Effect of intracerebroventricular administration of the GABAB-receptor agonist baclofen on operant feeding in satiated pigs

Mapping GABAergic projections that mediate feeding

Neuroscience offers important insights into the pathogenesis and treatment of obesity by investigating neural circuits underpinning appetite and feeding. Gamma-aminobutyric acid (GABA), one of the most abundant neurotransmitters in the brain, and its associated receptors represent an array of pharmacologically targetable mediators of appetite signalling. Targeting the GABAergic system is therefore an increasingly investigated approach to obesity treatment. However, the many GABAergic projections that control feeding have yet to be collectively analysed. This review provides a comprehensive analysis of the relationship between GABAergic signalling and appetite by examining both foundational studies and the results of newly emerging chemogenetic/optogenetic experiments. A current snapshot of these efforts to map GABAergic projections influencing appetite is provided, and potential avenues for further investigation are provided.

Deletion of astrocytic BMAL1 results in metabolic imbalance and shorter lifespan in mice

Disruption of the circadian cycle is strongly associated with metabolic imbalance and reduced longevity in humans. Also, rodent models of circadian arrhythmia, such as the constitutive knockout of the clock gene Bmal1, leads to metabolic disturbances and early death. Although astrocyte clock regulates molecular and behavioral circadian rhythms, its involvement in the regulation of energy balance and lifespan is unknown. Here, we show that astrocyte-specific deletion of Bmal1 is sufficient to alter energy balance, glucose homeostasis, and reduce lifespan. Mutant animals displayed impaired hypothalamic molecular clock, age-dependent astrogliosis, apoptosis of hypothalamic astrocytes, and increased glutamate and GABA levels. Importantly, modulation of GABAA-receptor signaling completely restored glutamate levels, delayed the reactive gliosis as well as the metabolic phenotypes and expanded the lifespan of the mutants. Our results demonstrate that the astrocytic clock can influence many aspects of brain function and neurological disease and suggest astrocytes and GABAA receptor as pharmacological targets to prevent the metabolic dysfunctions and shortened lifespan associated with alterations of circadian rhythms.

Total and acylated ghrelin plasma levels as potential long-term response markers in alcohol-dependent patients receiving high-dose of the GABA-B receptor agonist baclofen

The orexigenic hormone ghrelin is involved in the regulation of food intake and energy balance. Previous findings suggest its involvement in the modulation of mesolimbic reward pathways, thus potentially being relevant in the pathophysiology of substance use disorders such as alcohol dependence. In the present study, we assessed plasma levels of total and acylated ghrelin within the BACLAD trial, where alcohol-dependent patients received individually titrated high-dose baclofen (30-270 mg/d) within a randomized, placebo-controlled design. Plasma levels of total ghrelin and acylated ghrelin were measured at baseline, during treatment with individually titrated high-dose baclofen and after termination of the study medication within a timeframe of up to 20 weeks. Multivariate longitudinal non-parametric analysis revealed that plasma levels of total ghrelin significantly decreased in the group of abstinent patients receiving high-dose baclofen. In addition, plasma levels of total ghrelin correlated negatively with days of abstinence during treatment with high-dose baclofen. Plasma levels of acylated ghrelin increased during the study in the group of relapsed patients under baclofen and placebo treatment. These findings suggest that the long-term response to baclofen treatment in alcohol use disorder (AUD) might be monitored by assessing total and acylated ghrelin plasma levels.

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 white rice containing enriched gamma-aminobutyric acid on blood pressure

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter with beneficial effects including antihypertension and antistress properties. In this study, we examined the effects of GABA-enriched white rice (GABA rice) on blood pressure (BP) in 39 mildly hypertensive adults in a randomized, double-blind, placebo-controlled study. The participants were divided into a test group (n = 22) who consumed rice with 11.2 mg GABA/100 g of rice and a placebo group (n = 17) who consumed rice with 2.7 mg GABA/100 g of rice. For 8 weeks, the participants took 150 g of either the GABA rice or the placebo rice. Hematological examinations were performed on both groups at 0, 4, and 8 weeks after the start of rice consumption. Home BP was self-measured two times daily, morning and evening, from 1 weeks before to 2 weeks after the intervention. Although the hospital BP and evening BP measurements of the participants showed no significant change, consumption of the GABA rice improved the morning BP compared with the placebo rice after the 1^st week and during the 6^th and 8^th weeks. These results showed the possibility that the GABA rice improves morning hypertension.

