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 he ld 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 i ntermediate duration or number. In agreement with previous research, sensit ivity to variation in time was greater than variation in number. Rats recei ved injections of the specific D-2 agonist, quinpirole (0.08 mg/kg), or sal ine. Quinpirole significantly attenuated control by both time and number, b ut it did not increase behavioral estimates of time or number. In Experimen t 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 numbe r 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 quinp irole on both rime and number processing is consistent with the idea that t he 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 i n a temporal bisection procedure. Quinpirole (at doses between 0.02 and 0.0 8 mg/kg) reduces sensitivity to time and number in a bisection procedure wi thout altering the speed of the internal clock. (C) 2001 Elsevier Science I nc. All rights reserved.