Ultrastructure of Fos-labeled neurons relating to nociceptive primary afferent and substance P terminals in rat spinal superficial laminae.

The presence of Fos-labeled neurons at ultrastructural level was confirmed in the spinal superficial laminae following an injection of formalin into rat hindpaw in the present study. The Fos-like immunoreactive products were found exclusively in regions associated with the euchromatin in the nuclei of Fos-labeled neurons. By the methods used-of anterograde transport of horseradish peroxidase conjugated to wheat-germ agglutinin and immunocytochemistry-it was observed that some Fos-labeled neuronal bodies received synaptic contacts from, or were apposed directly to, small diameter primary afferent terminals in the spinal superficial laminae. By means of double-labeled immunocytochemistry, a direct apposition was often observed and a synaptic relationship was occasionally found between Fos-labeled neuronal bodies and substance P-like immunoreactive terminals.