Chronic thalamotomy increases pain-related behavior in rats.

The thalamus has been traditionally considered as the 'chief organ' by which pain is perceived (Head H, Holmes G. Sensory disturbances from cerebral lesions. Brain 1911;34:102-254). However, several clinical and experimental observations led to a challenge of this traditional view. In this report, we demonstrate that chronic thalamic lesions, instead of producing hypoalgesia, increased pain reactivity in rats. Different groups of rats were subjected to either subtotal, lateral or medial thalamic lesions. Their reactions to nociceptive stimuli were then assessed for a period of 1-2 months. Rats in the different groups showed an increased reactivity to acute mechanical and thermal nociceptive stimuli and an increase in the pain scores of the formalin test. These results suggest an important role of the thalamus in pain modulation in addition to that of nociceptive transmission.