Leptin-regulated endocannabinoids are involved in maintaining food intake.

Leptin is the primary signal through which the hypothalamus senses nutritional state and modulates food intake and energy balance. Leptin reduces food intake by upregulating anorexigenic (appetite-reducing) neuropeptides, such as alpha-melanocyte-stimulating hormone, and downregulating orexigenic (appetite-stimulating) factors, primarily neuropeptide Y. Genetic defects in anorexigenic signalling, such as mutations in the melanocortin-4 (ref. 5) or leptin receptors, cause obesity. However, alternative orexigenic pathways maintain food intake in mice deficient in neuropeptide Y. CB1 cannabinoid receptors and the endocannabinoids anandamide and 2-arachidonoyl glycerol are present in the hypothalamus, and marijuana and anandamide stimulate food intake. Here we show that following temporary food restriction, CB1 receptor knockout mice eat less than their wild-type littermates, and the CB1 antagonist SR141716A reduces food intake in wild-type but not knockout mice. Furthermore, defective leptin signalling is associated with elevated hypothalamic, but not cerebellar, levels of endocannabinoids in obese db/db and ob/ob mice and Zucker rats. Acute leptin treatment of normal rats and ob/ob mice reduces anandamide and 2-arachidonoyl glycerol in the hypothalamus. These findings indicate that endocannabinoids in the hypothalamus may tonically activate CB1 receptors to maintain food intake and form part of the neural circuitry regulated by leptin.

A metabolic defect promotes obesity in mice lacking melanocortin-4 receptors.

Melanocortin-4 receptor (Mc4r)-null mice exhibit late-onset obesity. To determine whether aberrant metabolism contributes to the obesity, food consumption by Mc4r-null mice was restricted to (pair-fed to) that consumed by wild-type (WT) mice. Pair-fed Mc4r-null females maintained body weights intermediate to that of WT and nonpair-fed Mc4r-null females, whereas pairfeeding normalized the body weights of Mc4r-null male mice. Fat pad and circulating leptin levels were elevated in both male and female pair-fed Mc4r-null mice compared with WT mice. Oxygen consumption of Mc4r-null mice with similar body weights as WT controls was reduced by 20%. Locomotor activity of young nonobese Mc4r-null males was significantly lower than that of WT males; however, locomotion of young nonobese females was normal. Core body temperature of Mc4r-null mice was normal, and they responded normally to cold exposure. Young nonobese Mc4r-null females were unable to induce uncoupling protein 1 (UCP1) in brown adipose tissue in response to peripheral leptin administration, whereas UCP1 mRNA was increased by 60% in the WT females. These results indicate that Mc4r deficiency enhances caloric efficiency, similar to that seen in the agouti obesity syndrome and in melanocortin-3 receptor-null mice.

Neurobiological responses to ethanol in mutant mice lacking neuropeptide Y or the Y5 receptor.

We have previously shown that voluntary ethanol consumption and resistance are inversely related to neuropeptide Y (NPY) levels in NPY-knockout (NPY -/-) and NPY-overexpressing mice. Here we report that NPY -/- mice on a mixed C57BL/6Jx129/SvEv background showed increased sensitivity to locomotor activation caused by intraperitoneal (ip) injection of 1.5 g/kg of ethanol, and were resistant to sedation caused by a 3.5-g/kg dose of ethanol. In contrast, NPY -/- mice on an inbred 129/SvEv background consumed the same amount of ethanol as wild-type (WT) controls at 3%, 6%, and 10% ethanol, but consumed significantly more of a 20% solution. They exhibited normal locomotor activation following a 1.5-g/kg injection of ethanol, and displayed normal sedation in response to 2.5 and 3.0 g/kg of ethanol, suggesting a genetic background effect. Y5 receptor knockout (Y5 -/-) mice on an inbred 129/SvEv background showed normal ethanol-induced locomotor activity and normal voluntary ethanol consumption, but displayed increased sleep time caused by 2.5 and 3.0 g/kg injection of ethanol. These data extend previous results by showing that NPY -/- mice of a mixed C57BL/6Jx129/SvEv background have increased sensitivity to the locomotor activation effect caused by a low dose of ethanol, and that expression of ethanol-related phenotypes are dependent on the genetic background of NPY -/- mice.

Effects of neuropeptide Y deficiency on hypothalamic agouti-related protein expression and responsiveness to melanocortin analogues.

Central administration of neuropeptide Y (NPY) potently induces feeding and its abundance in the hypothalamus increases when energy stores fall. Consequently, NPY is considered to be a physiological effector of feeding behavior. Surprisingly, NPY-deficient (NPY-/-) mice feed and grow normally with ad libitum access to food and manifest a normal hyperphagic response after fasting, suggesting that other feeding effectors may compensate for the lack of NPY. Agouti-related protein (AgRP), a melanocortin receptor antagonist, can also stimulate feeding behavior when administered centrally and is coexpressed in a majority of hypothalmamic NPY-ergic neurons, making AgRP a candidate compensatory factor. To test this possibility, we evaluated AgRP mRNA and protein expression, as well as responsiveness to centrally administered AgRP in NPY-/- mice. These studies demonstrate that hypothalamic AgRP mRNA and immunoreactivity are upregulated with fasting and that these increases are not affected by NPY deficiency. Interestingly, NPY-/- mice are hypersensitive to central administration of AgRP(83-132), yet exhibit a normal response to centrally administered MTII, a melanocortin receptor agonist. These data suggest that if AgRP compensates for the lack of NPY in NPY-/- mice, it is not at the level of AgRP synthesis and may instead involve alterations in the postsynaptic signaling efficacy of AgRP. Moreover, the effects of AgRP are not limited to its actions at the melanocortin-4 receptor (MC4R), because MC4R-deficient (MC4R-/-) mice manifest a significant response to centrally administered AgRP. These data imply that AgRP has additional targets in the hypothalamus.