Lisa Tanzer

Gonadal Steroid Regulation of Hamster Facial Nerve Regeneration: Effects of Dihydrotestosterone and Estradiol

We have demonstrated, in a series of experiments, the therapeutic potential of androgens in facial motoneuron regeneration in the adult hamster. Initial work utilized testosterone propionate (TP) as the form of androgen given to adult hamster at the time of facial nerve crush axotomy at its exit from the stylomastoid foramen. TP is capable of being enzymatically converted to estrogen. Thus, the effects of TP on the regenerative properties of facial motoneurons could be due to androgens, estrogens, or both. Recent studies of androgen receptor (AR) mRNA levels suggest that androgens and estrogens work synergistically to regulate AR expression in these motoneurons. In this study, we examined the ability of dihydrotestosterone propionate (DHTP), a nonaromatizable androgen which cannot be converted to estrogen, and estradiol (E2) to alter facial nerve regeneration, using fast axonal transport of radioactively labeled proteins to assess facial nerve regeneration. Adult gonadectomized male hamsters were subjected to right facial nerve crush axotomy, with the left side serving as control, and divided into three groups. One-third of the animals received 1 subcutaneous implant of DHTP, one-third received 1 subcutaneous implant of E2, and the remaining third was sham implanted. Postoperative survival times were 4 and 7 days. As expected, DHTP treatment resulted in an approximately 40% increase in the rate of regeneration, with an associated prolongation in the delay time before sprouting occurred. These effects were slightly greater than previously observed with TP, as might be predicted given the more potent physiological effects observed with DHTP compared to TP. Surprisingly, E2 treatment also resulted in an increase in the rate of regeneration (30%), with minimal effects on the delay time before sprout formation occurred. The results argue for a synergistic role for androgens and estrogens in augmenting peripheral nerve regeneration in the model system used in this study.

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

Neurotherapeutic action of testosterone on hamster facial nerve regeneration: temporal window of effects

Neurotherapeutic or neuroprotective effects of gonadal steroids on the injured nervous system have been demonstrated in our laboratory and others. We have previously demonstrated that testosterone propionate (TP) administered systemically at supraphysiological levels accelerates both recovery from facial paralysis and regeneration rates following facial nerve injury in the hamster. Initial temporal studies of steroidal enhancement of functional recovery from facial paralysis established that steroid exposure is necessary during the first postoperative week. Furthermore, accumulated evidence suggests that TP manifests its effects on neuronal regeneration in the immediate postoperative or preregenerative phase by altering the cellular stress response. The purpose of this study was to identify the effective temporal window of TP exposure sufficient to enhance regenerative properties of injured facial motoneurons and functional recovery from facial paralysis induced by facial nerve injury. Adult castrated male hamsters received a right facial nerve crush axotomy at the stylomastoid foramen and were divided into (1) short term, (2) delayed, (3) continuous, and (4) no TP treatment groups. Short term and continuous groups were implanted with 1 subcutaneous (sc) TP capsule each immediately after axotomy, with the capsule removed at 30 min, 2, 4, or 6 h in short-term groups and allowed to remain for the duration of the experiment in the continuous group. In the delayed TP group, 1 sc TP capsule was implanted 6 h after axotomy and allowed to remain for the duration of the experiment. For regeneration rate studies, postoperative times ranged from 4 to 7 days. For the behavioral studies, observations were made for 26 days postaxotomy. The results point to a critical 6-h interval immediately after injury when TP enhances nerve outgrowth distances and augments behavioral recovery.

Androgen Regulates Neuritin mRNA Levels in an In Vivo Model of Steroid-Enhanced Peripheral Nerve Regeneration

