Sensory (nociceptive) stimulation evokes Fos expression in the subthalamus of hemiparkinsonian rats

Objective: In an effort to understand cell activity patterns and sensorimotor integration in Parkinson's disease, we have explored the expression of the Fos protein in the subthalamus after sensory ( nociceptive) stimulation of hemiparkinsonian Sprague-Dawley rats [6-hydroxydopamine [6OHDA]-lesioned]. Fos is a marker for neuronal activity in most areas of the brain and the subthalamus is a major driving force of the basal ganglia and target for surgical intervention in parkinsonian patients. Methods: The medial forebrain bundle (major tract carrying dopaminergic nigrostriatal axons) was injected with either 6OHDA or saline (controls). A week later, some rats were subjected to mechanical stimulation ( pinching; activating nociceptive pathways) of the hindpaw for 2 hours, while others received no stimulation. Thereafter, brains were processed using routine tyrosine hydroxylase (TH; marker for dopaminergic cells) or Fos immunocytochemistry. Results: In the cases that had saline injections combined with mechanical stimulation or with no stimulation, as well as those that had 6OHDA lesions combined with no stimulation, there were no Fos(+) cells in the subthalamus. However, in the cases that had 6OHDA lesions combined with mechanical stimulation, there were many Fos(+) cells within the subthalamus of both sides, particularly on the ipsilateral side. Discussion: Our results indicate that after an increase in sensory (nociceptive) activity, via mechanical stimulation, there is an induction of Fos expression in the subthalamus of 6OHDA-lesioned cases. We suggest that activating nociceptive pathways exacerbates the abnormal cell activity in the basal ganglia generated by the hemiparkinsonian condition.