Neuroprotective effect of testosterone treatment on motoneuron recruitment following the death of nearby motoneurons

The Effects of Neuroactive Steroids on Myelin in Health and Disease

Myelin plays a pivotal role in the efficient transmission of nerve impulses. Disruptions in myelin integrity are associated with numerous neurological disorders, including multiple sclerosis. In the central nervous system (CNS), myelin is formed by oligodendrocytes. Remyelination refers to the re-formation of the damaged myelin sheath by newly formed oligodendrocytes. Steroids have gained attention for their potential modulatory effects on myelin in both health and disease. Steroids are traditionally associated with endocrine functions, but their local synthesis within the nervous system has generated significant interest. The term "neuroactive steroids" refers to steroids that can act on cells of the nervous system. In the healthy state, neuroactive steroids promote myelin formation, maintenance, and repair by enhancing oligodendrocyte differentiation and maturation. In pathological conditions, such as demyelination injury, multiple neuroactive steroids have shown promise in promoting remyelination. Understanding the effects of neuroactive steroids on myelin could lead to novel therapeutic approaches for demyelinating diseases and neurodegenerative disorders. This review highlights the potential therapeutic significance of neuroactive steroids in myelin-related health and diseases. We review the synthesis of steroids by neurons and glial cells and discuss the roles of neuroactive steroids on myelin structure and function in health and disease. We emphasize the potential promyelinating effects of the varying levels of neuroactive steroids during different female physiological states such as the menstrual cycle, pregnancy, lactation, and postmenopause.

EMG Testing throughout behavioral recovery after rat sciatic nerve crush injury results in exuberant motoneuron dendritic hypertrophy

Background

Peripheral nerve injury (PNI) is the most common type of nerve trauma yet, while injured motoneurons exhibit a robust capacity for regeneration, behavioral recovery is protracted and typically poor. Neurotherapeutic approaches to PNI and repair have primarily focused on the enhancement of axonal regeneration, in terms of rate, axonal sprouting, and reconnection connectivity. Both electrical stimulation (ES) and treatment with androgens [e.g., testosterone propionate (TP)] have been demonstrated to enhance axonal sprouting, regeneration rate and functional recovery following PNI. To date, very little work has been done to examine the effects of ES and/or TP on dendritic morphology and organization within the spinal cord after PNI.

Objective

The objective of the current study was to examine the impact of treatment with TP and ES, alone or in combination, on the dendritic arbor of spinal motoneurons after target disconnection via sciatic nerve crush injury in the rat.

Methods

Rats received a crush injury to the sciatic nerve. Following injury, some animals received either (1) no further treatment beyond implantation with empty Silastic capsules, (2) electrical nerve stimulation immediately after injury, (3) implantation with Silastic capsules filled with TP, or (4) electrical nerve stimulation immediately after injury as well as implantation with TP. All of these groups of axotomized animals also received bi-weekly electromyography (EMG) testing. Additional groups of intact untreated animals as well as a group of injured animals who received no further treatment or EMG testing were also included. Eight weeks after injury, motoneurons innervating the anterior tibialis muscle were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions.

Results

After nerve crush and ES and/or TP treatment, motoneurons innervating the anterior tibialis underwent marked dendritic hypertrophy. Surprisingly, this dendritic hypertrophy occurred in all animals receiving repeated bi-weekly EMG testing, regardless of treatment. When the EMG testing was eliminated, the dendritic arbor extent and distribution after nerve crush in the treated groups did not significantly differ from intact untreated animals.

Conclusions

The ability of repeated EMG testing to so dramatically affect central plasticity following a peripheral nerve injury was unexpected. It was also unexpected that gonadal steroid hormones and/or ES, two neurotherapeutic approaches with demonstrated molecular/behavioral changes consistent with peripheral improvements in axonal repair and target reconnection, do not appear to impact central plasticity in a similar manner. The significance of peripheral EMG testing and resulting central plasticity reorganization remains to be determined.

Testosterone levels in men with chronic migraine

Chronic migraine is a frequent and debilitating condition affecting 14% of the general population. This prospective observational pilot study investigated whether men with chronic migraine have lower than expected total serum testosterone levels. We identified 14 men ages 26-51 at our Institution who fulfilled the ICHD-3b criteria for chronic migraine and obtained serum total testosterone levels. The mean total testosterone level in our 14 patients was 322 ng/dL (range: 120-542 ng/dL) which is in the lower 5% of the reference range for our laboratory (300-1080 ng/dL). Men with chronic migraine had lower total testosterone levels compared to published agematched normative median values by a median difference of 62 ng/dL (P=0.0494). This finding suggests that hypothalamic regulation is altered in patients with chronic migraine. Further studies are warranted to determine whether testosterone supplementation in men with chronic migraine reduces the number of headaches or the associated symptoms of hypogonadism.

Neuroprotective Effects on the Morphology of Somatic Motoneurons Following the Death of Neighboring Motoneurons: A Role for Microglia?

