Spontaneous and auditory-evoked activity of medial agranular cortex as a function of arousal state in the freely moving rat: interaction with locus coeruleus activity.

To characterize the electrophysiological properties of neurons in the medial agranular frontal cortex (Fr2) with respect to arousal level and locus coeruleus (LC) activity, we recorded spontaneous and auditory-evoked single unit activity in these areas simultaneously during different states of arousal in the rat. In the low-arousal state, as determined by EEG, 14/56 Fr2 neurons showed a tonic increase in spontaneous firing rate and 9/56 presented a clear inhibitory response to tone (onset latency 37 ms, duration 200 ms). The inhibitory response was not clear during the high-arousal state. Cross-correlation analysis of pairs of Fr2 and LC units, excluding activity during the actual tone, also showed a negative correlation from 120 ms before, to 170 ms after, Fr2 discharge in 5/63 pairs, only during low arousal. Significantly, 4/5 of the Fr2 neurons having this negative correlation with LC were included in that population which showed a tonic increase in spontaneous firing rate and inhibited to tone during low arousal. LC neurons, on the other hand, all showed excitation to tone stimulation (peak latency 30 ms), and response amplitude was not affected by changes in arousal level. The negative correlation in spontaneous activity, as well as their differential responses to tone, suggests an interaction between a select population of Fr2 neurons and the LC during the low-arousal state. Future studies with lesion or pharmacological manipulations would be necessary to confirm the presence of this interaction.