Differential effects of axotomy on immature and mature hamster facial neurons: a time course study of initial nucleolar and nuclear changes

Abstract

The early nuclear and nucleolar responses at 0.5, 1, 2 and 4 days after axotomy were observed in neurons just before and after the completion of nuclear maturation. Axotomy of hamster facial motor neurons at a postnatal age of 15 days did not produce any changes within the nucleus that were significantly different from those of control cells. In addition, no significant changes were evident in the adults at 0.5 and 1 day after axotomy. At postoperative days 2 and 4, however, the adult neurons showed enlargement of the nucleus and nucleolus. Nucleolonemal strands became more rounded and distinct, and the large cluster of granules located centrally in the nucleolus disaggregated. The irregularly distributed clumps of nucleolus-associated chromatin dispersed to form a thin shell about the nucleolar periphery. In adults at postoperative day 4, the nucleoplasmic granules became more homogeneous and less distinctly outlined than normal. The peak of both nucleolar and nuclear responses coincided at 2 days after injury in the adult, i.e. 2 days before the previously documented chromatolytic peak at 4 days after injury. These studies on the ultrastructural level support our previous hypothesis that the 15-day neurons are synthesizing at peak capacity related to their rapid growth phase and cannot be stimulated further by axotomy. The adult neurons, however, do undergo a metabolic reorganization for regenerative synthesis, and the nucleolar and nuclear changes observed are indicative of transcriptive alterations involving the underlying genome.

Gonadal Steroid Attenuation of Developing Hamster Facial Motoneuron Loss by Axotomy: Equal Efficacy of Testosterone, Dihydrotestosterone, and 17-β Estradiol

“…Testosterone is neuroprotective after facial nerve axotomy on P7 Previous work using the developing hamster facial nerve injury paradigm has established that significant cell death occurs as a result of injury during the first postnatal week LaVelle, 1958, 1984;Jones and LaVelle, 1985, 1986, 1987Clark et al, 1992Clark et al, , 1993. Based on that work and the results of the AR immunocytochemistry time-course study presented above, P7 was chosen as the best age to test the ability of gonadal steroids to rescue injured developing FMNs from axotomy-induced cell death.…”

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

“…In the electron microscopy/ stereology experiment, the postoperative time of 12 hours was selected. This timepoint was chosen based on previous electron microscope studies of the nucleolar reaction in FMN, as well as our recent in situ hybridization study of the effects of axotomy on stable ribosomal RNA levels in FMN (Jones and LaVelle, 1986;Kinderman and Jones, 1993). Each group contained 5 animals, for a total of 10 hamsters.…”

“…Three to five days after castration (Kujawa et al, 1991), each animal was anesthetized by intraperitoneal injection of Nembutal (50 mg/ml, 0.18 ml/100 g of body weight), and the right facial nerve was severed at its exit from the stylomastoid foramen and proximal to the posterior auricular branch, with the left side serving as contralateral, unoperated control (Jones and LaVelle, 1986). Immediately after axotomy, half of the animals were implanted with one Silastic capsule containing 100% crystalline testosterone propionate (TP; 0.062 inch i.d.…”

“…It is unknown, however, which aspects of ribosome biogenesis are altered by axotomy and what effects steroid hormone treatment coincident with injury might have on the nucleolar reaction. The morphologic evidence presented in the hamster FMN system suggests that axotomy induces changes in rRNA transcription, rather than stability (Jones and LaVelle, 1986;Clark et al, 1991). In this study, we used molecular and stereologic approaches to determine the effects of both axotomy and steroid treatment on rRNA transcription and processing in axotomized hamster FMN.…”

“…Evidence obtained from numerous systems has demonstrated that the nucleolus, site of polymerase I-directed ribosomal RNA transcription and processing, is one of the earliest cellular organelles to be affected by axon disconnection from the periphery (Watson, 1968;Lieberman, 1971;Jones and LaVelle, 1986). Ultrastructural analysis of axotomized FMN nucleoli in developing and adult hamsters suggests that a key step in the ability of these neurons to survive injury and successfully regenerate occurs at some point in rRNA transcription, processing, or both (Kinderman and LaVelle, 1976;Jones and LaVelle, 1986;Clark et al, 1991). Steroid hormones have also been shown to regulate ribosomal gene expression and nucleolar ultrastructure (Cohen and Pfaff, 1981;Whelly, 1985;Jones et al, 1990).…”