Involvement of the Cav3.2 T-type calcium channel in thalamic neuron discharge patterns

A Thalamocortical Perspective on Sleep Spindle Alterations in Neurodevelopmental Disorders

Purpose of Review

Neurodevelopmental disorders are a group of conditions that affect the development and function of the nervous system, typically arising early in life. These disorders can have various genetic, environmental, and/or neural underpinnings, which can impact the thalamocortical system. Sleep spindles, brief bursts of oscillatory activity that occur during NREM sleep, provide a unique in vivo measure of the thalamocortical system. In this manuscript, we review the development of the thalamocortical system and sleep spindles in rodent models and humans. We then utilize this as a foundation to discuss alterations in sleep spindle activity in four of the most pervasive neurodevelopmental disorders-intellectual disability, attention deficit hyperactivity disorder, autism, and schizophrenia.

Recent Findings

Recent work in humans has shown alterations in sleep spindles across several neurodevelopmental disorders. Simultaneously, rodent models have elucidated the mechanisms which may underlie these deficits in spindle activity. This review merges recent findings from these two separate lines of research to draw conclusions about the pathogenesis of neurodevelopmental disorders.

Summary

We speculate that deficits in the thalamocortical system associated with neurodevelopmental disorders are exquisitely reflected in sleep spindle activity. We propose that sleep spindles may represent a promising biomarker for drug discovery, risk stratification, and treatment monitoring.

Intrinsic Morphologic and Physiologic Development of Human Derived Retinal Ganglion Cells In Vitro

Purpose

Human retinal ganglion cells (hRGC) derived from human pluripotent stem cells are promising candidates to model, protect, and replace degenerating RGCs. Here, we examined intrinsic morphologic and physiologic development of hRGCs.

Methods

We used CRISPR-Cas9 to selectively express tdTomato under the RGC-specific promoter, BRN3B. Human pluripotent stem cells were chemically differentiated into hRGCs and cultured up to 7 weeks. We measured soma area, neurite complexity, synaptic protein, axon-related messenger RNA and protein, and voltage-dependent responses.

Results

Soma area, neurite complexity, and postsynaptic density protein 95 increased over time. Soma area and neurite complexity increased proportionally week to week, and this relationship was dynamic, strengthening between 2 and 3 weeks and diminishing by 4 weeks. Postsynaptic density 95 localization was dependent on culture duration. After 1 to 2 weeks, postsynaptic density 95 localized within somas but redistributed along neurites after 3 to 4 weeks. Axon initial segment scaffolding protein, Ankyrin G, expression also increased over time, and by 7 weeks, Ankyrin G often localized within putative axons. Voltage-gated inward currents progressively developed, but outward currents matured by 4 weeks. Current-induced spike generation increased over time but limited by depolarization block.

Conclusions

Human RGCs develop up to 7 weeks after culture. Thus, the state of hRGC maturation should be accounted for in designing models and treatments for optic neuropathies.

Translational Relevance

We characterized hRGC morphologic and physiologic development towards identifying key time points when hRGCs express mechanisms that may be harnessed to enhance the efficacy of neuroprotective and cell replacement therapies.

Somatosensory Thalamic Activity Modulation by Posterior Insular Stimulation: Cues to Clinical Application Based on Comparison of Frequencies in a Cat Model

BACKGROUND

The posterior insula (PI) has been proposed as a potential neurostimulation target for neuropathic pain relief as it represents a key-structure in pain processing. However, currently available data remain inconclusive as to efficient stimulation parameters.

OBJECTIVE

As frequency was shown to be the most correlated parameter to pain relief, this study aims to evaluate the potential modulatory effects of low frequency (LF-IS, 50 Hz) and high-frequency (HF-IS, 150 Hz) posterior insular stimulation on the activity of somatosensory thalamic nuclei.

MATERIALS AND METHODS

Epidural bipolar electrodes were placed over the PI of healthy adult cats, and extracellular single-unit activities of nociceptive (NS), nonnociceptive (NN), and wide dynamic range (WDR) thalamic cells were recorded within the ventral posterolateral nucleus and the medial division of the thalamic posterior complex. Mean discharge frequency and burst firing mode were analyzed before and after either LF-IS or HF-IS.

RESULTS

LF-IS showed a significant thalamic modulatory effects increasing the firing rate of NN cells (p ≤ 0.03) and decreasing the burst firing of NS cells (p ≤ 0.03), independently of the thalamic nucleus. Conversely, HF-IS did not induce any change in firing properties of the three recorded cell types.

CONCLUSION

These data indicate that 50 Hz IS could be a better candidate to control neuropathic pain.

