Neuronal responses in vestibular nuclei to dorsal raphe electrical activation.

The effects of dorsal raphe (DR) electrical stimulation on the neuronal activity of vestibular nuclei were studied in anaesthetized rats. The aim was to establish whether the central systems classically involved in nociceptive functions are able to influence vestibular secondary neurons. DR activation induced modifications of the firing in 70% of the tested neurons, the percentage being similar in the lateral (LVN), superior (SVN), and spinal (SpVN) vestibular nuclei. Three different types of responses were recorded: long-lasting modifications (generally enhancements) of the mean firing rate (43%), short-latency response patterns (14%), both (43%). Short-latency response patterns were more numerous in LVN than in SVN. Iontophoretic applications of 5-HT antagonists Methysergide and Ketanserin blocked long-lasting effects but were scarcely effective on the short-latency response patterns evoked by DR stimulation. It is concluded that DR exerts a double control on secondary vestibular neurons: a generalised excitatory influence by serotoninergic fibers and a specific action mostly targeted on LVN, by nonserotoninergic pathways.