Following crush injury to the facial nerve in Syrian hamsters, treatment with androgens enhances axonal regeneration rates and decreases time to recovery. It has been demonstrated in vitro that the ability of androgen to enhance neurite outgrowth in motoneurons is dependent on neuritin-a protein that is involved in the re-establisment of neuronal connectivity following traumatic damage to the central nervous system and that is under the control of several neurotrophic and neuroregenerative factors--and we have hypothesized that neuritin is a mediator of the ability of androgen to increase peripheral nerve regeneration rates in vivo. Testosterone treatment of facial nerve-axotomized hamsters resulted in an approximately 300% increase in neuritin mRNA levels 2 days post-injury. Simultaneous treatment with flutamide, an androgen receptor blocker that is known to prevent androgen enhancement of nerve regeneration, abolished the ability of testosterone to upregulate neuritin mRNA levels. In a corroborative in vitro experiment, the androgen dihydrotestosterone induced an approximately 100% increase in neuritin mRNA levels in motoneuron-neuroblastoma cells transfected with androgen receptors, but not in cells without androgen receptors. These data confirm that neuritin is under the control of androgens, and suggest that neuritin is an important effector of androgen in enhancing peripheral nerve regeneration following injury. Given that neuritin has now been shown to be involved in responses to both central and peripheral injuries, and appears to be a common effector molecule for several neurotrophic and neurotherapeutic agents, understanding the neuritin pathway is an important goal for the clinical management of traumatic nervous system injuries.

ANDROGENIC REGULATION OF THE CENTRAL GLIA RESPONSE FOLLOWING NERVE DAMAGE

Current research on the effects of gonadal steroids on the brain and spinal cord indicates that these agents have profound trophic effects on many aspects of neuronal functioning, including cell survival, growth and metabolism, elaboration of processes, synaptogenesis, and neurotransmission (Jones et al., 1985; Luine, 1985; Nordeen et al., 1985; Matsumoto et al., 1988a,b; Gould et al., 1990). Since many of the aspects of normal neuronal functioning altered by gonadal steroids are affected by injury to the nervous system, we initiated a series of experiments designed to exploit the trophic capabilities of steroids as therapeutic agents in neuronal injury and repair (Kujawa et al., 1989, 1991; Kujawa and Jones, 1990). Three steroid-sensitive model systems were used for these studies: the hamster facial motoneuron, the rat sciatic motoneuron, and the hamster rubrospinal motoneuron. The results of our initial series of experiments suggest that androgens, and possibly estrogens, act either directly or indirectly on the injured motoneuron and enhance elements of the neuronal reparative response that are critical to successful recovery of function. Recently, we discovered that gonadal steroids may also modulate the central glia response to nerve damage. In this review, a summary of our data identifying a therapeutic role for androgens in enhancing the reparative response of motoneurons to injury is presented. This is followed by a discussion of the effects of androgens on the glial response to injury.

Accelerating functional recovery after rat facial nerve injury: Effects of gonadal steroids and electrical stimulation

Objective We investigated the combined effects of electrical stimulation and testosterone propionate on overall recovery time in rats with extracranial crush injuries to the facial nerve. Study Design Male rats underwent castration 3 to 5 days prior to right facial nerve crush injury and electrode implantation. Rats were randomly assigned to two groups: crush injury + testosterone or crush injury with electrical stimulation + testosterone. Recovery was assessed by daily subjective examination documenting vibrissae orientation/movement, semi-eye blink, and full eye blink. Results Milestones of early recovery were noted to be significantly earlier in the groups with electrical stimulation, with/without testosterone. The addition of testosterone to electrical stimulation showed significant earlier return of late recovery parameters and complete overall recovery. Conclusion Electrical stimulation may decrease cell death or promote sprouting to accelerate early recovery. Testosterone may affect the actual rate of axonal regeneration and produce acceleration in functional recovery. By targeting different stages of neural regeneration, the synergy of electrical stimulation and testosterone appears to have promise as a neurotherapeutic strategy for facial nerve injury.

Inhibition of the Accelerative Effects of Testosterone on Hamster Facial Nerve Regeneration by the Antiandrogen Flutamide