Anterior nucleus of paraventricular thalamus mediates chronic mechanical hyperalgesia

Pain-related diseases are the top leading causes of life disability. Identifying brain regions involved in persistent neuronal changes will provide new insights for developing efficient chronic pain treatment. Here, we showed that anterior nucleus of paraventricular thalamus (PVA) plays an essential role in the development of mechanical hyperalgesia in neuropathic and inflammatory pain models in mice. Increase in c-Fos, phosphorylated extracellular signal-regulated kinase, and hyperexcitability of PVA neurons were detected in hyperalgesic mice. Direct activation of PVA neurons using optogenetics and pharmacological approaches were sufficient to induce persistent mechanical hyperalgesia in naive animals. Conversely, inhibition of PVA neuronal activity using DREADDs (designer receptors exclusively activated by designer drugs) or inactivation of PVA extracellular signal-regulated kinase at the critical time window blunted mechanical hyperalgesia in chronic pain models. At the circuitry level, PVA received innervation from central nucleus of amygdala, a known pain-associated locus. As a result, activation of right central nucleus of amygdala with blue light was enough to induce persistent mechanical hyperalgesia. These findings support the idea that targeting PVA can be a potential therapeutic strategy for pain relief.

Sleep Spindles: Mechanisms and Functions

Sleep spindles are burstlike signals in the electroencephalogram (EEG) of the sleeping mammalian brain and electrical surface correlates of neuronal oscillations in thalamus. As one of the most inheritable sleep EEG signatures, sleep spindles probably reflect the strength and malleability of thalamocortical circuits that underlie individual cognitive profiles. We review the characteristics, organization, regulation, and origins of sleep spindles and their implication in non-rapid-eye-movement sleep (NREMS) and its functions, focusing on human and rodent. Spatially, sleep spindle-related neuronal activity appears on scales ranging from small thalamic circuits to functional cortical areas, and generates a cortical state favoring intracortical plasticity while limiting cortical output. Temporally, sleep spindles are discrete events, part of a continuous power band, and elements grouped on an infraslow time scale over which NREMS alternates between continuity and fragility. We synthesize diverse and seemingly unlinked functions of sleep spindles for sleep architecture, sensory processing, synaptic plasticity, memory formation, and cognitive abilities into a unifying sleep spindle concept, according to which sleep spindles 1) generate neural conditions of large-scale functional connectivity and plasticity that outlast their appearance as discrete EEG events, 2) appear preferentially in thalamic circuits engaged in learning and attention-based experience during wakefulness, and 3) enable a selective reactivation and routing of wake-instated neuronal traces between brain areas such as hippocampus and cortex. Their fine spatiotemporal organization reflects NREMS as a physiological state coordinated over brain and body and may indicate, if not anticipate and ultimately differentiate, pathologies in sleep and neurodevelopmental, -degenerative, and -psychiatric conditions.

Marine Toxins and Nociception: Potential Therapeutic Use in the Treatment of Visceral Pain Associated with Gastrointestinal Disorders

Visceral pain, of which the pathogenic basis is currently largely unknown, is a hallmark symptom of both functional disorders, such as irritable bowel syndrome, and inflammatory bowel disease. Intrinsic sensory neurons in the enteric nervous system and afferent sensory neurons of the dorsal root ganglia, connecting with the central nervous system, represent the primary neuronal pathways transducing gut visceral pain. Current pharmacological therapies have several limitations, owing to their partial efficacy and the generation of severe adverse effects. Numerous cellular targets of visceral nociception have been recognized, including, among others, channels (i.e., voltage-gated sodium channels, VGSCs, voltage-gated calcium channels, VGCCs, Transient Receptor Potential, TRP, and Acid-sensing ion channels, ASICs) and neurotransmitter pathways (i.e., GABAergic pathways), which represent attractive targets for the discovery of novel drugs. Natural biologically active compounds, such as marine toxins, able to bind with high affinity and selectivity to different visceral pain molecular mediators, may represent a useful tool (1) to improve our knowledge of the physiological and pathological relevance of each nociceptive target, and (2) to discover therapeutically valuable molecules. In this review we report the most recent literature describing the effects of marine toxin on gastrointestinal visceral pain pathways and the possible clinical implications in the treatment of chronic pain associated with gut diseases.

