1Neurophysiology, Clinical Sciences Centre, Imperial College London, London, United Kingdom
2Clinical Sciences Centre, MRC, London, United Kingdom
Serotonin neurons play a central role in a variety of behavioural and cognitive functions, particularly in processing aversive stimuli. It has been suggested that serotonergic dorsal raphe neurons may encode a prediction error rule for aversive stimuli and act as an opponent system to the midbrain dopamine system, which encodes a prediction error rule for appetitive stimuli. We examined the hypothesis whether nociceptive footshocks phasically activate neurochemically-identified serotonin neurons in the dorsal raphe nuclei.
In the past, serotonin neurons were typically identified by their physiological properties: Serotonin neurons fire in a slow clock-like pattern and have broad action potentials. Recent studies using juxtacellular labelling demonstrated that not all of these presumed serotonin neurons were 5HT-positive. Therefore, we combined electrophysiological recording and the juxtacellular labelling method to identify the neurons. We recorded from neurons within the dorsal raphe nuclei in anaesthetised rats. During the recording nociceptive footshocks were administered, and then the individual neurons were labelled juxtacellularly, and then identified by immunohistochemistry.
Results showed a variety of different neurons within the dorsal raphe nuclei, which can be grouped due to their electrophysiological and histological properties. Group I consists of 5-HT-positive cells with typical clock-like firing that were phasically activated by the footshocks. A second group included 5-HT-positive neurons that were either inhibited or showed no reaction to the footshocks. A majority of these showed stereotypic bursting pattern. The last group contains 5-HT negative neurons, which showed ambiguous reactions to the administration of nociceptive stimuli.
This study gives evidence that at least some serotonergic dorsal raphe neurons are phasically activated by nociceptive stimuli.