Effects of chronic systemic administration of the GABAB receptor agonist baclofen on food intake and body weight in rats

Effects of chronic systemic administration of the GABAB receptor agonist baclofen on food intake and body weight in rats

European Journal of Pharmacology 635 (2010) 129–134 Contents lists available at ScienceDirect European Journal of Pharmacology j o u r n a l h o m e...

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European Journal of Pharmacology 635 (2010) 129–134

Contents lists available at ScienceDirect

European Journal of Pharmacology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / e j p h a r

Behavioural Pharmacology

Effects of chronic systemic administration of the GABAB receptor agonist baclofen on food intake and body weight in rats Sunit M. Patel a, Ivor S. Ebenezer a,b,⁎ a b

Neuropharmacology Research Group, School of Pharmacy and Biomedical Sciences, University of Portsmouth, Portsmouth, PO 1 2DT, England, United Kingdom Institute of Biomedical and Biomolecular Sciences, University of Portsmouth, Portsmouth, England, United Kingdom

a r t i c l e

i n f o

Article history: Received 21 December 2009 Accepted 1 March 2010 Available online 19 March 2010 Keywords: GABAB receptor Food intake Baclofen Chronic Body weight gain Tolerance

a b s t r a c t The effects of daily administration of physiological saline of baclofen (1 and 4 mg/kg, i.p.) for 27 days were investigated on food intake and body weight in non-deprived rats in Experiment 1. Baclofen (1 and 4 mg/kg) significantly increased daily short-term food intake when measured at 30 min (F(2,15) = 11.011, P b 0.01) and 90 min (F(2,15) = 7.3801, P b 0.01) over the 27 day experimental period.. Tolerance did not develop to the short-term hyperphagic effects of baclofen. Baclofen (1 mg/kg) had no significant effects on body weight gain of the rats compared with controls. By contrast, baclofen (4 mg/kg) significantly (P b 0.05) decreased the body weight gain of the animals. In Experiment 2, the effect of daily administration of baclofen (4 mg/kg, i.p.) for 24 days was investigated on 24 h food intake in rats measured after the first, eight, fifteenth and twenty second injections. The 24 h food intake of the animals was not significantly different from those of control rats on any of the measurement days (F(1,14) = 1.602, ns). However, the body weight gain of the rats chronically treated with baclofen (4 mg/kg) was significantly reduced. (F(1,14) = 14.011, P b 0.01). The observations that chronic administration of baclofen (4 mg/kg) stimulates short-term food intake without affecting long term (24 h) feeding, but decreases body weight gain, suggest that baclofen may act through different mechanisms to influence food intake and body weight. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Gamma-amino butyric acid (GABA) is the major inhibitory neurotransmitter in mammalian central nervous system and acts at three pharmacologically distinct receptor subtypes, namely the ionotropic GABAA and GABAC receptors, and the metabotropic GABAB receptor (Olsen, 2002; Bowery and Smart, 2006; Bowery, 2006). GABA is found in high concentrations in areas considered to be involved in the control of feeding behaviour, such as the lateral, ventromedial and arcuate nuclei of the hypothalamus (Decavel and Van den Pol, 1990; Backberg et al., 2003) and, almost 2 decades ago, it was demonstrated that intracerebroventricular (icv) injections of the GABAB receptor agonist baclofen elicited feeding in satiated pigs and non-deprived rats by an action at central GABAB receptors (Ebenezer, 1990; Ebenezer and Baldwin, 1990). Recent experiments from our laboratory have suggested that endogenous GABA, acting at central GABAB receptors, may play a physiological role in the regulation of feeding behaviour (Patel and Ebenezer, 2004). It has also been shown that systemic administration of baclofen generally increases feeding

