Selective involvement of dopamine in the nucleus accumbens in the feeding response elicited by muscimol injection in the nucleus raphe dorsalis of sated rats

Pharmacological targeting of glutamatergic neurons within the brainstem for weight reduction

Food intake and body weight are tightly regulated by neurons within specific brain regions, including the brainstem, where acute activation of dorsal raphe nucleus (DRN) glutamatergic neurons expressing the glutamate transporter Vglut3 (DRN^Vglut3) drive a robust suppression of food intake and enhance locomotion. Activating Vglut3 neurons in DRN suppresses food intake and increases locomotion, suggesting that modulating the activity of these neurons might alter body weight. Here, we show that DRN^Vglut3 neurons project to the lateral hypothalamus (LHA), a canonical feeding center that also reduces food intake. Moreover, chronic DRN^Vglut3 activation reduces weight in both leptin-deficient (ob/ob) and leptin-resistant diet-induced obese (DIO) male mice. Molecular profiling revealed that the orexin 1 receptor (Hcrtr1) is highly enriched in DRN Vglut3 neurons, with limited expression elsewhere in the brain. Finally, an orally bioavailable, highly selective Hcrtr1 antagonist (CVN45502) significantly reduces feeding and body weight in DIO. Hcrtr1 is also co-expressed with Vglut3 in the human DRN, suggesting that there might be a similar effect in human. These results identify a potential therapy for obesity by targeting DRN^Vglut3 neurons while also establishing a general strategy for developing drugs for central nervous system disorders.

GABA-A and GABA-B receptors mediate feeding elicited by the GABA-B agonist baclofen in the ventral tegmental area and nucleus accumbens shell in rats: reciprocal and regional interactions

Food intake is significantly increased following administration of GABA-B and GABA-A agonists into the nucleus accumbens (NAC) shell and ventral tegmental area (VTA) with receptor-selective antagonist pretreatment capable of blocking these responses within sites. Regional interactions in feeding studies have been evaluated by administering an antagonist in one site of interest prior to administration of the feeding-active agonist in a second site of interest and have identified important relationships, particularly for opioid-opioid interactions. To evaluate whether regional and reciprocal VTA and NAC shell interactions occur for GABA-mediated feeding, the present study examined whether feeding elicited by the GABA-B agonist, baclofen, microinjected into the NAC shell was dose-dependently blocked by pretreatment with either the GABA-B antagonist, saclofen, or the GABA-A antagonist, bicuculline, into the VTA, and then whether VTA baclofen-induced feeding was dose-dependently blocked by NAC shell pretreatment of either saclofen or bicuculline in rats. Rats were stereotaxically implanted with bilateral pairs of cannulae aimed at the VTA and NAC shell and were assessed for food intake following vehicle and baclofen (200 ng) in each site. Baclofen produced similar magnitudes of increased food intake following VTA and NAC shell treatment. Baclofen administration in the VTA and NAC shell was preceded 20 min earlier with administration of bicuculline (0, 7.5, 75, 150, 300 ng) or saclofen (0, 0.5, 1.5, 3, 5 μg) into the other site with intake measured 1, 2 and 4h after agonist treatment. VTA saclofen dose-dependently and significantly blocked feeding elicited by NAC shell baclofen. Correspondingly, NAC shell saclofen dose-dependently and significantly blocked feeding elicited by VTA baclofen, indicating a robust and bidirectional GABA-B/GABA-B receptor interaction between sites. Whereas VTA bicuculline significantly blocked the increased feeding elicited by NAC shell baclofen, NAC shell bicuculline reduced but did not block feeding elicited by VTA baclofen, indicating a unidirectional interaction GABA-B/GABA-A receptor interaction between sites. Unlike within-site receptor specificity governing the ability of GABA agonist mediation of food intake, the present study demonstrates that GABA, like opioids, employs a distributed brain network in mediating its ingestive effects, and that under certain circumstances, uses multiple receptor subtypes to underlie its regional effects.

Deep brain stimulation in the treatment of obesity

Obesity is a growing global health problem frequently intractable to current treatment options. Recent evidence suggests that deep brain stimulation (DBS) may be effective and safe in the management of various, refractory neuropsychiatric disorders, including obesity. The authors review the literature implicating various neural regions in the pathophysiology of obesity, as well as the evidence supporting these regions as targets for DBS, in order to explore the therapeutic promise of DBS in obesity.