Cav3.2 T-type calcium channels shape electrical firing in mouse Lamina II neurons

The T-type calcium channel, Cav3.2, is necessary for acute pain perception, as well as mechanical and cold allodynia in mice. Being found throughout sensory pathways, from excitatory primary afferent neurons up to pain matrix structures, it is a promising target for analgesics. In our study, Cav3.2 was detected in ~60% of the lamina II (LII) neurons of the spinal cord, a site for integration of sensory processing. It was co-expressed with Tlx3 and Pax2, markers of excitatory and inhibitory interneurons, as well as nNOS, calretinin, calbindin, PKCγ and not parvalbumin. Non-selective T-type channel blockers slowed the inhibitory but not the excitatory transmission in LII neurons. Furthermore, T-type channel blockers modified the intrinsic properties of LII neurons, abolishing low-threshold activated currents, rebound depolarizations, and blunting excitability. The recording of Cav3.2-positive LII neurons, after intraspinal injection of AAV-DJ-Cav3.2-mcherry, showed that their intrinsic properties resembled those of the global population. However, Cav3.2 ablation in the dorsal horn of Cav3.2^GFP-Flox KI mice after intraspinal injection of AAV-DJ-Cav3.2-Cre-IRES-mcherry, had drastic effects. Indeed, it (1) blunted the likelihood of transient firing patterns; (2) blunted the likelihood and the amplitude of rebound depolarizations, (3) eliminated action potential pairing, and (4) remodeled the kinetics of the action potentials. In contrast, the properties of Cav3.2-positive neurons were only marginally modified in Cav3.1 knockout mice. Overall, in addition to their previously established roles in the superficial spinal cord and in primary afferent neurons, Cav3.2 channel appear to be necessary for specific, significant and multiple controls of LII neuron excitability.

Inhibition of human N- and T-type calcium channels by an ortho-phenoxyanilide derivative, MONIRO-1

BACKGROUND AND PURPOSE

Voltage-gated calcium channels are involved in nociception in the CNS and in the periphery. N-type (Ca_v 2.2) and T-type (Ca_v 3.1, Ca_v 3.2 and Ca_v 3.3) voltage-gated calcium channels are particularly important in studying and treating pain and epilepsy.

EXPERIMENTAL APPROACH

In this study, whole-cell patch clamp electrophysiology was used to assess the potency and mechanism of action of a novel ortho-phenoxylanilide derivative, MONIRO-1, against a panel of voltage-gated calcium channels including Ca_v 1.2, Ca_v 1.3, Ca_v 2.1, Ca_v 2.2, Ca_v 2.3, Ca_v 3.1, Ca_v 3.2 and Ca_v 3.3.

KEY RESULTS

MONIRO-1 was 5- to 20-fold more potent at inhibiting human T-type calcium channels, hCa_v 3.1, hCa_v 3.2 and hCa_v 3.3 (IC₅₀ : 3.3 ± 0.3, 1.7 ± 0.1 and 7.2 ± 0.3 μM, respectively) than N-type calcium channel, hCa_v 2.2 (IC₅₀ : 34.0 ± 3.6 μM). It interacted with L-type calcium channels Ca_v 1.2 and Ca_v 1.3 with significantly lower potency (IC₅₀  > 100 μM) and did not inhibit hCa_v 2.1 or hCa_v 2.3 channels at concentrations as high as 100 μM. State- and use-dependent inhibition of hCa_v 2.2 channels was observed, whereas stronger inhibition occurred at high stimulation frequencies for hCa_v 3.1 channels suggesting a different mode of action between these two channels.

CONCLUSIONS AND IMPLICATIONS

Selectivity, potency, reversibility and multi-modal effects distinguish MONIRO-1 from other low MW inhibitors acting on Ca_v channels involved in pain and/or epilepsy pathways. High-frequency firing increased the affinity for MONIRO-1 for both hCa_v 2.2 and hCa_v 3.1 channels. Such Ca_v channel modulators have potential clinical use in the treatment of epilepsies, neuropathic pain and other nociceptive pathophysiologies.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Presynaptic and extrasynaptic regulation of posterior nucleus of thalamus

