Calcium Dynamics and Electrophysiological Properties of Cerebellar Purkinje Cells in SCA1 Transgenic Mice

Abstract

Cerebellar Purkinje cells (PCs) from spinocerebellar ataxia type 1 (SCA1) transgenic mice develop dendritic and somatic atrophy with age. Inositol 1,4,5-trisphosphate receptor type 1 and the sarco/endoplasmic reticulum Ca(2+) ATPase pump, which regulate [Ca(2+)](i), are expressed at lower levels in these cells compared with the levels in cells from wild-type (WT) mice. To examine PCs in SCA1 mice, we used whole-cell patch clamp recording combined with fluorometric [Ca(2+)](i) and [Na(+)](i) measurements in cerebellar slices. PCs in SCA1 mice had Na(+) spikes, Ca(2+) spikes, climbing fiber (CF) electrical responses, parallel fiber (PF) electrical responses, and metabotropic glutamate receptor (mGluR)-mediated, PF-evoked Ca(2+) release from intracellular stores that were qualitatively similar to those recorded from WT mice. Under our experimental conditions, it was easier to evoke the mGluR-mediated secondary [Ca(2+)](i) increase in SCA1 PCs. The membrane resistance of SCA1 PCs was 3.3 times higher than that of WT cells, which correlated with the 1.7 times smaller cell body size. Most SCA1 PCs (but not WT) had a delayed onset (about 50--200 ms) to Na(+) spike firing induced by current injection. This delay was increased by hyperpolarizing prepulses and was eliminated by 4-aminopyridine, which suggests that this delay was due to enhancement of the A-like K(+) conductance in the SCA1 PCs. In response to CF stimulation, most PCs in mutant and WT mice had rapid, widespread [Ca(2+)](i) changes that recovered in <200 ms. Some SCA1 PCs showed a slow, localized, secondary Ca(2+) transient following the initial CF Ca(2+) transient, which may reflect release of Ca(2+) from intracellular stores. Thus, with these exceptions, the basic physiological properties of mutant PCs are similar to those of WT neurons, even with dramatic alteration of their morphology and downregulation of Ca(2+) handling molecules.

Aminopyridines Correct Early Dysfunction and Delay Neurodegeneration in a Mouse Model of Spinocerebellar Ataxia Type 1

“…This plateau was not observed in control cells (7 of 43 SCA1 cells vs 0 of 25 cells; p ϭ 0.041, Fisher's test). A similar plateau potential was previously described in most PCs from 3-to 6-monthold SCA1 mice (Inoue et al, 2001) and is suggestive of fast activation and inactivation of an A-type potassium conductance (IK A ) (Hounsgaard and Midtgaard, 1988;Klee et al, 1995). Accordingly, when recorded, this plateau current was reversibly abolished by low doses of 4-aminopyridine and could be recruited by a hyperpolarizing prepulse (Fig.…”

“…4b). Interestingly, the proportion of PCs displaying this irregular plateau potential is higher in older SCA1 mice, as if its occurrence reflected the progression of the disease and was correlated with disease severity: 16% of PCs display the plateau in presymptomatic mice, 45% in early symptomatic SCA1 mice (5 of 11 vs 0 of 10 in controls; p ϭ 0.035, Fisher's test), and Ͼ80% in 3-to 6-month-old SCA1 mice (Inoue et al, 2001). IK A peak amplitude was isolated in PCs from presymptomatic SCA1 mice by the subtraction of potassium currents evoked by a depolarization to Ϫ20 mV after hyperpolarized (Ϫ80 mV) and depolarized (Ϫ40 mV) conditioning steps (200 ms) from a holding potential of Ϫ70 mV.…”

“…The animals develop motor abnormalities before histopathological changes are visible, suggesting a possible early neuronal dysfunction. While previous electrophysiological studies of SCA1 mice were performed at a late stage with major morphological alterations of PCs (Inoue et al, 2001), we focused on anomalies already present in presymptomatic mice and attempted to test whether correction of early neuronal dysfunction of PCs delayed later cell atrophy.…”

AFG3L2 supports mitochondrial protein synthesis and Purkinje cell survival

“…After the electrophysiological characterization, the PCs were labeled by dye injection via the recording electrode. All parameter values that we determined for the electrophysiological properties of control PCs ( Figure 4) were well in the range that has been described previously for PCs in rat and mouse brain slices and in vivo preparations (19)(20)(21)(22)(23)(24). The statistical comparison of these electrophysiological parameters between PCs of Afg3l2 PC-KO mice and their control littermates revealed no differences (Figure 4).…”

Targeting potassium channels to treat cerebellar ataxia

“…Addressing intrinsic dendritic hyperexcitability is likely an important aspect of altered physiology which must be addressed in order to sustain benefit in the treatment of SCA, consistent with the critical role that intrinsic dendritic excitability plays in regulating synaptic integration 59. It is important to note that we and others have demonstrated that in association with dendritic degeneration, ATXN1[82Q] Purkinje neurons display increased subthreshold‐activated potassium channel currents2, 60 that affect spiking. Nevertheless, agents that reduce dendritic excitability through targeting these or other subthreshold‐activated potassium channels are beneficial in maintaining improvements in motor dysfunction at a stage of disease associated with dendritic degeneration.…”