The Role of GABAB Receptors in Morphine Self-Administration

BACKGROUND

There is only little information about the effects of GABA receptors agonist and antagonist on morphine self-administration. Present study was designed to assess role of GABAB receptors in the regulation of morphine-reinforced self-administration.

METHODS

THIS STUDY WAS PERFORMED IN FOUR GROUPS OF RATS: (1) Saline group, which received saline in the self-administration session. (2) Morphine group, which received morphine in saline solution in the self-administration session. (3) Baclofen + Morphine group, which received both baclofen 20 min before self- administration test and morphine in the self-administration session. (4) Phaclofen + Morphine group, which received both phaclofen 20 min before self- administration test and morphine in the self-administration session. The number of lever pressing and self-infusion were recorded.

RESULTS

Morphine significantly increased the number of active lever pressing dose dependently in self-administration session in comparative with saline group. Administration of baclofen, 20 min before morphine self-administration produced significant decrease in the initiation of morphine self-administration during all session. Conversely, pre-treatment of phaclofen increased the number of active lever pressing and self-infusion in this test.

CONCLUSION

Our results indicated a short-term treatment by baclofen, reduced morphine-maintenance response in a dose-dependent manner, suggesting that GABAB receptor agonists could be useful for reversing the neuroadaptations related to opiates.

Glutamate and GABA in Appetite Regulation

Appetite is regulated by a coordinated interplay between gut, adipose tissue, and brain. A primary site for the regulation of appetite is the hypothalamus where interaction between orexigenic neurons, expressing Neuropeptide Y/Agouti-related protein, and anorexigenic neurons, expressing Pro-opiomelanocortin cocaine/Amphetamine-related transcript, controls energy homeostasis. Within the hypothalamus, several peripheral signals have been shown to modulate the activity of these neurons, including the orexigenic peptide ghrelin and the anorexigenic hormones insulin and leptin. In addition to the accumulated knowledge on neuropeptide signaling, presence and function of amino acid neurotransmitters in key hypothalamic neurons brought a new light into appetite regulation. Therefore, the principal aim of this review will be to describe the current knowledge of the role of amino acid neurotransmitters in the mechanism of neuronal activation during appetite regulation and the associated neuronal-astrocytic metabolic coupling mechanisms. Glutamate and GABA dominate synaptic transmission in the hypothalamus and administration of their receptors agonists into hypothalamic nuclei stimulates feeding. By using (13)C High-Resolution Magic Angle Spinning Nuclear Magnetic Resonance spectroscopy based analysis, the Cerdán group has shown that increased neuronal firing in mice hypothalamus, as triggered by appetite during the feeding-fasting paradigm, may stimulate the use of lactate as neuronal fuel leading to increased astrocytic glucose consumption and glycolysis. Moreover, fasted mice showed increased hypothalamic [2-(13)C]GABA content, which may be explained by the existence of GABAergic neurons in key appetite regulation hypothalamic nuclei. Interestingly, increased [2-(13)C]GABA concentration in the hypothalamus of fasted animals appears to result mainly from reduction in GABA metabolizing pathways, rather than increased GABA synthesis by augmented activity of the glutamate-glutamine-GABA cycle.