The posterior nucleus of thalamus (PO) is a higher-order nucleus involved in sensorimotor processing, including nociception. An important characteristic of PO is its wide range of activity profiles that vary across states of arousal, thought to underlie differences in somatosensory perception subject to attention and degree of consciousness. Furthermore, PO loses the ability to downregulate its activity level in some forms of chronic pain, suggesting that regulatory mechanisms underlying the normal modulation of PO activity may be pathologically altered. However, the mechanisms responsible for regulating such a wide dynamic range of activity are unknown. Here, we test a series of hypotheses regarding the function of several presynaptic receptors on both GABAergic and glutamatergic afferents targeting PO in mouse, using acute slice electrophysiology. We found that presynaptic GABA_B receptors are present on both GABAergic and glutamatergic terminals in PO, but only those on GABAergic terminals are tonically active. We also found that release from GABAergic terminals, but not glutamatergic terminals, is suppressed by cholinergic activation and that a subpopulation of GABAergic terminals is regulated by cannabinoids. Finally, we discovered the presence of tonic currents mediated by extrasynaptic GABA_A receptors in PO that are heterogeneously distributed across the nucleus. Thus we demonstrate that multiple regulatory mechanisms concurrently exist in PO, and we propose that regulation of inhibition, rather than excitation, is the more consequential mechanism by which PO activity can be regulated.NEW & NOTEWORTHY The posterior nucleus of thalamus (PO) is a key sensorimotor structure, whose activity is tightly regulated by inhibition from several nuclei. Maladaptive plasticity in this inhibition leads to severe pathologies, including chronic pain. We reveal here, for the first time in PO, multiple regulatory mechanisms that modulate synaptic transmission within PO. These findings may lead to targeted therapies for chronic pain and other disorders.

Genetic Tracing of Cav3.2 T-Type Calcium Channel Expression in the Peripheral Nervous System

Characterizing the distinct functions of the T-type ion channel subunits Ca_v3.1, 3.2 or 3.3 has proven difficult due to their highly conserved amino-acid sequences and the lack of pharmacological blockers specific for each subunit. To precisely determine the expression pattern of the Ca_v3.2 channel in the nervous system we generated two knock-in mouse strains that express EGFP or Cre recombinase under the control of the Ca_v3.2 gene promoter. We show that in the brains of these animals, the Ca_v3.2 channel is predominantly expressed in the dentate gyrus of the hippocampus. In the peripheral nervous system, the activation of the promoter starts at E9.5 in neural crest cells that will give rise to dorsal root ganglia (DRG) neurons, but not sympathetic neurons. As development progresses the number of DRG cells expressing the Ca_v3.2 channel reaches around 7% of the DRG at E16.5, and remains constant until E18.5. Characterization of sensory neuron subpopulations at E18.5 showed that EGFP⁺ cells are a heterogeneous population consisting mainly of TrkB⁺ and TrkC⁺ cells, while only a small percentage of DRG cells were TrkA⁺. Genetic tracing of the sensory nerve end-organ innervation of the skin showed that the activity of the Ca_v3.2 channel promoter in sensory progenitors marks many mechanoreceptor and nociceptor endings, but spares slowly adapting mechanoreceptors with endings associated with Merkel cells. Our genetic analysis reveals for the first time that progenitors that express the Ca_v3.2 T-type calcium channel, defines a sensory specific lineage that populates a large proportion of the DRG. Using our Ca_v3.2-Cre mice together with AAV viruses containing a conditional fluorescent reporter (tdTomato) we could also show that Cre expression is largely restricted to two functionally distinct sensory neuron types in the adult ganglia. Ca_v3.2 positive neurons innervating the skin were found to only form lanceolate endings on hair follicles and are probably identical to D-hair receptors. A second population of nociceptive sensory neurons expressing the Ca_v3.2 gene was found to be positive for the calcitonin-gene related peptide but these neurons are deep tissue nociceptors that do not innervate the skin.

Ion channels, ion channel receptors, and visceral hypersensitivity in irritable bowel syndrome

Ion channels are expressed throughout the gastrointestinal system and regulate nearly every aspect of digestion, including fluid secretion and absorption, motility, and visceral sensitivity. It is therefore not surprising that in the setting of functional bowel disorders, such as irritable bowel syndrome (IBS), ion channels are often altered in terms of expression level and function and are a target of pharmacological intervention. This is particularly true of their role in driving abdominal pain through visceral hypersensitivity (VH), which is the main reason IBS patients seek medical care. In the study by Scanzi et al., in the current issue of this journal, they provide evidence that the T-type voltage-gated calcium channel (Ca_v ) Ca_v 3.2 is upregulated in human IBS patients, and is necessary for the induction of an IBS-like disease state in mice. In this mini-review, we will discuss the contribution of specific ion channels to VH in IBS, both in human patients and rodent models. We will also discuss how Ca_v 3.2 may play a role as an integrator of multiple environmental stimuli contributing toward VH.

Genetic alteration of the metal/redox modulation of Cav3.2 T-type calcium channel reveals its role in neuronal excitability

KEY POINTS

In this study, we describe a new knock-in (KI) mouse model that allows the study of the H191-dependent regulation of T-type Cav3.2 channels. Sensitivity to zinc, nickel and ascorbate of native Cav3.2 channels is significantly impeded in the dorsal root ganglion (DRG) neurons of this KI mouse. Importantly, we describe that this H191-dependent regulation has discrete but significant effects on the excitability properties of D-hair (down-hair) cells, a sub-population of DRG neurons in which Cav3.2 currents prominently regulate excitability. Overall, this study reveals that the native H191-dependent regulation of Cav3.2 channels plays a role in the excitability of Cav3.2-expressing neurons. This animal model will be valuable in addressing the potential in vivo roles of the trace metal and redox modulation of Cav3.2 T-type channels in a wide range of physiological and pathological conditions.

