Age-dependent inhibition of suckling by cholecystokinin

The effects of exogenous cholecystokinin (10--40 U/kg body wt) on neonatal rat suckling were studied in 5-, 10-, 15-, and 20-day-old rats that were equipped with tongue cannulas. Milk (0.1 ml over 10 s) was delivered through the tongue cannula once every minute, providing a rat was still suckling. Cholecystokinin, a well-established inhibitor of adult rat feeding, did not reduce milk intake volume of 5- and 10-day-old rats. It did, however, depress milk intake of 15- and especially of 20-day-old rats. Despite its effect on milk intake, cholecystokinin never lengthened the latency to attach to the nipple and, paradoxically, reduced it on certain occasions. The implications of these findings for the events controlling various aspects of suckling behavior throughout the course of the nursing-suckling period in rats and for the mechanism of action of cholecystokinin are discussed.