Differential modulation of gonadotropin responses to kisspeptin by aminoacidergic, peptidergic, and nitric oxide neurotransmission

Kisspeptins (Kp), products of the Kiss1 gene, have emerged as essential elements in the control of GnRH neurons and gonadotropic secretion. However, despite considerable progress in the field, to date limited attention has been paid to elucidate the potential interactions of Kp with other neurotransmitters known to centrally regulate the gonadotropic axis. We characterize herein the impact of manipulations of key aminoacidergic (glutamate and GABA), peptidergic (NKB, Dyn, and MCH), and gaseous [nitric oxide (NO)] neurotransmission on gonadotropin responses to Kp-10 in male rats. Blockade of ionotropic glutamate receptors (of the NMDA and non-NMDA type) variably decreased LH responses to Kp-10, whereas activation of both ionotropic and metabotropic receptors, which enhanced LH and FSH release per se, failed to further increase gonadotropin responses to Kp-10. In fact, coactivation of metabotropic receptors attenuated LH and FSH responses to Kp-10. Selective activation of GABA(A) receptors decreased Kp-induced gonadotropin secretion, whereas their blockade elicited robust LH and FSH bursts and protracted responses to Kp-10 when combined with GABA(B) receptor inhibition. Blockade of Dyn signaling (at κ-opioid receptors) enhanced LH responses to Kp-10, whereas activation of Dyn and NKB signaling modestly reduced Kp-induced LH and FSH release. Finally, MCH decreased basal LH secretion and modestly reduced FSH responses to Kp-10, whereas LH responses to Kp-10 were protracted after inhibition of NO synthesis. In summary, we present herein evidence for the putative roles of glutamate, GABA, Dyn, NKB, MCH, and NO in modulating gonadotropic responses to Kp in male rats. Our pharmacological data will help to characterize the central interactions and putative hierarchy of key neuroendocrine pathways involved in the control of the gonadotropic axis.

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 GABAB receptor agonists and antagonists on glycemia regulation in mice

γ-Aminobutyric acid (GABA) inhibits insulin secretion through GABA(B) receptors in pancreatic β-cells. We investigated whether GABA(B) receptors participated in the regulation of glucose homeostasis in vivo. BALB/c mice acutely pre-injected with the GABA(B) receptor agonist baclofen (7.5mg/kg, i.p.) presented glucose intolerance and diminished insulin secretion during a glucose tolerance test (GTT, 2g/kg body weight, i.p.). The GABA(B) receptor antagonist 2-hydroxysaclofen (15 mg/kg, i.p.) improved the GTT and reversed the baclofen effect. Also a slight increase in insulin secretion was observed with 2-hydroxysaclofen. In incubated islets 1.10(-5)M baclofen inhibited 20mM glucose-induced insulin secretion and this effect was reversed by coincubation with 1.10(-5)M 2-hydroxysaclofen. In chronically-treated animals (18 days) both the receptor agonist (5mg/kg/day i.p.) and the receptor antagonist (10mg/kg/day i.p.) induced impaired GTTs; the receptor antagonist, but not the agonist, also induced a decrease in insulin secretion. No alterations in insulin tolerance tests, body weight and food intake were observed with the treatments. In addition glucagon, insulin-like growth factor I, prolactin, corticosterone and growth hormone, other hormones involved in glucose metabolism regulation, were not affected by chronic baclofen or 2-hydroxysaclofen. In islets obtained from chronically injected animals with baclofen, 2-hydroxysaclofen or saline (as above), GABA(B2) mRNA expression was not altered. Results demonstrate that GABA(B) receptors are involved in the regulation of glucose homeostasis in vivo. Treatment with receptor agonists or antagonists, given acutely or chronically, altered glucose homeostasis and insulin secretion alerting to the need to evaluate glucose metabolism during the clinical use of these drugs.

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.

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.

The effects of intraperitoneal administration of the GABA(B) receptor agonist baclofen on food intake in CFLP and C57BL/6 mice

The effects of the GABA(B) receptor agonist baclofen were investigated on food intake in non-deprived CFLP and C57BL/6 mice. In Experiment 1, baclofen (1-8 mg /kg) administered i.p. to CFLP mice, produced a dose-related increase in food intake. The 4 and 8 mg/kg doses produced significant increases in cumulative feeding when measure 120 min after administration (at least P < 0.05, in each case). In Experiment 2, baclofen (1-10 mg/kg), administered intraperitoneally (i.p.) to C57BL/6 mice, also produced a dose-related increase in food intake. The 4 mg/kg dose of baclofen significantly increased cumulative food intake at 60 min (P < 0.05), while the 2 and 4 mg/kg doses significantly increased cumulative food intake at 120 min (P < 0.01, in each case). The 10mg/kg dose was without effect. These data show that systemic administration of the GABA(B) agonist baclofen produces an increase in food consumption in two different strains of mice and extend previous observations made in rat to another rodent species.

