Single-unit recordings at dorsal raphe nucleus in the awake-anesthetized rat: spontaneous activity and responses to cutaneous innocuous and noxious stimulations

Abstract

In this study, we recorded the single-unit activity of the dorsal raphe nucleus (DRN) in rats tested first awake and, a few days later, anesthetized with sodium pentobarbital and recorded again. This was achieved by means of a small chronically implanted device supporting a 25 micron platinum-iridium wire as the recording electrode. In both the awake and anesthetized conditions, and in agreement with most of the studies performed at the DRN level, we found that a vast majority of the units, displaying small amplitude and long-duration action potentials, possessed a low level of spontaneous activity (0.2-4 Hz). Among these units, found in greater number under pentobarbital, it was possible to establish that this activity was regular or irregular, in accordance with the literature reports. However, as opposed to these studies, we determined that the 'regularity' was relative, only noticeable in more or less prolonged phases of activity. In particular, we never recorded the so-called 'clock-like' activity, largely reported as an unambiguous criterion for selecting the serotoninergic neurons. In both the awake and anesthetized conditions, the responses of the DRN neurons to peripheral mechanical innocuous and noxious stimulations were observed in only one-half of the units recorded and were weak in comparison to other results that we obtained at the nucleus raphe magnus level in previous studies. When present, these responses were excitation or inhibition, occurring during or after the stimulus application. These results question the direct involvement of the DRN in acute nociception.

Serotonergic Neuronal Activity in the Dorsal Raphe Nucleus of Selectively Bred Alcohol‐Preferring and Alcohol‐Nonpreferring Rats and Unselected Wistar Rats

“…Presynaptic inhibition may be less in P rats compared with NP and Wistar rats, which results in more 5-HT release. For single-spiking 5-HT neurons, the range of firing frequencies reported here in paralyzed Wistar rats (0.4-3.6 spikeshec) is similar to that reported in freely moving rats (0.2-4.0 spikeshec) (Montagne-Clavel et al, 1995). However, the mean firing frequency of single-spiking 5-HT neurons in paralyzed Wistar rats (1.8 ?…”

Organization of Monosynaptic Inputs to the Serotonin and Dopamine Neuromodulatory Systems

“…For instance, dopamine is associated with positive reinforcement and promoting approach/exploration behavior,whereas serotonin is associated with negative reinforcement and behavioral inhibition (Daw et al, 2002; Dayan and Huys, 2008; Deakin and Graeff, 1991; den Ouden et al, 2013; Soubrie, 1986). This idea of opponencybetween the two systemshas been supported by the observation that serotonin neurons are activated by noxious stimuli(Montagne-Clavel et al, 1995; Schweimer and Ungless, 2010) while dopamine neurons are activated by reward. It remains to be determined how these observations can be reconciled withrecent studies showing that many DR neurons (which likely included serotonin neurons) were excited by reward(Miyazaki et al, 2011; Nakamura et al, 2008; Ranade and Mainen, 2009).…”

Release of Serotonin in the Rat Locus Coeruleus: Effects of Cardiovascular, Stressful and Noxious Stimuli

“…These findings suggest that serotonergic neurons are implicated in the modulation of locus coeruleus neurons by sensory stimuli. In this respect it might be of interest to note that, in the dorsal raphe nucleus of conscious rats, -50% of the studied neurons have been shown to be excited by noxious pinch (Montagne-Clavel et al, 1995). Future work will have to clarify the exact origin of the locus coeruleus serotonergic neurons activated by sensory stimuli.…”