Determinants of Action Potential Propagation in Cerebellar Purkinje Cell Axons

Consumption of alcoholic beverages produces alterations in motor coordination and equilibrium that are responsible for millions of accidental deaths. Studies indicate that ethanol produces these alterations by affecting the cerebellum, a brain region involved in the control of motor systems. Purkinje neurons of the cerebellar cortex have been shown to be particularly important targets of ethanol. However, its mechanism of action at these neurons is poorly understood. We hypothesized that ethanol could modulate Purkinje neuron function by altering the excitatory input provided by the climbing fiber from the inferior olive, which evokes a powerful all-or-none response denoted as the complex spike. To test this hypothesis, we performed whole-cell patch-clamp electrophysiological and Ca 2ϩ imaging experiments in acute slices from rat cerebella. We found that ethanol potently inhibits the late phase of the complex spike and that this effect is the result of inhibition of type-1 metabotropic glutamate receptor-dependent responses at the postsynaptic level. Moreover, ethanol inhibited climbing fiber long-term depression, a form of synaptic plasticity that also depends on activation of these metabotropic receptors. Our findings identify the climbing fiber3 Purkinje neuron synapse as an important target of ethanol in the cerebellar cortex and indicate that ethanol significantly affects cerebellar circuits even at concentrations as low as 10 mM (legal blood alcohol level in the United States is below 0.08 g/dl ϭ 17 mM).

Section: Functional Implicationsmentioning

confidence: 50%

Alcohol Potently Modulates Climbing Fiber→Purkinje Neuron Synapses: Role of Metabotropic Glutamate Receptors

Carta¹,

Mameli²,

Valenzuela³

2006

“…Innervation of single DCN neurons by multiple Purkinje neurons is likely crucial for the alterations reported here to be consequential for the DCN firing given that, in individual neurons, axonal propagation of action potentials, particularly of spikelets, which are more often of stout stature, is unreliable at the high frequencies attained within complex spikes (Khaliq and Raman, 2005;Monsivais et al, 2005). Intriguingly, Kv3.3 channels thus far appear not to be enriched at nodes of Ranvier, in contrast to Kv3.1 (Chang et al, 2007), offering a potential explanation for the conduction failures and suggesting this is not the subcellular locus of Kv3.3 function.…”

Section: Perturbed Complex Spikes: Potential Impact On Deep Nuclear Nmentioning

confidence: 97%

Purkinje-Cell-Restricted Restoration of Kv3.3 Function Restores Complex Spikes and Rescues Motor Coordination inKcnc3Mutants

