APPLICATION OF THE GABA ANTAGONIST BICUCULLINE TO THE PREMOTOR CORTEXREDUCES THE ABILITY TO WITHHOLD REACHING MOVEMENTS BY WELL-TRAINED MONKEYS IN VISUALLY GUIDED REACHING TASK

1. A gamma-aminobutyric acid (GABA) antagonist, bicuculline methiodide (BMI, 10 mu g/mu l, 1 mu l),was locally injected into a total of 32 s ites in the right premotor cortex (PM) of two rhesus monkeys that had been well-trained in a visually guided reaching task (VR) for similar to 3 yr. The monkey initiated the task by pressing a central hold leve r with its left hand, and this was followed by waiting (1 s), warning (central green square on a computer monitor, 0.5 s), cue (right, upper , or left square), delay (2-5 s), and go (central green square changes to red, <1.2 s) periods. In the go period, the monkey released the ho ld lever and reached out to one of three target levers (left, upper, o r right) that had been indicated 2-5 s previously in the cue period. 2 . At three sites in the dorsal part of the PM, after the local applica tion of BMI, reaching movements of the left forelimb, which were not p art of the trained-reaching, occurred 200-300 ms after the onset of a burst of neuronal activity at the BMT injection site. This induced-rea ching, which was designated a ''forced-reaching'' movement, occurred w hile the monkeys were pressing the hold lever before the cue appeared- i.e., during the waiting or warning period. No reaching occurred when the burst did not appear. Furthermore, trajectories and electromyogram s of the forelimbs during the forced-reaching movements were similar t o those in the trained-reaching movements in the VR task. 3. These res ults suggest that restricted sites in the dorsal PM of monkeys are inv olved in the initiation and/or execution of trained-reaching movements and that GABAergic inhibition at these sites normally suppresses this initiation/execution unless it is required. By relaxing GABAergic sup pression, the dorsal PM might send a command to a neuronal system that is associated with trained reaching to recruit the system, thereby in itiating and/or executing the trained reaching.