Feelings of hunger are triggered, in part, when the empty stomach releases a hormone called ghrelin (GRELLin), which acts on neurons in the hypothalamus in a process called synaptic remodeling. The same process underlies learning and memory in other parts of the brain, leading researchers to study whether ghrelin contributes to these "higher" functions. In the March issue of Nature Neuroscience, Tamas Horvath of Yale University and colleagues identified ghrelin and its receptors in the hippocampus-a brain area in which synaptic remodeling brings about navigational memory and recall of place and context.
In rats treated with ghrelin, the investigators showed an increase in dendritic spines-points at which neurons make connections-as well as an increase in a cellular form of memory, called longterm potentiation, in the hippocampus. Ghrelin improved rats' performances on a memory-taxing maze test. It also boosted the memory of mice compared to "knock-out" animals missing the ghrelin receptor and mice with a condition mimicking Alzheimer's disease.
The authors note that aging and obesity-both associated with low ghrelin levels-influence the progression of Alzheimer's disease. Horvath said the findings raise the hope that ghrelin supplementation can improve memory in Alzheimer's and other dementias. The work also raises caution for therapies aimed at controlling appetite by inhibiting ghrelin, the study concludes.