Serotonergic neurons in the nucleus centralis superior (NCS) of freely moving cats fire most rapidly during active waking (mean 2.94 spikes/s), slow down during slow-wave sleep (mean 1.38 spikes/s), and are least active during REM sleep (mean 0.46 spikes/s). Their activity does not increase with transient muscle movements but decreases just before and during sleep spindles. Most NCS serotonergic neurons are excited by sudden sounds or lights. A serotonin-like drug reduces their firing by about 44%. A subset of these neurons shows much smaller changes across sleep-wake states and is inhibited, rather than excited, by sensory stimuli. These patterns are compared with serotonergic neurons in other brain regions.
In unanesthetized and unrestrained cats, serotonergic neurons in several raphe nuclei showed a strong negative correlation between their spontaneous firing rate during waking and their response to systemic injections of the serotonin agonists 5-MeODMT or LSD. The authors propose that both the baseline activity of these neurons and the size of their response to serotonin-like drugs can be explained by the density of autoreceptors on each neuron.