Baclofen pretreatment attenuates the suppressant effect of intraperitoneal administration of cholecystokinin (CCK) on food intake in rats

Role of leptin in the regulation of food intake in fasted mice

Leptin is well acknowledged as an anorexigenic hormone that plays an important role in feeding control. Hypothalamic GABA system plays a significant role in leptin regulation on feeding and metabolism control. However, the pharmacological relationship of leptin and GABA receptor is still obscure. Therefore, we investigated the effect of leptin or combined with baclofen on the food intake in fasted mice. We detected the changes in hypothalamic c-Fos expression, hypothalamic TH, POMC and GAD67 expression, plasma insulin, POMC and GABA levels to demonstrate the mechanisms. We found that leptin inhibit fasting-induced increased food intake and activated hypothalamic neurons. The inhibitory effect on food intake induced by leptin in fasted mice can be reversed by pretreatment with baclofen. Baclofen reversed leptin's inhibition on c-Fos expression of PAMM in fasted mice. Therefore, these results indicate that leptin might inhibit fasting-triggered activation of PVN neurons via presynaptic GABA synaptic functions which might be partially blocked by pharmacological activating GABA-B. Our findings identify the role of leptin in the regulation of food intake.

Neuroendocrine response to GABA-B receptor agonism in alcohol-dependent individuals: Results from a combined outpatient and human laboratory experiment

Gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the nervous system, plays an important role in biobehavioral processes that regulate alcohol seeking, food intake, and stress response. The metabotropic GABA-B receptor has been investigated as a potential therapeutic target for alcohol use disorder, by using orthosteric agonists (e.g., baclofen) and positive allosteric modulators. Whether and how pharmacological manipulation of the GABA-B receptor, in combination with alcohol intake, may affect feeding- and stress-related neuroendocrine pathways remains unknown. In the present randomized, double-blind, placebo-controlled study, thirty-four alcohol-dependent individuals received baclofen (30 mg/day) or placebo in a naturalistic outpatient setting for one week, and then performed a controlled laboratory experiment which included alcohol cue-reactivity, fixed-dose priming, and self-administration procedures. Blood samples were collected, and the following neuroendocrine markers were measured: ghrelin, leptin, amylin, glucagon-like peptide-1 (GLP-1), insulin, prolactin, thyroid-stimulating hormone, growth hormone, cortisol, and adrenocorticotropic hormone (ACTH). During the outpatient phase, baclofen significantly increased blood concentrations of acyl-ghrelin (p = 0.01), leptin (p = 0.01), amylin (p = 0.004), and GLP-1 (p = 0.02). Significant drug × time-point interaction effects for amylin (p = 0.001) and insulin (p = 0.03), and trend-level interaction effects for GLP-1 (p = 0.06) and ACTH (p = 0.10) were found during the laboratory experiment. Baclofen, compared to placebo, had no effect on alcohol drinking in this study (p's ≥ 0.05). Together with previous studies, these findings shed light on the role of the GABAergic system and GABA-B receptors in the shared neurobiology of alcohol-, feeding-, and stress-related behaviors.

