Characterization of acute somatosensory pain transmission in P/Q-type Ca(2+) channel mutant mice, leaner

Differential cerebellar GABAA receptor expression in mice with mutations in CaV2.1 (P/Q-type) calcium channels

Ataxia is the predominant clinical manifestation of cerebellar dysfunction. Mutations in the human CACNA1A gene, encoding the pore-forming α1 subunit of CaV2.1 (P/Q-type) calcium channels, underlie several neurological disorders, including Episodic Ataxia type 2 and Familial Hemiplegic Migraine type 1 (FHM1). Several mouse mutants exist that harbor mutations in the orthologous Cacna1a gene. The spontaneous Cacna1a mutants Rolling Nagoya (tg(rol)), Tottering (tg) and Leaner (tg(ln)) mice exhibit behavioral motor phenotypes, including ataxia. Transgenic knock-in (KI) mouse strains with the human FHM1 R192Q and S218L missense mutations have been generated. R192Q KI mice are non-ataxic, whereas S218L KI mice display a complex behavioral phenotype that includes cerebellar ataxia. Given the dependence of γ-aminobutyric acid type A (GABAA) receptor subunit functioning on localized calcium currents, and the functional link between GABAergic inhibition and ataxia, we hypothesized that cerebellar GABAA receptor expression is differentially affected in Cacna1a mutants and contributes to the ataxic phenotype. Herein we quantified functional GABAA receptors and pharmacologically dissociated cerebellar GABAA receptors in several Cacna1a mutants. We did not identify differences in the expression of GABAA receptor subunits or in the number of functional GABAA receptors in the non-ataxic R192Q KI strain. In contrast, tg(rol) mice had a ∼15% decrease in the number of functional GABAA receptors, whereas S218L KI mice showed a ∼29% increase. Our data suggest that differential changes in cerebellar GABAA receptor expression profile may contribute to the neurological phenotype of cerebellar ataxia and that targeting GABAA receptors might represent a feasible complementary strategy to treat cerebellar ataxia.

GABAA Receptor Expression in the Forebrain of Ataxic Rolling Nagoya Mice

The human CACNA1A gene encodes the pore-forming α₁ subunit of Ca_V2.1 (P/Q-type) calcium channels and is the locus for several neurological disorders, including episodic ataxia type 2 (EA2), spinocerebellar ataxia type 6 (SCA6) and Familial Hemiplegic Migraine type 1 (FHM1). Several spontaneous mouse Cacna1a mutant strains exist, among them Rolling Nagoya (tg^rol), carrying the R1262G point mutation in the mouse Cacna1a gene. tg^rol mice display a phenotype of severe gait ataxia and motor dysfunction of the hind limbs. At the functional level, the R1262G mutation results in a positive shift of the activation voltage of the Ca_V2.1 channel and reduced current density. γ-Aminobutyric acid type A (GABA_A) receptor subunit expression depends critically on neuronal calcium influx, and GABA_A receptor dysfunction has previously been described for the cerebellum of tg^rol and other ataxic Cacna1a mutant mice. Given the expression pattern of Ca_V2.1, it was hypothesized that calcium dysregulation in tg^rol might affect GABA_A receptor expression in the forebrain. Herein, functional GABA_A receptors in the forebrain of tg^rol mice were quantified and pharmacologically dissociated using [³H] radioligand binding. No gross changes to functional GABA_A receptors were identified. Future cell type-specific analyses are required to identify possible cortical contributions to the psychomotor phenotype of tg^rol mice.

Pharmacological Inhibition of Voltage-gated Ca(2+) Channels for Chronic Pain Relief

