Ketamine “unlocks” the reduced clock-speed effects of cocaine following extended training: Evidence for dopamine–glutamate interactions in timing and time perception

The present study examined the clock-speed modulating effects of acute cocaine administration in groups of male rats that received different amounts of baseline training on a 36-s peak-interval procedure prior to initial drug injection. After injection of cocaine (10, 15, or 20 mg/kg, ip), rats that had received a minimal amount of training (e.g., ⩽30 sessions) prior to drug administration displayed a horizontal leftward shift in their timing functions indicating that the speed of the internal clock was increased. In contrast, rats that had received an extended amount of training (e.g., ⩾180 sessions) prior to cocaine (15 mg/kg, ip) administration did not produce this “classic” curve-shift effect, but instead displayed a general disruption of temporal control following drug administration. Importantly, when co-administered with a behaviorally ineffective dose of ketamine (10 mg/kg, ip) the ability of cocaine to modulate clock speed in rats receiving extended training was restored. A glutamate “lock/unlock” hypothesis is used to explain the observed dopamine–glutamate interactions as a function of timing behaviors becoming learned habits.