Analysis of licking responses in rats: effects of cholecystokinin and bombesin.

A detailed analysis of the licking response of rats to milk was made. The effects of cholecystokinin (CCK) and bombesin (BBS) on the consummatory response were compared. The normal licking response pattern was found not to change as the meal progressed; no predictor was found to indicate when the meal would terminate. Licks normally occurred at about six per second, but there were a few licks that occurred at about three per second, with no licking occurring in the intermediate range; thus a distinctive bimodal frequency distribution of the interlick interval was formed. Cholecystokinin was found to reduce the intake of milk and the related licking indexes as expected, but it also induced an unusual licking pattern: The contact duration was shortened, the interlick interval was lengthened, and there was an increase in the proportion of licks with long interlick intervals, the effects that accentuated the bimodality of the distribution of interlick intervals. Bombesin reduced the intake and the related licking indexes as expected; however, it did not induce any unusual licking characteristics as seen with CCK at comparable effective doses. The CCK effect may be mediated partially by a change in the motor aspect of the consummatory response; however, BBS reduced intake by simply decreasing the number of licks and shortening the intake duration.