Effects of the dopamine D2 agonist, quinpirole, on time and number processing in rats.

In Experiment 1, rats were trained to discriminate discrete sound sequences that varied in time or number. On time trials, the number of sounds was held constant at 4 and the duration of the sound sequence was either 2 or 8 s. On number trials, the duration of the sound sequence was held constant at 4 s, and the number of sounds was either 2 or 8. Psychophysical functions for time and number were obtained by presenting unreinforced sequences of intermediate duration or number. In agreement with previous research, sensitivity to variation in time was greater than variation in number. Rats received injections of the specific D2 agonist, quinpirole (0.08 mg/kg), or saline. Quinpirole significantly attenuated control by both time and number, but it did not increase behavioral estimates of time or number. In Experiment 2, rats were given different dosages of quinpirole (0.02, 0.04 or 0.06 mg/kg). The steepness of the psychophysical functions for both time and number was reduced in a dose-related fashion without any evidence of an increase in the estimation of time or number. The similarity of the effect of quinpirole on both time and number processing is consistent with the idea that the same internal mechanism is used for timing and counting. However, it is not consistent with the idea that D2 dopamine agonists selectively increase the rate of the internal clock, which is assumed to underlie performance in a temporal bisection procedure. Quinpirole (at doses between 0.02 and 0.08 mg/kg) reduces sensitivity to time and number in a bisection procedure without altering the speed of the internal clock.