The lateral hypothalamus and ventromedial hypothalamus are the appetite and satiety centers in the brain, respectively. Substantial data support targeting these regions with DBS for the purpose of appetite suppression and weight loss. However, reward sensation associated with highly caloric food has been implicated in overconsumption as well as obesity, and may in part explain the failure rates of conservative management and bariatric surgery. Thus, regions of the brain's reward circuitry, such as the nucleus accumbens, are promising alternatives for DBS in obesity control.

The authors conclude that deep brain stimulation should be strongly considered as a promising therapeutic option for patients suffering from refractory obesity.

GABA receptor subtype antagonists in the nucleus accumbens shell and ventral tegmental area differentially alter feeding responses induced by deprivation, glucoprivation and lipoprivation in rats

GABA(A) and GABA(B) receptor agonists stimulate feeding following microinjection into the nucleus accumbens shell and ventral tegmental area, effects blocked selectively and respectively by GABA(A) and GABA(B) receptor antagonists. GABA antagonists also differentially alter opioid-induced feeding responses elicited from these sites. Although GABA agonists and antagonists have been shown to modulate feeding elicited by deprivation or glucoprivation, there has been no systematic examination of feeding elicited by homeostatic challenges following GABA antagonists in these sites. Therefore, the present study examined the dose-dependent ability of GABA(A) (bicuculline, 75-150 ng) and GABA(B) (saclofen, 1.5-3 microg) antagonists administered into the nucleus accumbens shell or ventral tegmental area upon feeding responses elicited by food deprivation (24 h), 2-deoxy-D-glucose-induced glucoprivation (500 mg/kg) or mercaptoacetate-induced lipoprivation (70 mg/kg). A site-specific effect of GABA receptor antagonism was observed for deprivation-induced feeding in that both bicuculline and saclofen administered into the nucleus accumbens shell, but not the ventral tegmental area, produced short-term (1-4 h), but not long-term (24-48 h) effects upon deprivation-induced intake without meaningfully altering body weight recovery. In contrast to the relative inability of GABA receptor antagonism in both sites to alter 2-deoxy-D-glucose-induced intake, mercaptoacetate-induced intake was eliminated by saclofen and significantly reduced by bicuculline in the nucleus accumbens shell and eliminated by both bicuculline and saclofen in the ventral tegmental area. These data reinforce the findings that GABA(A) and GABA(B) receptors in the nucleus accumbens shell and ventral tegmental area are not only important in the modulation of pharmacologically induced feeding responses, but also participate in differentially mediating the short-term feeding response to food deprivation in the nucleus accumbens shell as well strongly modulating lipoprivic, but not glucoprivic feeding responses in both sites.

GABA(A) receptor modulation of the rewarding and aversive effects of ethanol

Ethanol has been shown to exert many of its biochemical and behavioral effects through an interaction with the gamma-aminobutyric acid (GABA) receptor system. This review focuses on a subset of studies that has used self-administration, as well as place and taste conditioning, procedures to investigate a role for the GABA(A) receptor system in modulating the rewarding and aversive effects of ethanol. Potential advantages and disadvantages of each procedure are also discussed. A significant amount of evidence supports the suggestion that GABA(A) receptors are important modulators of the motivational effects of ethanol, although most of the findings have been obtained from studies examining oral ethanol self-administration. Relatively fewer studies have investigated ethanol place and taste conditioning. All self-administration studies reviewed used rats, whereas most conditioning studies used mice. Results of these studies show that GABA(A) antagonists and inverse agonists reduce ethanol self-administration under limited-access conditions. The effect of GABA(A) agonists on ethanol self-administration is less clear due to their bidirectional effects. GABA(A) receptor antagonists have been shown to increase ethanol-induced conditioned place preference and conditioned taste aversion in mice and decrease ethanol-induced conditioned taste aversion in rats. Issues related to interpretation and integration of these findings across models and species are considered. The integration of data from self-administration and conditioning procedures is necessary to define the role of GABA(A) receptors in modulating the rewarding and aversive effects of ethanol and may lead to the development of pharmacotherapies that target GABA(A) receptors to treat alcoholism in human beings.

Specific changes in food intake elicited by blockade or activation of glutamate receptors in the nucleus accumbens shell

Blockade of non-N-methyl-D-aspartic acid (NMDA) ionotropic glutamate receptors in the nucleus accumbens shell (AcbSh) with 6,7-dinitroquinoxaline-2,3-dione (DNQX) elicits intense feeding in satiated rats. In order to determine whether or not this feeding is part of a general behavioral activation, we observed the effect of intra-AcbSh DNQX injections on intake of solid food, liquid food, and water, and on gnawing behavior. In addition, we investigated the possibility that activation of a subset of these receptors with (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) could suppress feeding. DNQX significantly increased intake of solid and liquid food, but did not significantly affect water intake or gnawing behavior. Furthermore, injections of AMPA into the AcbSh suppressed deprivation-induced feeding and intake of a palatable 5% sucrose solution without affecting water intake in water-deprived rats. Taken together, these data suggest that DNQX is acting on a system specifically involved with the regulation of food intake.