Neurotransmitters in key neurons of the hypothalamus that regulate feeding behavior and body weight

During the last two decades attention has been focussed on the role of different neuropeptides in hypothalamic control of feeding behavior. Several hypothalamic peptides that participate in the control of ingestive behavior are produced in neuronal cell bodies of the arcuate nucleus and/or the lateral hypothalamic area. Apart from producing orexigenic or anorexigenic compounds of peptidergic nature, these neurons also produce excitatory and inhibitory amino acid neurotransmitters. The role of GABA and glutamate in regulating energy balance has received less attention in comparison to neuropeptides. The arcuate nucleus-median eminence area, a region with a weak blood-brain barrier, contains at least two neuronal cell populations that exert opposing actions on energy balance. The majority of the neurons located in the ventromedial aspect of the arcuate nucleus, which produce the orexigenic peptides neuropeptide Y (NPY) and agouti-related peptide (AGRP), contain in addition the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD) and the vesicular GABA transporter (VGAT), thereby supporting their GABAergic nature. Some neurons producing pro-opiomelanocortin (POMC)- and cocaine- and amphetamine-regulated transcript (CART), located in the ventrolateral division of the arcuate nucleus have recently been reported to contain the vesicular glutamate transporter 2 (VGLUT2), a marker for glutamatergic neurons, and the acetylcholine (ACh) synthesizing enzyme choline acetyltransferase (ChAT) as well as the vesicular ACh transporter (VAChT), supporting also a cholinergic phenotype. In the lateral hypothalamic area, hypocretin/orexin neurons express VGLUT1 or VGLUT2, but not GAD, whereas some melanin-concentrating hormone (MCH) cells contain GAD. These observations support the view that several classical transmitters, relatively neglected feeding transmitters candidates, are present in key neurons that regulate body weight and consequently may represent important orexigenic/anorexigenic mediators that convey information to other neurons within the hypothalamus as well as from the hypothalamus to other brain regions that participate in regulation of energy balance.

Lack of effects of GHB precursors GBL and 1,4-BD following i.c.v. administration in rats

Gamma-hydroxybutyrate (GHB) is used therapeutically and recreationally worldwide. Since the scheduling of GHB by the USA and the United Nations in 2000-2001, the recreational use of GHB precursors has reportedly increased. The aim of this study was to examine if potency differences of GHB and GHB-like compounds are due to their blood-brain barrier permeability. The effects of peripheral and central administration of GHB, GHB precursors gamma-butyrolactone (GBL) and 1,4-butanediol (1,4-BD), and the gamma-aminobutyric acid (GABA)(B) receptor agonist baclofen on schedule-controlled responding were examined in rats. GHB and baclofen were 276- and 253-fold more potent, respectively, after intracerebroventricular (i.c.v.) administration than after intraperitoneal (i.p.) administration, whereas GBL and 1,4-BD, up to a dose of 1780 microg were without effect after i.c.v. administration. These data suggest that GBL and 1,4-BD are not metabolically converted to GHB in the brain, that enhanced brain penetration cannot account for potency differences between compounds, and that baclofen, like GHB, can readily cross the blood-brain barrier.