ABSTRACT

Cav3.2 channels are T-type voltage-gated calcium channels that play important roles in controlling neuronal excitability, particularly in dorsal root ganglion (DRG) neurons where they are involved in touch and pain signalling. Cav3.2 channels are modulated by low concentrations of metal ions (nickel, zinc) and redox agents, which involves the histidine 191 (H191) in the channel's extracellular IS3-IS4 loop. It is hypothesized that this metal/redox modulation would contribute to the tuning of the excitability properties of DRG neurons. However, the precise role of this H191-dependent modulation of Cav3.2 channel remains unresolved. Towards this goal, we have generated a knock-in (KI) mouse carrying the mutation H191Q in the Cav3.2 protein. Electrophysiological studies were performed on a subpopulation of DRG neurons, the D-hair cells, which express large Cav3.2 currents. We describe an impaired sensitivity to zinc, nickel and ascorbate of the T-type current in D-hair neurons from KI mice. Analysis of the action potential and low-threshold calcium spike (LTCS) properties revealed that, contrary to that observed in WT D-hair neurons, a low concentration of zinc and nickel is unable to modulate (1) the rheobase threshold current, (2) the afterdepolarization amplitude, (3) the threshold potential necessary to trigger an LTCS or (4) the LTCS amplitude in D-hair neurons from KI mice. Together, our data demonstrate that this H191-dependent metal/redox regulation of Cav3.2 channels can tune neuronal excitability. This study validates the use of this Cav3.2-H191Q mouse model for further investigations of the physiological roles thought to rely on this Cav3.2 modulation.

Suppression of Sleep Spindle Rhythmogenesis in Mice with Deletion of CaV3.2 and CaV3.3 T-type Ca(2+) Channels

STUDY OBJECTIVES

Low-threshold voltage-gated T-type Ca(2+) channels (T-channels or CaV3 channels) sustain oscillatory discharges of thalamocortical (TC) and nucleus Reticularis thalami (nRt) cells. The CaV3.3 subtype dominates nRt rhythmic bursting and mediates a substantial fraction of spindle power in the NREM sleep EEG. CaV3.2 channels are also found in nRt, but whether these contribute to nRt-dependent spindle generation is unexplored. We investigated thalamic rhythmogenesis in mice lacking this subtype in isolation (CaV3.2KO mice) or in concomitance with CaV3.3 deletion (CaV3.double-knockout (DKO) mice).

METHODS

We examined discharge characteristics of thalamic cells and intrathalamic evoked synaptic transmission in brain slices from wild-type, CaV3.2KO and CaV3.DKO mice through patch-clamp recordings. The sleep profile of freely behaving CaV3.2KO and CaV3.DKO mice was assessed by polysomnographic recordings.

RESULTS

CaV3.2 channel deficiency left nRt discharge properties largely unaltered, but additional deletion of CaV3.3 channels fully abolished low-threshold whole-cell Ca(2+) currents and bursting, and suppressed burst-mediated inhibitory responses in TC cells. CaV3.DKO mice had more fragmented sleep, with shorter NREM sleep episodes and more frequent microarousals. The NREM sleep EEG power spectrum displayed a relative suppression of the σ frequency band (10-15 Hz), which was accompanied by an increase in the δ band (1-4 Hz).

CONCLUSIONS

Consistent with previous findings, CaV3.3 channels dominate nRt rhythmogenesis, but the lack of CaV3.2 channels further aggravates neuronal, synaptic, and EEG deficits. Therefore, CaV3.2 channels can boost intrathalamic synaptic transmission, and might play a modulatory role adjusting the relative presence of NREM sleep EEG rhythms.

Ion Homeostasis in Rhythmogenesis: The Interplay Between Neurons and Astroglia

Proper function of all excitable cells depends on ion homeostasis. Nowhere is this more critical than in the brain where the extracellular concentration of some ions determines neurons' firing pattern and ability to encode information. Several neuronal functions depend on the ability of neurons to change their firing pattern to a rhythmic bursting pattern, whereas, in some circuits, rhythmic firing is, on the contrary, associated to pathologies like epilepsy or Parkinson's disease. In this review, we focus on the four main ions known to fluctuate during rhythmic firing: calcium, potassium, sodium, and chloride. We discuss the synergistic interactions between these elements to promote an oscillatory activity. We also review evidence supporting an important role for astrocytes in the homeostasis of each of these ions and describe mechanisms by which astrocytes may regulate neuronal firing by altering their extracellular concentrations. A particular emphasis is put on the mechanisms underlying rhythmogenesis in the circuit forming the central pattern generator (CPG) for mastication and other CPG systems. Finally, we discuss how an impairment in the ability of glial cells to maintain such homeostasis may result in pathologies like epilepsy and Parkinson's disease.

