The sham intake of concentrated, but not weak, milk solutions requires up to three sham-feeding tests for intake to reach maximum (7). It is well known that the sham intake of concentrated (0.8 M) sucrose requires three or more sham-feeding tests to reach its maximum (4, 6, 17), but it is not known if this occurs with weaker sucrose solutions. We investigated this question by measuring the sham intake of seven concentrations of sucrose (0.025, 0.05, 0.1, 0. 2, 0.4, 0.6, and 0.8 M) during five sham-feeding tests. Sham intake of the three highest concentrations required up to three sham-feeding tests to reach maximum. Sham intake of the four lowest concentrations reached maximum in the first sham-feeding test. Our results show that the type of negative feedback that controls the intake of sucrose depends on its concentration. With weak solutions, intake is limited by a single direct, physiological, negative-feedback signal. When concentrated solutions are ingested, intake is controlled jointly by a direct physiological signal operating in conjunction with a labile one that loses its effectiveness with sham-feeding experience.

We describe at the microstructural level the impact of unconditioned and conditioned negative feedback on the licking behavior of the rat. Seven groups of rats were trained to ingest one of seven different concentrations of sucrose (0.025, 0.05, 0.1, 0.2, 0.4, 0.6, and 0.8 M sucrose) under real-feeding conditions until intake was stable. They were then given three sham-feeding tests with the same solution. We compared the size of the clusters (SC) and number of clusters (NC) during the 17-min period when intake rate was declining in the real-feeding test with SC and NC during the corresponding 17-min period in the following sham-feeding test. Intake increased significantly over the three sham-feeding tests with the three highest concentrations, indicating the extinction of conditioned negative feedback. With these three solutions, we compared the microstructure of licking behavior in the first with that in the third sham-feeding test to determine if conditioned negative feedback affected SC or NC or both. The effect of both unconditioned and conditioned negative feedback on licking behavior was to decrease the NC without significantly affecting their size. We conclude that negative feedback derived from the accumulation of sucrose in the gastrointestinal tract decreases the probability of initiating a bout of licking during a pause. It has no effect on the ability to continue a bout of licking once it has begun.

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