We have previously demonstrated that systemic administration of testosterone propionate (TP) to adult hamsters accelerates the rate of facial nerve regeneration following crush axotomy of the facial nerve at its exit from the stylomastoid foramen. In this study, we utilized flutamide, a potent nonsteroidal antiandrogen, in conjunction with radioisotopic labeling procedures for the assessment of facial nerve regeneration rates to test the hypothesis that TP exerts its accelerative effects on facial nerve regeneration through a receptor-mediated mechanism. Castrated adult male hamsters were subjected to right facial nerve crush axotomies and divided into three groups of axotomized animals: castrate plus one subcutaneous TP implant plus daily injections of flutamide, castrate plus one subcutaneous TP implant plus vehicle injections, and castrate only plus sham implant and vehicle injections. There were two postoperative timepoints: 4 and 7 days. In agreement with previous studies, systemic administration of TP resulted in an approximately 26% increase in the rate of regeneration of the fastest growing population of axons. Exposure to flutamide completely abolished the TP-induced accelerative effects on facial nerve regeneration rate. As a bioassay for the effectiveness of systemic administration of flutamide by subcutaneous injection, seminal vesicle weights were collected from all groups at the end of the postoperative time and compared as a percentage of the seminal vesicle weights of intact (nongonadectomized) male control animals. Castration greatly reduced seminal vesicle weights, whereas exogenous TP restored the seminal vesicle weights to those of the intact male. Flutamide blocked the effects of exogenous TP on seminal vesicle weights and reduced them to castrate levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of electrical stimulation and gonadal steroids on rat facial nerve regenerative properties

PURPOSE The neurotherapeutic effects of nerve electrical stimulation and gonadal steroids have independently been demonstrated. The purpose of this study was to investigate the therapeutic potential of a combinatorial treatment strategy of electrical stimulation and gonadal steroids on peripheral nerve regeneration. METHODS Following a facial nerve crush axotomy in gonadectomized adult male rats, testosterone propionate (TP), dihydrotestosterone (DHT), or estradiol (E(2)) was systemically administered with/without daily electrical stimulation of the proximal nerve stump. Facial nerve outgrowth was assessed at 4 and 7 days post-axotomy using radioactive labeling. RESULTS Administration of electrical stimulation alone reduced the estimated delay in sprout formation but failed to accelerate the overall regeneration rate. Conversely, TP treatment alone accelerated the regeneration rate by approximately 10% but had no effect on the sprouting delay. Combining TP with electrical stimulation, however, maintained the enhanced rate and reduced the sprouting delay. DHT treatment alone failed to alter the regeneration rate but combining it with electrical stimulation increased the rate by 10%. E(2) treatment alone increased the regeneration rate by approximately 5% but with electrical stimulation, there was no additional effect. CONCLUSIONS Electrical stimulation and gonadal steroids differentially enhanced regenerative properties. TP, an aromatizable androgen, augmented regeneration most, suggesting a synergism between androgenic and estrogenic effects. Therapeutically, combining electrical stimulation with gonadal steroids may boost regenerative properties more than the use of either treatment alone.

Motoneuron injury and repair: New perspectives on gonadal steroids as neurotherapeutics.

In this review, we will summarize recent work from our laboratory on the role of gonadal steroids as neuroprotective agents in motoneuron viability following cell stress. Three motoneuron models will be discussed: developing axotomized hamster facial motoneurons (FMNs); adult axotomized mouse FMNs; and immortalized, cultured mouse spinal motoneurons subjected to heat shock. New work on two relevant motoneuron proteins, the survival of motor neuron protein, and neuritinor candidate plasticity-related gene 15, indicates differential steroid regulation of these two proteins after axotomy. The concept of gonadal steroids as cellular stress correction factors and the implications of this for acute neurological injury situations will be presented as well.

Testosterone treatment attenuates the effects of facial nerve transection on glial fibrillary acidic protein (GFAP) levels in the hamster facial motor nucleus.

Testosterone propionate (TP) administration coincident with facial nerve injury accelerates the recovery rate from facial muscle paralysis in the hamster. One mechanism by which TP could augment peripheral nerve regeneration is through glial fibrillary acidic protein the facial motor nucleus. In a previous study, axotomy alone induces increases in GFAP mRNA, with TP significantly attenuating the axotomy-induced increases in GFAP mRNA. In the present study, immunoblotting techniques were used to extend our previous GFAP mRNA studies to the protein level. Castrated male hamsters were subjected to a right facial nerve transection, with half of the animals receiving subcutaneous implants of 100% crystalline TP. The left facial motor nucleus of each animal served as an internal control. Postoperative survival times include Days 4, 7, and 14. In non–TP-treated animals, facial nerve transections alone increased GFAP levels at all time points, relative to internal controls. As previously observed at the mRNA level, TP treatment attenuated but did not eliminate the axotomy-induced increase in GFAP levels at all time points tested. These results suggest that the regulatory actions of gonadal steroids on GFAP expression manifested in parallel at the mRNA/protein levels.