⁎ Corresponding author. Neuropharmacology Research Group, School of Pharmacy and Biomedical Sciences, University of Portsmouth, Portsmouth, PO 1 2DT, England, United Kingdom. E-mail address: [email protected] (I.S. Ebenezer). 0014-2999/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ejphar.2010.03.014

in non-deprived rats (Ebenezer and Pringle, 1992, Ebenezer, 1995; Ebenezer and Patel, 2004; Higgs and Barber, 2004; Buda-Levin et al., 2005; Patel and Ebenezer, 2008a) and mice (Ebenezer and Prabhaker, 2007). Baclofen readily crosses the blood brain barrier (Faigle and Keberle, 1972) and evidence that the hyperphagia produced by systemic administration of the drug is mediated by a central action of the GABAB receptor agonist has been provided (Ebenezer and Patel, 2004). Pharmacological and immunohistochemical studies have implicated a number of brain areas, such as the medial raphe nucleus, the nucleus accumbens and the arcuate nucleus of the hypothalamus, as possible central sites where the drug may act to mediate its hyperphagic actions (Wirtshafter et al., 1993, Stratford and Kelly, 1997, Ward et al., 2000, Backberg et al., 2003). Most of the studies on the effects of baclofen on food intake have focussed on the effects of acute administration of the GABAB receptor agonist on short-term feeding responses (Ebenezer, 1990, 1995, 1996; Ebenezer and Pringle, 1992; Ebenezer and Patel, 2004; Ebenezer and Prabhaker, 2007; Stratford and Kelly, 1997; Ward et al., 2000; Higgs and Barber, 2004). Recently, however, we have reported on the effects of chronic injections of baclofen (2 mg/kg, administered i.p. once daily), on short- and long term feeding and on body weight in nondeprived rats (Patel and Ebenezer, 2008b). We found that (i) baclofen (2 mg/kg., i.p.) elicits a short-term (60 min) hyperphagia in the animals over the treatment period, (ii) tolerance did not occur to the short-term hyperphagia over the 27 day experimental period,

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(iii) long term (24 h) food intake following acute and chronic exposure to baclofen (2 mg/kg, i.p.) was similar to those of control rats, suggesting that the animals were able to regulate their daily intake quite accurately, and (iv) chronic exposure to baclofen (2 mg/kg administered i.p. once daily) does not affect body weight. We concluded that chronic systemic administration of baclofen increases short-term food intake without affecting long term food intake and body weight (Patel and Ebenezer, 2008b). By contrast, Rothwell et al. (1985) reported that the effects of chronic administration of baclofen (2 mg/kg, s.c. twice daily) significantly decreases body weight gain in rats over a 15 day test period. The authors did not measure food intake in their study. These findings on body weight gain are not consistent with our results, and one option mooted for these difference was that Rothwell et al. (1985) administered a 2 mg/kg dose of baclofen twice daily whereas, in our study, baclofen (2 mg/kg) was administered once daily (see Patel and Ebenezer, 2008b). Thus, the possibility exists that higher doses of baclofen than the 2 mg/kg once daily dose used in the Patel and Ebenezer (2008b) study may have different effects on short and long term food intake and body weight gain in rats. In the present study we investigated the effect of a higher dose of baclofen (i.e. 4 mg/kg, i.p.) on food intake and body weight gain in non-deprived rats. We chose a dose of 4 mg/kg as (i) it was the sum of the two 2 mg/kg doses used in the Rothwell et al.(1985)study,and(ii)ithasbeenpreviouslyshowntoreliablyincrease food intake ((Ebenezer andPringle, 1992; Ebenezer, 1995, Ebenezer and Patel, 2004)). We also investigated the effects of a low dose of baclofen (1 mg/kg)onshort-termfoodintakeandbodyweightgaininthisstudy.

