Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Neuronal Voltage-Gated Calcium Channels: Structure, Function, and Dysfunction

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

Calcium-Permeable Ion Channels in Pain Signaling

Physiological roles of the intermediate conductance, Ca2+-activated potassium channel Kcnn4. Vol. 279 (2004) 47681–47687

Most cardiovascular researchers are familiar with intermediate-conductance KCa3.1 and small-conductance KCa2.3 channels because of their contribution to endothelium-derived hyperpolarization (EDH). However, to immunologists and neuroscientists these channels are primarily known for their role in lymphocyte activation and neuronal excitability. KCa3.1 is involved in the proliferation and migration of T cells, B cell, mast cells, macrophages, fibroblasts and dedifferentiated vascular smooth muscle cells and is, therefore, being pursued as a potential target for use in asthma, immunosuppression, and fibroproliferative disorders. In contrast, the three KCa2 channels (KCa2.1, KCa2.2 and KCa2.3) contribute to the neuronal medium afterhyperpolarization and, depending on the type of neuron, are involved in determining firing rates and frequencies or in regulating bursting. KCa2 activators are accordingly being studied as potential therapeutics for ataxia and epilepsy while KCa2 channel inhibitors like apamin have long been known to improve learning and memory in rodents. Given this background, we review the recent discoveries of novel KCa3.1 and KCa2.3 modulators and critically assess the potential of KCa activators for the treatment of diabetes and cardiovascular diseases by improving endothelium-derived hyperpolarizations.

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Endothelial small-conductance and intermediate-conductance KCa channels: an update on their pharmacology and usefulness as cardiovascular targets.

Encoding sensory input requires the expression of postsynaptic ion channels to transform key features of afferent input to an appropriate pattern of spike output. Although Ca2+-activated K+ channels are known to control spike frequency in central neurons, Ca2+-activated K+ channels of intermediate conductance (KCa3.1) are believed to be restricted to peripheral neurons. We now report that cerebellar Purkinje cells express KCa3.1 channels, as evidenced through single-cell RT-PCR, immunocytochemistry, pharmacology, and single-channel recordings. Furthermore, KCa3.1 channels coimmunoprecipitate and interact with low voltage-activated Cav3.2 Ca2+ channels at the nanodomain level to support a previously undescribed transient voltage- and Ca2+-dependent current. As a result, subthreshold parallel fiber excitatory postsynaptic potentials (EPSPs) activate Cav3 Ca2+ influx to trigger a KCa3.1-mediated regulation of the EPSP and subsequent after-hyperpolarization. The Cav3-KCa3.1 complex provides powerful control over temporal summation of EPSPs, effectively suppressing low frequencies of parallel fiber input. KCa3.1 channels thus contribute to a high-pass filter that allows Purkinje cells to respond preferentially to high-frequency parallel fiber bursts characteristic of sensory input.

https://www.pnas.org/content/pnas/109/7/2601.full.pdf

Intermediate conductance calcium-activated potassium channels modulate summation of parallel fiber input in cerebellar Purkinje cells

The expression pattern and functional roles for calcium-activated potassium channels of the KCa2.x family and KCa1.1 have been extensively examined in central neurons. Recent work indicates that intermediate conductance calcium-activated potassium channels (KCa3.1) are also expressed in central neurons of the cerebellum and spinal cord. The current study used immunocytochemistry and GFP linked to KCNN4 promoter activity in a transgenic mouse to determine the expression pattern of KCa3.1 channels in rat or mouse neocortex, hippocampus, thalamus, and cerebellum. KCa3.1 immunolabel and GFP expression were closely matched and detected in both excitatory and inhibitory cells of all regions examined. KCa3.1 immunolabel was localized primarily to the somatic region of excitatory cells in cortical structures but at the soma and over longer segments of dendrites of cells in deep cerebellar nuclei. More extensive labeling was apparent for inhibitory cells at the somatic and dendritic level with no detectable label associated with axon tracts or regions of intense synaptic innervation. The data indicate that KCa3.1 channels are expressed in the CNS with a differential pattern of distribution between cells, suggesting important functional roles for these calcium-activated potassium channels in regulating the excitability of central neurons.