Partial depletion of spinal motoneuron populations induces dendritic atrophy in neighboring motoneurons, and treatment with testosterone protects motoneurons from induced dendritic atrophy. We explored a potential mechanism for this induced atrophy and protection by testosterone, examining the microglial response to partial depletion of motoneurons. Motoneurons innervating the vastus medialis muscles of adult male rats were killed by intramuscular injection of cholera toxin-conjugated saporin; some saporin-injected rats were treated with testosterone. Microglia were later visualized via immunohistochemical staining, classified as monitoring or activated, and counted stereologically. Partial motoneuron depletion increased the number of activated microglia in the quadriceps motor pool, and this increase was attenuated with testosterone treatment. The attenuation in microglial response could reflect an effect of testosterone on suppressing microglia activation, potentially sparing motoneuron dendrites. Alternatively, testosterone could be neuroprotective, sparing motoneuron dendrites, secondarily resulting in reduced microglial activation. To discriminate between these hypotheses, following partial motoneuron depletion, rats were treated with minocycline to inhibit microglial activation. Motoneurons innervating the ipsilateral vastus lateralis muscle were later labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed. Reduction of microglial activation by minocycline did not prevent induced dendritic atrophy following partial motoneuron depletion. Further, reduction of microglial activation by minocycline treatment resulted in dendritic atrophy in intact animals. Together, these findings indicate that the neuroprotective effect of testosterone on dendrites following motoneuron death is not due to inhibiting microglial activation, and that microglial activity contributes to the normal maintenance of dendritic arbors.

Protective Effects of Estradiol and Dihydrotestosterone following Spinal Cord Injury

Spinal cord injury (SCI) results in lesions that destroy tissue and disrupt spinal tracts, producing deficits in locomotor and autonomic function. We previously demonstrated that motoneurons and the muscles they innervate show pronounced atrophy after SCI, and these changes are prevented by treatment with testosterone. Here, we assessed whether the testosterone active metabolites estradiol and dihydrotestosterone have similar protective effects after SCI. Young adult female rats received either sham or T9 spinal cord contusion injuries and were treated with estradiol, dihydrotestosterone, both, or nothing via Silastic capsules. Basso-Beattie-Bresnahan locomotor testing was performed weekly and voiding behavior was assessed at 3 weeks post-injury. Four weeks after SCI, lesion volume and tissue sparing, quadriceps muscle fiber cross-sectional area, and motoneuron dendritic morphology were assessed. Spontaneous locomotor behavior improved after SCI, but hormone treatments had no effect. Voiding behavior was disrupted after SCI, but was significantly improved by treatment with either estradiol or dihydrotestosterone; combined treatment was maximally effective. Treatment with estradiol reduced lesion volume, but dihydrotestosterone alone and estradiol combined with dihydrotestosterone were ineffective. SCI-induced decreases in motoneuron dendritic length were attenuated by all hormone treatments. SCI-induced reductions in muscle fiber cross-sectional areas were prevented by treatment with either dihydrotestosterone or estradiol combined with dihydrotestosterone, but estradiol treatment was ineffective. These findings suggest that deficits in micturition and regressive changes in motoneuron and muscle morphology seen after SCI are ameliorated by treatment with estradiol or dihydrotestosterone, further supporting a role for steroid hormones as neurotherapeutic agents in the injured nervous system.

Neuroprotective effects of testosterone metabolites and dependency on receptor action on the morphology of somatic motoneurons following the death of neighboring motoneurons

Partial depletion of spinal motoneuron populations induces dendritic atrophy in neighboring motoneurons, and treatment with testosterone is neuroprotective, attenuating induced dendritic atrophy. In this study we examined whether the protective effects of testosterone could be mediated via its androgenic or estrogenic metabolites. Furthermore, to assess whether these neuroprotective effects were mediated through steroid hormone receptors, we used receptor antagonists to attempt to prevent the neuroprotective effects of hormones after partial motoneuron depletion. Motoneurons innervating the vastus medialis muscles of adult male rats were selectively killed by intramuscular injection of cholera toxin-conjugated saporin. Simultaneously, some saporin-injected rats were treated with either dihydrotestosterone or estradiol, alone or in combination with their respective receptor antagonists, or left untreated. Four weeks later, motoneurons innervating the ipsilateral vastus lateralis muscle were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. Compared with intact normal animals, partial motoneuron depletion resulted in decreased dendritic length in remaining quadriceps motoneurons. Dendritic atrophy was attenuated with both dihydrotestosterone and estradiol treatment to a degree similar to that seen with testosterone, and attenuation of atrophy was prevented by receptor blockade. Together, these findings suggest that neuroprotective effects on motoneurons can be mediated by either androgenic or estrogenic hormones and require action via steroid hormone receptors, further supporting a role for hormones as neurotherapeutic agents in the injured nervous system. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 691-707, 2017.

