NIDDK scientists and their collaborators have discovered that a protein called M3 muscarinic acetylcholine receptor helps regulate how much animals eat. The findings, which appear in the 8 March 2001, issue of Nature, highlight just how complex the brain chemistry governing food intake is.
The researchers, led by Jürgen Wess, a section chief in NIDDK's Laboratory of Bioorganic Chemistry, showed that mice lacking M3 muscarinic acetylcholine receptors ate much less, weighed less, and had far fewer fat deposits than mice with normal levels of M3 receptors. The mice deficient in M3 receptors also had lower levels of leptin, insulin, and melanin-concentrating hormone, all hormones that act with others in the brain's hypothalamus to influence eating.
M3 muscarinic acetylcholine receptors are found in some glands, in the stomach, gut and bladder, and throughout the brain, but their role in the brain has been unclear. They, like many other other receptors, respond to acetylcholine, a critical substance that transmits nerve impulses. By showing that functioning M3 receptors are required for animals to eat normally, Wess says, "We've found a new role for an old neurotransmitter. The study shows for the first time that acetylcholine acting through M3 muscarinic receptors has a role in stimulating food intake."
To better understand the M3 receptor's physiological roles, Wess' group, in collaboration with Chu-Xia Deng's lab in NIDDK's Genetics of Development and Disease Branch, generated a line of mice that lacked functional M3 receptors. The altered mice were as healthy, fertile, and long-lived as normal mice, and the two groups started life at similar weights. But by the time the altered mice reached adulthood, they weighed 25% less than mice with intact M3 receptors, and they ate 30% fewer calories per day than the normal mice did. The mice that ate less weren't sick, and they behaved like normal mice, indicating that illness played no role in their reduced appetites, says Wess.
A key part of the group's study was learning how the lack of M3 receptors affects the levels of other hormones involved in the regulation of food intake. Masahisa Yamada (now at RIKEN Brain Science Institute in Japan) did most of this work while he was a postdoctoral fellow in Wess' lab.
Wess and his colleagues have found that, in normal mice, M3 receptors are abundant in the hypothalamus, the part of the brain that stimulates and suppresses eating.
More than a dozen molecules are known to influence food intake in the hypothalamus. Levels of blood leptin and insulin, for instance, are first sensed by neurons in a section of the hypothalamus called the arcuate nucleus. High levels of these hormones in normal animals then set in motion additional signals in other areas of the hypothalamus that cause the animals to reduce food intake. Different hypothalamic signaling molecules like neuropeptide Y, agouti-related protein, and melanin-concentrating hormone trigger eating when their levels are high in normal animals.
Wess and his colleagues observed that blood leptin and insulin levels were quite low in the mice lacking M3 receptors, a finding that is not surprising given that lean animals usually produce reduced amounts of these hormones. However, the researchers were surprised that the altered mice produced less melanin-concentrating hormone in the hypothalamus than normal mice. "This was completely unexpected," says Wess.
Generally, levels of hypothalamic melanin-concentrating hormone go up in hungry animals, thus encouraging them to eat. Also, it usually rises when leptin and insulin levels are low, as they are in the altered mice. When the scientists gave the altered mice melanin-concentrating hormone, their appetites increased and they ate the same amount as normal mice. This result supports studies by other labs that show melanin-concentrating hormone is required for animals to eat normally.
Because the same cells in the hypothalamus that contain melanin-concentrating hormone also make M3 receptors, Wess speculates that acetylcholine stimulates the production of melanin-concentrating hormone by first acting on the M3 receptors. These discoveries reveal yet another pathway in the brain that influences eating, Wess adds.
Citation: Nature 410:207-212, 2001.