Today's neurologists have few therapies to offer patients suffering the limb ataxia, imbalance, and visual impairment resulting from diseases of the cerebellum. Such patients are common in the veteran population and becoming more so due to traumatic brain injuries suffered in recent military operations. Recent developments, however, suggest that highly effective drugs can be developed: Two drugs that influence neuronal potassium channels have been shown to ameliorate cerebellar deficits in humans and mice, and may provide starting points to develop more efficacious and tolerable drugs. However, the sources of their therapeutic effects are debatable. Likewise, a new theory that ataxia can originate in disturbed rhythmicity of cerebellar Purkinje cells suggests procedures for drug development, but the theory is controversial and requires more investigation. This project will lay a better groundwork for drug development by clarifying the origins of cerebellar motor dysfunction and mechanism of one existing treatment. We focus on ocular motility abnormalities in mice carrying mutations of the CACNA1A gene of the P/Q (CaV2.1) calcium channel, but the results should be applicable to other causes and manifestations of cerebellar dysfunction.
Specific Aim 1 involves eye movement recordings in the CACNA1A mutant tottering, and in normal mice following pharmacological inhibition of the cerebellar flocculus, to determine the cause of the abnormal vertical eye positions found in ataxic mice. The result will determine whether mice can be used to test treatments for downbeat nystagmus, a human manifestation of cerebellar disease that results in blurred vision.
Specific Aim 2 tests the theory that ataxia in calcium channel mutants originates in disturbed Purkinje cell rhythmicity, through recordings of Purkinje cells in the flocculus and anterior vermis of normal mice and the CACNA1A mutant rocker. One goal of these recordings is to determine whether rocker Purkinje cells exhibit, as the rhythmicity theory predicts, rhythmicity intermediate between that of normal animals and the more severely affected CACNA1A mutant, tottering. Another goal of the recordings is to reassess whether CACNA1A mutant Purkinje cells modulate their firing normally in response to natural stimulation. This claim is central to the rhythmicity theory, but the experiments that support the claim may have been affected by sampling bias.
Specific aim 3 assesses the therapeutic mechanisms of 4- aminopyridine, one of the drugs that has recently been shown to provide some benefits in cerebellar disorders. We will record eye movements in tottering following systemic and intrafloccular injections of 4- aminopyridine. Results of these experiments will allow us to test published speculations that aminopyridines exert their beneficial effects by restoring normal Purkinje cell activity.
Disorders of the cerebellum cause loss of coordination (ataxia), imbalance, and blurred vision. They arise from many causes, including (but not limited to) traumatic brain injury (TBI), damage during tumor resection, stroke, inflammatory diseases (such as multiple sclerosis), chemicals (including medications and alcohol), degeneration due to genetic errors, other degenerative processes whose causes have yet to be discovered, and inherited mutations of genes that ultimately control neuronal activity patterns. Although no quantitative data are available, neurologists like the applicant recognize that patients with chronic cerebellar disorders are common in the veteran population. The high prevalence arises because there are so many causes of cerebellar dysfunction, and because they are generally compatible with long life and may begin early (i.e., during the years of military service or shortly after). The prevalence of veterans with chronic cerebellar dysfunction due to TBI is increasing due to Operation Enduring Freedom and Operation Iraqi Freedom (OEF/OIF), since cerebellar dysfunction is often prominent in TBI [9, 42]. Unfortunately, while in rare instances the cause of a progressive cerebellar damage can be discovered and arrested, neurologists like the applicant have had almost no treatments to offer to improve the symptoms of damage that has already occurred. Cerebellar dysfunction is notoriously resistant to physical therapy and physical therapy strategies generally concentrate on teaching patients to avoid specific types of movements . Likewise, except in very special circumstances (such as the use of the drug baclofen to arrest ocular oscillations due to injuries of the vestibulocerebellum ), there have been no pharmacological treatments for cerebellar dysfunction. Recent developments, however, suggest that better therapies might be developed. Certain types of progressive cerebellar dysfunction are now known to arise from defects of genes involved in cerebellar signaling - defects that could potentially be corrected with a medication. Also, a class of drugs (aminopyridines) has recently been found to improve specific symptoms of various cerebellar disorders. The existence of these drugs supports the hope that it should be possible to develop more effective, tolerable, and practical drug therapies. The search for better treatments would be aided by a better understanding of how specific diseases lead to the symptoms of cerebellar disease, and the mechanisms by which the drugs we do have produce their benefits. The overall goal of this project is to facilitate development of better therapies for cerebellar dysfunction by advancing this understanding of the mechanisms of motor dysfunction and of the drugs that have already been shown to produce some benefit. The project focuses on the cerebellar disorders arising from mutations of the genes that specify the structure of neuronal calcium channels, because: 1) Such "channelopathy" diseases form one of the largest groups of heritable, degenerative, cerebellar disorders in which the cause of the disease is known;2) mouse models of the diseases exist and have already been the subject of many studies;3) it is particularly reasonable to hope that drugs may be developed that could compensate for the effects of these mutations on neuronal function;4) previous promising work on drug therapies has been conducted in these diseases but has left many questions;5) there are reasons to believe that therapies developed for these disorders may be used to treat cerebellar dysfunction arising from other causes. The application stresses the eye movement functions of the cerebellum because: 1) Eye movement control is probably the best understood aspect of cerebellar function, and thus the area in which we can best hope to determine how a disease leads to abnormal movements and how a drug ameliorates the abnormalities;2) eye movements are affected early in many cerebellar disorders and thus a particularly promising and important target for a drug therapy;3) we can build upon the research that has already been done on the cause and treatment of eye movement abnormalities in the channelopathy diseases. The specific goals of the application include: 1)To determine whether one of the eye movement abnormalities of mice with cerebellar disorders is the appropriate experimental model for a somewhat different eye movement abnormality that occurs in the human cerebellar patients;2) to test a new theory of how channel mutations cause cerebellar dysfunction by testing assumptions underlying, and predictions of, that theory;3) to test theories of the mechanism of action of 4-aminopyridine, which has recently been shown to improve cerebellar function in mice and humans.