Myelinated axons in the mammalian CNS are uniquely designed to support rapid and efficient saltatory impulse propagation. Myelin loss due to genetic mutations or autoimmune disease, as in the case of multiple sclerosis (MS) can result in increased motor neurons fatigue and nerve impulse conduction failure. This disorder can eventually lead to axonal degeneration and irreversible damage to motor and sensory function. Interestingly, some of MS patients show hearing acuity loss with normal cochlear function, referred as auditory neuropathy. Most studies related to demyelination have focused on the PNS and spinal cord axons, however, little is known about how loss of myelin sheaths affects the synaptic transmission at the single synapse level in mammalian central nervous system (CNS). The object of this proposal is to investigate the properties of single auditory axons and the nerve terminals after demyelination by studying the electrophysiological properties of a single synapse in the auditory brainstem in the CNS. Mutant rats (the Long-Evans Shaker;LES) that lack compact CNS myelination, simulate some of the hallmark properties of MS and they have symptoms that often mimic those of MS patients. My main hypothesis is that axons that lose myelin have a disrupted distribution of ion channels, action potentials timing, and an altered handling of Ca2+ and Na+ ion influx. Because of abnormal axonal Ca2+ and Na+ ion concentrations, the nerve terminal associated with the axon has abnormal synaptic properties. To test this hypothesis I will perform electrophysiological recordings and fluorescent dye imaging studies in the calyx of Held synapse of the auditory brainstem. This nerve terminal is involved in a special neuronal circuit that computes the location of sound sources for the auditory system. The calyx synapse therefore provides us with a more accessible model to directly examine the effects of demyelination on homeostatic mechanisms and synaptic transmission in CNS nerve terminals.
Specific Aims : I plan to first conduct electrophysiological and imaging experiments to study the properties of the LES rats. I will compare the axonal and synaptic properties of neural tissue obtained from the LES pups that show a severe behavior phenotype (homozygote) or are normal (heterozygote). I will study the timing and firing probability of the action potential (AP) during tetanic stimulation at several different frequencies, and the excitatory postsynaptic currents (EPSC) of the normal and mutant pups. And, I will examine the re-distribution of ion channels and transporters of demyelinated axons and nerve terminals. Next, I will directly study Ca2+ and Na+ ion concentration changes in the axon and nerve terminal using Ca2+ and Na+ ion sensitive fluorescent dyes.
The proposed research will give us insights into the basic biological processes triggered by demyelination in auditory nerve fibers, as well as this proposal has a broad relevance for the entire field of neurodegenerative diseases. The proposed study will lay the foundations for the development of novel therapeutic strategies for preventing a permanent childhood hearing loss and an adult hearing impairment due to the consequences of demyelinating diseases like MS or auditory neuropathy.
|Kim, Sei Eun; Turkington, Karl; Kushmerick, Christopher et al. (2013) Central dysmyelination reduces the temporal fidelity of synaptic transmission and the reliability of postsynaptic firing during high-frequency stimulation. J Neurophysiol 110:1621-30|
|Kim, Jun Hee; Renden, Robert; von Gersdorff, Henrique (2013) Dysmyelination of auditory afferent axons increases the jitter of action potential timing during high-frequency firing. J Neurosci 33:9402-7|
|Kim, Jun Hee; von Gersdorff, Henrique (2012) Suppression of spikes during posttetanic hyperpolarization in auditory neurons: the role of temperature, I(h) currents, and the Na(+)-K(+)-ATPase pump. J Neurophysiol 108:1924-32|
|Kim, Jun Hee; Kushmerick, Christopher; von Gersdorff, Henrique (2010) Presynaptic resurgent Na+ currents sculpt the action potential waveform and increase firing reliability at a CNS nerve terminal. J Neurosci 30:15479-90|