[Role of the ventrobasal complex of the thalamus in nociception and pain: data obtained in the normal rat and in a model of clinical pain].

Recent anatomical, electrophysiological, neuropharmacological and behavioural studies have provided new elements for the understanding of the role of the thalamus in nociceptive and pain mechanism. Data presented here demonstrate that the thalamic ventrobasal complex (VB), which receives direct afferents from the spinothalamic tract in the rat and monkey, plays a role in the sensory-discriminatory component of pain in these two species. Apart from the electrophysiological aspect, we discuss the effects of analgesic compounds on neuronal responses observed at this level and modifications in a nociceptive reaction threshold after lesions of this structure in the non-anesthetized freely moving animal. Data obtained in the normal rat are compared with those obtained under the same experimental conditions in a clinical pain model: the arthritic rat. In these animals the capacity of the VB neurons to respond to somatic stimuli is profoundly modified, many of them being activated by moderate stimuli from inflamed joints (lateral pressure, movements). Spinal tracts transmitting messages from these joints appear to differ (at least in part) from those transmitting nociceptive messages in the normal rat. Finally, at similar doses, morphine is much more effective in these animals than in the normal rat. Results of these studies show that nociception and clinical pain are not always exactly dependent on the same systems.