Chronic pain is a major therapeutic problem as the current treatment options are unsatisfactory with low efficacy and deleterious side effects. Voltage-gated Ca2+ channels (VGCCs), which are multi-complex proteins consisting of α1, β, γ, and α2δ subunits, play an important role in pain signaling. These channels are involved in neurogenic inflammation, excitability, and neurotransmitter release in nociceptors. It has been previously shown that N-type VGCCs (Cav2.2) are a major pain target. U.S. FDA approval of three Cav2.2 antagonists, gabapentin, pregabalin, and ziconotide, for chronic pain underlies the importance of this channel subtype. Also, there has been increasing evidence that L-type (Cav1.2) or T-type (Cav3.2) VGCCs may be involved in pain signaling and chronic pain. In order to develop novel pain therapeutics and to understand the role of VGCC subtypes, discovering subtype selective VGCC inhibitors or methods that selectively target the inhibitor into nociceptors would be essential. This review describes the various VGCC subtype inhibitors and the potential of utilizing VGCC subtypes as targets of chronic pain. Development of VGCC subtype inhibitors and targeting them into nociceptors will contribute to a better understanding of the roles of VGCC subtypes in pain at a spinal level as well as development of a novel class of analgesics for chronic pain.

Ionotropic glutamate receptors and voltage-gated Ca²⁺ channels in long-term potentiation of spinal dorsal horn synapses and pain hypersensitivity

Over the last twenty years of research on cellular mechanisms of pain hypersensitivity, long-term potentiation (LTP) of synaptic transmission in the spinal cord dorsal horn (DH) has emerged as an important contributor to pain pathology. Mechanisms that underlie LTP of spinal DH neurons include changes in the numbers, activity, and properties of ionotropic glutamate receptors (AMPA and NMDA receptors) and of voltage-gated Ca²⁺ channels. Here, we review the roles and mechanisms of these channels in the induction and expression of spinal DH LTP, and we present this within the framework of the anatomical organization and synaptic circuitry of the spinal DH. Moreover, we compare synaptic plasticity in the spinal DH with classical LTP described for hippocampal synapses.

Progress in genetic studies of pain and analgesia

Interindividual variability in pain sensitivity and the response to analgesic manipulations remains a considerable clinical challenge as well as an area of intense scientific investigation. Techniques in this field have matured rapidly so that much relevant data have emerged only in the past few years. Our increasing understanding of the genetic mediation of these biological phenomena have nonetheless revealed their surprising complexity. This review provides a comprehensive picture and critical analysis of the field and its prospects.

Hypoalgesic behaviors of P/Q-type voltage-gated Ca2+ channel mutant mouse, rolling mouse Nagoya

Rolling mouse Nagoya (tg(rol)) is a spontaneously occurring P/Q-type voltage-gated Ca2+ channel (VGCC) mutant mouse. A P/Q-type VGCC with the tg(rol) mutation has lower voltage sensitivity of activation, and mice with a homozygous genotype (tg(rol)/tg(rol)) but not with a heterozygous genotype (tg(rol)/+) show impaired motor coordination of the hind limbs. To investigate the roles of P/Q-type VGCC in pain sensing mechanisms, behavioral responses of adult tg(rol) mice to thermal, mechanical and chemical nociceptive stimuli were examined by the plantar, tail-flick, von Frey and formalin tests. The latency of the withdrawal response to thermal stimuli in the plantar or tail-flick tests was significantly longer in tg(rol)/tg(rol) mice than in tg(rol)/+ and wild-type (+/+) mice, and in tg(rol)/+ mice than in +/+ mice. The withdrawal response to mechanical stimuli in the von Frey test was lower in tg(rol)/tg(rol) mice than in +/+ mice. Although the licking time during the first 5 min after the formalin injection was similar among all of the three genotypes, that during 5-60 min was significantly shorter in tg(rol)/tg(rol) mice than in tg(rol)/+ and +/+ mice, and in tg(rol)/+ mice than in +/+ mice. Artificial inflammation induced by injection of complete Freund's adjuvant (CFA) into a hind paw significantly enhanced the withdrawal response recorded in the plantar and von Frey tests regardless of the mouse genotype. The CFA-enhanced response in the tg(rol)/tg(rol) mice was similar to the response in +/+ mice without the CFA injection. These results suggest that tg(rol) mutant mice show hypoalgesic responses caused by a lower sensitivity to nociceptive thermal, mechanical and chemical stimuli. It is concluded that the P/Q-type VGCC has a pro-nociceptive role and that the tg(rol) mutant mouse may be a useful tool to investigate the role of the P/Q-type VGCC in pain sensing mechanisms.

