Gene expressions of cholecystokinin (CCK) and CCK receptors, and its satiety effect in young and old male rats. 
Cholecystokinin (CCK), one of the first discovered gastrointestinal hormones, which stimulates pancreatic enzyme secretion and induces gallbladder contraction, is one of the most abundant neurotransmitter peptides in the brain and is implicated in satiety via CCK-A receptors.  We compared the suppressive effect of central administration of CCK on food intake in young and old rats.  The suppressive effect on food intake was enhanced in old rats.  To examine the mechanism of this enhanced suppression, we measured the mRNA levels of CCK, CCK-A and CCK-B receptors in the cerebral cortex and the hypothalamus of young and old male rats.  The mRNA level of CCK-A receptors in the hypothalamus decreased with age, whereas the mRNA levels of CCK-B receptors in the hypothalamus and cerebral cortex did not.  The mRNA level of CCK in the cerebral cortex decreased significantly in old rats, although the decrease in the hypothalamus was not significant.  Therefore, the enhanced sensitivity to CCK of old rats could not be explained by changes in gene expressions of CCK and CCK receptors.  Moreover, the effects of aging on the gene expressions of CCK-A and CCK-B receptors were different.