Median and dorsal raphe injections of the 5-HT1A agonist, 8-OH-DPAT, and the GABAA agonist, muscimol, increase voluntary ethanol intake in Wistar rats

Low doses of the selective 5-HT1A agonist 8-OH-DPAT increase ethanol intake in a limited access paradigm following peripheral injection. This may be due to a reduction in 5-HT neurotransmission following activation of raphe somatodendritic autoreceptors. In order to test this hypothesis, and to determine the effects of selective reductions in raphe 5-HT activity, experiments examined the effects of injecting 8-OH-DPAT into the dorsal raphe (0, 0.02, 0.1, 1 and 2.5 micrograms) or the median raphe (0, 0.1, 1 and 5 micrograms) in rats trained to drink 12% ethanol for 40 min each day. The effects of the GABAA agonist, muscimol, on ethanol intake were also examined. Ethanol intake was increased at the highest dose of 8-OH-DPAT following injection into either site, with no change in water intake. Thus, the effects of 8-OH-DPAT are selective for ethanol. The selective 5-HT1A antagonist, (+)-WAY100135 (0.3, 1 and 3 mg/kg), blocked the effect of 8-OH-DPAT, showing that activation of 5-HT1A receptors underlies the ethanol drinking induced by 8-OH-DPAT. These results are consistent with the idea that reduced 5-HT function increases ethanol intake. Several behavioral mechanisms for this effect are discussed. Muscimol (50-100 ng) also increased ethanol drinking. Following injection into the median raphe, muscimol also stimulated water intake. These effects are probably due to non-specific behavioural activation induced by this treatment. However, the effect of muscimol in the dorsal raphe was specific for ethanol since water intake was not altered.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAA receptors in the amygdala: role in feeding in fasted and satiated rats

The purpose of this study was to clarify further the site of action in the amygdala as well as functional characteristics of feeding in response to two GABA receptor agonists. Guide cannulae for microinjection were implanted stereotaxically in the rat just above the central nucleus of the amygdala (CNA). Microinjections of 0.05, 0.25, 0.5 or 1.0 nmol muscimol, a GABAA-selective receptor agonist, produced a dose- and time-dependent decrease of food intake in both the satiated and fasted rat. The bilateral injection of muscimol into the amygdala was more effective than a unilateral injection during the first 2 h, although the overall effects were similar. Microinjection of 0.1 nmol bicuculline methiodide, a GABAA receptor antagonist, into the CNA significantly blocked this inhibitory effect of 0.05 and 0.5 nmol muscimol again in both the satiated and fasted rat. Doses of 0.05, 0.5, 5.0 and 10.0 nmol of the selective GABAB agonist, baclofen, injected into homologous sites in the CNA did not alter food intake. These findings support the viewpoint that the amygdala and its central nucleus comprise a pivotal region involved in the mechanisms underlying the control of feeding behavior. Further, it is envisaged that hypophagic or anorexic responses are induced through the activation of GABAA receptors by the presynaptic release of GABA from neurons which form a component of the anatomical system for hunger and satiety.

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

Nonserotonergic control of nucleus accumbens dopamine metabolism by the median raphe nucleus

Injections of the GABA agonist muscimol into the median raphe nucleus (MR) have been shown to result in an acceleration of dopamine metabolism within the nucleus accumbens. To examine whether serotonergic mechanisms play a role in this effect, muscimol or its vehicle was injected into the MR of either control subjects or of rats that had received prior injections of the serotonin-depleting agent p-chlorophenylalanine (PCPA). Although PCPA treatments produced massive depletions of forebrain serotonin, they failed to alter the effect of muscimol infusions on dopamine metabolism. This finding suggests that the effects of intra-MR injections of muscimol on accumbens dopamine turnover do not result entirely from an interaction between serotonergic and dopaminergic systems.

