Relationship between the concentration of cholecystokinin-like immunoreactivity in plasma and food intake in male rats.

In food-deprived male rats IP injection of cholecystokinin octapeptide (CCK-8, 5 micrograms), ingestion of food or ejaculation caused a comparable increase in plasma concentrations of CCK-8 and inhibited food intake. IV injection of 0.1 microgram CCK-8 interrupted ongoing feeding and greatly increased plasma CCK-8 levels. Osmotic minipumps delivering 0.5 micrograms CCK-8/h implanted IP reduced meal size and caused a modest increase in plasma CCK-8 levels. Injection of 5 micrograms CCK-8 IP produced an abrupt but transient increase in plasma CCK-8 concentrations whereas plasma concentrations of CCK-8 increased gradually with feeding. Injection of 5 micrograms CCK-8 IP, but not feeding, caused a marked increase in plasma oxytocin levels. The suppression of feeding, but not the increase in oxytocin, induced by IP CCK-8 was reversed by ICV injection of the CCK antagonist proglumide in a dose (100 micrograms) which failed to affect food intake if injected IP. Deprivation of food decreased and feeding increased the concentration of CCK-like immunoreactivity in the CSF. It is suggested that CCK-8 inhibits feeding in physiological doses by a specific mechanism in which peripheral as well as central neural CCK is involved.