Effects of Immobilization Stress on the Background Impulse Activity of Locus Coeruleus Neurons

We studied in rats changes in the impulse background activity (BA) of locus coeruleus (LC) neurons after short- and long-term immobilization stress; distributions of LC neurons by the level of regularity of their BA, dynamics of spike trains, and pattern of histograms of interspike intervals (ISI) were taken into account. We also calculated the means of the main BA statistical indices. Both short- and long-lasting immobilizations resulted in drops in the mean frequency of background discharges of LC neurons to about half of the initial value. Two-hour-long immobilization evoked statistically significant shifts in the distribution of LC neurons by the level of regularity of their BA, while after longer (15 h) immobilization this distribution nearly returned to the initial pattern. Short-lasting immobilization exerted no significant effect on the dynamic characteristics of BA; statistically significant changes in this respect developed only after longer stress. After 15-h-long immobilization, we also observed a noticeable increase in the number of neurons with polymodal ISI distributions. Therefore, stress results in significant modifications of the temporal parameters of the BA of LC neurons; characteristics of the BA of these neurons should be considered neuronal correlates of the stress state.