Intense noxious thermal stimuli ≥48°C activate serotonergic neurons much more strongly in the lateral paragigantocellular nucleus than in the raphe magnus nucleus.
The present study was designed to record electrophysiological responses to graded noxious thermal stimuli of serotonergic and nonserotonergic neurons in the lateral paragigantocellular reticular (LPGi) and the raphe magnus (RMg) nuclei in rats. All of the neurons recorded were juxtacellularly filled with neurobiotin and identified with double immunofluorescent labeling for both neurobiotin and serotonin. Under halothane anesthesia (0.75%), noxious thermal stimuli ≥48°C activated almost all (88%) of the serotonergic neurons located within the LPGi (n = 16). The increase in firing was clear (3.4 ± 0.3 spike/s: mean of responses above the population median) and sustained during the whole application of strong thermal noxious stimuli, with a high mean threshold (48.3 ± 0.3°C) and large receptive fields. Recording of serotonergic neurons in the RMg (n = 21) demonstrated that the proportion of strongly activated (>2 spike/s) neurons (19% vs 59% for the LPGi) as well as the magnitude of the activation (2.1 ± 0.4 spike/s: mean of responses above the population median) to thermal noxious stimuli were significantly lower than in the LPGi (P < .05). Within the boundaries of both the LPGi and the RMg (B3 group), nonserotonergic neurons were also predominantly excited (75%) by noxious stimuli, and the resulting activation (7.9 ± 1.2 spike/s) was even greater than that of serotonergic neurons. Thermal noxious stimuli–induced activation of LPGi serotonergic cells probably plays a key role in serotonin-mediated modulations of cardiac baroreflex and transmission of nociceptive messages occurring under such intense noxious conditions.
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aUniversité Pierre et Marie Curie, Paris 6, Site Pitié-Salpêtrière, Paris, France
bUniversité Paris Descartes, Paris, France
cCentre de Psychiatrie et Neurosciences, INSERM UMR 894, Paris, France
*Corresponding author. Address: INSERM/UPMC, UMR 894, Site Pitiè-Salpêtrière, 91 Boulevard de l’Hôpital, F-75013 Paris, France.