Participation of low-threshold Ca2+ spike in the Purkinje cells complex spike

T-type calcium channels as therapeutic targets in essential tremor and Parkinson's disease

Neuronal action potential firing patterns are key components of healthy brain function. Importantly, restoring dysregulated neuronal firing patterns has the potential to be a promising strategy in the development of novel therapeutics for disorders of the central nervous system. Here, we review the pathophysiology of essential tremor and Parkinson's disease, the two most common movement disorders, with a focus on mechanisms underlying the genesis of abnormal firing patterns in the implicated neural circuits. Aberrant burst firing of neurons in the cerebello-thalamo-cortical and basal ganglia-thalamo-cortical circuits contribute to the clinical symptoms of essential tremor and Parkinson's disease, respectively, and T-type calcium channels play a key role in regulating this activity in both the disorders. Accordingly, modulating T-type calcium channel activity has received attention as a potentially promising therapeutic approach to normalize abnormal burst firing in these diseases. In this review, we explore the evidence supporting the theory that T-type calcium channel blockers can ameliorate the pathophysiologic mechanisms underlying essential tremor and Parkinson's disease, furthering the case for clinical investigation of these compounds. We conclude with key considerations for future investigational efforts, providing a critical framework for the development of much needed agents capable of targeting the dysfunctional circuitry underlying movement disorders such as essential tremor, Parkinson's disease, and beyond.

T-type calcium channels contribute to NMDA receptor independent synaptic plasticity in hippocampal regular-spiking oriens-alveus interneurons

KEY POINTS

Regular-spiking interneurons in the hippocampal stratum oriens exhibit a form of long-term potentiation of excitatory transmission that is independent of NMDA receptors but requires co-activation of Ca²⁺ -permeable AMPA receptors and group I metabotropic glutamate receptors. We show that T-type Ca²⁺ channels are present in such interneurons. Blockade of T-type currents prevents the induction of long-term potentiation, and also interferes with long-lasting potentiation induced either by postsynaptic trains of action potentials or by pairing postsynaptic hyperpolarization with activation of group I metabotropic receptors. Several Ca²⁺ sources thus converge on the induction of NMDA receptor independent synaptic plasticity.

ABSTRACT

NMDA receptor independent long-term potentiation (LTP) in hippocampal stratum oriens-alveus (O/A) interneurons requires co-activation of postsynaptic group I metabotropic glutamate receptors (mGluRs) and Ca²⁺ -permeable AMPA receptors. The rectification properties of such AMPA receptors contribute to the preferential induction of LTP at hyperpolarized potentials. A persistent increase in excitatory transmission can also be triggered by exogenous activation of group I mGluRs at the same time as the interneuron is hyperpolarized, or by postsynaptic trains of action potentials in the absence of presynaptic stimulation. In the present study, we identify low-threshold transient (T-type) channels as a further source of Ca²⁺ that contributes to synaptic plasticity. T-type Ca²⁺ currents were detected in mouse regular-spiking O/A interneurons. Blocking T-type currents pharmacologically prevented LTP induced by high-frequency stimulation of glutamatergic axons, or by application of the group I mGluR agonist dihydroxyphenylglycine, paired with postsynaptic hyperpolarization. T-type current blockade also prevented synaptic potentiation induced by postsynaptic action potential trains. Several sources of Ca²⁺ thus converge on NMDA receptor independent LTP induction in O/A interneurons.

Contributions of T-type voltage-gated calcium channels to postsynaptic calcium signaling within Purkinje neurons

Low threshold voltage-gated T-type calcium channels have long been implicated in the electrical excitability and calcium signaling of cerebellar Purkinje neurons although the molecular composition, localization, and modulation of T-type channels within Purkinje cells have only recently been addressed. The specific functional roles that T-type channels play in local synaptic integration within Purkinje spines are also currently being unraveled. Overall, Purkinje neurons represent a powerful model system to explore the potential roles of postsynaptic T-type channels throughout the nervous system. In this review, we present an overview of T-type calcium channel biophysical, pharmacological, and physiological characteristics that provides a foundation for understanding T-type channels within Purkinje neurons. We also describe the biophysical properties of T-type channels in context of other voltage-gated calcium channel currents found within Purkinje cells. The data thus far suggest that one specific T-type isoform, Ca_v3.1, is highly expressed within Purkinje spines and both physically and functionally couples to mGluR1 and other effectors within putative signaling microdomains. Finally, we discuss how the selective potentiation of Ca_v3.1 channels via activation of mGluR1 by parallel fiber inputs affects local synaptic integration and how this interaction may relate to the overall excitability of Purkinje neuron dendrites.

Functional integration of calcium regulatory mechanisms at Purkinje neuron synapses

Cerebellar Purkinje neurons receive synaptic inputs from three different sources: the excitatory parallel fibre and climbing fibre synapses as well as the inhibitory synapses from molecular layer stellate and basket cells. These three synaptic systems use distinct mechanisms in order to generate Ca(2+) signals that are specialized for specific modes of neurotransmitter release and post-synaptic signal integration. In this review, we first describe the repertoire of Ca(2+) regulatory mechanisms that generate and regulate the amplitude and timing of Ca(2+) fluxes during synaptic transmission and then explore how these mechanisms interact to generate the unique functional properties of each of the Purkinje neuron synapses.