animals were kept on a 12 h light–dark cycle (lights were turned on at 8.30 h and were turned off at 20.30 h) and given 2 training sessions when they were allowed free access to their normal pelleted food (food composition similar to that used in Experiment 1) and water in experimental cages over a 24 h period. During experimental sessions that followed, rats in Group 1 (n = 8) received saline and those in Group 2 (n = 8) received baclofen (4 mg/kg). Saline and baclofen were administered i.p. Treatments were given once daily over a 24 day period. On treatment days 1, 8, 15 and 22 the rats were placed separately in experimental cages immediately after i.p. injection of saline (Group 1) or baclofen (Group 2) and cumulative food intake measured 24 h later. Body weight for each rat was also recorded. 2.3. Drugs (±) Baclofen was purchased from Sigma Biochemicals, Dorset, UK. The drug was dissolved in physiological saline solution (0.9% w/v, NaCl) to give an injection volume of 0.1 ml/100 g body weight. Physiological saline solution was used in control groups. 2.4. Body weight The body weight data obtained in Experiments 1 and 2 for each rat were expressed as a percentage of the animal's body weight recorded on the first day of the experiment. 2.5. Statistics

2. Materials and methods The protocols used in this study were approved by the Ethical Review Committee at the University of Portsmouth, U.K. 2.1. Experiment 1: Effects of repeated administration of baclofen on short-term (30 and 90 min) food intake and body weight gain in rats Adult male Wister rats (n = 18; starting body weights: 410–480 g; age = 16 weeks at start of experiment) were housed in cages in groups of 3 where they had free access to food and water at all times. The animals were maintained on a 12 h light/dark cycle (lights on at 8.30 h and lights off at 20.30 h). The rats were divided into 3 equal groups of similar body weights and were given 4 training sessions when they were allowed free access to their normal pelleted food (food composition: (a) Percentage mass: protein 20%, oil 4.5%, carbohydrate 60%, fibre 5%, ash, 7% + traces of vitamins and metals, (b) percentage energy: protein 27.3%, oil 11.48% and carbohydrate 61.2%, and (c) energy density: 3.6 kcal/g) and water in experimental cages measuring 32 × 25 × 10 cm. The food was presented to the rats in shallow cylindrical cups, as described previously (Ebenezer, 1990). During experimental sessions that followed, the rats were injected i.p. with either saline (Group 1; n = 6) or baclofen (1 or 4 mg/kg; Groups 2 and 3; n = 6 in each group) at 11.00 h and placed separately into experimental cages immediately after injection with free access to food and water and cumulative food intake measured at 30 and 90 min. This was repeated on a daily basis over a period of 27 days, except week-ends. During week-ends, the rats were given the appropriate treatments of saline or baclofen, but food intake was not measured. Body weight for each rat was recorded between 09.00 and 09.30 h during the experimental period. 2.2. Experiment 2: Effects of repeated administration of baclofen on long term (24 h) food intake and body weight gain in rats Adult male Wistar rats (n = 16; b. starting body weights: 250– 310 g; age = 9 weeks at start of experiment) were divided into 2 equal groups of similar body weights and housed in groups of 4. The