Effects of the GABAB receptor agonist baclofen on primary drinking in rats

The effects of subcutaneous (s.c.) administration of the GABA(B) receptor agonist baclofen were investigated on primary drinking in rats. Baclofen (1-4 mg/kg) produced a dose-related reduction in cumulative water intake in 16 h water-deprived rats during the 120 min measurement period (Experiment 1). The suppressant effect of baclofen (2mg/kg) on water intake 16 h water-deprived rats was significantly attenuated by pretreatment with the GABA(B) receptor antagonist CGP 35348 (3-aminopropyl (diethoxymethyl)-phosphinic acid; 50mg/kg; s.c., Experiment 2.), indicating that the hypodipsic effects of the drug in thirsty rats are mediated by an action at GABA(B) receptors. Experiment 3 was undertaken to investigate the effects of baclofen on volemic drinking induced in rats pretreated with propylene glycol. S.C. administration of polyethylene glycol induces volemic drinking in rats by reducing extracellular fluid. Baclofen (2mg/kg, s.c.) significantly reduced the volemic drinking in rats pretreated with polyethylene glycol (30% w/v solution). Experiment 4 was conducted to investigate the effects of baclofen on osmotic drinking in non-deprived rats pretreated with hypertonic sodium chloride (NaCl) solution. Hypertonic NaCl will draw out intracellular fluid to stimulate osmotic drinking. Baclofen (2mg/kg; s.c.) significantly reduced osmotic drinking in rats pretreated with 1 ml hypertonic NaCl (16% w/v). The results of this study indicate that (i) the hypodipsic effect of baclofen in water-deprived rats is mediated by an action at GABA(B) receptors and (ii) baclofen suppresses both volemic and osmotic drinking.

Effects of intraperitoneal administration of the GABA B receptor agonist baclofen on food intake in rats measured under different feeding conditions

The effects of intraperitoneal (i.p.) administration of the GABA(B) receptor agonist baclofen were assessed in rats under different feeding conditions. In Experiment 1, it was observed that baclofen (1-4 mg/kg) significantly (at least, P<0.05) increased cumulative food intake in non-deprived rats during the 120 min measurement period during the early light phase of the light-dark cycle. By contrast, during the early dark phase of the light-dark cycle in non-deprived rats, the 1mg/kg doses of baclofen significantly increased cumulative feeding at 30, 60 and 120 min (at least P<0.05), the 2mg/kg dose significantly increased feeding at 30 and 60 min (at least P<0.05) and the 4 mg/kg dose had no effects on feeding. In Experiment 2, baclofen (1-4 mg/kg) was found to produce no significant effects on food intake in rats that were food-deprived for 22 h. In Experiment 3, the effects of baclofen were investigated on food intake in 16 h food-deprived rats that had received an oral preload for 2h prior to drug administration. Baclofen (1-4 mg/kg) significantly increased cumulative food consumption (at least, P<0.05) only during the first 30 min after administration in these animals. The results of this study indicate that the effects of baclofen on food intake may be related to the state of hunger or satiety of the animals and the time during the light-dark cycle when the drug is administered.

Baclofen, raclopride, and naltrexone differentially affect intake of fat/sucrose mixtures under limited access conditions

This study assessed the effects of the opioid antagonist naltrexone, the dopamine 2-like (D2) antagonist raclopride, and the GABA(B) agonist baclofen on consumption of fat/sucrose mixtures (FSM) using a limited access protocol. Sixty male Sprague-Dawley rats were grouped according to two schedules of access (Daily [D] or Intermittent [I]) to an optional FSM. Each FSM was created by whipping 3.2% (L), 10% (M), or 32% (H) powdered sugar into 100% vegetable shortening in a w/w manner (n=10 per group). One-hour intakes of the IL and IM groups were significantly greater than intakes of the respective DL and DM groups, thus fulfilling our operational definition of binge-type eating in these groups. Baclofen reduced intakes of the L and M mixtures regardless of access schedule, but failed to reduce intake of the H mixture. Naltrexone reduced intake in all groups, but potency was greater in IL rats than in DL rats. Furthermore, potency was attenuated in Intermittent rats, but enhanced in Daily rats, at higher sucrose concentrations. Raclopride reduced intake in the DL and stimulated intake in the IL groups, reduced intake in both M groups, and was without effect in both H groups. These results indicate that fat/sucrose mixtures containing relatively low concentrations of sucrose allow distinctions to be made between: 1) intakes stimulated by different access schedules and 2) opioid and dopaminergic modulation of those intakes. These results also suggest that brief bouts of food consumption involving fatty, sugar-rich foods may prove to be particularly resistant to pharmacological intervention.