N/A

N/A

Electroacupuncture improves restraint stress-induced delay of gastric emptying via central glutaminergic pathways in conscious rats

Acupuncture has been used for treating functional gastrointestinal (GI) disorders. Animal studies demonstrated that acupuncture improves various stress-induced physiological responses. We investigated the effects of electroacupuncture (EA) at ST-36 (Zusanli; lower limb) on stress-induced delay of gastric emptying. Solid food gastric emptying in 90 min was significantly delayed by restraint stress (27.3 +/- 2.1%, n = 8), compared to that of controls (64 +/- 2.1%, n = 8). Restraint stress-induced delay of gastric emptying was significantly restored by the intracisternal (IC)-injection of GABA(A) receptor antagonist, bicuculline methiodide (46.5 +/- 3.1%; n = 6) and GABA(B) receptor antagonist, phaclofen (48 +/- 3.3%; n = 6). Delayed gastric emptying induced by restraint stress was significantly improved by EA at ST-36 (49.7 +/- 1.4%). The stimulatory effect of EA on stress-induced delay of gastric emptying was prevented by pretreatment with IC-injection of glutamate receptor antagonist, kynurenic acid (30.1 +/- 2.1%). In conclusion, restraint stress-induced delay of gastric emptying is mediated via central GABA(A) and GABA(B) receptors. EA at ST-36 stimulates glutaminergic neurons in the brainstem resulting in improvement of stress-induced delay of gastric emptying.

Baclofen decreases feeding in non-human primates

This study examined how the GABA(b) agonist baclofen (0.5-5.6 mg/kg, p.o.), reported to increase food intake in rodents, affected the appetitive and consummatory aspects of feeding of non-human primates. Baboons had access to food 24 h each day, but they had to complete a two-phase operant procedure in order to eat. Responding on one lever during a 30-min appetitive phase was required before animals could start a consumption phase, where responding on another lever led to food delivery, i.e., a meal. Responding during the appetitive phase resulted in presentations of food-related stimuli only. Baclofen increased the latency to the first meal and decreased both appetitive and consummatory behavior. At the largest dose, baclofen induced emesis, indicating that the effects were due to malaise rather than a specific motivational action. In contrast, the positive control diazepam (GABA(a) agonist, 1.0-2.0 mg/kg, i.m.) decreased the latency to the first meal and increased both appetitive (P < 0.07) and consummatory behavior. Although the baclofen-induced decrease in appetitive behavior replicates data obtained in rodents, the baclofen-induced decreases in consummatory behavior do not. The findings suggest that the effects of large doses of baclofen in non-human primates may, in part, be due to non-specific behavioral disruptions.

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.

Intracerebroventricular injection of muscimol, baclofen or nipecotic acid stimulates food intake in layer-type, but not meat-type, chicks

This study was designed to compare the effects of muscimol (GABA(A) agonist), baclofen (GABA(B) agonist) and nipecotic acid (GABA uptake inhibitor) on food intake in two chicken strains (meat-type and layer-type chicks). The intracerebroventricular (ICV) injection of all GABA agents induced hyperphagia in layer-type chicks. However, in broiler chicks, there were similar tendencies with muscimol and nipecotic acid but not significantly different. Conversely, ICV injection of baclofen depressed feeding of broiler chicks. These results suggest that there are some differences in central GABAergic systems between these strains of chicks, but GABAergic systems have an important role in the regulation of food intake in neonatal chicks.

Intracerebroventricular administration of GABA-A and GABA-B receptor antagonists attenuate feeding and sleeping-like behavior induced by L-pipecolic acid in neonatal chicks

It has been demonstrated that L-pipecolic acid (L-PA), a major metabolic intermediate of L-lysine (L-Lys) in the mammalian and chicken brain, is involved in the functioning of the GABAergic system. A previous study has shown that intracerebroventricular (i.c.v.) injection of L-PA suppressed feeding and induced sleep-like behavior in neonatal chicks; however, the precise relationship between the GABAergic system and L-PA has not been clarified. In the present study, the role of the GABA-A or GABA-B receptors in the suppression of food intake and induction of sleeping-like behavior by L-PA was investigated. Chicks were injected i.c.v. with the GABA-A antagonist picrotoxin or GABA-B antagonist CGP54626 along with L-PA. Although suppression of food intake by L-PA was restored partially by co-injection with CGP54626, but not picrotoxin, sleep-like behavior induced by L-PA was suppressed significantly by both antagonists. These results suggested that L-PA activated both GABA-A and GABA-B receptors, and GABA-B receptors alone contributed to food intake whereas both receptors contributed to sleep-like behavior.

