Effects of acute low-dose combined treatment with rimonabant and sibutramine on appetite and weight gain in rats

In view of its potential advantages, drug polytherapy is currently attracting significant interest in the field of obesity research. In this context, concurrent manipulation of serotonergic and cannabinoid pathways in rodents has been found to reduce food and fluid intake in both an additive or synergistic manner. To further assess the value of this polytherapeutic approach, the current study examined the acute effects of low-dose combinations of the cannabinoid CB1 receptor antagonist/inverse agonist rimonabant (0.5 mg/kg) and the dual serotonin- and noradrenaline-reuptake inhibitor sibutramine (0.125 and 0.25 mg/kg) in male rats. Ethological analysis was used to generate comprehensive behavioural profiles, including the behavioural satiety sequence (BSS). Findings confirmed that, although neither drug given alone significantly altered food intake, feeding behaviour or weight gain, rimonabant per se tended to reduce consumption and time spent feeding while significantly increasing scratching and grooming responses. However, none of these effects of the CB1 receptor antagonist/inverse agonist was significantly altered by the presence of either dose of sibutramine. In striking contrast to recent reports of acute low-dose interactions (enhanced appetite suppression and reduced side-effects) between rimonabant and naloxone, present results would not appear to support the clinical potential of rimonabant/sibutramine polytherapy for obesity.

Endogenous opioids and cannabinoids: system interactions in the regulation of appetite, grooming and scratching

Growing evidence suggests substantial crosstalk between endogenous opioid and cannabinoid systems in the regulation of appetite. Not only is cannabinoid-induced hyperphagia abolished by opioid receptor antagonists (and vice versa), but several laboratories have reported supra-additive anorectic responses following co-administration of opioid and CB1 receptor antagonists. In the present study, videoanalysis has been used to characterise the acute effects of sub-anorectic doses of rimonabant (0.25, 0.75 mg/kg) and naloxone (0.1 mg/kg), alone and in combination, on mash intake, ingestive and non-ingestive behaviour, and post-treatment weight gain in male rats. The results confirmed that, when administered alone, none of these treatments significantly altered mash consumption, various measures of feeding behaviour, or weight gain. Although most non-ingestive behaviours were also unaffected, 0.75 mg/kg rimonabant induced compulsive scratching and grooming. However, when naloxone was given in combination with either dose of rimonabant, both food intake and time spent feeding were significantly decreased while the behavioural satiety sequence (BSS) was accelerated. On further analysis, the co-treatment reductions in food intake and feeding behaviour were found to be of an additive rather than supra-additive nature. Intriguingly, the co-administration of naloxone also virtually abolished the compulsive scratching response to the higher dose of rimonabant. Findings are discussed in relation to current views on the molecular bases of opioid-cannabinoid system interactions and the unexpected 'dual' advantage (reduction in appetite plus attenuation of side-effect) of low-dose combinations of opioid and cannabinoid CB1 receptor antagonists.

Grooming, scratching and feeding: role of response competition in acute anorectic response to rimonabant in male rats

RATIONALE

Although the CB1 receptor antagonist/inverse agonist rimonabant acutely suppresses food intake in rodents, the behavioural specificity of this effect remains unclear.

OBJECTIVES

To profile the behavioural effects of rimonabant in a free-feeding context.

MATERIALS AND METHODS

Videoanalysis was employed to characterise the effects of acute rimonabant (1.5 and 3.0 mg/kg, IP) on the behaviour of non-deprived male rats exposed to palatable mash. Data were also collected on post-treatment weight gain, and, as prolonged appetite suppression has been found after single dosing with compounds of this series, rats were reassessed (drug-free) for food intake 7 days after initial testing.

RESULTS

Both doses of rimonabant not only decreased mash consumption (44-55%) but also reduced 24-h weight gain. Although videoanalysis confirmed the inhibitory effects of rimonabant on feeding behaviour, it also revealed concurrent reductions in locomotion, rearing and sniffing as well as substantial (up to tenfold) and dose-dependent increases in grooming and scratching. Timecourse analyses further revealed that rimonabant dose-dependently induced frequent episodes of atypical scratching that waned over the test but which were succeeded by prolonged and behaviourally disruptive grooming. Finally, as groups did not differ in mash consumption on retest, any prolonged anorectic effect of acute rimonabant dissipates within 7 days of treatment.

CONCLUSIONS

The anorectic response to rimonabant in male rats would appear to be due largely to response competition. This parsimonious conclusion is supported by the less profound (although still significant) increases in scratching and grooming observed in rats treated with a sub-anorectic dose (0.5 mg/kg) of the compound.

Atypical anxiolytic-like response to naloxone in benzodiazepine-resistant 129S2/SvHsd mice: role of opioid receptor subtypes

RATIONALE

Mice of many 129 substrains respond to environmental novelty with behavioural suppression and high levels of anxiety-like behaviour. Although resistant to conventional anxiolytics, this behavioural phenotype may involve stress-induced release of endogenous opioids.

OBJECTIVES

To assess the effects of opioid receptor blockade on behavioural reactions to novelty stress in a chlordiazepoxide-resistant 129 substrain.

MATERIALS AND METHODS

Experiment 1 contrasted the effects of the broad-spectrum opioid receptor antagonist naloxone (1.0-10.0 mg/kg) in C57BL/6JOlaHsd and 129S2/SvHsd mice exposed to the elevated plus-maze. Experiments 2-4 examined the responses of 129S2/SvHsd mice to the mu-selective opioid receptor antagonist beta-funaltrexamine (2.5-10.0 mg/kg), the delta-selective antagonist naltrindole (2.5-10.0 mg/kg) and the kappa-selective antagonist nor-binaltorphimine (2.5-5.0 mg/kg).

RESULTS

129 mice displayed higher levels of anxiety-like behaviour and lower levels of general exploration relative to their C57 counterparts. Although naloxone failed to alter the behaviour of C57 mice, both doses of this antagonist produced behaviourally selective reductions in open-arm avoidance in 129 mice. Surprisingly, none of the more selective opioid receptor antagonists replicated this effect of naloxone: beta-funaltrexamine was devoid of behavioural activity, naltrindole suppressed rearing (all doses) and increased immobility (10 mg/kg), while nor-binaltorphimine (5 mg/kg) nonspecifically increased percent open arm entries.

CONCLUSIONS

Recent evidence suggests differential involvement of opioid receptor subtypes in the anxiolytic efficacy of diverse compounds including conventional benzodiazepines. The insensitivity of 129 mice to the anxiolytic action of chlordiazepoxide, coupled with their atypical anxiolytic response to naloxone (but not more selective opioid receptor antagonists), suggests an abnormality in anxiety-related neurocircuitry involving opioid-GABA interactions.