Stimulation of the nucleus paraventricularis thalami suppresses scratching and biting behaviour of arthritic rats and exerts a powerful effect on tests for acute pain.

In 20 arthritis-sensitive Wistar rats, an electrode was implanted in the paraventricular thalamic nucleus (PV). In the tail-flick and hot-plate test, PV was stimulated continuously for a period of 30 min, during which every 5 min either a tail-flick or hot-plate test was performed. Stimulation of PV induced important prolongations of the reaction times on tail-flick and hot-plate tests. However, there was no significant correlation between the results of the two tests. The effect of stimulation was immediate and did not outlast the stimulation period. PV stimulation did not give rise to tolerance, either within the stimulation period of 30 min, or during the successive testing days. After the termination of these acute pain tests, rats were inoculated with Mycobacterium butyricum. All animals developed polyarthritis. PV stimulation had a profound suppressive effect on the scratching and biting behaviour displayed by the animals. This reduction in scratching and biting behaviour was specific for PV stimulation since stimulation of another target, the ventrolateral thalamic nucleus (4 animals), had no effect on these behaviours. The attenuation of the scratching and biting behaviour was not a result of a general motor impairment, since other behaviours increased (running and sniffing) or did not change. There was no significant correlation between the results of scratching behaviour with those of the tail-flick and hot-plate test.