NMDA and dopamine D-2 receptors in the caudate-putamen are not involved incontrol of motor readiness in rats

Rationale: In reaction time (RT) paradigms, in which a variable preparation interval preceded the imperative stimulus, RT become shorter as a function of increasing time from the start of a trial until presentation of the imp erative stimulus. The shortening of RT as the preparatory foreperiod elapse s reflects increasing motor readiness, however, the underlying neurochemica l mechanisms are still poorly defined. Objective: The present study investi gated in rats whether signals transmitted via the N-methyl-D-aspartate (NMD A) subtype of glutamate receptors and via dopamine D-2 receptors in the cau date-putamen (CPu) are involved in motor readiness. Methods: A simple RT ta sk demanding conditioned lever release was used, in which the upcoming rewa rd magnitude (5 pellets or 1 pellet) was signalled in advance by discrimina tive stimuli and the imperative stimulus was subsequently presented after a variable foreperiod (200, 500 or 800 ms). Results: In intact rats, RT of c onditioned responses was shortened with foreperiod lengthening and with exp ectancy of the high reward magnitude, but there was no interaction between both factors. Bilateral infusion of the competitive NMDA antagonist DL-2-am ino-5-phosphonovaleric acid (APV) (2, 10 mug in 0.5 mul/side), of the prefe rential dopamine D-2 antagonist haloperidol (5, 12.5 mug in 0.5 mul/side) o r infusion of vehicle (0.5 mul/side) into the central subregion of the CPu had no effect on progressive RT shortening with increasing foreperiod. Conc lusion: The present data provide no clues to suggest that motor readiness r elies on stimulation of dopamine D-2 and NMDA receptors in the central CPu.