Firing properties of two types of nucleus raphe dorsalis neurons during the sleep-waking cycle and their responses to sensory stimuli.

Spontaneous activity of the nucleus raphe dorsalis (NRD) neurons during the sleep-waking cycle and effects of sensory stimuli upon NRD neurons were studied in cats. Seventy-one neurons recorded within the NRD were classified into two groups with the use of the coefficient of variation of firing intervals during waking (W): 41 regularly firing (clock-like) and 30 irregularly firing (non-clock-like) neurons. The majority of clock-like and one-third of non-clock-like neurons showed a decrease in their firing rate during slow-wave sleep (SWS) compared with W. All neurons of both types displayed their lowest level of activity during paradoxical sleep. During the late phase of SWS, many clock-like neurons reduced their firing prior to the occurrence of pontogeniculo-occipital waves, whereas non-clock-like neurons did not show such a specific property. Clock-like neurons were totally unresponsive to nociceptive and non-nociceptive somesthetic stimuli, while about half of the non-clock-like neurons were driven by these stimuli. Half of the clock-like and one-third of the non-clock-like neurons were driven by click stimulation, and the majority of them showed an excitatory response. Some of the clock-like and non-clock-like neurons exhibited inhibitory and excitatory response to flash stimulation, respectively. The results of this experiment show that two types of neurons do exist in the NRD and suggest that they play a functionally different role in the brain.