Saccharin increases the effectiveness of glucose in stimulating ingestion in rats but has little effect on negative feedback

To determine if oropharyngeal stimulation contributes to the magnitude of the negative feedback signal during ingestion, we tested rats on a series of solutions that varied in taste intensity but not in nutritive or osmotic properties. To do this, Na saccharin was added to a standard 0.2 M glucose solution in concentrations of 0, 0.25, 0.5, 1, 2, 4, and 8 mM. Nondeprived rats were given free access to these solutions in ascending order of concentration in 30-min tests in test cages with a lickometer that recorded the time of each lick to the nearest msec. The cumulative licking curve for each rat on each test, obtained by cumulating the number of licks in each successive minute during the test, was fit by the least squares method to an exponential function. This provides an estimate of the initial rate of licking and of the rate of decline in the rate of licking. The results were that the volume ingested increased from 6 to 26 ml and the initial rate of licking increased from 50 to 230 licks/min/min over the saccharin concentration range, but the estimates of the rate of decline of the rate of licking remained constant at about 0.06 licks/min over this range. Thus, we found no evidence for an oropharyngeal contribution to the decline in the rate of licking. Changes in the volume ingested depended solely on the initial avidity with which the rats ingested the solution.