Endocannabinoid levels in rat limbic forebrain and hypothalamus in relation to fasting, feeding and satiation: stimulation of eating by 2-arachidonoyl glycerol

Abstract

Endocannabinoids are implicated in appetite and body weight regulation. In rodents, anandamide stimulates eating by actions at central CB1 receptors, and hypothalamic endocannabinoids may be under the negative control of leptin. However, changes to brain endocannabinoid levels in direct relation to feeding or changing nutritional status have not been investigated. We measured anandamide and 2-arachidonoyl glycerol (2-AG) levels in feeding-associated brain regions of rats, during fasting, feeding of a palatable food, or after satiation. Endocannabinoid levels were compared to those in rats fed ad libitum, at a point in their daily cycle when motivation to eat was absent. Fasting increased levels of anandamide and 2-AG in the limbic forebrain and, to a lesser extent, of 2-AG in the hypothalamus. By contrast, hypothalamic 2-AG declined as animals ate. No changes were detected in satiated rats. Endocannabinoid levels in the cerebellum, a control region not directly involved in the control of food intake, were unaffected by any manipulation. As 2-AG was most sensitive to variation during feeding, and to leptin regulation in a previous study, we examined the behavioural effects of 2-AG when injected into the nucleus accumbens shell, a limbic forebrain area strongly linked to eating motivation. 2-AG potently, and dose-dependently, stimulated feeding. This effect was attenuated by the CB1 receptor antagonist SR141716. These findings provide the first direct evidence of altered brain levels of endocannabinoids, and of 2-AG in particular, during fasting and feeding. The nature of these effects supports a role for endocannabinoids in the control of appetitive motivation.

Figure 1

Endocannabinoid levels in the limbic forebrain, hypothalamus and cerebellum of rats in relation to fasting, feeding and satiation. Data for anandamide levels (A), and 2-arachidonoyl glycerol levels (B) are shown, and are expressed as pmol or nmol, respectively, per gramme of wet tissue weight. All values are means±s.e.mean of n=4. ^* P<0.05 and ^** P<0.01, significantly different from control by ANOVA followed by the Bonferroni test.

Figure 2

Dose-related increases in food intake over one hour following bilateral accumbens shell 2-arachidonoyl glycerol infusion. All values are mean±s.e.mean from eight rats. ^** P<0.01, ^*** P<0.0001, significant difference from vehicle; ^†† P<0.01, significant attenuation of the hyperphagic effects of 0.5 μg 2-AG by pre-treatment with the CB1 antagonist SR141716 (0.5 mg kg⁻¹, s.c.).