PII S0024-3205(98)00322-1
PHARMACOLOGY LETTERS
Accelerated Communication
C.N.R. Center for Neuropharmacology, Cagliari, Italy "Bernard B. Brodie" Department of Neuroscience, University of Cagliari, Italy Department of Cytomorphology, University of Cagliari, Italy
(Submitted April 14, 1998; accepted May 1, 1998; received in final form June 2, 1998)
Introduction
Over the last decade, research in the cannabinoid field has benefited by substantial advances. The identification of a) a central receptor for D 9-tetrahydrocannabinol (D 9-THC; the major psychoactive constituent of marijuana) (1), and b) anandamide and other endogenous ligands to this receptor (2,3) indicates the existence of a brain cannabinoid system, which may constitute the neuroanatomical substrate underlying 1) the psychotropic effects of D 9-THC and, even more importantly, 2) unravelled physiological functions.
The recent availability of a selective and potent antagonist of the brain cannabinoid CB1 receptor, SR 141716 (4), constitutes a unique tool for investigations in this field. SR 141716 has been reported to antagonize several neuropharmacological effects of cannabinoid receptor agonists (e.g., 4-9), and possess intrinsic effects (5,6,9). Since SR 141716 binds specifically to the cannabinoid receptor without any significant affinity for a large number of other receptors (4), its intrinsic efficacy may be due to the antagonism of an endogenous tone of the cannabinoid system.
Marijuana has been reported to stimulate appetite in humans and cannabinoids to enhance feeding in laboratory animals (see 10). Thus, appetite control may be one of the physiological processes regulated by endogenous cannabinoids and cannabinoid receptors. This hypothesis was tested in the present study by evaluating whether the repeated administration of SR 141716 alters food intake and body weight in non-obese Wistar rats.
Methods
Animals - Male Wistar rats (Harlan Nossan, Correzzana, MI, Italy), approximately 3 months old and weighing 280-330 g at the start of the study, were used. Rats were individually housed under an inverted 12-h light-dark cycle (lights on at 21:00), at a constant temperature of 22±2°C and relative humidity of 60%. Standard rat chow (MIL Morini, San Polo d'Enza, RE, Italy) and tap water were available ad libitum throughout the study. Rats were habituated to handling and i.p. injection.
Procedure - Daily food and water intakes and body weight were recorded daily at 08:30-08:45. Food and water intakes were expressed as g/kg and ml/kg body weight, respectively, while body weight was expressed as percent of the body weight monitored on the last day prior to the start of drug treatment. Rats were divided into 3 groups matched for food and water intake during the 3 days preceding the start of drug treatment and treated with 0 (n=6), 2.5 (n=7) and 10 (n=6) mg/kg SR 141716, respectively. SR 141716 (Sanofi Recherche, Montpellier, France) was suspended in saline with 0.1% Tween 80 and injected i.p. in a volume of 2 ml/kg once a day (between 08:30 and 08:45) for 14 consecutive days.
Data analyses - Data were analyzed by two-way ANOVA with between-group factor (doses) and within-group factor with repeated measurements (days); multiple comparisons were evaluated by the Newman-Keuls test. Body weight changes were also analyzed by piecewise linear regression with breakpoint.
Results
Fig. 1 shows the effect of SR 141716 on daily food (top panel) and water (center panel) intake and on body weight (bottom panel) over the course of the study; results of the ANOVAs are also reported in Fig. 1.
On the first day of treatment, 2.5 and 10 mg/kg SR 141716 inhibited food intake by 25 and 50%, respectively, in comparison to vehicle-treated rats; however, on continuing treatment, tolerance to the feeding suppressant effect of SR 141716 developed within 5 days.
Water was not affected by SR 141716 treatment; only a modest reduction, which did not reach statistically significance, was observed in both SR 141716-treated groups on the first day of treatment.
Body weight paralleled the initial reduction of food intake; indeed, it was reduced up to 3.3 (on day 2) and 6.9% (on day 4) at the doses of 2.5 and 10 mg/kg SR 141716, respectively. However, in contrast to the rapid return of food intake to baseline values, body weight of SR 141716-treated rats grew constantly slower than that of vehicle-treated rats throughout the entire treatment period, as indicated by the different slope of the growth curves of SR 141716-treated rats and control animals.
Fig. 1: Effect of SR 141716 on daily food (top panel) and water (center panel) intake and body weight (bottom panel) in rats. SR 141716 was administered i.p. once daily at the doses of 0 (n=6; (open circle)), 2.5 (n=7;(filled circle) ) and 10 (n=6; (open square)) mg/kg. The dotted vertical lines comprise the 14-day treatment period. Each point indicates the mean ± SEM of n subjects. Results of ANOVAs for treatment and post-treatment phases are also reported. *p<0.05 in comparison to vehicle-treated rats (Newman-Keuls test). In the bottom panel, a) the dotted horizontal line represents the 100% of the body weight monitored on the last day prior to the start of drug treatment, and b) solid lines indicate the growth curves as assessed by piecewise linear regression (in the 2.5 and 10 mg/kg SR 141716-treated groups, the calculated slope break points coincided with drug treatment interruption).
After treatment discontinuation, food intake was significantly higher in the rats of the 10 mg/kg SR 141716 group compared to vehicle-treated rats; moreover, body weight of both groups previously treated with SR 141716 increased at equal or even higher rate than in control rats.
Discussion
The results of the present study demonstrate that the recently synthesized, cannabinoid CB1 receptor antagonist, SR 141716, reduces appetite and body weight in non-obese Wistar rats. These results, later duplicated in genetically obese Zucker rats (this laboratory, in preparation), are in agreement with recent data indicating that SR 141716 decreases sucrose and regular food intake in rodents (11,12). The results of the present study are predictive of a physiological role of the brain cannabinoid receptor in the control of appetite and body weight. Accordingly, SR 141716 might exert its anorectic effect via antagonism of an endogenous cannabinoid tone that normally stimulates feeding or, alternatively, behaving as an inverse agonist at the cannabinoid CB1 receptor.
The results of this study also indicate that tolerance to the anorectic effect of SR 141716 developed rather rapidly. Nevertheless, body weight loss in SR 141716-treated rats persisted well beyond the drug effect on food intake. Thus, if the initial reduction of body weight was the likely result of the severe anorexia induced by SR 141716 on days 1 and 2 of treatment, the longer lasting effect of SR 141716 on body weight might be caused by increasing energy expenditure. This conclusion is consistent with clinical data showing that marijuana may increase body weight even when no appetite stimulation is produced (13).
Finally, SR 141716 might be clinically useful in the treatment of obese patients.
Acknowledgements
The authors are grateful to Mrs Marinella Boi and M.Elena Vincis for animal care, Mr Hugh Sugden for language editing of the manuscript and Dr Philippe Soubrie Sanofi Recherche, Montpellier, France, for the supply of SR 141716.
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Corresponding author: Prof. Gian Luigi Gessa, Department of Neuroscience, University of Cagliari, Via Porcell 4, I-09124, Cagliari, Italy, Tel: +39 70 675-8417, Fax: +39 70 657-237, e-mail: lgessa@unica.it