Neuroplasticity and Repair in Rodent Neurotoxic Models of Spinal Motoneuron Disease

Retrogradely transported toxins are widely used to set up protocols for selective lesioning of the nervous system. These methods could be collectively named "molecular neurosurgery" because they are able to destroy specific types of neurons by using targeted neurotoxins. Lectins such as ricin, volkensin, or modeccin and neuropeptide- or antibody-conjugated saporin represent the most effective toxins used for neuronal lesioning. Some of these specific neurotoxins could be used to induce selective depletion of spinal motoneurons. In this review, we extensively describe two rodent models of motoneuron degeneration induced by volkensin or cholera toxin-B saporin. In particular, we focus on the possible experimental use of these models to mimic neurodegenerative diseases, to dissect the molecular mechanisms of neuroplastic changes underlying the spontaneous functional recovery after motoneuron death, and finally to test different strategies of neural repair. The potential clinical applications of these approaches are also discussed.

Female Rats Demonstrate Improved Locomotor Recovery and Greater Preservation of White and Gray Matter after Traumatic Spinal Cord Injury Compared to Males

The possibility of a gender-related difference in recovery after spinal cord injury (SCI) remains a controversial subject. Current empirical animal research lacks sizable test groups to definitively determine whether significant differences exist. Evaluating locomotor recovery variances between sexes following a precise, clinically relevant spinal cord contusion model can provide valuable insight into a possible gender-related advantage in outcome post-SCI. In the current study, we hypothesized that by employing larger sample sizes in a reproducible contusive SCI paradigm, subtle distinctions in locomotor recovery between sexes, if they exist, would be elucidated through a broad range of behavioral tests. During 13 weeks of functional assessment after a thoracic (T8) contusive SCI in rat, significant differences owing to gender existed for the Basso, Beattie, and Bresnahan score and CatWalk hindlimb swing, support four, and single stance analyses. Significant differences in locomotor performance were noticeable as early as 4 weeks post-SCI. Stereological tissue-volume analysis determined that females, more so than males, also exhibited greater volumes of preserved gray and white matter within the injured cord segment as well as more spared ventral white matter area at the center of the lesion. The stereological tissue analysis differences favoring females directly correlated with the female rats' greater functional improvement observed at endpoint.

Neuroprotective effects of testosterone on motoneuron and muscle morphology following spinal cord injury

Treatment with testosterone is neuroprotective/neurotherapeutic after a variety of motoneuron injuries. Here we assessed whether testosterone might have similar beneficial effects after spinal cord injury (SCI). Young adult female rats received either sham or T9 spinal cord contusion injuries and were implanted with blank or testosterone-filled Silastic capsules. Four weeks later, motoneurons innervating the vastus lateralis muscle of the quadriceps were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. Soma volume, motoneuron number, lesion volume, and tissue sparing were also assessed, as were muscle weight, fiber cross-sectional area, and motor endplate size and density. Contusion injury resulted in large lesions, with no significant differences in lesion volume, percent total volume of lesion, or spared white or gray matter between SCI groups. SCI with or without testosterone treatment also had no effect on the number or soma volume of quadriceps motoneurons. However, SCI resulted in a decrease in dendritic length of quadriceps motoneurons in untreated animals, and this decrease was completely prevented by treatment with testosterone. Similarly, the vastus lateralis muscle weights and fiber cross-sectional areas of untreated SCI animals were smaller than those of sham-surgery controls, and these reductions were both prevented by testosterone treatment. No effects on motor endplate area or density were observed across treatment groups. These findings suggest that regressive changes in motoneuron and muscle morphology seen after SCI can be prevented by testosterone treatment, further supporting a role for testosterone as a neurotherapeutic agent in the injured nervous system.

Neuroprotective effects of testosterone on dendritic morphology following partial motoneuron depletion: efficacy in female rats

Motoneuron loss is a significant medical problem, capable of causing severe movement disorders and even death. We have previously demonstrated that partial depletion of motoneurons induces dendritic atrophy in remaining motoneurons, with a concomitant reduction in motor activation. Treatment of male rats with testosterone attenuates the regressive changes following partial motoneuron depletion. To test whether testosterone has similar effects in females, we examined potential neuroprotective effects in motoneurons innervating muscles of the quadriceps of female rats. Motoneurons were selectively killed by intramuscular injection of cholera toxin-conjugated saporin. Simultaneously, some saporin-injected rats were given implants containing testosterone or left untreated. Four weeks later, surviving motoneurons were labeled with cholera toxin-conjugated HRP, and dendritic arbors were reconstructed in three dimensions. Compared to normal females, partial motoneuron depletion resulted in decreased dendritic length in remaining quadriceps motoneurons, and this atrophy was greatly attenuated by testosterone treatment. These findings suggest that testosterone has neuroprotective effects on morphology in both males and females, further supporting a role for testosterone as a neurotherapeutic agent in the injured nervous system.