Thalamic Kv 7 channels: pharmacological properties and activity control during noxious signal processing

BACKGROUND AND PURPOSE

The existence of functional K(v)7 channels in thalamocortical (TC) relay neurons and the effects of the K(+)-current termed M-current (I(M)) on thalamic signal processing have long been debated. Immunocytochemical evidence suggests their presence in this brain region. Therefore, we aimed to verify their existence, pharmacological properties and function in regulating activity in neurons of the ventrobasal thalamus (VB).

EXPERIMENTAL APPROACH

Characterization of K(v)7 channels was performed by combining in vitro, in vivo and in silico techniques with a pharmacological approach. Retigabine (30 μM) and XE991 (20 μM), a specific K(v)7 channel enhancer and blocker, respectively, were applied in acute brain slices during electrophysiological recordings. The effects of intrathalamic injection of retigabine (3 mM, 300 nL) and/or XE991 (2 mM, 300 nL) were investigated in freely moving animals during hot-plate tests by recording behaviour and neuronal activity.

KEY RESULTS

K(v)7.2 and K(v)7.3 subunits were found to be abundantly expressed in TC neurons of mouse VB. A slow K(+)-current with properties of IM was activated by retigabine and inhibited by XE991. K(v)7 channel activation evoked membrane hyperpolarization, a reduction in tonic action potential firing, and increased burst firing in vitro and in computational models. Single-unit recordings and pharmacological intervention demonstrated a specific burst-firing increase upon I(M) activation in vivo. A K(v)7 channel-mediated increase in pain threshold was associated with fewer VB units responding to noxious stimuli, and increased burst firing in responsive neurons.

CONCLUSIONS AND IMPLICATIONS

K(v)7 channel enhancement alters somatosensory activity and may reflect an anti-nociceptive mechanism during acute pain processing.

Alleviation of neuropathic pain by regulating T-type calcium channels in rat anterior cingulate cortex

Background

It has been demonstrated that administration of T-type calcium channel (TCC) inhibitors could relieve the neuropathic pain by intraperitoneally or intrathecally. TCCs are not only expressed in dorsal root ganglia or dorsal horn, but also in some of the pain associated brain regions. In the present study, we sought to investigate whether modulating TCCs in the anterior cingulate cortex (ACC) could alleviate the neuropathic pain.

Results

(1) Ca_v3.2 was up regulated in rat ACC after chronic constriction injury (CCI). (2) T-type calcium current intensity was increased in CCI animal model. (3) TCC inhibitor reduced miniature excitatory postsynaptic currents frequency of ACC neurons in CCI animal model. (4) TCC inhibitor suppressed the firing rate of ACC neurons in CCI animal model. (5) Both mechanical and thermal allodynia were partially relieved by ACC microinjection with TCC inhibitor.

Conclusions

TCCs in the ACC may be contributing to the maintenance of neuropathic pain, and the neuropathic pain can be alleviated by inhibiting the neuronal activity of ACC through modulating the TCCs.

Thalamic reticular cells firing modes and its dependency on the frequency and amplitude ranges of the current stimulus

The neurons of the Thalamic Reticular Nucleus (TRNn) respond to inputs in two activity modes called burst and tonic firing and both can be observed in different physiological states. The functional states of the thalamus depend in part on the properties of synaptic transmission between the TRNn and the thalamocortical and corticothalamic neurons. A dendrite can receive inhibitory and excitatory postsynaptic potentials. The novelties presented in this paper can be summarized as follows: First, it shows, through a computational simulation, that the burst and tonic firings observed in the TRNn soma could be explained as a product of random synaptic inputs on the distal dendrites, the tonic firings are generated by random excitatory stimuli, and the burst firings are generated by two different types of stimuli: inhibitory random stimuli, and a combination of inhibitory (from TRNn) and excitatory (from corticothalamic and thalamocortical neurons) random stimuli; second, according to in vivo recordings, we have found that the burst observed in the TRNn soma has graduate properties that are proportional to the stimuli frequency; and third, a novel method for showing in a quantitative manner the accelerando-decelerando pattern is proposed.

