Differences in the responses of raphe nuclei to repetitive somatosensory stimulation

The responses of single units in raphe (R.) nuclei dorsalis, magnus, pallidus, and obscurus to repetitive stimulation of peripheral nerves were studied in the chloralose-anesthetized cat. Low-intensity electrical stimuli (1.5T) which activated the large-diameter fibers were applied to the common peroneal and lateral gastrocnemius nerves at 0.25, 0.5, 1 and 2 Hz. Responses evoked at each frequently were compared with the control response evoked at 0.1 Hz. All units isolated demonstrated response decrements during periods of stimulation greater than or equal to 0.5 Hz. The long-term effects of repetitive stimulation, however, varied among the four nuclei. After 150 stimulus presentations at 0.5, 1, or 2 Hz, sensory responsiveness decreased in R. dorsalis but was enhanced in caudal R. obscurus units. Changes in the responsiveness of the other two nuclei were not significantly different from control. Responses to twin pulses indicated that R. neurons are not well suited to relay rapid, repetitive stimuli. The functional significance of these observations has implications for the role of the raphe in habituation and in the modulation of sensory traffic to higher centers. Raphe responses are also contrasted to the known responses of reticular formation neurons to repetitive stimulation.