CURRENT RESEARCH 
Click below to access abstracts:
1. Activators of Ca2+-dependent K+ channels as potential therapeutic agents in episodic ataxia type-2
2. Contribution of the electrogenic Na/K ATPase pump to the intrinsic firing of Purkinje cells
3. Insights from an animal model of Rapid-onset Dystonia-Parkinsonism
4. Mapping the functional connectivity between cerebellar granule cells and Purkinje cells
5. Feed-forward inhibition and cerebellar function
6. The role of cerebellar Purkinje cells in episodic dyskinesia
Click here to access our latest poster, presented at Einstein's Annual Departmental Poster Session.
1. Activators of Ca2+-dependent K+ channels as potential therapeutic agents in episodic ataxia type-2
Karina Alvina and Kamran Khodakhah
Episodic ataxia type 2 (EA2) is a hereditary neurological disease characterized by recurrent periods of dyskinesia and ataxia. EA2 is caused by mutations in the pore-forming subunit of the P/Q-type Ca2+ channel. In cerebellar Purkinje cells these mutations reduce the Ca2+ current density. A consequence of this smaller Ca2+ current is the reduced activation of small-conductance Ca2+-activated K+ channels (SK channels) and thus irregular pacemaking in Purkinje cells. In cerebellar slices it has been shown that the regularity of pacemaking can be restored in P/Q-type Ca2+ channel mutant mice by pharmacological activation of SK channels with EBIO. Moreover, chronic perfusion of EBIO into the cerebellum of the P/Q channel mutant mice tottering reduces ataxia and the frequency of the stress-induced episodes of dyskinesia. We investigated the potential efficacy of systemic administration of another SK channel activator, chlorzoxazone (CHZ), in reducing ataxia and episodes of dyskinesia in the tottering mice, an animal model of EA2. We find that oral administration of CHZ effectively reduced ataxia and also the frequency of stress-induced episodes of dyskinesia. We also compared the efficacy of CHZ with that of 4-aminopyridine (4-AP). 4-AP has been recently suggested as a potential treatment for EA2. While 4-AP and CHZ were equally effective in reducing the frequency of stress-induced episodes of dyskinesia, CHZ was considerably better in reducing ataxia in the tottering mice. We thus propose CHZ as a potential therapeutic agent for the treatment of EA2. [back to top]
2. Contribution of the electrogenic Na/K ATPase pump to the intrinsic firing of Purkinje cells
D. Paola Calderon, Mary D. Womack, and Kamran Khodakhah
Purkinje cells are the main cells of the cerebellar cortex and it is thought that the information required for motor coordination is encoded in their activity. In order to understand the role of Purkinje cells in motor coordination, it is important to learn how synaptic inputs and the conductances responsible for intrinsic firing affect the rate and pattern of activity of these cells. Although the contribution of ionic channels in regulation of spontaneous firing has been studied, little is known about the effect of the electrogenic Na/K-ATPase pump on it. Considering that the sodium load provided every action potential by the activation of voltage-dependent sodium channels makes a significant contribution to the Na/K ATPase pump current and that Purkinje cells fire at high frequencies, a high activity of the sodium pump in these cells is expected. Therefore, we examined whether the outward current generated by the activity of the pump is significant enough to contribute to regulation of membrane excitability. Additionally, we determined whether the amplitude of the pump current would proportionally scale with increases of the firing rate and if this effect acts as a negative feedback mechanism to regulate the rate of spontaneous activity. We found that spontaneous activity of Purkinje cells is exquisitely sensitive to dysfunction of sodium pumps. Furthermore, preliminary data supports the notion that the outward current of the sodium pump acts as a negative feedback mechanism to regulate the rate of spontaneous activity. [back to top]
3. Insights from an animal model of Rapid-onset Dystonia-Parkinsonism
D. Paola Calderon and Kamran Khodakhah
Rapid onset Dystonia Parkinsonism (RDP) is an inherited autosomal dominant movement disorder characterized by the rapid onset of combined dystonia and parkinsonism. The symptoms include limb and cranial dystonic spasms, bradykinesia, slow gait, dysarthria and postural instability which appear rapidly over hours to weeks after severe stress. The disease is caused by mutations in the ?3 isoform of the Na/K ATPase, which reduce the sodium pump's activity. Even though the genetic basis of this disorder is known, it is currently unclear how dysfunction of the sodium pump results in RDP. Furthermore, the anatomical brain structures that malfunction in this disease have not been identified. We used a pharmacological approach to generate an animal model of RDP. We partially blocked sodium pumps in specific brain regions by local chronic perfusion of ouabain, a selective blocker of the sodium pump. We targeted structures that cause dystonia and/or Parkinsonism such as the cerebellum and basal ganglia. We found that perfusion of ouabain into the vermis of mouse cerebellum resulted in dystonic postures while the blockade of basal ganglia sodium pumps resulted in symptoms indicative of Parkinsonism. When both structures were perfused with ouabain concurrently, distinct symptoms were observed which were different from the symptoms seen when these structures were perfused separately. This finding suggests an interaction between the cerebellum and the basal ganglia. Moreover, similar to that seen in RDP patients, stress resulted in the appearance of dystonia in mice in which ouabain were perfused in both structures concurrently. Previous experiments suggest that abnormal cerebellar electrical activity results in dystonia. We examined this activity by performing EEGs in the cerebellum. These experiments revealed that dysfunction of sodium pumps causes cerebellar hyperactivity with the dystonic postures. Our data thus suggest an adverse interaction between Basal ganglia and Cerebellum and point to cerebellar hyperactivity as a potential correlate of dystonia in this disorder. [back to top]
