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Intrinsic plasticity of cerebellar Purkinje cells (PCs) has recently been demonstrated in cerebellar local circuits; however, its physiological impact on cerebellar learning and memory remains elusive. Here, we suggest that intrinsic plasticity of PCs is tightly involved in motor memory consolidation based on findings from PC-specific STIM1 knockout male mice, which show severe memory consolidation deficiency in vestibulo-ocular reflex memory. Gain-up training of the vestibulo-ocular reflex produced a decrease in the synaptic weight of PCs in both the WT and KO groups. However, intrinsic plasticity was impaired only in the knockout mice. Furthermore, the observed defects in the intrinsic plasticity of PCs led to the formation of aberrant neural plasticity in the vestibular nucleus neurons. Our results suggest that synergistic modulation of intrinsic and synaptic plasticity in PCs is required for the changes in downstream plasticity in the vestibular nucleus, and thereby contributing to the long-term storage of motor memory.SIGNIFICANCE STATEMENTSynaptic plasticity is a well-known mechanism for learning and memory. Although plasticity of excitability, intrinsic plasticity, of the cerebellar Purkinje cell has been reported in both directions (potentiation and depression), the physiological role of intrinsic plasticity still remains ambiguous. In this study, we suggest that both synaptic and intrinsic plasticity are required for successful memory consolidation in cerebellar eye movement learning. Despite successful induction and maintenance of synaptic plasticity, we found deficits of memory consolidation when there were defects in intrinsic plasticity. Our results suggest that intrinsic plasticity of cerebellar Purkinje cell has a significant role in motor memory consolidation.

Control of Ca2+flux between the cytosol and intracellular Ca2+stores is essential for maintaining normal cellular function. It has been well established in both neuronal and non-neuronal cells that stromal interaction molecule 1 (STIM1) initiates and regulates refilling Ca2+into the ER. Here, we describe a novel, additional role for STIM1, the regulation of free cytosolic Ca2+, and the consequent control of spike firing in neurons. Among central neurons, cerebellar Purkinje neurons express the highest level of STIM1, and they fire continuously in the absence of stimulation, making somatic Ca2+homeostasis of particular importance. By using Purkinje neuron-specific STIM1 knock-out (STIM1PKO) male mice, we found that the deletion of STIM1 delayed clearance of cytosolic Ca2+in the soma during ongoing neuronal firing. Deletion of STIM1 also reduced the Purkinje neuronal excitability and impaired intrinsic plasticity without affecting long-term synaptic plasticity. In vestibulo-ocular reflex learning, STIM1PKOmale mice showed severe deficits in memory consolidation, whereas they were normal in memory acquisition. Our results suggest that STIM1 is critically involved in the regulation of the neuronal excitability and the intrinsic plasticity of the Purkinje neurons as well as cerebellar memory consolidation.SIGNIFICANCE STATEMENTStromal interaction molecule 1 (STIM1), which regulates the refilling of ER Ca2+, has been investigated in several systems including the CNS. In addition to a previous study showing that STIM1 regulates dendritic ER Ca2+refilling and mGluR1-mediated synaptic transmission, we provide compelling evidence describing a novel role of STIM1 in spike firing Purkinje neurons. We found that STIM1 regulates cytosolic Ca2+clearance of the soma during spike firing, and the interruption of this cytosolic Ca2+clearing disrupts neuronal excitability and cerebellar memory consolidation. Our results provide new insights into neuronal functions of STIM1 from single neuronal Ca2+dynamics to behavior level.

Long-term depression (LTD) at the parallel fiber (PF)-to-cerebellar Purkinje cell (PC) synapse is implicated in the output of PCs, the sole output of the cerebellar cortex. In addition to synaptic plasticity, intrinsic excitability is also one of the components that determines PC output. Although long-term potentiation of intrinsic excitability (LTP-IE) has been suggested, it has yet to be investigated how PF–PC LTD modifies intrinsic excitability of PCs. Here, we show that pairing of the PF and climbing fiber (CF) for PF–PC LTD induction evokes LTD-IE in cerebellar PCs from male C57BL/6 mice. Interestingly, this intrinsic plasticity showed different kinetics from synaptic plasticity, but both forms of plasticity share Ca(2+) signaling and protein kinase C pathway as their underlying mechanism. Although small-conductance Ca(2+)-activated K(+) channels play important roles in LTP-IE, no direct implication has been found. After PF–PC LTD induction, neither the temporal summation of dendritic EPSP nor the power of spike frequency adaptation is changed, indicating that cerebellar LTD executes the information processing in a quantitative way without quality changes of synaptic integration and generation of output signals. Our results suggest that LTD-IE may have a synergistic effect with synaptic depression on the total net output of neurons by amplifying the modification of PF synaptic transmission. SIGNIFICANCE STATEMENT Although the output of Purkinje cells (PCs) is a critical component of cerebellum-dependent learning and memory, the changes of PC excitability when synaptic LTD occurs are unclear. Here, we show that the induction of PF–PC LTD evokes LTD-IE in PCs. Our observation complements previous intrinsic plasticity phenomenon of long-term potentiation of intrinsic excitability (LTP-IE), providing evidence for the idea that intrinsic plasticity has bidirectionality as synaptic plasticity. LTD-IE occurs together with synaptic LTD and both phenomena are dependent on the Ca(2+) signaling pathway. Furthermore, our findings raise the prospect that this synaptic and intrinsic plasticity acts synergistically in PCs to modify neuronal activity in the same direction when learning occurs.

In the cerebellum, synaptic strength at the synapses between parallel fibers and Purkinje cells is best known to be modulated via metabotropic glutamate receptor 1 (mGluR1)-dependent cerebellar long-term depression (LTD). An increase in intracellular calcium levels plays an important role in inducing mGluR1-dependent cerebellar LTD. Downstream of mGluR1, there are two major sources of calcium: transient receptor potential canonical (TRPC) channels and inositol trisphosphate receptors (IP3R). IP3R triggers a calcium release from the intracellular calcium store. Here, we show that TRPC channels mediate mGluR1-evoked slow currents to regulate cerebellar LTD in Sprague Dawley rats. We found that the inhibition of TRPC channels blocks the induction of cerebellar LTD. Moreover, we show that processes known to underlie cerebellar LTD induction, such as increases in intracellular calcium concentration, the activation of protein kinase C, and the internalization of GluR2, are also hindered by blocking TRPC. These results suggest that the mGluR1-evoked activation of TRPC channels is required for the induction of cerebellar LTD.

The increase of cytosolic free Ca2+([Ca2+]c) due to NMDA receptor activation is a key step for spinal cord synaptic plasticity by altering cellular signal transduction pathways. We focus on this plasticity as a cause of persistent pain. To provide a mechanism for these classic findings, we report that [Ca2+]cdoes not trigger synaptic plasticity directly but must first enter into mitochondria. Interfering with mitochondrial Ca2+uptake during a [Ca2+]cincrease blocks induction of behavioral hyperalgesia and accompanying downstream cell signaling, with reduction of spinal long-term potentiation (LTP). Furthermore, reducing the accompanying mitochondrial superoxide levels lessens hyperalgesia and LTP induction. These results indicate that [Ca2+]crequires downstream mitochondrial Ca2+uptake with consequent production of reactive oxygen species (ROS) for synaptic plasticity underlying chronic pain. These results suggest modifying mitochondrial Ca2+uptake and thus ROS as a type of chronic pain therapy that should also have broader biologic significance.