The data from these experiments were analysed by 2 way analysis of variance (ANOVA) with repeated measures on treatment and time (days) and by post-hoc Dunnett's test (Winer, 1971). 3. Results 3.1. Experiment 1: Effects of repeated administration of baclofen on short-term (30 and 90 min) food intake and body weight in rats The effects of chronic administration of baclofen (1 and 4 mg/kg) on food intake measured at 30 min after administration are shown in Fig. 1A and B respectively. Statistical analysis of the results revealed that there was significant main effects of treatment (F(2,15) = 11.011, P b 0.01) and time (days) (F(19,285) = 2.007, P b 0.01), and treatment× time interaction (F(38,285) = 1.683, P b 0.01). Baclofen (1 mg/kg) produced a significant increase in food consumption on Day 1 and this hyperphagic response generally persisted for the remainder of the study without indication of attenuation (see Fig. 1A). Baclofen (4 mg/kg) was without effect on food intake in the “baclofen naive” rats on treatment Day 1 during this measurement period. However, from Day 2 onwards there was a hyperphagic response to this dose of baclofen at 30 min that persisted during the remaining 25 days without evidence of attenuation (see Fig. 1B). Statistical analysis (2 way ANOVA with repeated measures on treatment and time) of the food intake data between the 2 baclofentreated groups at 30 min revealed no significant effects of treatment (F(1,10) =0.448, ns). However, there was a significant effect of time (days) (F(19,190) = 2.372, P b 0.01) which probably reflects the observation that the 4 mg/kg dose of baclofen was without effect on food intake on treatment Day 1. The treatment× time interaction was just below significance (F(19,190) = 1.629, P b 0.054, ns) at 30 min. Fig. 1C and D show the effects of baclofen (1 and 4 mg/kg) on cumulative food intake measured at 90 min after administration. Statistical analysis of the results revealed that there was a significant main effect of treatment (F(2,15) = 7.38, P b 0.01), but no significant effects of time (days) (F(19,285) = 1.36, ns) or treatment × time interaction (F(38,285) = 1.360, ns). Both doses of baclofen produced significant increases in food intake during this measurement period

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on experimental Day 1 and this persisted without evidence of attenuation for the remaining 26 days (see Fig. 1C and D). Statistical analysis (2 way ANOVA with repeated measures on treatment and

Fig. 2. Effects of daily intraperitoneal injections of physiological saline or baclofen (1 and 4 mg/kg) over a period of 27 days on body weight in rats (n = 6 for each group; body weight = 410–480 g and age = 16 weeks at start of experiment). See text for further details. Vertical lines represent ± S.E.M. *P b 0.05 (baclofen groups vs. saline control group).

time) of the food intake data between the 2 baclofen-treated groups at 90 min revealed no significant effects of treatment F(1,10) = 2.181, ns) time (days) (F(19,190) = 1.593, ns) and treatment × time interaction (F(19,190) = 1.190, ns). The 4 mg/kg dose of baclofen produced a short lasting ataxia in 4 of the animals that lasted for between 10 and 20 min after administration. It was most apparent during the first day of treatment and gradually subsided over the next 2–5 treatment days. The effects of chronic administration of baclofen (1 and 4 mg/kg) on body weight gain are shown in Fig. 2. Statistical analysis of the results showed that there were significant main effects of treatment (F(2,15) = 9.821, P b 0.01) and time (F(19,285) = 171.232, P b 0.01) and a small but significant treatment × time interaction (F(38,285) = 1.55, P b 0.05). Although adult rats in the weight range 410–480 g were used in this study, the control animals continued to gain weight over the experimental period. The rats treated chronically with baclofen (1 mg/kg) showed no significant effects of treatment on body weight gain. By contrast, the rats treated with baclofen (4 mg/kg) displayed a loss of body weight from their starting weight baseline during the first 9 days of the experiment. Thereafter, they began to gain weight, but at a slower rate than the control animals. Post-hoc tests revealed that there was a significant reduction in the weight gain in the animals treated chronically with baclofen (4 mg/kg) compared with salinetreated controls during the first 18 days of the experiment (P b 0.05) (see Fig. 2). Thereafter, until the end of the experiment, the mean weight gains of the baclofen-treated rats were still lower than those of control animals, but did not reach statistical significance. 3.2. Experiment 2: Effects of repeated administration of baclofen on long term (24 h) food intake and body weight in rats

Fig. 1. Effects of daily intraperitoneal injections of physiological saline or baclofen (1 and 4 mg/kg) over a period of 27 days on food intake in rats (n = 6 for each group; body weight = 410–480 g and age = 16 weeks at start of experiment). Cumulative food intake wasmeasured30 min(AandB)or90 min(CandD)afterinjectionofsalineorbaclofen(1or 4 mg/kg). See text for further details. Vertical lines represent ± S.E.M. *P b 0.05.

