Intracellular activity and morphology of the prefrontal neurons related to visual attention task in behaving monkeys

Dopamine modulates neuronal activities related to motor performance in the monkey prefrontal cortex

Effects of iontophoretically applied dopamine were investigated in prefrontal neurons of the monkey which showed activity changes during a visual reaction time task. The task consisted of an initial waiting phase (3.0 s), a warning phase (green lamp, 1.5-3.5 s), a lever-release GO phase (red lamp) and a reward phase. The neurons (n = 99) showed their activity changes during the warming (n = 31), GO (n = 43) or reward (n = 25) phase. Among those, dopamine predominantly influenced the GO phase-related activities (39/43) and the activity changes were enhanced by dopamine. Further, fluphenazine attenuated the GO phase-related activity changes. Results suggest that prefrontal dopamine may be involved in modulations of neuronal activities related to motor performance.

Laminar distributions of neurons sensitive to acetylcholine, noradrenaline and dopamine in the dorsolateral prefrontal cortex of the monkey

Sensitivities of neurons to acetylcholine (ACh), noradrenaline (NA) and dopamine (DA) were investigated at different depths of the dorsolateral prefrontal cortex (PFC) in awake or halothane-anesthetized macaque monkeys, using microiontophoretic techniques with multi-barreled electrodes. The laminar locations of tested neurons (n = 403) were estimated by reconstructing electrode tracks based on the microlesion made by passing a current through the recording barrel, which contained a carbon fiber. Iontophoretically applied drugs induced excitatory or inhibitory responses. Neurons excited by ACh (n = 105) were located mainly in layers III and V, and those inhibited by ACh (n = 126) were in layers III and IV. The majority of the NA-sensitive neurons (n = 123) were NA-inhibited neurons (n = 100), and were found most often in layers III and IV. The ratio of DA-sensitive neurons (excited, n = 74; inhibited, n = 63) to tested neurons was higher in the deep layers than in the superficial ones. These results indicate that sensitivities of the PFC neurons to ACh, NA and DA are not uniform between cortical layers, suggesting that each of these substances may predominantly influence the neuronal activity of particular layers of the monkey PFC.

Prefrontal unit activity of macaque monkeys during auditory and visual reaction time tasks

During a simple reaction time task using auditory or visual stimuli, a total of 96 single units were recorded from the dorsolateral prefrontal cortex of macaque monkeys. These monkeys were trained to depress a lever for a fixed period which produced a tone burst or a small spot of light. After a variable period, the stimulus intensity changed, and then, the monkey released the lever. Eighty-one cue-related units were classified into 3 types according to their decay time; that is, phasic, tonic and mixed. Phasic units (n = 19) showed a transient increase of discharge rate with a relatively short peak latency (70-300 ms). Of these, 17 units responded exclusively to either visual or auditory stimuli and two to both. Tonic units (n = 55) showed enhanced or suppressed activity, with longer latencies, which was sustained as long as the cue period continued. The temporal pattern of the discharge in 23 tonic units was found to be similar for both the auditory and visual cues. Seven mixed-type units showed combined phasic and tonic patterns. Lever release-related units (n = 15) were activated only during the period of lever release with no distinction in cue modality. It is suggested that the dorsolateral prefrontal cortex receives sensory inputs fairly discretely on the phasic-type neurons and that these sensory activities are transmitted to the tonic-type neurons which lead to an initiation of the lever release behavior.