Our bodies naturally generate stress hormones on a daily basis, we need them to survive. During periods of stress, the levels of these hormones can go up and that’s generally a good thing. It helps to mediate the stress response. There are a host of different actions of stress hormones that help to modulate and dampen the overall physiology of the stress response. One of the actions that we’ve been especially interested in is that stress hormone cortisol in humans has an effect on the transport of sugar or glucose into cells. Many cells in the body can use glucose as an energy source, but the brain is absolutely dependent on glucose. It can’t use any other energy supply.
Gluco corticoids, the general name for these stress hormones, can inhibit the transport of sugar or glucose into brain cells, which depend on glucose for energy. There are different concentrations of receptors for these gluco corticoids in different parts of the brain, making some parts of the brain thought to be particularly vulnerable to having their supply of sugar, or glucose or energy, cut off or shut down. The part of the brain that we were very interested in was the hippocampus which has the highest concentration of these gluco corticoids or stress hormone receptors. The hippocampus also performs a very important function for us, it is a central player in a particular memory system that we have. It’s called a declarative memory system or explicit memory system and it can be thought of as a kind of shopping list memory function. You go to the grocery store and you’ve forgotten your list, this is what enables you to accurately tick off at least a fair number of things that you intended to buy. Our hypothesis going into the study was that giving these gluco corticoids to people might inhibit the availability of glucose for those cells which would be needed to assist these people to do particular kinds of memory tasks.
There are different kinds of short-term memory and short can be very, very short or it can be over intermediate intervals. Generally speaking, when you and I are able to recall a factor event over an interval that’s anywhere from a few minutes to hours or days, that’s using this declarative memory system. Other kinds of memory, like our ability to remember how to ride a bicycle, is a completely different sort of thing. You might not even be able to put it into words and yet clearly we remember that in some sense. We were looking at the kind of memory for factor events where you can potentially, consciously call it up and say, “Ah!” this or that.
Insulin is the hormone that we all naturally produce in response to eating a meal, or in response to having some kind of sugar load in our body which a meal will produce. Insulin again is a good thing, if you don’t have it you’ve got diabetes. The experimental interest that we have in insulin is that it can effectively promote gluco transport in certain parts of the brain a little bit better than in other parts of the brain, based again on the availability of receptors for insulin in these different brain areas. What we’ve found is that giving controlled gluco infusions or giving controlled insulin infusions we’ve been able to facilitate or improve memory performance in Alzheimer’s disease.
Source: Ivanhoe Broadcast News, PO Box 865, Orlando, Florida, 32802