The results are illustrated in Fig. 3. ANOVA revealed that there was no significant effects of treatment (F(1,14) = 1.601, ns), time (F(3,42) = 1.981, ns) and treatment × time interaction (F(3,42) = 1.769, ns) and indicate that the 24 h cumulative food intake following acute (Day 1) and chronic (Days 8, 15 and 22) administration of baclofen was not significantly different from those of control rats. Fig. 4 shows that there were significant decreases in the body weight gain of the rats treated with baclofen (4 mg/kg) compared with saline-treated controls over the 24 day period of the experiment (F(1,14) = 14.011, P b 0.01). The animals treated with baclofen displayed a short lasting loss of body weight from their starting weight

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Fig. 3. Effects chronic administration of physiological saline or baclofen (4 mg/kg , i.p.) on 24 h food intake following the first, eight, fifteenth and twenty second injections in rats (body weight: 250–310 g and age = 9 weeks at start of experiment). The animals received saline (n = 8) or baclofen (1 or 4 mg/kg; n = 8 for each group) each day for 24 days. See text for further details. Vertical lines represent + S.E.M. *P b 0.05.

baselines following the first, second and third injections. Thereafter, they gained weight at a steady rate but post-hoc tests revealed that the gain in body weight was significantly less than those of the control rats throughout the duration of the experiment (at least P b 0.05; see Fig. 4). Thus, after 24 days of treatment, the body weight of the saline control rats had increased by 21.0 ± 1.9% while the body weight gain of the rats treated with baclofen was 12.8 ± 1.7% (P b 0.01). 4. Discussion We have previously demonstrated that baclofen (2 mg/kg, i.p.) given chronically to rats once daily over a period of 27 days increases short-term (60 min) but not long term (24 h ) food consumption and does not affect body weight (Patel and Ebenezer, 2008b). However, Rothwell et al. (1985) reported that the effects of chronic administration of baclofen (2 mg/kg, s.c. twice daily) significantly decreased body weight gain in rats over a 15 day test period, raising the possibility that a higher dose of baclofen than the dose used in the Patel and Ebenezer (2008a,b) study may differently effect short- and long term food intake when administered chronically. The present study was therefore undertaken to extend previous observations (Patel and Ebenezer, 2008b) by investigating the effects of chronic i.p. administration of a low (1 mg/kg) and a high (4 mg/kg) dose of the GABAB receptor agonist baclofen on short-term (30 and 90 min) and

Fig. 4. Effects of daily intraperitoneal injections of physiological saline or baclofen (4 mg/kg) over a period of 24 days on body weight in young adult rats (n = 8 for each group; body weight = 250–310 g and age = 9 weeks at start of experiment). See text for further details. Vertical lines represent ± S.E.M. *P b 0.05.

