Locus coeruleus-to-dorsal raphe input examined by electrophysiological and morphological methods

In order to examine the hypothesis that the locus coeruleus (LC) projects directly to the nucleus raphe dorsalis (DR), electrical stimulation was applied to the LC of rats while recording from single neurons in the region of the DR. Slow firing units of the DR were not influenced by the stimulation, although faster firing units in the nearby substantia grisea centralis (SGC) were. These latter cells become oscillatory in their firing rates during LC stimulation. In parallel studies a retrograde transport technique was employed to obtain morphological evidence regarding projections to DR. Placements of horseradish peroxidase precisely in the DR resulted in very sparse labeling in the LC, although positive transport occurred to other areas. The results indicate that the LC does not project directly to slow firing DR neurons, but does influence faster firing cells in the region of the SGC, probably by complex routes. Suggestions are made for the integration of these findings with earlier fluorescence studies.