SK2 channels in cerebellar Purkinje cells contribute to excitability modulation in motor-learning-specific memory traces.

Giorgio Grasselli,Gerco C Beekhof,Martijn Schonewille,Nora Bradford,Silas E Busch,Heather K Titley,Lisa Van Beers,Lindsey Jay,Henk-Jan Boele,Chris I De Zeeuw,Christian Hansel

doi:10.1371/journal.pbio.3000596

Abstract

Neurons store information by changing synaptic input weights. In addition, they can adjust their membrane excitability to alter spike output. Here, we demonstrate a role of such “intrinsic plasticity” in behavioral learning in a mouse model that allows us to detect specific consequences of absent excitability modulation. Mice with a Purkinje-cell–specific knockout (KO) of the calcium-activated K+ channel SK2 (L7-SK2) show intact vestibulo-ocular reflex (VOR) gain adaptation but impaired eyeblink conditioning (EBC), which relies on the ability to establish associations between stimuli, with the eyelid closure itself depending on a transient suppression of spike firing. In these mice, the intrinsic plasticity of Purkinje cells is prevented without affecting long-term depression or potentiation at their parallel fiber (PF) input. In contrast to the typical spike pattern of EBC-supporting zebrin-negative Purkinje cells, L7-SK2 neurons show reduced background spiking but enhanced excitability. Thus, SK2 plasticity and excitability modulation are essential for specific forms of motor learning.

Highlights

The association of learning with changes in the membrane excitability of neurons was first described in invertebrate mollusks such as Hermissenda and Aplysia [1,2,3,4,5] but is found in the vertebrate hippocampus [6,7,8] and in the cerebellar cortex and nuclei [9,10,11,12]
Using the previously reported constitutive SK2-KO as a null reference [48], a pharmacological analysis of the sensitivity to the SK channel blocker apamin demonstrated that, while this toxin causes a dramatic increase of evoked spike frequency in control Purkinje cells, it does not affect evoked Purkinje cell firing in L7-SK2 or in SK2-KO mice, confirming the absence of functional SK channels in L7-SK2 Purkinje cells (Fig 1C; relative increase in spike frequency at 10–15 min: control, 211.0 ± 20.2%, n = 10; L7-SK2: 106.0 ± 11.4%, n = 4; SK2-KO: 108.0 ± 5.7%, n = 4; Kruskal– Wallis: p = 0.0018; post hoc comparison: p = 0.013 for control versus L7-SK2, p = 0.013 for control versus SK2-KO, p = 1.00 for L7-SK2 versus SK2-KO)
Measuring the total length and complexity of the dendritic arbor by Golgi staining and subsequent Sholl analysis in the new L7-SK2 strain (Fig 1F–1H), we did not observe any difference in dendritic length between control (2.9 ± 0.3 mm, n = 7) and L7-SK2 mice (2.8 ± 0.2 mm, n = 10; p = 0.97; Fig 1G), nor any difference in dendritic branching complexity as tested by Sholl analysis, again confirming that SK2 channels do not play a major role in Purkinje cell morphogenesis

Summary

Introduction

The association of learning with changes in the membrane excitability of neurons was first described in invertebrate mollusks such as Hermissenda and Aplysia [1,2,3,4,5] but is found in the vertebrate hippocampus [6,7,8] and in the cerebellar cortex and nuclei [9,10,11,12]. Despite significant progress in the field, it has been difficult to comprehensively describe the cellular mechanisms underlying vertebrate behavioral learning. This holds for relatively simple forms of cerebellum-dependent motor learning, such as delay eyeblink conditioning (EBC) [14, 15] and adaptation of the vestibulo-ocular reflex (VOR) [16,17,18]. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Methods

Results

Conclusion