Changes in osmolality modulate voltage-gated calcium channels in trigeminal ganglion neurons

Voltage-gated calcium channels (VGCCs) participate in many important physiological functions. However whether VGCCs are modulated by changes of osmolarity and involved in anisotonicity-induced nociception is still unknown. For this reason by using whole-cell patch clamp techniques in rat and mouse trigeminal ganglion (TG) neurons we tested the effects of hypo- and hypertonicity on VGCCs. We found that high-voltage-gated calcium current (I(HVA)) was inhibited by both hypo- and hypertonicity. In rat TG neurons, the inhibition by hypotonicity was mimicked by Transient Receptor Potential Vanilloid 4 receptor (TRPV4) activator but hypotonicity did not exhibit inhibition in TRPV4(-/-) mice TG neurons. Concerning the downstream signaling pathways, antagonism of PKG pathway selectively reduced the hypotonicity-induced inhibition, whereas inhibition of PLC- and PI3K-mediated pathways selectively reduced the inhibition produced by hypertonicity. In summary, although the effects of hypo- and hypertonicity show similar phenotype, receptor and intracellular signaling pathways were selective for hypo- versus hypertonicity-induced inhibition of I(HVA).

Motor Deficits in Homozygous and Heterozygous P/Q-Type Calcium Channel Mutants

P/Q-type voltage-dependent Ca2+channels (VDCCs) are highly expressed in the cerebellum, and mutations of these channels are associated with disrupted motor function. Several allelic variants of the α1A pore-forming subunit of P/Q-type VDCCs have been described, and mice homozygous for these mutations exhibit gait ataxia, as do α1A knockout mice. Here we report that heterozygous α1A mutants also have a motor phenotype. Mice heterozygous for the leaner and α1A knockout mutations exhibit impaired motor learning in the vestibulo-ocular reflex (VOR), suggesting that subtle disruption of P/Q Ca2+currents is sufficient to disrupt motor function. Basal VOR and optokinetic reflex performance were normal in the heterozygotes but severely impaired in the leaner and α1A knockout homozygotes.

Pain sensitivity in mice lacking the Ca(v)2.1alpha1 subunit of P/Q-type Ca2+ channels

The role of voltage-gated Ca(2+) (Ca(V)) channels in pain mechanisms has been the object of intense investigation using pharmacological approaches and, more recently, using mutant mouse models lacking the Ca(V)alpha(l) pore-forming subunit of N-, R- and T-type channels. The role of P/Q-type channels in nociception and pain transmission has been investigated by pharmacological approaches but remains to be fully elucidated. To address this issue, we have analyzed pain-related behavioral responses of null mutant mice for the Ca(V)2.1alpha(1) subunit of P/Q-type channels. Homozygous null mutant Ca(V)2.1alpha(1)-/- mice developed dystonia at 10-12 days after birth and did not survive past weaning. Tested at ages where motor deficit was either absent or very mild, Ca(V)2.1alpha(1)-/- mice showed reduced tail withdrawal latencies in the tail-flick test and reduced abdominal writhes in the acetic acid writhing test. Adult heterozygous Ca(V)2.1alpha(1)+/- mice did not show motor deficits in the rotarod and activity cage tests and did not show alterations in pain responses in the tail-flick test and the acetic acid writhing test. Strikingly, they showed a reduced licking response during the second phase of formalin-induced inflammatory pain and a reduced mechanical allodynia in the chronic constriction injury model of neuropathic pain. Our findings show that P/Q-type channels play an antinociceptive role in sensitivity to non-injurious noxious thermal stimuli and a pronociceptive role in inflammatory and neuropathic pain states, pointing to an important role of Ca(V)2.1 channels in central sensitization.

Function and dysfunction of synaptic calcium channels: insights from mouse models

In the past few years several spontaneous or engineered mouse models with mutations in Ca2+ channel genes have become available, providing a powerful approach to defining Ca2+ channel function in vivo. There have been recent advances in outlining the phenotypes and in the functional analysis of mouse models with mutations in genes encoding the pore-forming subunits of Ca(V)2.1 (P/Q-type), Ca(V)2.2 (N-type) and Ca(V)2.3 (R-type) Ca2+ channels, the channels involved in controlling neurotransmitter release at mammalian synapses. These data indicate that Ca(V)2.1 channels have a dominant and efficient specific role in initiating fast synaptic transmission at central excitatory synapses in vivo, and suggest that the Ca(V)2.1 channelopathies are primarily synaptic diseases. The different disorders probably arise from disruption of neurotransmission in specific brain regions: the cortex in the case of migraine, the thalamus in the case of absence epilepsy and the cerebellum in the case of ataxia.

