This paper shows that leptin injections into the hippocampus improves memory in a T-maze footshock avoidance and step down inhibitory avoidance tasks. It caught my eye because I just finished a course in behavioral neuroscience, but I have never for the life of me thought the two were related.

Leptin is a hormone that is released by fat cells that tells your brain that you're good on food and that you should stop eating. Leptin insufficiency causes obesity. Before you get excited though you should know that leptin is not a diet drug because if you have too much of it your brain stops listening.

An example of your brain ignoring leptin can be found in individuals with diabetes and obesity. Obese or diabetic individuals often get leptin resistance. Individuals with diabetes also have some cognitive deficits.

This paper shows that if you inject leptin into the hippocampus (part of the brain involved in learning and memory) of a mouse, it improves memory retention. The two tasks that they used were a T-maze footshock avoidance task -- wherein mice are taught to choose one direction at a T junction by shocking them if they choose the other one -- and a step down avoidance task -- wherein mice are taught to get the hell off a platform before they get their feet shocked. Both improved retention.

Interestingly, the improvement shows a U shape. When you get to high doses of leptin the effect vanishes.

Here are the figures for both effects:

As I was taught so well in my class, you always have to check with these studies that the tasks assigned actually REQUIRE the part of the brain they were modifying. This one does. Otherwise, who knows what you are changing.

Anyway I just thought this study was sort of cool. I have never heard of a link between leptin and memory. They also make this link between ABeta mice and leptin. ABeta is a protein whose accumulation has been linked to Alzheimer's. Mice that create to much ABeta show cognitive deficits. They show that an ABeta overproducing mouse also showed improvement when treated with leptin -- and link that result to the observation that leptin decreases ABeta. I am not sure what to make of that, but there is probably a metabolic story related to ABeta production.

Incidentally here is the press release where I found this article.

UPDATE: Evil Monkey at Neurotopia expands nicely on the Leptin-ABeta association and the implications for Alzheimer's disease.