Abstract
The present study was undertaken to investigate the hyperphagic responses to the 5-HT 1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), in young and adult rats fed either a powder diet or pellets. In the young rats, 8-OH-DPAT (500 μg/kg s.c.) increased the consumption of pellets - but not powder - during the 2 h following drug administration. On the other hand, 8-OH-DPAT did not promote hyperphagia in adult rats presented with either pellets or a powdered diet. The influence of the 5-HT 1A agonist on midbrain serotonin (5-hydroxytryptamine, 5-HT) turnover was examined. Administration of 8-OH-DPAT (500 μg/kg s.c.) induced similar decreases in 5-HT turnover, as reflected by the ratio of 5-hydroxyindoleacetic acid (5-HIAA) to 5-HT, in young and adult rath 1 h after administration. Nevertheless, some metabolic responses to 8-OH-DPAT were found to be influenced by age. Young and adult rats were injected with a low dose of 8-OH-DPAT (50 μg/kg s.c.) to specifically test the presynaptic regulation of 5-HT turnover. Again, midbrain 5-HIAA to 5-HT ratios were decreased to the same extent in both young and adult rats. The results suggest that (i) gnawing may be an important parameter in the food consumption that is triggered by a high dose of 8-OH-DPAT, (ii) analysis of the presynaptic effects of 8-OH-DPAT on 5-HT turnover cannot solely explain the influence of the agonist on feeding behavior.
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