Calcium as a trigger for cerebellar long-term synaptic depression

Cerebellar long-term depression (LTD) is a form of long-term synaptic plasticity that is triggered by calcium(Ca2+) signals in the postsynaptic Purkinje cell. This Ca2+comes both from IP3-mediated release from intracellular Ca2+ stores, as well as from Ca2+ influx through voltage-gated Ca2+ channels. The Ca2+ signal that triggers LTD occurs locally within dendritic spines and is due to supralinear summation of signals coming from these two Ca2+ sources. The properties of this postsynaptic Ca2+signal can explain several features of LTD, such as its associativity, synapse specificity, and dependence on thetiming of synaptic activity, and can account for the slow kinetics of LTD expression. Thus, from a Ca2+ signaling perspective, LTD is one of the best understood forms of synaptic plasticity.

Transient receptor potential canonical channels regulate the induction of cerebellar long-term depression

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 (IP(3)R). IP(3)R 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.

Dendritic calcium signaling in cerebellar Purkinje cell

The Purkinje cells in the cerebellum are unique neurons that generate local and global Ca(2+) signals in response to two types of excitatory inputs, parallel fiber and climbing fiber, respectively. The spatiotemporal distribution and interaction of these synaptic inputs produce complex patterns of Ca(2+) dynamics in the Purkinje cell dendrites. The Ca(2+) signals originate from Ca(2+) influx through voltage-gated Ca(2+) channels and Ca(2+) release from intracellular stores that are mediated by the metabotropic glutamate receptor signaling pathway. These Ca(2+) signals are essential for the induction of various forms of synaptic plasticity and for controlling the input-output relationship of Purkinje cells. In this article we review Ca(2+) signaling in Purkinje cell dendrites.

T-type calcium channel as a new therapeutic target for tremor

Voltage-gated calcium channels play an important role in many physiological and pathological processes. Accumulating studies suggest that the T-type calcium channel is a potential target for the treatment of various neurological disorders, such as epilepsy, insomnia, and neuropathic pain. Here, we highlight recent advances in our understanding of T-type calcium channel regulation and their implications for tremor disorders. Several T-type calcium channel blockers effectively suppressed experimental tremors that have been suggested to originate from either the cerebellum or basal ganglia. Among T-type calcium channel blockers that have been used clinically, the anti-tremor efficacy of zonisamide garnered our attention. Based on both basic and clinical studies, the possibility is emerging that T-type calcium channel blockers that transit into the central nervous system may have therapeutic potentials for tremor disorders.

P/Q-type and T-type calcium channels, but not type 3 transient receptor potential cation channels, are involved in inhibition of dendritic growth after chronic metabotropic glutamate receptor type 1 and protein kinase C activation in cerebellar Purkinje cells

The development of a neuronal dendritic tree is modulated both by signals from afferent fibers and by an intrinsic program. We have previously shown that chronic activation of either type 1 metabotropic glutamate receptors (mGluR1s) or protein kinase C (PKC) in organotypic cerebellar slice cultures of mice and rats severely inhibits the growth and development of the Purkinje cell dendritic tree. The signaling events linking receptor activation to the regulation of dendritic growth remain largely unknown. We have studied whether channels allowing the entry of Ca(2+) into Purkinje cells, in particular the type 3 transient receptor potential cation channels (TRPC3s), P/Q-type Ca(2+) channels, and T-type Ca(2+) channels, might be involved in signaling after mGluR1 or PKC stimulation. We show that the inhibition of dendritic growth seen after mGluR1 or PKC stimulation is partially rescued by pharmacological blockade of P/Q-type and T-type Ca(2+) channels, indicating that activation of these channels mediating Ca(2+) influx contributes to the inhibition of dendritic growth. In contrast, the absence of Ca(2+) -permeable TRPC3s in TRPC3-deficient mice or pharmacological blockade had no effect on mGluR1-mediated and PKC-mediated inhibition of Purkinje cell dendritic growth. Similarly, blockade of Ca(2+) influx through glutamate receptor δ2 or R-type Ca(2+) channels or inhibition of release from intracellular stores did not influence mGluR1-mediated and PKC-mediated inhibition of Purkinje cell dendritic growth. These findings suggest that both T-type and P/Q-type Ca(2+) channels, but not TRPC3 or other Ca(2+) -permeable channels, are involved in mGluR1 and PKC signaling leading to the inhibition of dendritic growth in cerebellar Purkinje cells.

Axon initial segment Ca2+ channels influence action potential generation and timing

Although action potentials are typically generated in the axon initial segment (AIS), the timing and pattern of action potentials are thought to depend on inward current originating in somatodendritic compartments. Using two-photon imaging, we show that T- and R-type voltage-gated Ca(2+) channels are colocalized with Na(+) channels in the AIS of dorsal cochlear nucleus interneurons and that activation of these Ca(2+) channels is essential to the generation and timing of action potential bursts known as complex spikes. During complex spikes, where Na(+)-mediated spikelets fire atop slower depolarizing conductances, selective block of AIS Ca(2+) channels delays spike timing and raises spike threshold. Furthermore, AIS Ca(2+) channel block can decrease the number of spikelets within a complex spike and can even block single, simple spikes. Similar results were found in cortex and cerebellum. Thus, voltage-gated Ca(2+) channels at the site of spike initiation play a key role in generating and shaping spike bursts.