long term (24 h) feeding and changes in the body weight in freefeeding, non-deprived rats. Systemic administration of these doses of baclofen has been previously shown to reliably increase food intake in rats (Ebenezer and Pringle, 1992; Ebenezer, 1995, Ebenezer and Patel, 2004) in acute experiments. A higher dose of 8 mg/kg was considered unsuitable because we have previously found that this dose is very sedative and causes the animals to go into a sleep-like state within 15 min of administration (Ebenezer and Pringle, 1992). The results from Experiment 1 show that both the 1 mg/kg and 4 mg/kg doses of baclofen produce short-term hyperphagia throughout the 27 day treatment period without the development of tolerance. These results are in agreement with previous findings with a 2 mg/kg dose of baclofen (Patel and Ebenezer, 2008b). By contrast, tolerance can rapidly develop to other pharmacological actions of the drug, such as ataxia, sedation, hypothermia and analgesia (Patel and Ebenezer, 2008a,b; Ebenezer and Prabhaker, 2007; Kroin et al., 1993; Malcangio et al., 1995; Lehmann et al., 2003). It is feasible that tolerance does not occur to the short-term hyperphagic effect of baclofen (1–4 mg/kg) because it does not down-regulate the GABAB receptors involved in the regulation of the feeding response during the 27 day experimental time period. However, it is possible that more prolonged exposure to the drug than that used in the present study may down-regulate these receptors and produce tolerance to its effects on short-term food intake. Baclofen has been reported to produce sedation, muscle relaxation, and ataxia in rat, mouse and pig (Ebenezer, 1990; Ebenezer and Pringle, 1992; Patel and Ebenezer, 2008a) and nausea in monkeys (Foltin, 2005), and it is possible that these behaviours may compete with the effect of the drug on food consumption (see Patel and Ebenezer, 2008a) Some of the rats given the 4 mg/kg dose of baclofen in this study displayed signs of ataxia that was most apparent for about 10–20 min after injection during the first day of treatment. Administration of the 4 mg/kg dose of baclofen did not increase feeding at 30 min on Day 1 of the experiment (see Fig. 1), and it is likely that the ataxia observed in some of the animals may have interfered with their ability to feed (see Patel and Ebenezer, 2008a). However, as the ataxia subsided, the food consumption of the rats increased and by 90 min cumulative food intake was significantly higher than controls. It is noteworthy that with continued administration of the drug, tolerance rapidly developed to the ataxia over the next 2–5 days and that food intake was significantly increased by Day 2 of the experiment at 30 min following administration of the 4 mg/kg dose of baclofen (see Fig. 1). We have previously shown that baclofen increases short-term food intake in pigs but has no effect in the amount of food consumed by the animals in 24 h when compared with vehicle treatment (Ebenezer and Baldwin, 1990). More recently, we have reported that baclofen (2 mg/kg; i.p.) increases short-term food intake in rats but has no effect on 24 h intake either after acute or chronic administration (Patel and Ebenezer, 2008b). Krolczyk et al. (2005) have also shown that there was no effects on daily 24 h food consumption nor body weight gain in rats injected with baclofen (2 mg/kg, i.p.) once a day over a period of 24 days. In agreement with these observations, we have found that baclofen (4 mg/kg, i.p.) does not affect 24 h food intake after the first, eight, fifteenth and twenty second injections (see Fig. 3). The present results indicate that, despite the short-term hyperphagic effects of the 4 mg/kg dose of baclofen, the animals are able to accurately regulate their daily (24 h) food consumption after acute and chronic treatment with the drug. It has been previously reported that chronic administration of baclofen (2 mg/kg; i.p. once daily for 24 days (Krolczyk et al., 2005) or 27 days (Patel and Ebenezer, 2008b)) to rats did not produce significant changes in their body weights compared with control animals. In agreement with these findings, the results from the present study show that chronic administration of the 1 mg/kg dose of baclofen did not produce changes in body weight gain of the animals

