Increased firing of locus coeruleus neurons associated with preparatory set in rats.

The activity of locus coeruleus (LC) neurons was studied during the performance of a reaction-time task with warning. Seventeen cells were tentatively identified as LC neurons using two criteria: (1) Relative location from the mesencephalic trigeminal (Me5) neurons, which have large-amplitude bursting discharges synchronized with masticatory movements. (2) The presence of an antidromic response to the stimulation of the dorsal pathway. Of these 17 cells, 16 neurons showed intense and consistent firing during the period between the warning stimulus and the response. Neither the stimuli nor the lever pressing movements alone were accompanied by such a change, suggesting that increased firing occurs in association with preparatory set. The one remaining neuron of the 17 cells showed no change with the task performance. It is supposed that only the 16 homogeneous neurons belong to the LC.