Chemical coding of GABA(B) receptor-immunoreactive neurones in hypothalamic regions regulating body weight

Gamma-aminobutyric acid (GABA) interacts with hypothalamic neuronal pathways regulating feeding behaviour. GABA has been reported to stimulate feeding via both ionotropic GABA(A) and metabotropic GABA(B) receptors. The functional form of the GABA(B) receptor is a heterodimer consisting of GABA(B) receptor-1 (GABA(B)R1) and GABA(B) receptor-2 (GABA(B)R2) proteins. Within the heterodimer, the GABA-binding site is localized to GABA(B)R1. In the present study, we used an antiserum to the GABA(B)R1 protein in order to investigate the cellular localization of GABA(B)R1-immunoreactive neurones in discrete hypothalamic regions implicated in the control of body weight. The colocalization of GABA(B)R1 immunoreactivity with different chemical messengers that regulate food intake was analysed. GABA(B)R1-immunoreactive cell bodies were found in the periventricular, paraventricular (PVN), supraoptic, arcuate, ventromedial hypothalamic, dorsomedial hypothalamic, tuberomammillary nuclei and lateral hypothalamic area (LHA). Direct double-labelling showed that glutamic acid decarboxylase (GAD)-positive terminals were in close contact with GABA(B)R1-containing cell bodies located in all these regions. In the ventromedial part of the arcuate nucleus, GABA(B)R1-immunoreactive cell bodies were found to contain neuropeptide Y, agouti-related peptide (AGRP) and GAD. In the ventrolateral part of the arcuate nucleus, GABA(B)R1-immunoreactive cell bodies were shown to contain pro-opiomelanocortin and cocaine- and amphetamine-regulated transcript. In the LHA, GABA(B)R1 immunoreactivity was present in both melanin-concentrating hormone- and orexin-containing cell populations. In the tuberomammillary nucleus, GABA(B)R1-immunoreactive cell bodies expressed histidine decarboxylase, a marker for histamine-containing neurones. In addition, GAD and AGRP were found to be colocalized in some nerve terminals surrounding GABA(B)R1-immunoreactive cell bodies in the parvocellular part of the PVN. The results may provide a morphological basis for the understanding of how GABA regulates the hypothalamic control of food intake and body weight via GABA(B) receptors.

Effects of subcutaneous administration of the gamma-aminobutyric acid(A) receptor agonist muscimol on water intake in water-deprived rats

The effects of the gamma-aminobutyric acid(A) (GABA(A)) receptor agonist muscimol were investigated on water intake in rats that had been deprived of water for 16 h. Muscimol (0.5-2.0 mg/kg sc) produced a dose-related inhibition of water consumption in both male (n=8) and female (n=8) rats, with maximal suppression of drinking occurring during the first 30 min after administration. Doses of 1 and 2 mg/kg produced significant decreases in water intake (P<.01), while a lower dose of 0.5 mg/kg was without effect. The hypodipsic effect of muscimol (1.0 mg/kg sc) was abolished by pretreatment of the animals with the GABA(A) receptor antagonist bicuculline (1 mg/kg sc). Furthermore, muscimol (2 mg/kg sc) did not produce aversion in a two-bottle conditioned taste aversion test, indicating that the suppressant effects of muscimol on water intake are not due to drug-induced malaise. The results suggest that systemic administration of muscimol produces a behaviourally specific suppression of primary drinking in rats by a GABA(A) receptor-mediated mechanism. Moreover, this action of muscimol appears to be independent of the gender of the animals.

