The serial reaction time task in the rat: effects of D1 and D2 dopamine-receptor antagonists

Sequential behaviour, probably reflecting procedural learning, has intensively been investigated in humans and monkeys using so-called serial reaction time tasks (SRTT), where serial stimuli are either presented in a random or sequential fashion. Learning of sequences is typically inferred from faster reaction times to such sequences as compared to random blocks of stimuli. Work with such tasks has shown that sequential behaviour seems to be mediated by specific brain systems, including the basal ganglia and the neurotransmitter dopamine. We have recently developed a rat version of the human serial reaction time task, in which rats have to respond to visual stimuli in one of four spatial locations by nose-poking in order to obtain food reward under a fixed ratio schedule (FR13). Here, we used a test version where random and sequential condition phases (10 min each) were alternated within-sessions. In support of our previous work, we found that well-trained (i.e. skilled) rats display superior performance under sequential than random conditions, namely, faster reaction times and higher response accuracies. Furthermore, we investigated the effects of selective dopamine-receptor blockade, by systemically administering SKF 83566, a D1 antagonist (.05-.15 mg/kg), or raclopride, a D2 antagonist (.05-.20 mg/kg), in two separate experiments. Both antagonists impaired responding to the conditioned visual stimuli in a dose-related way, i.e. they decreased, or even blocked, nose-poke rates. In those rats, which kept responding, the speeding of reaction times during sequential conditions was no longer observed with the D1 antagonist, whereas the enhancements in accuracy were preserved, or even enhanced as compared to vehicle. The D2 antagonist also impaired instrumental behaviour, but did not alter sequence effects on accuracy or reaction times. In contrast to responses to the conditioned stimuli, reaction times to the unconditioned stimuli (food pellets) were not substantially affected by either drug. These results are discussed with respect to methodological factors, and the possible role of dopamine for instrumental behaviour, in general, and sequential behaviour, in specific.