Blocking the R-type (Cav2.3) Ca2+ channel enhanced morphine analgesia and reduced morphine tolerance

Morphine is the drug of choice to treat intractable pain, although prolonged administration often causes undesirable side-effects including analgesic tolerance. It is speculated that voltage-dependent Ca(2+) channels (VDCCs) play a key role in morphine analgesia and tolerance. To examine the subtype specificity of VDCCs in these processes, we analysed mice lacking N-type (Ca(v)2.2) or R-type (Ca(v)2.3) VDCCs. Systemic morphine administration or exposure to warm water swim-stress, known to induce endogenous opioid release, resulted in greater analgesia in Ca(v)2.3(-/-) mice than in controls. Moreover, Ca(v)2.3(-/-) mice showed resistance to morphine tolerance. In contrast, Ca(v)2.2(-/-) mice showed similar levels of analgesia and tolerance to control mice. Intracerebroventricular (i.c.v.) but not intrathecal (i.t.) administration of morphine reproduced the result of systemic morphine in Ca(v)2.3(-/-) mice. Furthermore, i.c.v. administration of an R-type channel blocker potentiated morphine analgesia in wild-type mice. Thus, the inhibition of R-type Ca(2+) current could lead to high-efficiency opioid therapy without tolerance.

Effects of familial hemiplegic migraine type 1 mutations on neuronal P/Q-type Ca2+ channel activity and inhibitory synaptic transmission

Inhibitory synapses play key roles in the modulatory circuitry that regulates pain signaling and generation of migraine headache. A rare, dominant form of this common disease, familial hemiplegic migraine type 1 (FHM1), arises from missense mutations in the pore-forming alpha1A subunit of P/Q-type Ca2+ channels. These channels are normally vital for presynaptic Ca2+ entry and neurotransmitter release at many central synapses, raising questions about effects of FHM1 mutations on neuronal Ca2+ influx and inhibitory and excitatory neurotransmission. We have expressed the four original FHM1 mutant channels in hippocampal neurons from alpha1A knockout mice. Whole-cell recordings indicated that FHM1 mutant channels were less effective than wild-type channels in their ability to conduct P/Q-type current, but not generally different from wild type in voltage-dependent channel gating. Ca2+ influx triggered by action potential waveforms was also diminished. In keeping with decreased channel activity, FHM1 mutant channels were correspondingly impaired in supporting the P/Q-type component of inhibitory neurotransmission. When expressed in wild-type inhibitory neurons, FHM1 mutant channels reduced the contribution of P/Q-type channels to GABAergic synaptic currents, consistent with a competition of mutant and endogenous channels for P/Q-specific slots. In all cases, N-type channels took up the burden of supporting transmission and homeostatic mechanisms maintained overall synaptic strength. The shift to reliance on N-type channels greatly increased the susceptibility to G protein-coupled modulation of neurotransmission, studied with the GABAB agonist baclofen. Thus, mutant-expressing synapses might be weakened in a heightened state of neuromodulation like that provoked by triggers of migraine such as stress.

Posttranslational mechanisms of peripheral sensitization

The sensation of pain can be dramatically altered in response to injury or disease. This sensitization can occur at the level of the primary sensory neuron, and can be mediated by multiple biochemical mechanisms, including, but not limited to, changes in gene transcription, changes in translation, stability, or subcellular localization of translated proteins, and posttranslational modifications. This review focuses on posttranslational modifications to ion channels expressed in primary sensory neurons that form the machinery driving peripheral sensitization and pain hypersensitivity. Studies published to date show strong evidence for modulation of ion channels involved in transduction and transmission of nociceptive inputs coincident with biophysical and behavioral sensitization. The roles of phosphorylation and oxidation/reduction reactions of voltage-dependent sodium, potassium, and calcium channels are discussed, as well as phosphorylation-mediated modulation of sensory transduction channels.

