Abstract
Recent evidence suggests ghrelin may up-regulate the number of spine synapses. However, it is not completely understood whether an increased number of synapses are expressed on existing spines or accommodated in newly generated spines. We examined if ghrelin might have promoted the generation of new dendritic spines. Localization of polymerized actin (F-actin), highly expressed in dendritic spines, was assayed using phalloidin, a mushroom toxin that has a high affinity to F-actin. Alexa 488-conjugated phalloidin was visualized and relative changes in fluorescing puncta were quantified using a confocal microscope. Ghrelin was applied to cultured hippocampal slices for either 60 min or 23 h. Ghrelin increased the phalloidin fluorescent signals. The antagonist of the ghrelin receptor, D-Lys3-GHSR-6, blocked the ghrelin's effect of increasing the phalloidin signal, suggesting that the ghrelin's effect was mediated by the ghrelin receptor (GHSR1a). The ghrelin-mediated increase in phalloidin signals remained elevated while ghrelin was present in the culture media for 23 h. However, removal of ghrelin from culture media restored the phalloidin signal to control level. Our results suggest ghrelin may have a stimulating effect on the generation or remodeling of dendritic spines, and the spine change persists in the presence of ghrelin. The serum ghrelin level is high when the stomach is empty, and the ghrelin level remains high until metabolic demands are fulfilled. Thus, ghrelin may be involved in food-related and appetite-related learning in the hippocampus.