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compared with controls (see Fig. 2). Sato et al. (2007) have also reported that lean mice chronically exposed to baclofen in their drinking water for 5 weeks displayed no differences from vehicle control animals in body weight gain. By contrast, chronic administration of the 4 mg/kg dose to rats caused significant reductions in body weight gain compared with saline-treated controls (see Figs 2 and 4). In Experiment 1, adult rats with starting body weights in the range 410–485 g, were used to (i) reduce excessive changes in body weight gain and food intake over the chronic treatment period as would occur if younger animals were used, and (ii) emulate the conditions used by Patel and Ebenezer (2008b) in a previous study with a 2 mg/kg dose of baclofen. Statistical analysis of the data revealed that there was a significant reduction in weight gain in the animals treated chronically with baclofen (4 mg/kg) compared with saline-treated controls during the first 18 days of the study (see Fig. 2). Thereafter, until the end of the 27 day treatment period, the mean weight gains of the baclofen-treated rats were still lower than those of control animals, but did not reach statistical significance. Interestingly, the animals treated with baclofen displayed a loss in body weight compared to their baseline starting weights during the first 9 days of drug treatment. In Experiment 2, younger rats, with starting body weights in the range 250–310 g were used. Treatment with baclofen (4 mg/kg) caused significant decreases in body weight gain of these rats compared with the control animals throughout the 24 day treatment period (see Fig. 4). In addition, the animals treated with the GABAB receptor agonist displayed a short lasting loss of body weight from their starting baselines weight following the first, second and third injections. There are some differences in the effects of baclofen (4 mg/kg) on body weight of the animals in the 2 experiments. Thus, the results from Experiment 1 (see Fig. 2) suggest that tolerance may occur after 18 days of treatment, whereas the results from Experiment 2 (see Fig. 4) indicate that tolerance does not occur with chronic treatment over a 24 day period. These differences may be due to the fact that in Experiment 2 younger and more rapidly growing animals were used compared with the older and fully grown rats used in the Experiment 1. Nevertheless, while these differences need more investigation, the results indicate that, unlike the 1 and 2 mg/kg doses (this study; Patel and Ebenezer, 2008b), chronic administration of a 4 mg/kg dose of baclofen causes an initial reduction in body weight from baseline and decreases the rate at which the animals gain weight. As the daily (24 h) food consumption of the rats chronically treated with baclofen was similar to those of the control animals, the loss in body weight cannot be attributed to a reduction in energy intake. Over two decades ago, Rothwell et al. (1985) reported that chronic administration of baclofen (2 mg/kg, s.c. twice daily) significantly decreased body weight gain in rats over a 15 day test period. In their study, the authors did not measure daily food intake. We have shown that baclofen (1 or 2 mg/kg once a day) has no effect on body weight in rats (Fig. 2; Patel and Ebenezer, 2008b).However, the results from this study have shown that a 4 mg/kg dose of baclofen does reduce body weight gain in both older and younger rats (see Figs 2 and 4). As the half-life of baclofen is approximately 4 h in the rat (Popova et al., 1995), it may be that a more prolonged effect of the drug is required to reduce weight gain, as would be achieved by giving 4 mg/kg of the drug once a day (this study) or 2 mg/kg twice a day (Rothwell et al. 1985). The mechanism(s) involved has still to be established. Rothwell et al. (1985) have speculated that baclofen may decrease body weight by an action in the ventromedial nucleus of the hypothalamus (VMH) to stimulate thermogenesis in brown adipose tissue (BAT) secondary to activation of sympathetic nervous system activity. Thus, Rothwell et al. (1985) have shown that baclofen microinjected into the ventromedial nucleus of the hypothalamus of urethane-anaesthetised rats activates brown fat metabolism. On the other hand, it is possible that other mechanisms may also be involved in the reduction of body weight gain observed after chronic

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administration of baclofen (4 mg/kg). There is no direct evidence that baclofen may act peripherally to affect body weight. However, immunohistochemical studies have indicated that GABAB receptors are associated with neuropeptides-Y (NPY) and proopiomelanocortin (POMC) neurones within the arcuate nucleus of the hypothalamus (see Backberg et al., 2003). It is therefore possible that an action of baclofen in this area of the brain may affect metabolic rate by modulating either NPY and or POMC neurones which can lead to a decrease in body weight gain (see Morton et al., 2006). In conclusion, the main observations of this study are that (i) chronic administration of a low dose of baclofen (i.e. 1 mg/kg) can (a) stimulates short-term feeding without affecting body weight gain, (ii) chronic administration of a higher dose of baclofen (i.e. 4 mg/kg) can (a) stimulates short-term feeding without affecting long term (24 h) feeding, and (b) decrease body weight gain. These results extend previous observations (Patel and Ebenezer, 2008b) and suggest that the GABAB receptor agonist may act through different mechanisms to influence food intake and body weight. These findings may have important implications for future work on the role of GABA and GABAB receptors in the regulation of food intake and energy homeostasis.

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