Electrophysiological diversity of the dorsal raphe cells across the sleep–wake cycle of the rat

Abstract

Through their widespread projections to the entire brain, dorsal raphe cells participate in many physiological functions and are associated with neuropsychiatric disorders. In previous studies, the width of action potentials was used as a criterion to identify putative serotonergic neurons, and to demonstrate that cells with broad spikes were more active in wakefulness, slowed down their activity in slow wave sleep and became virtually silent during paradoxical sleep. However, recent studies reported that about half of these presumed serotonergic cells were not immunoreactive for tyrosine hydroxylase. Here, we re-examine the electrophysiological properties of dorsal raphe cells across the sleep-wake cycle in rats by the extracellular recording of a large sample of single units (n = 770). We identified two major types of cells, which differ in spike waveform: a first population characterized by broad, mostly positive spikes, and a second one displaying symmetrical positive-negative spikes with a large distribution of spike durations (0.6-3.2 ms). Although we found classical broad-spike cells that were more active in wakefulness, we also found that about one-third of these cells increased or did not change their firing rate during sleep compared with wakefulness. Moreover, 62% of the latter cells were active in paradoxical sleep when most of raphe cells were silent. Such a diversity in the neuronal firing behaviour is important in the light of the recent controversy regarding the neurochemical identity of dorsal raphe cells exhibiting broad spikes. Our results also suggest that the dorsal raphe contains subpopulations of neurons with reciprocal activity across the sleep-wake cycle.

Prominent Burst Firing of Dopaminergic Neurons in the Ventral Tegmental Area during Paradoxical Sleep

“…A U-shaped aluminum piece, fixed to a flexible carriage (GFG Co., Pierre-Benite, France) fastened to the stereotaxic apparatus, was positioned above the VTA and lambda suture. This U-piece was then embedded in dental cement with the EEG screws and EMG wires and their six-pin connector, as already described Urbain et al, 2000Urbain et al, , 2002Urbain et al, , 2004Urbain et al, , 2006, leaving a well inside the U-piece. At the end of the surgery, the well was closed with bone wax and wound-healing cream (Madecassol, Serdex, France) was poured out on all the borders of the implant to facilitate healing.…”

Midbrain Raphe Stimulation Improves Behavioral and Anatomical Recovery from Fluid-Percussion Brain Injury

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Serotonin increases synaptic activity in olfactory bulb glomeruli

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