with a maximal effect evident at the 15 mg/kg dose of PPA. Pretreatment with 2.0 mg/kg prazosin reversed the anorexic activity of PPA. Pretreatment with RS-17053 (0.1-2.5 mg/kg) did not alter either baseline feeding or the anorexic action of PPA. These results suggest that PPA does not act via the alpha 1a-Adrenergic receptor subtype to suppress food intake.

Antisense oligodeoxynucleotides to G-protein alpha-subunit subclasses identify a transductional requirement for the modulation of normal feeding dependent on G alpha OA subunit

A variety of G-protein-coupled receptors are proposed to participate in the modulation of ingestive behavior and in the mode of action of antiobesity drugs. In the present study, we investigated the involvement of G-protein alpha-subunit subclasses (molecular transducers of multiple chemical signals) in the control of ingestive behavior. We report here that the chronic intracerebroventricular (i.c.v.) microinfusion for 72 h (via osmotic minipumps) with antisense phosphothio-oligodeoxynucleotides corresponding to G-protein alpha-subunitO common (to OA and OB) and OA subclasses decrease the nighttime food intake without affecting water intake in rats. Computerized analyses of the microstructure of feeding indicate that the G alpha OA antisense depresses feeding by reducing meal frequency, while meal size and meal duration increased slightly, but not significantly. The effects of G alpha O common and G alpha OA antisense on feeding are specific since the chronic i.c.v. microinfusion of sense to G alpha O common or G alpha OA, antisense to the related subclass G alpha OB, and antisense to other G-protein alpha-subunits (G alpha S, G alpha Q, G alpha 11 and G alpha i common) had no effect on food or water intake. The observed effects by G alpha O common and G alpha OA antisense imply a direct action in the central nervous system since the chronic subcutaneous microinfusion of G alpha O common and G alpha OA antisense in doses equivalent to two-fold higher relative to those administered centrally had no effect on food intake. The chronic microinfusion of G alpha O common antisense drastically decreased the levels of G alpha O protein detected in immunoblots of hypothalamic ventromedial nuclei. The results suggest that the G-protein alpha-subunit subclass G alpha OA is critical for the integrative modulation of normal feeding behavior, and that changes in its activity may be associated with modifications of feeding. These studies also show a novel approach to study the molecular basis of specific behaviors by manipulating elements of the transductional systems.

Conditioned taste aversion in rats induced by the alpha 1-adrenoceptor agonist cirazoline

Recent studies have indicated that alpha 1-adrenoceptor agonists such as phenylpropanolamine (PPA), cirazoline, amidephrine, and SK&F-89748 suppress food intake in rats. These compounds activate alpha 1-adrenoceptors within the paraventricular hypothalamic nucleus (PVN) and may excite efferent fibers that inhibit feeding. Studies of the effects of alpha 1-agonists suggest a specificity for feeding behavior, but no study to date has evaluated whether these agonists may suppress feeding behavior by the induction of malaise. Accordingly, the present experiment examined the ability of systemically administered cirazoline (0.1, 0.2, and 0.4 mg/kg, IP) to induce conditioned taste aversion (CTA) to a saccharin solution. Significant CTA was noted for 0.2 and 0.4 mg/kg cirazoline but not for 0.1 mg/kg cirazoline, compared to a vehicle treatment. The ED50 for cirazoline-induced aversion was computed to be 0.3 mg/kg, which contrasts with an ED50 value of 0.09 mg/kg for the effect of cirazoline on food intake (computed in other studies). More importantly, a 0.1 mg/kg dose of cirazoline, which is slightly greater than that of the ED50 value for suppression of feeding, did not induce significant CTA in the present study. These results suggest that malaise is not a prominent factor in the suppressive activity of cirazoline on food intake and advocate the use of cirazoline as an effective appetite suppressant.