Ribosomal RNA transcriptional activation and processing in hamster facial motoneurons: effects of axotomy with or without exposure to testosterone.

A key step in the ability of neurons to survive injury and successfully regenerate involves ribosomal RNA production. Testosterone propionate (TP), augments facial nerve regeneration in the adult hamster. TP modulates the nucleolar reaction in injured facial motoneurons, such that mature ribosome levels increase more rapidly and in greater magnitude than with injury only. In this study, molecular and electron microscopic stereologic approaches were used to determine the effects of axotomy and steroid treatment on ribosomal transcription and processing in facial motoneurons. Castrated adult male hamsters were subjected to right facial nerve transection at the stylomastoid foramen. Half the animals were subcutaneously implanted with one Silastic TP capsule, with the remainder sham implanted. For the in situ hybridization experiments, postoperative survival times were 0.5, 2, or 6 hours. In situ hybridization with a ribosomal DNA probe specific to the external transcribed spacer region located at the 5' end of the ribosomal gene was accomplished. Transcriptional activation of the rRNA gene occurred rapidly, within 2 hours, after injury only. Unexpectedly, TP treatment did not alter the time course or magnitude of rRNA transcriptional activity. For the electron microscope experiments, the postoperative time of 12 hours was selected. Stereologic analysis of 3 nucleolar subcomponents, fibrillar centers (site of rRNA transcription), nucleolonema (site of rRNA processing), and granular material (site of preribosome storage), was accomplished. TP decreased the nucleolonemal strands and the granular material, relative to injury only. These results suggest that, although rRNA transcription is rapidly activated by axotomy, rRNA processing is temporarily stalled. TP does not affect the early, axotomy-induced transcriptional activation of the ribosomal gene, but may, instead, prevent the subsequent disruption in rRNA processing. An hypothesis for the molecular mechanism by which steroids augment the regenerative capabilities of injured facial motoneurons is presented.