Raphe unit activity in freely moving cats: effects of phasic auditory and visual stimuli

The effects of phasic auditory or visual stimuli upon the single unit activity of serotonergic neurons within the dorsal raphe nucleus (DRN) were studied in freely moving cats. The predominant response to auditory stimulation (86% of the cells) was excitation, with a mean latency of 40 +/- 3 ms (S.E.M.) and a mean duration of 64 +/- 4 ms. This was typically followed by a longer period (206 +/- 32 ms) with unit activity below the baseline level. This did not appear to be a stimulus-induced inhibition of unit activity, however, since its duration closely corresponded to the normal interspike interval for that particular neuron. The response to repetitive auditory stimulation showed no evidence of habituation and was even present during sleep. A similar response, although generally of lesser magnitude, was evoked by a phasic visual stimulation in 64% of the cells tested. The mean latency for the response to visual stimulation was 53 +/- 4 ms, the mean duration of excitation was 76 +/- 7 ms, and the mean duration of the subsequent suppressed period was 239 +/- 37 ms. The response to the visual stimulus also showed no evidence of habituation. These data indicate that serotonergic neurons of the DRN are driven, with similar temporal characteristics, by stimuli in two different sensory modalities. We hypothesize that these similar effects are attributable to a common excitatory input.