GABA-receptor activation in the globus pallidus and entopeduncular nucleus: opposite effects on reaction time performance in the cat.

The possible role of GABAergic mechanisms in the control of the basal ganglia output structures, the globus pallidus (GP) and the entopeduncular nucleus (EP), was studied in cats performing a conditioned flexion movement triggered by an auditory stimulus. The effects of discrete unilateral microinjections of low doses of the GABAA receptor agonist (muscimol 5-100 ng/0.5 microliter) and antagonist (bicuculline methiodide 25-150 ng/0.5 microliter) in the GP and the EP were tested on the motor performance of eight animals trained to release a lever in a simple reaction time (RT) schedule after an auditory stimulus. Control injections in neighboring structures did not induce any effect except with five- to tenfold higher doses in the closest injection sites. The dose of 20 ng muscimol injected into the ventral and medial part of the GP produced an arrest of the performance after a few unsuccessful trials (over the RT reinforcement limit of 500 ms), while muscimol injected in sites located in the lateral GP resulted in a dose-dependent lengthening in RTs, with a concomitant increase in the force change latency. In most of the subjects, the force exerted on the lever was higher after muscimol than after vehicle injection. Force change velocity was then significantly increased. In contrast, muscimol injected in the ventral and rostral region of the EP produced a decrease in RTs or a complete cessation of responding after a high number of anticipatory responses (release of the lever before the trigger stimulus). No significant changes in the force change latency could be observed while there was a non-significant tendency for the force levels to be lowered. Bicuculline injections in the EP were found to increase RTs with a concomitant increase in force change latency and a slowness of velocity, while no significant effect was observed following injections in the GP. These results suggest that a balance between GABAergic activity in the two output nuclei of the basal ganglia, the GP and the EP, is crucial for the correct initiation and execution of the conditioned motor task.