The orexigenic effect of peripheral ghrelin differs between rats of different age and with different baseline food intake, and it may in part be mediated by the area postrema.

Ghrelin is mainly secreted during fasting. While an orexigenic effect of peripherally injected ghrelin has been reported, reproducing this effect has often proven difficult. Here, we hypothesized that ghrelin's effect to increase food intake may depend on the experimental conditions (e.g., age of animals). We therefore investigated the effect of an IP ghrelin injection (100 microg/kg) on food intake in rats of different age and at different times during the light-dark cycle, i.e. with different levels of baseline food intake. Ghrelin injected at dark onset in ad libitum fed young rats (body weight [BW] 92 g) slightly increased feeding while no such effect was observed in 12 h food deprived rats (BW 150 g). In the middle of the light phase, ghrelin significantly increased feeding up to 2 h after injection in ad libitum fed rats (BW 130 g; food intake 1 h after injection: NaCl 0.4 +/- 0.2 g versus ghrelin 1.2 +/- 0.3 g [p < 0.05]). In various subsequent experiments, older rats (BW 300-490 g) tested under the same conditions did not respond to a single ghrelin injection. However repeated ghrelin injection (15 microg/kg/day once daily at light onset) over 10 days significantly increased food intake in rats (BW 400-460 g) starting from day 4 of the experiment (24 h food intake: NaCl approx. 19.5 g, ghrelin 22.5 g). Interestingly, the latter effect was completely abolished in rats lesioned in the area postrema (AP). Cumulative food intake was also increased in SHAM but not in AP-X animals (e.g., after 7 days: SHAM/NaCl 135.1 +/- 5.3 g versus SHAM/ghrelin 149.7 +/- 3.5 g [p < 0.05], AP-X/NaCl 127.2 +/- 16.4 versus AP-X/ghrelin 127.9 +/- 5.3). We conclude that ghrelin's effect to increase food intake can best be demonstrated when basal food intake is low. Ghrelin increases feeding mainly in young, fast growing animals. Ghrelin may therefore link the high energy needs to body growth in young individuals. In older animals, peripheral ghrelin increased feeding when injected repeatedly over several days. At least under these conditions, ghrelin's effect was mediated by the AP/NTS region. Using repeated administration, ghrelin might be an interesting tool to increase feeding in patients suffering from wasting diseases such as cancer anorexia.