Retrograde response of the rat facial motor nucleus to muscle inflammationelicited by phytohaemagglutinin

To investigate whether motoneurons react to signals deriving from target in flammation, we studied the facial motor nucleus after injections of phytoha emagglutinin in the snout of adult rats. This plant lectin is a tool widely used to induce proliferation and activation of T lymphocytes, and we obser ved marked lymphocyte infiltration in the injected facial muscles. Retrogra de labelling of motoneurons was not detected after peripheral injections of fluorochrome-conjugated phytohaemagglutinin. Nitric oxide synthase, reveal ed by NADPH-diaphorase histochemistry, OX-42-immunoreactive microglia, and expression of the cell death repressor gene bcl-2, investigated with nonrad ioactive in situ hybridization and immunohistochemistry, were evaluated in the facial nucleus. Daily phytohaemagglutinin injections for 4 days, mimick ing repeated muscle exposure to inflammatory stimuli, resulted after 2-day survival in NADPH-diaphorase induction in motoneurons and marked activation of the surrounding microglia. Quantitative image analysis of NADPH-diaphor ase staining, and OX-42 immunoreactivity and microglial cell counts indicat ed highly significant increases with respect to saline-injected control cas es. The occurrence of a neuroprotective retrograde response was evaluated m onitoring bcl-2 expression. Following single phytohaemagglutinin administra tion, bcl-2 mRNA was significantly upregulated at 6 h in facial motoneurons and returned to basal levels at 24 h. Bcl-2 immunoreactivity was markedly upregulated at 24 h and was still significantly higher than in controls at 7 days, when concomitant NADPH-diaphorase induction in motoneurons and micr oglia activation was also observed. No degenerative features were observed in motoneurons after phytohaemagglutinin injections at the examined time-po ints. The data point out that local muscle inflammation retrogradely elicit s gene activation in motoneurons and their microenvironment.