Differential effects of axotomy on immature and mature hamster facial neurons: a tritiated-uridine autoradiographic study

Gonadal steroid attenuation of developing hamster facial motoneuron loss by axotomy: equal efficacy of testosterone, dihydrotestosterone, and 17-beta estradiol

In the hamster facial nerve injury paradigm, we have established that androgens enhance both functional recovery from facial nerve paralysis and the rate of regeneration in the adult, through intrinsic effects on the nerve cell body response to injury and via an androgen receptor (AR)-mediated mechanism. Whether these therapeutic effects of gonadal steroids encompass neuroprotection from axotomy-induced cell death is the focus of the present study. Virtually 100% of adult hamster facial motoneurons (FMNs) survive axotomy at the stylomastoid foramen (SMF), whereas, before postnatal day 15 (P15), developing FMNs undergo substantial axotomy-induced cell death. The first part of the present study focuses on determining when ARs are first expressed in developing hamster FMNs. Using AR immunocytochemistry, it was found that males express ARs by P2 and females by P4, which is the earliest demonstration of AR expression in mammalian motoneurons reported thus far in the literature. The second half examines the neuroprotective effects of testosterone propionate, 17-beta estradiol, and dihydrotestosterone on FMNs of P7 hamsters after facial nerve transection at the SMF. The results demonstrate that androgens and estrogens are equally able to rescue approximately 20% of FMNs from axotomy-induced cell death, with the effects permanent. This study is the first to investigate the effects of both androgens and estrogens on axotomy-induced cell death in one system and, with our previously published work, to validate the hamster FMN injury paradigm as a model of choice in the investigation of both neurotherapeutic and neuroprotective actions of gonadal steroids.

Suppressive silver methods—a tool for identifying axotomy-induced neuron degeneration

Suppressive silver methods evolved from empirical observations about 50 years ago that argyrophilia of normal nerve fibers can be suppressed by a short period of oxidation of tissue sections, whereas degenerating nerve fibers in the same preparations were still clearly visible. Based on this property, suppressive silver impregnation became the main technique for investigating pathways in the central nervous system until the early 1970s. Suppressive silver methods were also found to visualize degenerating nerve cell bodies, in addition to degenerating nerve fibers. This possibility has given these methods an important place among current tools for identifying neuronal degeneration in trauma, disease and toxicity. In this article we demonstrate and review the usefulness of suppressive silver methods in identifying neurons undergoing degeneration as a result of peripheral or central axon injury in immature animals. The documentation is based on previously published data from experiments in which silver impregnation was used to demonstrate degeneration of motoneurons following pure motor axon injury or mixed peripheral nerve injury, as well as on new results on degeneration-induced argyrophilia in the inferior olive following cerebellar lesions. We find that silver precipitates resulting from these injuries are localized either to the entire neuronal cytoplasm, to a few (typically two) intranuclear bodies, or to both sites. The findings are discussed in relation to morphological features of apoptosis, necrosis and retrograde neuronal responses. We suggest that suppressive silver methods allow visualization of different processes of neuronal degeneration, and therefore may be a useful adjunct for identifying axotomy-induced neuronal degeneration.

Metabolic changes in axotomized fetal and early postnatal hamster facial motoneurons: an autoradiographic study

The developing facial neurons of a series of hamsters ranging in age from the 14-day fetus to the 9 day postnatal were axotomized. Postoperative times were graded for each age so that the retrograde response could be observed before any significant amount of cell degeneration or death occurred. The incorporation of tritiated uridine was followed by the autoradiographic procedure. Although grain counts, relative to control values, were significantly reduced only in the axotomized fetus and at 24 hours postoperatively in 4-day postnatal animals, there was also a repression of isotopic incorporation in all the other axotomized animals. These results support data obtained from previous work with the hamsters which indicate that it is not until after the nerve cell nucleolus reaches full cytomorphic maturity (between 15 and 20 days postnatal age in hamster facial neurons) that the axotomized neurons respond with significantly increased incorporation levels of isotope over that of control neurons.

Testosterone effects on ribosomal RNA levels in injured peripheral motor neurons: a preliminary report

We have previously demonstrated that administration of testosterone to hamsters during the early phases of axonal regeneration following facial nerve injury accelerates both the rate of regeneration of the fastest growing population of axons and the return of functional movement. We hypothesized from those studies that testosterone primes the neuronal cell body in such a way as to accelerate the "switch" from a normal to a reparative state. That hypothesis was tested in this study using ribosomal DNA (rDNA) probes in conjunction with in situ hybridization to map the molecular response of the polymerase I system to axotomy, with and without hormone exposure. Adult male hamsters were subjected to right facial nerve severance, with the left side serving as an internal control. Half the animals were administered testosterone propionate via subcutaneous implants. In situ hybridization using a genomic rDNA probe complementary to the 28S rRNA species was accomplished, and levels of rRNA in injured facial neurons assessed both qualitatively and quantitatively. Our initial findings indicate that testosterone markedly upregulates rRNA levels after injury, and support the hypothesis of an acceleration in the metabolic switch to a reparative state. This leads us, in turn, to suggest that this effect of testosterone on the ribosomal system is causally related to the increase in axonal regeneration rate and return of functional movement previously documented in this system.

Ultrastructural and morphometric analysis of nucleolar and nuclear changes during the early growth period in hamster facial neurons

In this study, progressive developmental changes in the nucleus and associated organelles, including the nucleolus, coiled bodies, nuclear envelope, and nucleoplasm, of hamster facial motor neurons were characterized by two parallel analyses: ultrastructural and morphometric. Golden hamsters (Mesocricetus auratus) used for this series were the 14-day fetus, newborn (less than 6 hr), and 1, 2, 3, 4, 5, 7, 9, 11 and 13 days postnatal ages, with 3 animals per group. Following anesthesia and perfusion fixation, facial nuclear groups were dissected and processed for electron microscopy. Electron micrographs and camera lucida tracings of nuclear profiles were collected and analyzed. The ultrastructural analysis revealed progressive changes in the nucleolus from a compact, segregated type to a reticulated form characteristic of actively protein-secreting cells. Nucleolar microbodies and fibrillar centers were seen at all ages; the latter structures appeared to decrease in size and increase with age in the series. The nucleolus-associated chromatin became less condensed, suggesting an increase in the incorporation of rDNA into the nucleolus proper. Coiled bodies, both free and attached to nucleoli, were found in varying frequencies. The nucleoplasm of neurons at the earliest stages contained large numbers of heterochromatin clumps, which decreased concomitantly with an increase in interchromatin granules and fibrils during the later stages. Nuclear envelope invaginations, polarized along one side of the nucleus, increased throughout the developmental period examined. These changes occurred in concert with a 61% increase in nuclear size and a 47% increase in the length of nuclear envelope. The sequence of nuclear changes observed during this early period of normal facial neuronal growth completes the study of a series of distinctly defined cytomorphic events in this cell type, the lability of which can be experimentally tested for their functional roles in neuronal development.