AM 251 produces sustained reductions in food intake and body weight that are resistant to tolerance and conditioned taste aversion

Abstract

The cannabinoid 1 (CB(1)) receptor has been implicated in the regulation of food intake. Here, we examine the effect of the CB(1) receptor antagonist AM 251 on food intake and body weight over a prolonged period. Further, we examine whether AM 251 produces conditioned taste aversion (CTA) and if sustained antagonism at central receptors contributes to its anorectic effect. The effect of AM 251 of food intake and body weight was examined in daily (1 mg kg(-1)) and 5-day (5 mg kg(-1)) dosing schedules. Matching reductions in food intake and body weight were observed in both paradigms. A single administration of AM 251 (5 mg kg(-1)) significantly reduced food intake for 4 days. Tolerance to the anorectic effects of AM 251 did not develop in either dosing strategy. Active avoidance of AM 251 (3; 5 mg kg(-1), i.p.) was examined using a CTA assay. Rats showed no evidence of CTA associated with AM 251. We investigated the sustained effect of AM 251 (5 mg kg(-1), i.p.) on CB(1) receptors in the hypothalamus using Delta(9)-tetrahydrocannabinol (8 mg kg(-1), i.p.) induced hypothermia. AM 251 initially blocked hypothermia, but this effect was not seen 2 or 4 days later. The results demonstrate that smaller, or infrequent, administrations of AM 251 can produce sustained reductions in food intake and body weight in rat. Reductions in food intake were sustained longer than AM 251 antagonized the effects of a CB(1) receptor agonist in the hypothalamus, and occurred independently of CTA.

Figure 1

(a) Daily food intake (mean±s.e.m.; g) expressed in grams. Vehicle-treated rats (black triangles), 1 mg kg⁻¹ day⁻¹ AM 251 (open circles), 5 mg kg⁻¹ every 5 days AM 251 (black circles). Arrows indicate where 5 mg kg⁻¹ treatments were given. Each rat received identical amounts of AM 251 per body weight over 15 days. Significant differences in food intake between vehicle- and AM 251-treated rats are shown by the bars, P<0.05, Newman–Keuls multiple comparison test. Note the rebound hyperphagia that developed after treatment ended. P<0.05. (b) Cumulative food intake (g) from day 1 to day 15 during treatment (mean±s.e.m.; g) with either vehicle (open bar) or AM 251 (1 mg kg⁻¹ day⁻¹, grey bar; 5 mg kg⁻¹ every 5 days, black bar). Note that the overall reduction in food intake was essentially the same between rats treated daily and rats treated every 5 days with AM 251 relative to vehicle-treated controls. P<0.001. (c) Changes in body weight (mean±s.e.m.; g) starting the day before treatment with either vehicle (black triangles), or AM 251 (1 mg kg⁻¹ day⁻¹, open circles; 5 mg kg⁻¹ every 5 days, black circles). Note that weight loss is similar in both groups of rats given AM 251, even though treatment ends on day 11 in rats treated every 5 days and on day 15 in rats treated daily. (d) Feeding efficiency (Δ body weight/food intake × 100 mean±s.e.m.) of vehicle (black triangles) and AM 251 (1 mg kg⁻¹ day⁻¹, open circles; 5 mg kg⁻¹ every 5 days, black circles) treated rats is shown in 5-day intervals during and after treatment. During treatment, feeding efficiency tended to be lower in AM 251-treated rats compared with vehicle-treated controls, suggesting that changes in metabolism and/or energy expenditure also contributed to the effect of AM 251 on body weight. However, such differences were only statistically significant in rats treated daily with AM 251 during days 5–10. P<0.05. Interestingly, feeding efficiency was significantly greater in rats treated daily and rats treated every 5 days with AM 251 relative to vehicle-treated rats after treatment ended, suggesting that rats treated with AM 251 compensated for decreases in feeding efficiency caused by AM 251. ^*P<0.05 (1 mg kg⁻¹ day⁻¹), ^#P<0.05 (5 mg kg⁻¹ every 5 days).

Figure 2

(a) The percentage of pellets eaten data shows the selection of flavours associated with either AM 251 (2.5 mg kg⁻¹; grey bars; 5 mg kg⁻¹; black bars) or LiCl (open bars) treatment, relative to the selection of vehicle- or saline-associated flavours, respectively, during each test trial. Each rat was permitted to eat 10 out of a possible 24 flavoured pellets. Note that CTA occurs on the first trial for all LiCl-treated rats, but does not occur in AM 251-treated rats, even after four training and testing trials, P<0.0001, unpaired t-test. Rats consistently chose saline-associated flavours over flavours associated with LiCl, whereas rats treated with AM 251 showed no preference for vehicle-associated flavours. (b) Differences in the latency (mean±s.e.m.; min) before the first pellet was eaten between AM 251 or its vehicle (black bars) and LiCl or saline (open bars)-treated rats during training trials 2–4. Note the significant increase in the average latency to flavours associated with LiCl treatment. After the first training trial, rats avoided the flavour associated with LiCl treatment. Rats that refused to eat any flavoured pellets were timed out after 20 min, resulting in a significant increase in the average first pellet latency associated with LiCl injection. P<0.01, paired t-test.

Figure 3

The difference in temperature, from baseline, over 75 min after THC administration (mean±s.e.m.; °C h) in rats treated with either vehicle (open bars) or AM 251 (black bars). Differences are shown as area under the curve. Note that AM 251 did not antagonize the effect of THC 48 or 96 h after administration. P<0.01, unpaired t-test.