The effects of acute multiple intraperitoneal injections of the GABAB receptor agonist baclofen on food intake in rats

This study was undertaken to examine the effects of acute repeated administration of the GABA(B) receptor agonist baclofen on food intake in rats. In Experiment 1, the effects of repeated intraperitoneal (i.p.) injections of the GABA(B) receptor agonist baclofen (1 and 2 mg/kg) at 2 h intervals were investigated on food intake in non-deprived male Wistar rats. Both doses of baclofen significantly increased food intake after the 1st injection (P<0.05), but had no effects on intake following the 2nd and 3rd injections. By contrast, in Experiment 2, diazepam (1 and 2 mg/kg, i.p.) significantly increased food intake (at least, P<0.05) after each of 3 injection separated by 2 h in non-deprived rats. These data show that tolerance occurs to the hyperphagic effects of baclofen with acute multiple injections, and may have important implications for future studies investigating the effects of GABA(B) receptor agonists on food intake and energy homeostasis.

The effects of chronic intraperitoneal administration of the GABA B receptor agonist baclofen on food intake in rats

This study was undertaken to examine the effects of repeated administration of the GABA(B) receptor agonist baclofen on food intake in male Wistar rats. In the 1st Experiment, the effects of daily administration of physiological saline and baclofen (2 mg/kg, i.p.) for 27 days were investigated on food intake and body weight in non-deprived rats (n=6 in each group). Baclofen significantly (P<0.05) increased cumulative food intake each day over the treatment period during the 60 min measurement period following administration. Tolerance did not develop to the short-term hyperphagic effect of baclofen over the course of the experiment. In addition, treatment with baclofen did not alter body weight of the animals over the 27 day treatment period when compared with the saline control rats. In the 2nd Experiment, the effects of acute and chronic administration of baclofen (2 mg/kg) were investigated on 24 h food intake in rats. The rats were injected daily for 21 days with either saline (n=6) or baclofen (n=6). Food intake was measured in 30 min time bins for 24 h on treatment Days 1, 12 and 21 following injection. The results showed that while baclofen produced short-term increases in food consumption following injection on treatment Days 1, 12 and 21, the daily (24 h) food intake of the animals was not significantly different from those of control rats. Thus, these data reveal that while chronic administration of baclofen (2 mg/kg) produces short-term increases in feeding without the development of tolerance, daily (24 h) food consumption is not affected. These findings are consistent with the observation that chronic administration of baclofen (2 mg/kg) had no effect on the body weight of these animals.

Effects of baclofen on operant performance for food pellets and vegetable shortening after a history of binge-type behavior in non-food deprived rats

Operant performance of non-food deprived rats (n=8) was assessed under progressive ratio (PR) and concurrent PR-fixed ratio schedules of food pellet and/or vegetable shortening reinforcement. Post operant baselines, rats were matched and divided into 2 groups based upon the schedule of shortening availability: High restriction binge group (H, 1-hr home cage shortening access each week on Monday, Wednesday, and Friday) and Low restriction (L, 1-hr shortening access daily). Chow and water were continuously available; only access to the shortening was restricted. After 8 weeks, operant performance was reassessed. Lever pressing for shortening increased in the H rats for all schedules, but was either unaffected or decreased in the L rats. Pellet responding under the concurrent schedules increased for both groups. The effects of four dosages of (R)-baclofen (0.3-1.8 mg/kg, i.p.) on operant performance were also assessed. For both groups, 1.0 mg/kg baclofen significantly reduced shortening responding relative to saline for all schedules except one, but had no or minimal effect on pellet responding. This suggests a specific effect of baclofen on responding maintained by fat. These results indicate that intermittent episodes of bingeing on fat can increase the reinforcing efficacy of fat and that GABAB receptor activation can attenuate this effect.