4. Mapping the functional connectivity between cerebellar granule cells and Purkinje cells
Maria-Johanna Dizon and Kamran Khodakhah
Sensory and cortical information entering the cerebellum is processed and integrated by Purkinje cells, the sole projection neurons of the cerebellar cortex encoding the timing signals that allow for motor coordination. Each Purkinje cell receives convergent inputs from granule cells, subject to modulation by inhibitory interneurons. Classic perspectives of a straightforward functional output of the stereotypical circuitry of the cerebellar cortex have been recently challenged in view of emerging evidence alluding to a more complex interplay within the mossy fiber-granule cell-Purkinje cell pathway shaping the output of Purkinje cells. Presently, we are mapping the spatial patterns of activation within this circuitry, thereby addressing several questions that remain under active debate: 1) Does activity in this pathway propagate in a dispersed or patchy manner?; 2) Do the ascending axon and parallel fiber regions of the granule cells provide differential input to Purkinje cells?; and 3) How do molecular layer inhibitory interneurons between granule cells and Purkinje cells modulate Purkinje cell output? In ongoing experiments, Purkinje cell electrical activity in response to glutamate uncaging in multiple underlying granule cell patches is monitored via single-unit extracellular recording in acute cerebellar slices. This is done with inhibition intact or blocked, and using both sagittal and coronal slice orientations. Both spatial and temporal aspects of the response are analyzed. [back to top]
5. Feed forward inhibition and cerebellar function
Sung-Min Park and Kamran Khodakhah
The cerebellum coordinates movement through its principal neuron, the Purkinje cell. During performance of motor tasks such as oscillatory eye movements, Purkinje cell firing rate encodes the direction and magnitude of movement. From a baseline rate of ~50 Hz, firing rate increases to ~250 Hz encode the magnitude of one direction and decreases to ~0 Hz encode the magnitude of the opposite direction. A subset of Purkinje cells-type II cells-exhibit firing rates that are a "mirror image" of type I Purkinje cell firing rates. This means as a type I Purkinje cell fires above baseline a type II cell fires proportionally below baseline. Type I and II cells may provide the complementary signals for coordinated agonist muscle contraction and antagonist muscle relaxation. Because Purkinje cell baseline firing rate is maintained by intrinsic conductances, extrinsic inhibition is responsible for firing rates below ~50 Hz. The source of this inhibition may come from molecular layer interneurons, which are activated by the same inputs that activate Purkinje cells. In principle then, a fundamental unit of input that activates a type I Purkinje cell throughout ~50 to 250 Hz may also, through interneurons, proportionally inhibit a type II Purkinje cell throughout ~50 to 0 Hz. Our previous work has shown that excitatory inputs to Purkinje cells linearly increase Purkinje cell firing rate. Here, we found that the strength of those same inputs, via interneurons, is also linearly encoded in lateral Purkinje cell firing rates from ~50 to 0 Hz. Thus, the cerebellum is capable of generating both type I and II signals from the same set of inputs. [back to top]
6. The role of cerebellar Purkinje Cells in episodic dyskinesia
Esra Tara and Kamran Khodakhah
Episodic channelopathies are characterized by a mild baseline phenotype interrupted by attacks of severe symptoms induced by the same set of triggers, namely alcohol, caffeine, and emotional or physical stress. Episodic ataxia type two (EA2) is one such disorder that arises from mutations in the gene encoding for the P/Q-type voltage-gated calcium channel. The tottering, a mouse model of EA2, carries a mutation in the same gene as human EA2 patients. As a result, the tottering exhibits an ataxic baseline phenotype with severe attacks of abnormal movements, known as dyskinesia, triggered by ethanol, caffeine and stress. The intermittent nature of these attacks and their appearance in response to the same triggers as in human patients make tottering a tractable model of an episodic neurological disorder. The mechanism by which mutations in the P/Q-type calcium channel lead to dyskinesia and how triggers induce such episodes is unknown. The cerebellum has been implicated in the pathogenesis of dyskinesia, however its contribution is yet to be determined. Purkinje cells are the principal neurons of the cerebellar cortex and the regularity of their pacemaking has been implicated in many forms of ataxia. We propose to investigate the physiological basis of the sensorimotor attacks in EA2 and the site of action of different triggers in tottering. Our goal is to determine if changes in the intrinsic activity of Purkinje cells can mediate dyskinesia, and if triggers' effect on the cerebellum can induce such episodes. To address this question, we performed extracellular recordings from single Purkinje cells in the awake tottering mouse in the presence and absence of dyskinesia attacks, and found that the regularity of Purkinje cell activity is significantly altered during such attacks triggered by physiological doses of caffeine, ethanol or stress. [back to top]
