Channels
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The antianginal drug ranolazine exerts voltage- and use-dependent block (UDB) of several Na+ channel isoforms, including Na(v) 1.4. We hypothesized that ranolazine will similarly inhibit the paramyotonia congenita Na(v) 1.4 gain-of-function mutations, R1448C, R1448H, and R1448P that are associated with repetitive action potential firing. Whole-cell Na+ current (I(Na)) was recorded from HEK293 cells expressing the hNa(v) 1.4 WT or R1448 mutations. ⋯ In computer simulations, 3 µM ranolazine inhibited the sustained and excessive firing of skeletal muscle action potentials that are characteristic of myotonia. Taken together, the data indicate that ranolazine interacts with the open state and stabilizes the inactivated state(s) of Na(v)1.4 channels, causes voltage- and use-dependent block of I(Na) and suppresses persistent I(Na). These data further suggest that ranolazine might be useful to reduce the sustained action potential firing seen in paramyotonia congenita.
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Gain-of-function mutations in CaV 2.1 (P/Q-type) Ca2+ channels cause familial hemiplegic migraine type 1 (FHM1), a subtype of migraine with aura. Knockin (KI) mice carrying FHM1 mutations show increased neuronal P/Q-type current and facilitation of induction and propagation of cortical spreading depression (CSD), the phenomenon that underlies migraine aura and may activate migraine headache mechanisms. ⋯ In contrast, blockade of N- or R-type Ca2+ channels has only a small inhibitory effect on CSD threshold and velocity of propagation. Our findings support a model in which Ca2+ influx through presynaptic P/Q-type Ca2+ channels with consequent release of glutamate from recurrent cortical pyramidal cell synapses and activation of NMDA receptors are required for initiation and propagation of the CSD involved in migraine.
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Voltage-gated calcium channels (VGCC) play obligatory physiological roles, including modulation of neuronal: functions, synaptic plasticity, neurotransmitter release and gene transcription. Dysregulation and maladaptive changes in VGCC expression and activities may occur in the sensory pathway under various pathological conditions that could contribute to the development of pain. In this review, we summarized the most recent findings on the regulation of VGCC expression and physiological functions in the sensory pathway, and in dysregulation and maladaptive changes of VGCC under pain-inducing conditions. The implications of: these changes in understanding the mechanisms of pain transduction and in new drug design are also discussed.
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Alpha-conotoxins Vc1.1 and RgIA are peptides from the venom of marine Conus snails that are currently in development as a treatment for neuropathic pain. We have reported previously that the alpha9alpha10 nicotinic acetylcholine receptor (nAChR) selective-conotoxins Vc1.1 and RgIA potently and selectively inhibit high voltage-activated (HVA) N-type calcium channel currents in dissociated neurons from rat dorsal root ganglia (DRG). Our data indicated that Vc1.1 does not interact directly with N-type Ca(2+) channels but inhibits them via GABA(B) receptor activation. ⋯ The GABA(B) receptor antagonist CGP55845 (1 microM) antagonized the effect of Vc1.1 and RgIA on the N-type calcium channels in alpha9 KO mice. RT-PCR and western blot analysis confirmed the absence of the alpha9 nAChR in mice carrying a null mutation for the nAChR alpha9 gene. These results demonstrate that the inhibition of N-type Ca(2+) channel channels by Vc1.1 and RgIA is not mediated by the expression of alpha9alpha10 nAChRs in DRG neurons.
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Alternative splicing is known to alter pharmacological sensitivities, kinetics, channel distribution under pathological conditions, and developmental regulation of VGSCs. Mutations that alter channel properties in Na(V)1.7 have been genetically implicated in patients with bouts of extreme pain classified as inherited erythromelalgia (IEM) or paroxysmal extreme pain disorder (PEPD). Furthermore, patients with IEM or PEPD report differential age onsets. ⋯ Our major findings demonstrate that although the 5A splice variant of the IEM channel had no functional impact, the 5A splice variant of the PEPD channel significantly hyperpolarized the activation curve, slowed deactivation and closed-state inactivation, shifted the ramp current activation to more hyperpolarized potentials, and increased ramp current amplitude. We hypothesize a D1/S3-S4 charged residue difference between the 5N (Asn) and the 5A (Asp) variants within the coding region of exon 5 may contribute to shifts in channel activation and deactivation. Taken together, the additive effects observed on ramp currents from exon 5 splicing and the PEPD mutation (I1461T) are likely to impact the disease phenotype and may offer insight into how alternative splicing may affect specific intramolecular interactions critical for voltage-dependent gating.