Functional expression of T-type Ca2+ channels in spinal motoneurons of the adult turtle

Voltage-gated Ca²⁺ (Ca_V) channels are transmembrane proteins comprising three subfamilies named Ca_V1, Ca_V2 and Ca_V3. The Ca_V3 channel subfamily groups the low-voltage activated Ca²⁺ channels (LVA or T-type) a significant role in regulating neuronal excitability. Ca_V3 channel activity may lead to the generation of complex patterns of action potential firing such as the postinhibitory rebound (PIR). In the adult spinal cord, these channels have been found in dorsal horn interneurons where they control physiological events near the resting potential and participate in determining excitability. In motoneurons, Ca_V3 channels have been found during development, but their functional expression has not yet been reported in adult animals. Here, we show evidence for the presence of Ca_V3 channel-mediated PIR in motoneurons of the adult turtle spinal cord. Our results indicate that Ni²⁺ and NNC55-0396, two antagonists of Ca_V3 channel activity, inhibited PIR in the adult turtle spinal cord. Molecular biology and biochemical assays revealed the expression of the Ca_V3.1 channel isotype and its localization in motoneurons. Together, these results provide evidence for the expression of Ca_V3.1 channels in the spinal cord of adult animals and show also that these channels may contribute to determine the excitability of motoneurons.

The role of T-type calcium channel genes in absence seizures

The thalamic relay neurons, reticular thalamic nucleus, and neocortical pyramidal cells form a circuit that sustains oscillatory burst firing, and is regarded as the underlying mechanism of absence seizures. T-type calcium channels play a key role in this circuit. Here, we review the role of T-type calcium channel genes in the development of absence seizures, and emphasize gain or loss of function mutations, and other variations that alter both quantity and quality of transcripts, and methylation status of isoforms of T-type calcium channel proteins might be of equal importance in understanding the pathological mechanism of absence seizures.

Gabapentin reverses central pain sensitization following a collagenase-induced intrathalamic hemorrhage in rats

PURPOSE

The treatment of central neuropathic pain remains amongst the biggest challenges for pain specialists. The main objective of this study was to assess gabapentin (GBP), amitriptyline (AMI), and carbamazepine (CARBA) for the treatment of a rodent central neuropathic pain model.

METHODS

Male Sprague Dawley rats were trained on the rotarod, Hargreaves, Von Frey and acetone behavioral tests, and baseline values were obtained prior to surgery. A stereotaxic injection of either a collagenase solution or saline was made in the right ventral posterolateral thalamic nucleus. The rats were tested on days 2, 4, 8, and 11 postsurgery. They were retested at regular intervals from day 15 to day 25 postsurgery, after oral administration of either the vehicle (n=7 and n=8 rats with intracerebral injections of collagenase and saline, respectively) or the different drugs (GBP [60 mg/kg], AMI [10 mg/kg], CARBA [100 mg/kg]; n=8 rats/drug).

RESULTS

A significant decrease in the mechanical thresholds and no change in heat threshold were observed in both hind limbs in the collagenase group, as we had previously shown elsewhere. Reversal of the mechanical hypersensitivity was achieved only with GBP (P<0.05). AMI and CARBA, at the dosages used, failed to show any effect on mechanical thresholds. Transient cold allodynia was observed in some collagenase-injected rats but failed to be statistically significant.

CONCLUSION

Intrathalamic hemorrhaging in the ventrolateral thalamic nucleus induced a bilateral mechanical allodynia, which was reversed by GBP but not AMI or CARBA.

T-type calcium channels: functional regulation and implication in pain signaling

Low-voltage-activated T-type Ca(2+) channels (T-channels), especially Cav3.2 among the three isoforms (Cav3.1, Cav3.2, and Cav3.3), are now considered to play pivotal roles in processing of pain signals. Cav3.2 T-channels are functionally modulated by extracellular substances such as hydrogen sulfide and ascorbic acid, by intracellular signaling molecules including protein kinases, and by glycosylation. Cav3.2 T-channels are abundantly expressed in both peripheral and central endings of the primary afferent neurons, regulating neuronal excitability and release of excitatory neurotransmitters such as substance P and glutamate, respectively. Functional upregulation of Cav3.2 T-channels is involved in the pathophysiology of inflammatory, neuropathic, and visceral pain. Thus, Cav3.2 T-channels are considered to serve as novel targets for development of drugs for treatment of intractable pain resistant to currently available analgesics.