Pre- and postsynaptic contributions of voltage-dependent Ca2+ channels to nociceptive transmission in rat spinal lamina I neurons

Activation of voltage-dependent Ca2+ channels (VDCCs) is critical for neurotransmitter release, neuronal excitability and postsynaptic Ca2+ signalling. Antagonists of VDCCs can be antinociceptive in different animal pain models. Neurons in lamina I of the spinal dorsal horn play a pivotal role in the processing of pain-related information, but the role of VDCCs to the activity-dependent Ca2+ increase in lamina I neurons and to the synaptic transmission between nociceptive afferents and second order neurons in lamina I is not known. This has now been investigated in a lumbar spinal cord slice preparation from young Sprague-Dawley rats. Microfluorometric Ca2+ measurements with fura-2 have been used to analyse the Ca2+ increase in lamina I neurons after depolarization of the cells, resulting in a distinct and transient increase of the cytosolic Ca2+ concentration. This Ca2+ peak was reduced by the T-type channel blocker, Ni2+, by the L-type channel blockers, nifedipine and verapamil, and by the N-type channel blocker, omega-conotoxin GVIA. The P/Q-type channel antagonist, omega-agatoxin TK, had no effect on postsynaptic [Ca2+]i. The NMDA receptor channel blocker D-AP5 reduced the Ca2+ peak, whereas the AMPA receptor channel blocker CNQX had no effect. Postsynaptic currents, monosynaptically evoked by electrical stimulation of the attached dorsal roots with C-fibre and Adelta-fibre intensity, respectively, were reduced by N-type channel blocker omega-conotoxin GVIA and to a much lesser extent, by P/Q-type channel antagonist omega-agatoxin TK, and the L-type channel blockers verapamil, respectively. No difference was found between unidentified neurons and neurons projecting to the periaqueductal grey matter. This is the first quantitative description of the relative contribution of voltage-dependent Ca2+ channels to the synaptic transmission in lamina I of the spinal dorsal horn, which is essential in the processing of pain-related information in the central nervous system.

Spinally delivered N-, P/Q- and L-type Ca2+-channel blockers potentiate morphine analgesia in mice

We studied the antinociceptive effects induced at the spinal level by N-, P/Q- and L-type voltage-dependent Ca2+-channel (VDCC) blockers given alone or in combination with morphine, the test responses being the algesic ones induced by acute thermal and mechanical stimuli. When given alone, intrathecal omega-agatoxin IVA (P/Q-type blocker) produced a potent dose-dependent inhibition in the tail-flick and tail-pressure over the dose range 0.33-33 pmol/mouse. Omega-conotoxin GVIA (N-type blocker) also produced dose-dependent inhibitions, but its antinociception against thermal stimuli was weaker than against mechanical stimuli. Calciseptine (L-type blocker) slightly reduced both nociceptive responses, but only at 33 pmol. At their subthreshold doses, intrathecal omega-agatoxin IVA, omega-conotoxin GVIA and calciseptine each significantly enhanced morphine analgesia in the tail-flick and tail-pressure tests, the rank order of potencies being N-> or =P/Q->L-type. These results indicate that combining a low-dose VDCC blocker, especially the N- or P/Q-type, with morphine may be a very useful way of minimizing the dose of morphine and may reduce side effects.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Insights from mouse models of absence epilepsy into Ca2+ channel physiology and disease etiology

1. Changes in intracellular Ca2+ ([Ca2+]i) levels provide signals that allow neurons to respond to a host of external stimuli. A major mechanism for elevating [Ca2+]i is the influx of extracellular Ca2+ through voltage-gated channels (Ca(V)) in the plasma membrane. Malfunction in Ca(V) due to mutations in genes encoding channel proteins are increasingly being implicated in causing disease conditions, termed channelopathies. 2. Seven spontaneous mutations with cerebellar ataxia and generalized absence epilepsy have been identified in mice (tottering, leaner, rolling Nagoya, rocker, lethargic, ducky, and stargazer), and these overlapping phenotypes are directly related to mutations in genes encoding the four separate subunits that together form the multimeric neuronal Ca(V) complex. 3. The discovery and systematic analysis of these animal models is helping to clarify how different mutations affect channel function and how altered channel function produces disease.