GABAergic control of food intake in the meat-type chickens

This study examined the effects of intracerebroventricular injections of gamma-aminobutyric acid (GABA) agonists on short-term food intake in meat-type cockerels. In Experiment 1, birds were injected with various doses of muscimol, a GABA(A) agonist. In Experiment 2, the birds received bicuculline, a GABA(A) antagonist, prior to injection of muscimol. In Experiment 3, the effect of varying doses of baclofen, a GABA(B) agonist, on food intake was determined. The intracerebroventricular injection of muscimol caused a dose-dependent increase in food intake. This effect was significantly attenuated by pretreatment with bicuculline. Food intake was not affected by the intracerebroventricular injection of baclofen. These results suggest that GABA acts within the brain of broilers at a GABA(A), but not GABA(B), receptor to increase voluntary food intake.

Effects of the 5-HT1A receptor agonist 8-OH-DPAT on operant food intake in food-deprived pigs

The effects of the 5-HT1A agonist 8-hydroxy-2 (di-n-propylamino)tetralin (8-OH-DPAT) were investigated on operant food intake in food-deprived pigs. In Experiment 1, 8-OH-DPAT (5-20 microg/kg) administered intravenously (i.v.) 15 min prior to the occurrence of feeding produced a dose-related decrease in operant food intake in pigs that had been fasted overnight. The effects were mainly apparent during the first 30 min after the start of the feeding period. In Experiment 2, 8-OH-DPAT (25 and 50 microg/kg, i.v.) administered 60 min prior to the occurrence of feeding in pigs that were fasted overnight also produced significant decreases in food intake. The effects were mainly apparent during the first 30-40 min after the start of the feeding period. In Experiment 3, 8-OH-DPAT (20 microg/kg, i.v.) significantly increased operant feeding in satiated pigs during the first 30 min after administration. These results show that 8-OH-DPAT has complex effects on feeding behaviour in pigs, increasing operant food intake in satiated pigs, while producing a reduction in food intake in food-deprived animals.

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

The aim of this study was to investigate whether pretreatment with the GABAB receptor agonist baclofen could prevent the inhibitory effect of systemically administered cholecystokinin (CCK) on food intake in rats. Baclofen (2 mg/kg, SC) administered 60 min prior to IP injection of CCK (5 micrograms/kg) significantly attenuated the suppressant effect of the peptide on feeding in nondeprived rats (Experiment 1) and rats that had been deprived of food for 22 h (Experiment 2). Baclofen had no significant effects on food intake when administered alone. The results suggest that the inhibitory effect of exogenous peripheral CCK on food intake may be dependent on an interaction with a GABAB-receptor mediated mechanism.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

Systemic administration of baclofen inhibits water intake in rats

1. The present experiments were carried out to investigate the effects of systemic administration of baclofen on water intake in rats. 2. Baclofen (1, 2 and 4 mg/kg, s.c.) inhibited water intake in 16 hr water-deprived rats in a dose-related manner, with maximal effects occurring during the first 30 min after administration. 3. Baclofen (0.25 and 2 mg/kg, s.c.) had no effects on water intake in non-deprived rats. 4. Baclofen (2 mg/kg) inhibited water intake elicited by i.p. injection of hypertonic NaCl in rats. 5. Baclofen (1 mg/kg) did not produce taste aversion in a taste aversion experiment. This indicates that the effects of baclofen on water intake is not due to an aversive effect of the drug.

The effect of systemic administration of baclofen on food intake in rats

The effects of systemic administration of the GABAB agonist, baclofen was investigated on food intake in non-fasted rats. Baclofen (1.0, 2.0 and 4.0 mg/kg, s.c.) produced a dose-related increase in food intake in a free-feeding paradigm during the first 90 min after administration, with maximum increases occurring at a dose of 2 mg/kg (Experiment 1). Baclofen (0.5 and 1.0 mg/kg, s.c.) also increased food intake in the 40 min post-drug recording period in non-fasted rats, trained to make operant responses for food on a fixed-ratio schedule (Experiment 2). These results demonstrate that systemic administration of baclofen can stimulate ingestive behaviour in satiated rats and suggest a possible role for a GABAB receptor-mediated mechanism in the control of food intake.