Dorsal raphe and nociceptive stimulations evoke convergent responses on the thalamic centralis lateralis and medial prefrontal cortex neurons.

There is evidence for the existence of a descending pain suppression system, but also there are data supporting the hypothesis for the modulation of pain at higher central nervous system levels. In the present study we give evidence for a possible ascending pain modulation pathway which involves the dorsal raphe (DR), the centralis lateralis nucleus (CL) of the thalamus and the medial prefrontal cortex (PFCx). Urethane-anesthetized rats were used. Simultaneous single unit recordings were done in the CL and PFCx regions under noxious and DR stimulations. Cells responding to both types of stimuli exhibit duration responses directly related to the duration of the stimuli. Thus, from our results we conclude a DR influence upon CL and PFCx structures that are involved in the coding of nociceptive information. A possible route for an ascending pain modulation path is proposed.