Catecholamine sensitivities of neurons related to a visual reaction time task in the monkey prefrontal cortex

Abstract

1. Using microiontophoretic techniques and conscious monkeys, sensitivities to noradrenaline (NA) and dopamine (DA) of neurons of the prefrontal cortex (PFC), which showed changes in activity during a visual reaction time task, were investigated. The visual reaction time task was initiated by the pressing of a lever and consisted of four phases: an initial waiting phase of 3.0 s, a warning phase (green light of variable duration of 1.5-3.5 s), a lever release go phase (red light), and a final reward phase. 2. A total of 153 neurons, which showed changes in activity during one or two phase(s) of the task, were sampled. Of these neurons, 39 changed their activity during the warning phase, 48 changed their activity during the go phase, 38 changed their activity during both the warning and the go phases, and 28 changed their activity during the reward phase. 3. Iontophoretically applied NA and DA (with a current of 30-70 nA, but usually with a current of 50 nA) induced excitatory and/or inhibitory responses in 141 of the 153 task-related neurons. NA induced responses in 99 neurons, and these responses were predominantly inhibitory (n = 90). DA induced excitatory (n = 62) and inhibitory (n = 30) responses in 92 neurons. Fifty neurons were sensitive to both NA and DA. 4. The neurons showing changes in activity during different phases of the task showed different sensitivities to NA and DA applied with 50 nA. The warning phase-related neurons were primarily sensitive to NA (36/39), the go phase-related neurons were primarily sensitive to DA (44/48), neurons related to both the warning and go phases were sensitive to both NA and DA (33/38), and the reward phase-related neurons were primarily sensitive to NA (23/28). 5. In the neurons that showed increased changes in activity during the warning phase, NA reduced the background activity to a greater extent than the activity during the warning phase and increased the ratio of the warning phase-related activity to the background activity. In the neurons that showed decreased changes during the warning phase, NA reduced the activity during the warning phase to a great extent than the background activity, and increased ratio of the background activity to the warning phase-related activity. Furthermore, the latency of onset of the change in activity tended to become shorter by application of NA. Thus, NA enhanced the change in activity during the warning phase, irrespective of whether the direction of the change was toward an increase or a decrease.(ABSTRACT TRUNCATED AT 400 WORDS)