A BEHAVIORAL PROBE OF THE GROWTH OF INTAKE POTENTIAL DURING THE INTER-MEAL INTERVAL IN THE RAT

The rat's willingness to ingest glucose after an initial intraoral intake test was probed by beginning a 2nd intraoral intake test at variable durations (1-120 min). In Experiment 1, after an initial meal of 12.5% glucose solution averaging 26.9 +/- 1.7 ml (SE(M), n = 10), the size of the 2nd (probe) meal of the same stimulus increased linearly from 4.0 +/- 0.9 ml after a 1-min delay to 15.4 +/- 2.7 ml after a 120-min delay. In Experiment 2, intraoral intake of a more concentrated (37.5%) glucose solution rose more slowly as a function of delay from 2.4 +/- 2.7 ml (SE(M), n = 12) to 4.9 a 0.6 ml. For each glucose concentration, the linear recovery function and a slope that depends on stimulus concentration are consistent with a role for gastric emptying during the delay in intake recovery. In Experiment 3, rats ingested 12.5% or 37.5% glucose to satiety in an initial test and received, after a variable delay, either the same or the other concentration as the probe stimulus. The same volumes were ingested at each delay whether the glucose concentration of the probe stimulus was the same dr was switched from that presented in the initial test. This result shows that the taste and caloric properties of the probe stimulus played no role in determining how much of it would be ingested. Rather, the satiety criterion for the probe test was reached on the basis of the volume ingested. The 2-meal intraoral test paradigm is useful for exploring meal pattern control mechanisms and has the special advantage of capturing the growth of intake potential over intervals shorter than the typical inter-meal interval of the rat feeding ad libitum.