Baclofen reduces fat intake under binge-type conditions

The GABA-B agonist baclofen reduces drug self-administration in rats and has shown promise clinically in the treatment of substance abuse. Baclofen generally does not reduce food intake in non-binge feeding protocols. In this study, baclofen was tested in a fat-binge protocol. Thirty male rats were divided into three groups (B: binge; FM: fat-matched; C: chow). B received a bowl of vegetable shortening for 2 h on Monday, Wednesday, and Friday (MWF) and continuous access to powdered chow (regular chow) in all phases. FM had continuous access to a regular chow+shortening mixture (FM chow) that provided the same proportion of shortening and regular chow that the B rats consumed in all phases. In addition, FM had the following: phase 1: no separate bowl of shortening; phase 2: 2-h MWF access to a separate bowl of shortening; phase 3, daily 2-h access to a separate bowl of shortening; C rats had continuous access to the regular chow in all phases. In addition, C had the following: phase 1: no separate bowl of shortening; phase 2: 2-h MWF access to a separate bowl of shortening; in phase 3, daily 2-h access to a separate bowl of shortening. Baclofen (1.0, 1.8 mg/kg, i.p.) reduced shortening intake regardless of access condition. Baclofen had no effect on, or stimulated, FM and regular chow intake. These results demonstrate that baclofen can reduce fat intake in rats under binge-type conditions. Furthermore, these results indicate that bingeing, as modeled in our protocol, is different from other forms of food intake and may share similarities with substance abuse.

The effects of intraperitoneal and intracerebroventricular administration of the GABAB receptor antagonist CGP 35348 on food intake in rats

In order to test the hypothesis that endogenous gamma-aminobutyric acid (GABA), acting at central GABAB receptors, plays a physiological role in the control of feeding behaviour, it was reasoned that blocking these receptors with a centrally active GABAB receptor antagonist should reduce food intake in hungry rats. In the present study, experiments were carried out to test this possibility using the GABAB receptor antagonist 3-aminopropyl-diethoxy-methyl-phosphinic acid (CGP 35348), which is water-soluble and can penetrate the blood-brain barrier from the systemic circulation. CGP 35348 (50 and 100 mg/kg, i.p.) had no effect on food intake in 22-h fasted rats, but a higher dose (i.e. 500 mg/kg., i.p.) significantly reduced cumulative food consumption. These findings are consistent with previous observations that high systemic doses of CGP 35348 are needed to block central GABAB receptors. However, to eliminate the possibility that the 500 mg/kg dose of CGP 35348 decreased food intake by a peripheral, rather than a central mode of action, further experiments were undertaken where the drug was given directly into the brain by the intracerebroventricular (i.c.v.) route. I.c.v. administration of CGP 35348 (5 and 10 microg) significantly decreased cumulative food intake food intake in rats that had been fasted for 22 h. By contrast, i.c.v. administration of CGP 35348 (10 microg) had no effect on water intake in 16-h water-deprived rats. The results indicate that CGP 35348 reduces food consumption in hungry rats by blocking central GABAB receptors in a behaviourally specific manner. These findings suggest that endogenous GABA acting at central GABAB receptors plays a physiological role in the regulation of feeding behaviour.

Effects of baclofen on feeding behaviour examined in the runway

The motivational mechanisms underlying the effects of systemic administration of the GABA-B agonist baclofen on feeding were examined using a runway. Food-deprived male hooded Lister rats were trained to traverse a runway for food reinforcement. Baclofen (1 mg/kg i.p.) significantly increased food intake and this was most evident on the final two blocks of testing. The 2 mg/kg dose of baclofen increased running speed without significantly altering intake. At the highest dose tested (4 mg/kg), no significant effects on either consummatory or appetitive measures were observed. These data suggest that low doses of baclofen enhance the consummatory phase of ingestion by attenuating the natural signals associated with onset of satiation. The data also suggest that baclofen has complex effects on appetitive behaviour that may interfere with its effects on consumption.