The calcium-binding protein parvalbumin modulates the firing 1 properties of the reticular thalamic nucleus bursting neurons

The reticular thalamic nucleus (RTN) of the mouse is characterized by an overwhelming majority of GABAergic neurons receiving afferences from both the thalamus and the cerebral cortex and sending projections mainly on thalamocortical neurons. The RTN neurons express high levels of the "slow Ca(2+) buffer" parvalbumin (PV) and are characterized by low-threshold Ca(2+) currents, I(T). We performed extracellular recordings in ketamine/xylazine anesthetized mice in the rostromedial portion of the RTN. In the RTN of wild-type and PV knockout (PVKO) mice we distinguished four types of neurons characterized on the basis of their firing pattern: irregular firing (type I), medium bursting (type II), long bursting (type III), and tonically firing (type IV). Compared with wild-type mice, we observed in the PVKOs the medium bursting (type II) more frequently than the long bursting type and longer interspike intervals within the burst without affecting the number of spikes. This suggests that PV may affect the firing properties of RTN neurons via a mechanism associated with the kinetics of burst discharges. Ca(v)3.2 channels, which mediate the I(T) currents, were more localized to the somatic plasma membrane of RTN neurons in PVKO mice, whereas Ca(v)3.3 expression was similar in both genotypes. The immunoelectron microscopy analysis showed that Ca(v)3.2 channels were localized at active axosomatic synapses, thus suggesting that the differential localization of Ca(v)3.2 in the PVKOs may affect bursting dynamics. Cross-correlation analysis of simultaneously recorded neurons from the same electrode tip showed that about one-third of the cell pairs tended to fire synchronously in both genotypes, independent of PV expression. In summary, PV deficiency does not affect the functional connectivity between RTN neurons but affects the distribution of Ca(v)3.2 channels and the dynamics of burst discharges of RTN cells, which in turn regulate the activity in the thalamocortical circuit.

Differential effects of methylphenidate and cocaine on GABA transmission in sensory thalamic nuclei

Methylphenidate (MPH) is widely used to treat children and adolescents diagnosed with attention deficit/hyperactivity disorder. Although MPH shares mechanistic similarities to cocaine, its effects on GABAergic transmission in sensory thalamic nuclei are unknown. Our objective was to compare cocaine and MPH effects on GABAergic projections between thalamic reticular and ventrobasal (VB) nuclei. Mice (P18-30) were subjected to binge-like cocaine and MPH acute and sub-chronic administrations. Cocaine and MPH enhanced hyperlocomotion, although sub-chronic cocaine-mediated effects were stronger than MPH effects. Cocaine and MPH sub-chronic administration altered paired-pulse and spontaneous GABAergic input differently. The effects of cocaine on evoked paired-pulse GABA-mediated currents changed from depression to facilitation with the duration of the protocols used, while MPH induced a constant increase throughout the administration protocols. Thalamic reticular nucleus GAD67 and VB Ca(V) 3.1 protein levels were measured using western blot to better understand their link to increased GABA release. Both proteins were increased by sub-chronic administration of cocaine. MPH showed effects on GABAergic transmission that seems less disruptive than cocaine. Unique effects of cocaine on postsynaptic VB calcium currents might explain deleterious cocaine effects on sensory thalamic nuclei. These results suggest that cocaine and MPH produced distinct presynaptic alterations on GABAergic transmission.

State-dependent properties of a new T-type calcium channel blocker enhance Ca(V)3.2 selectivity and support analgesic effects

T-type calcium channels encoded by the Ca(V)3.2 isoform are expressed in nociceptive primary afferent neurons where they contribute to hyperalgesia and thus are considered as a potential therapeutic target to treat pathological pain. Here we report that the small organic state-dependent T-type channel antagonist TTA-A2 efficiently inhibits recombinant and native Ca(V)3.2 currents. Although TTA-A2 is a pan Ca(V)3 blocker, it demonstrates a higher potency for Ca(V)3.2 compared to Ca(V)3.1. TTA-A2 selectivity for T-type currents was demonstrated in sensory neurons where it lowered cell excitability uniquely on neurons expressing T-type channels. In vivo pharmacology in Ca(V)3.2 knockout and wild type mice reveal that TTA-A2-mediated antinociception critically depends on Ca(V)3.2 expression. The pathophysiology of irritable bowel syndrome (IBS) was recently demonstrated to involve Ca(V)3.2 in a rat model of this disease. Oral administration of TTA-A2 produced a dose-dependent reduction of hypersensitivity in an IBS model, demonstrating its therapeutic potential for the treatment of pathological pain. Overall, our results suggest that the high potency of TTA-A2 in the depolarized state strengthen its analgesic efficacy and selectivity toward pathological pain syndromes. This characteristic would be beneficial for the development of analgesics targeting T-type channels, in particular for the treatment of pain associated with IBS.