Neuronal expression of the intermediate conductance calcium-activated potassium channel KCa3.1 in the mammalian central nervous system

Cerebellar Purkinje cells are contacted by up to ~150,000 parallel fibers from granule cells, of which only a subset will convey sensory information at any given time. Purkinje cells must then possess the means to respond effectively to meaningful parallel fiber input over background noise. Previous work has shown that parallel fiber excitatory postsynaptic potential (EPSP) summation can be shaped by feedforward synaptic inhibition and the hyperpolarization-activated current IH[1,2]. We now report that parallel fiber EPSPs activate T-type calcium channels that are linked to intermediate conductance calcium-activated potassium (IKCa) channels in Purkinje cells. This novel complex exerts a frequency-dependent suppression of temporal summation, such that only high frequency parallel fiber inputs undergoing presynaptic facilitation can elicit spike output from Purkinje cells.

Cerebellar slices were prepared from P18-30 rats and patch recordings obtained from PC somata at 32-35°C. PFs were activated using a monopolar stimulating electrode in the molecular layer or granule cell layer. Alternatively, the role of postsynaptic PC ion channels were selectively tested by injecting simulated EPSCs to evoke PF simulated EPSPs (simEPSPs) at the soma.

PF EPSPs below threshold for spike discharge were followed by an after hyperpolarization (AHP) of up to 2.5 mV and 250 ms. Application of blockers against high voltage activated Ca2+ channels (Cd2+, Agatoxin IVA), SK channels (apamin), or BK channels (TEA, iberiotoxin, paxilline) did not significantly affect the rate of simEPSP decay. However, T-type Ca2+ channel blockers (Ni2+, Mibefradil, kurtoxin) caused a ~35% decrease in the rate of simEPSP decay. Moreover, these effects were reproduced by application of IKCa channel blockers (TRAM-34, charybdotoxin). Immunofluorescent labeling for IKCa protein confirmed its expression in Purkinje cells somata and dendrites. Ni2+ and TRAM-34 sensitive outward currents were found in outside-out patches from PC somata, confirming current clamp data showing a functional link between Cav3 and IKCa channels. The outward current was further blocked by BAPTA (10 mM) but not EGTA (10 mM) in the internal patch solution, indicating that the Ca2+-IKca channel interaction resides within a nanodomain. To examine the effect of this interaction on temporal summation, PFs were stimulated at varying frequencies. For frequencies up to 25 Hz, no temporal summation was observed in control conditions. However, blocking either Cav3 or IKca channels caused significant summation for 25 Hz stimulations. This effect was seen in both the presence and absence of feedforward-inhibition. Application of TRAM-34 greatly altered the frequency response of PC to 50 and 100 Hz PF stimulation during tonic firing. Finally, the Cav3-IKCa complex selectively suppresses non-facilitating inputs while allowing smaller-amplitude, facilitating inputs to generate output.

Our current work is the first to demonstrate the expression of IKCa channels in central neurons, its association with Cav3 channels and the role of this Cav3-IKCa complex in controlling the response of PCs to PF inputs. The Cav3-IKCa complex creates a high pass filter that reduces the effectiveness of background activity and allows Purkinje cells to respond preferentially to parallel fiber input indicative of sensory input carried by mossy fibers.

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Mirna Kruskic

Papers

Intermediate conductance calcium-activated potassium channels modulate summation of parallel fiber input in cerebellar Purkinje cells

Neuronal expression of the intermediate conductance calcium-activated potassium channel KCa3.1 in the mammalian central nervous system

IKCa-Cav3 complex creates a high pass filter for parallel fiber input in cerebellar Purkinje cells