Hurlock

McMahon²,

Joho³

2008

The fast-activating/deactivating voltage-gated potassium channel Kv3.3 (Kcnc3) is expressed in various neuronal cell types involved in motor function, including cerebellar Purkinje cells. Spinocerebellar ataxia type 13 (SCA13) patients carrying dominant-negative mutations in Kcnc3 and Kcnc3-null mutant mice both display motor incoordination, suggested in mice by increased lateral deviation while ambulating and slips on a narrow beam. Motor skill learning, however, is spared. Mice lacking Kcnc3 also exhibit muscle twitches. In addition to broadened spikes, recordings of Kcnc3-null Purkinje cells revealed fewer spikelets in complex spikes and a lower intraburst frequency. Targeted reexpression of Kv3.3 channels exclusively in Purkinje cells in Kcnc3-null mice as well as in mice also heterozygous for Kv3.1 sufficed to restore simple spike brevity along with normal complex spikes and to rescue specifically coordination. Therefore, spike parameters requiring Kv3.3 function in Purkinje cells are involved in the ataxic null phenotype and motor coordination, but not motor learning.Key words: K channels; burst firing; motor dysfunction; cerebellum; Purkinje cell; complex spike IntroductionVoltage-gated potassium (Kv) channels of the Kv3 subfamily enable neurons to fire narrow action potentials at high frequencies beyond ϳ200 Hz. Four separate genes (Kcnc1-Kcnc4 ) encoding subunits Kv3.1-Kv3.4 are expressed in various combinations in CNS neurons in which they assemble into homotetrameric and heterotetrameric channels. Mice lacking the Kcnc3-encoded Kv3.3 subunit exhibit motor deficits manifested as increased lateral deviation while ambulating and increased slips while traversing a narrow beam (McMahon et al., 2004;Joho et al., 2006b). A role for Kv3.3 channels in motor coordination is also underscored by the recent finding that dominant-negative mutations found in the human KCNC3 gene correlate with the neurological disorder spinocerebellar ataxia 13 (Waters et al., 2006). Kv3-type channels are distinguished by exceptionally rapid kinetics of activation and deactivation, rendering them ideally suited to drive repolarization that is rapid, both in its onset and relief, so that the next action potential upstroke can occur with minimal delay in the absence of lingering afterhyperpolarization. High-frequency spiking is facilitated by both minimizing sodium channel inactivation through brief action potentials and by promoting sodium channel deinactivation through the excursion to negative potentials during the fast afterhyperpolarization. Indeed, the loss of Kv3-type channels results in broadened spikes accompanied by decelerated firing (Rudy and McBain, 2001). Unlike most other neuron types in which Kv3.3 subunits are expressed with other Kv3-type subunits, Purkinje cells express high levels of Kv3.3 and lower levels of Kv3.4 (Weiser et al., 1994;Martina et al., 2003). Loss of Kv3.3 results in a doubling of action potential duration (McMahon et al., 2004). Furthermore, blockade with tetraethylammonium (TEA) or BDS-I at a conc...

“…This pause persists even when the strength of olivary stimulation is adjusted so that it does not evoke a CS from the cell from which SSs are recorded (Bloedel and Roberts, 1971). Consequently, the CS-associated pause in SSs cannot be attributed to a membrane property of the Purkinje cell (Monsivais et al, 2005). More likely, it reflects the action of climbing fibers on one or more classes of cerebellar interneurons.…”

Section: Antiphasic Behavior Of Css and Sssmentioning

confidence: 99%

Functions of Interneurons in Mouse Cerebellum

Barmack¹,

Yakhnitsa²

2008

217

259

The output signal of Purkinje cells is conveyed by the modulated discharge of simple spikes (SSs) often ascribed to mossy fiber-granule cell-parallel fiber inputs to Purkinje cell dendrites. Although generally accepted, this view lacks experimental support. We can address this view by controlling afferent signals that reach the cerebellum over climbing and mossy fiber pathways. Vestibular primary afferents constitute the largest mossy fiber projection to the uvula-nodulus. The discharge of vestibular primary afferent mossy fibers increases during ipsilateral roll tilt. The discharge of SSs decreases during ipsilateral roll tilt. Climbing fiber discharge [complex spikes (CSs)] increases during ipsilateral roll tilt. These observations suggest that the modulation of SSs during vestibular stimulation cannot be attributed directly to vestibular mossy fiber afferents. Rather we suggest that interneurons driven by vestibular climbing fibers may determine SS modulation. We recorded from cerebellar interneurons (granule, unipolar brush, Golgi, stellate, basket, and Lugaro cells) and Purkinje cells in the uvula-nodulus of anesthetized mice during vestibular stimulation. We identified all neuronal types by juxtacellular labeling with neurobiotin. Granule, unipolar brush, stellate, and basket cells discharge in phase with ipsilateral roll tilt and in phase with CSs. Golgi cells discharge out of phase with ipsilateral roll tilt and out of phase with CSs. The phases of stellate and basket cell discharge suggests that their activity could account for the antiphasic behavior of CSs and SSs. Because Golgi cells discharge in phase with SSs, Golgi cell activity cannot account for SS modulation. The sagittal array of Golgi cell axon terminals suggests that they contribute to the organization of discrete parasagittal vestibular zones.