CCK-nitric oxide interaction in rat cortex, striatum and pallidum

We have chosen to study the effects of both nitric oxide (NO) and cholecystokinin neuromodulatory systems in some motor structures that are frequently involved in excitotoxic phenomena. In particular, 7-nitroindazole, a selective inhibitor of neuronal NO synthase, was administered in control and sulfated cholecystokinin octapeptide-treated rats. Cortical surface, striatal and pallidal depth bioelectric activities were examined through Fast Fourier Transform analysis. Cortical and pallidal recordings revealed an increase of rapid standard rhythms after the inhibition of neuronal NO synthase; in contrast, striatal depth recordings showed a marked increase of slow standard rhythms. All these effects were completely abolished by chronic pre-treatment with sulfated cholecystokinin octapeptide. The results suggest a functional co-operation between cholecystokinin and NO systems in the modulation of the bioelectric activity of all the motor structures examined, and the possibility of preventing excitotoxic damages induced by an anomalous balance between excitatory and inhibitory neurotransmitters in these areas.

Dopamine receptor sensitive effect of dizocilpine on feeding behaviour

The effect of intracerebroventricular administration of dizocilpine on feeding behaviour and adrenal corticrotropic hormone (ACTH)-induced anorexia in elevated plus maze was examined. Dizocilpine (10, 20 and 40 nmol/rat, i.c.v.) showed a dose-dependent increase in food intake in 16 h food deprived rats. Dopamine receptor antagonists such as SCH 23390 (0.25 and 0.5 mg/kg, i.p.), pimozide (0.5 and 1 mg/kg, i.p.) and haloperidol (0.25 and 0.5 mg/kg, i.p.) dose-dependently blocked dizocilpine (40 nmol)-induced potentiation of food intake. Brain dopamine depletion by pretreatment with reserpine (5 mg/kg, i.p.) and alpha-methyl-p-tyrosine (200 mg/kg, i.p.) decreased food intake in rats. Similarly, pretreatment with reserpine and alpha-methyl-p-tyrosine (AMPT) reversed the hyperphagic effect of dizocilpine (20 and 40 nmol). Intracerebroventricular administration of ACTH (5 microgram/rat) produced significant diminution of feeding duration and increased tasting latency and feeding latency in elevated plus maze which was reversed by dizocilpine (40 nmol). SCH 23390 (0.25 mg/kg), pimozide (0.5 mg/kg) and haloperidol (0.25 mg/kg) reversed the effect of dizocilpine on ACTH-induced behaviours in elevated plus maze. The present observations support and extend the hypothesis that endogenous excitatory aminoacids (EAAs) play a role in the control of food intake. Further, dizocilpine-induced hyperphagia and dizocilpine-induced reversal of ACTH effect on feeding behaviour in elevated plus maze involve DAergic mediation.

Food for thought: a critique on the hypothesis that endogenous cholecystokinin acts as a physiological satiety factor

This review evaluates the various lines of evidence supporting the hypothesis that cholecystokinin (CCK) released from the small intestine during feeding plays a physiological satiety. Issues considered include, the effects of systemic injection of CCK on consummatory and operant feeding, the role of the vagus nerve, the effects of CCKB receptor antagonists, and the neuroendocrine responses to exogenous CCK. A critical appraisal of this research indicates that while it is clearly demonstratable that exogenous peripheral CCK can alter food intake by acting on CCKA receptors, the mechanism involved may be more closely related to the induction if aversion and nausea, rather than satiety. With regard to peripheral endogenous CCK, the available evidence also does not seem to support a role for the hormone in satiety. In particular, it is doubtful whether plasma concentrations of CCK following a meal are sufficiently high to inhibit feeding. Moreover, CCKA receptor antagonist which do not cross the blood brain barrier fail to increase meal size, as would be expected if peripheral CCK was an effective satiety factor. In addition, the recent literature concerned with the possibility that CCK may have a direct action within the brain in the control of food intake has been reviewed. These studies show that CCK administered intracerebroventicularly, or by micoinjection into discrete brain regions, also inhibits feeding via a CCKA receptor mechanism. However, the physiological relevance of these findings have yet to be determined.