It is reasonable to hypothesize that the mechanisms of hunger might mediate some fraction of the short-term and long-term benefits to health and life span noted to occur as a result of calorie restriction. Which in turn suggests that strategies for the practice of calorie restriction that suppress hunger might be counterproductive. The hormone ghrelin is involved in the response to hunger, and like most proteins it is involved in a range of processes in metabolism. Evolution tends to result in reuse of protein machinery in many mechanisms. Researchers here report on the connection between ghrelin and memory function, which, like many of the interactions between body and brain, is quite indirect.
Ghrelin is produced in the stomach and secreted in anticipation of eating, and is known for its role to increase hunger. For example, ghrelin levels would be high if you were at a restaurant, looking forward to a delicious dinner that was going to be served shortly. Once it is secreted, ghrelin binds to specialized receptors on the vagus nerve - a nerve that communicates a variety of signals from the gut to the brain. Researchers recently discovered that in addition to influencing the amount of food consumed during a meal, the vagus nerve also influences memory function. The team hypothesized that ghrelin is a key molecule that helps the vagus nerve promote memory.
Using an approach called RNA interference to reduce the amount of ghrelin receptor, the researchers blocked ghrelin signaling in the vagus nerve of laboratory rats. When given a series of memory tasks, animals with reduced vagal ghrelin signaling were impaired in a test of episodic memory, a type of memory that involves remembering what, when, and where something occurred, such as recalling your first day of school. For the rats, this required remembering a specific object in a specific location.
The team also investigated whether vagal ghrelin signaling influences feeding behavior. They found that when the vagus nerve could not receive the ghrelin signal, the animals ate more frequently, yet consumed smaller amounts at each meal. Researchers think that these results may be related to the episodic memory problems. "Deciding to eat or not to eat is influenced by the memory of the previous meal. Ghrelin signaling to the vagus nerve may be a shared molecular link between remembering a past meal and the hunger signals that are generated in anticipation of the next meal."