Activation of mu opioid receptors in the nucleus raphe dorsalis blocks apomorphine-induced aggression in rats: serotonin appears not to be involved

The role of mu opioid receptors in the nucleus raphe dorsalis (DR) in the control of apomorphine-induced aggression was studied in rats. Administration in the DR of a selective mu opioid receptor agonist, (D-Ala2,N-Me-Phe4,Gly5-ol)-enkephalin (DAGO), in doses ranging from 0.01 to 1 microgram/0.5 microliter, dose-dependently reduced aggression caused by apomorphine 20 mg/kg intraperitoneally. 0.01 microgram DAGO significantly reduced the time spent by the animals in aggressive posture and 0.1 and 1 microgram markedly reduced both aggressive postures and attacks. 5 micrograms (in 0.5 microliter) naloxone in the DR completely antagonized the anti-aggressive effect of DAGO (0.1 microgram/0.5 microliter) injected in the same area. 0.1 and 1 microgram but not 0.01 microgram DAGO significantly increased serotonin (5-HT) metabolism in the striatum, a terminal area almost exclusively innervated by DR, indicating that the activity of 5-HT cells in the DR was modified by DAGO. In animals given 6 micrograms/3 microliters 5,7-dihydroxytryptamine in the DR 11 days before, in which striatal 5-HT levels were markedly depleted, no significant changes were found in the time spent by the apomorphine-treated animals in aggressive postures, numbers of attacks or anti-aggressive effect of 0.1 and 1 microgram DAGO administered in the DR. The study shows for the first time that activation of mu opioid receptors in the DR has a powerful anti-aggressive effect in one model of experimental aggression by a mechanism apparently not involving changes in the activity of 5-HT cells in this area.

Biochemical hypotheses on antidepressant drugs: a guide for clinicians or a toy for pharmacologists?

The development of knowledge about the mechanism of action of tricyclic and the so-called 'atypical' antidepressants (AD) is reviewed. The discovery of clinically active antidepressants with little or no effect on noradrenaline or serotonin uptake has disproved the widely accepted concept that inhibition of monoamine uptake is a prerequisite for antidepressant activity. Another serious objection to this hypothesis is that blockade of monoamine uptake occurs in a matter of minutes after administration while 2-3 weeks of repeated treatment are necessary for the clinical AD effect. Nevertheless, the effect of repeated treatment with AD is compatible with the hypothesis that changes in central monoamine transmission are involved in the clinical activity of these drugs. Major changes in monoamine function after repeated treatment with AD include: desensitization and reduced density of noradrenaline receptors coupled to the adenylcyclase system, opposite changes in the sensitivity of alpha 1 (increased) and alpha 2-adrenoreceptors (decreased), down regulation of serotonin2 receptors and complex changes in the behavioural and electrophysiological responsiveness to serotonin agonists, subsensitivity of presynaptic dopamine receptors and enhanced activity of the mesolimbic dopamine system, decreased and increased density of GABA-A and GABA-B receptors respectively and down regulation of [3H]benzodiazepine binding. It remains to be clarified whether some of these changes have larger roles than others or whether they all contribute to the AD activity. An important role of dopamine in the activity of AD drugs is suggested by findings in the forced swimming test, whereas both catecholamines seem to be involved in the attenuation of escape deficit provoked by inescapable shock (learned helplessness).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative studies of the ingestive behaviors produced by microinjections of muscimol into the midbrain raphe nuclei of the ventral tegmental area of the rat

Microinjection of the GABA-A agonist muscimol into the median (MR) or dorsal (DR) raphe nuclei or the ventral tegmental area (VTA) of non-deprived rats induced intense feeding and drinking in a dose-dependent and site-specific manner. Lower doses of muscimol were required to increase food intake, spillage and water intake with injections into the MR than with injections into the other two sites. These data demonstrate that the MR is a more sensitive site for the elicitation of ingestive behavior than either the DR or the VTA.

Eating caused by neuropeptide-Y injection in the paraventricular hypothalamus: response to (+)-fenfluramine and (+)-amphetamine in rats

(+)-Fenfluramine and (+)-amphetamine have been compared for their ability to reduce food intake in food-deprived rats or eating caused by injecting neuropeptide-Y in the paraventricular hypothalamus of free feeding rats. (+)-Fenfluramine at doses ranging from 0.625 to 5 mg kg-1 reduced eating caused by neuropeptide-Y more effectively than it did the food intake of food-deprived rats, whereas (+)-amphetamine (dose range 0.625-2.5 mg kg-1) reduced both types of eating to a similar extent. The results confirm that (+)-fenfluramine, although less potent than (+)-amphetamine in reducing eating by food-deprived rats, markedly reduces overeating caused by various endogenous substances or stress in free feeding rats. The physiological significance of the neuropeptide-Y-induced eating and its control by (+)-fenfluramine remains to be elucidated.