Fos immunoreactivity in the intermediolateral nucleus induced by tendon vibration of the m. triceps surae in rats pretreated with a nitric oxide blocker or precursor.

We investigated neuronal activation of the rat intermediolateral (IML) nucleus of the thoracolumbar spinal cord, initiated by Achilles tendon vibration, after intramuscular (m. triceps surae) administration of 7­nitroindazole (7­NI) or L­arginine (LA). The spindle afferent response to vibratory stimuli induced a distinct bilateral increase in the activation of c­Fos immunoreactivity in the spinal neurons in three groups of rats (tendon­vibrated, tendon­vibrated + 7­NI and tendon­vibrated + LA). The T5/T13 segments in tendon­vibrated +7­NI animals showed the highest increase of Fos­immunoreactive neurons. This increase was two times higher than that in tendon only­vibrated control rats and endon­vibrated + LA animals. The highest mean number of labelled neurons were observed in the IML nucleus and in layers 4 and 7 of the T5-L3 segments in tendon­vibrated and tendon­vibrated + 7­NI animals, and in the IML nucleus and layer 4 in tendon­vibrated + LA rats. The highest mean number of activated neurons was found ipsilaterally in the IML nucleus of the T5/T13 segment. These results indicate that decreased nitric oxide release after injection of 7­NI was accompanied by a potentiation of the early c­fos gene expression induced by muscle proprioceptive activity within the thoracolumbar region of the rat spinal cord. Thus, enhanced c­Fos immunoreactivity in the IML nucleus indicated that the sympathetic nervous system can exert a direct influence on the muscle spindles.