Calcium Currents In Sensory Neurons Research Articles

The Fabry disease-associated lipid Lyso-Gb3 enhances voltage-gated calcium currents in sensory neurons and causes pain

Fabry disease is an X-linked lysosomal storage disorder characterised by accumulation of glycosphingolipids, and accompanied by clinical manifestations, such as cardiac disorders, renal failure, pain and peripheral neuropathy. Globotriaosylsphingosine (lyso-Gb3), a deacylated form of globotriaosylceramide (Gb3), has emerged as a marker of Fabry disease. We investigated the link between Gb3, lyso-Gb3 and pain. Plantar administration of lyso-Gb3 or Gb3 caused mechanical allodynia in healthy mice. In vitro application of 100nM lyso-Gb3 caused uptake of extracellular calcium in 10% of sensory neurons expressing nociceptor markers, rising to 40% of neurons at 1μM, a concentration that may occur in Fabry disease patients. Peak current densities of voltage-dependent Ca2+ channels were substantially enhanced by application of 1μM lyso-Gb3. These studies suggest a direct role for lyso-Gb3 in the sensitisation of peripheral nociceptive neurons that may provide an opportunity for therapeutic intervention in the treatment of Fabry disease-associated pain.

Loss of T-type calcium current in sensory neurons of rats with neuropathic pain.

Pathophysiology in the primary sensory neuron may contribute to chronic neuropathic pain. Ca channels play a central role in neuronal processes, and sensory neurons are rich in low-voltage-activated calcium channels (LVACCs). However, the physiologic function of these channels is unknown. Their possible role in rebound burst firing makes them a candidate for increased excitability after neuropathic injury. This study uses pharmacological methods to isolate LVACC in cells from the dorsal root ganglia of neuropathic and sham-operated rats, including the blockade of high-voltage-activated Ca channels with fluoride and selective toxins. LVACCs were examined with conventional whole cell patch clamp electrophysiology techniques. After chronic constriction injury of the peripheral axon, LVACC was significantly reduced compared to sham rats as shown by a 60% reduction in peak current density and an 80% reduction in total calcium influx. A depolarizing shift in the voltage dependence of activation and an increase in the rate of deactivation and inactivation appear to cause this reduction of LVACC. Either Ni2+ or mibefradil, blockers of LVACC, applied in the bath to normal dorsal root ganglion cells during current clamp significantly and reversibly increased excitability. These results suggest that loss of LVACC may contribute to decreased spike frequency adaptation and increased excitability after injury to sensory neurons. Through decreased Ca2+ influx, the cell becomes less stable and more likely to initiate or transmit bursts of action potentials. Consequently, modulation of Ca2+ currents at the dorsal root ganglion may be a potential method of therapeutic intervention.

Inhibitory effect of 1,1,1-trichloroethane on calcium channels of neurons.

Peripheral neuropathy may remain for some time after 1,1,1-trichloroethane exposure. A variety of Ca2+ channels gives sensory neurons many kinds of transmitting sensory information. We measured calcium currents in sensory neurons from neonatal rat dorsal root ganglion using whole-cell patch-clamp recordings. Trichloroethane reversibly reduced the low-voltage-activated (LVA) and high-voltage-activated (HVA) calcium. The half-inhibitory concentration (IC50) of the HVA and LVA currents was 5.76 x 10(-3) M and 3.99 x 10(-3) M, respectively. The Hill coefficient of the HVA and LVA currents was 0.61 and 1.04, respectively. In assessing voltage dependence for activation and inactivation of calcium currents, only the HVA calcium current was inactivated at greater negative potentials. This may be one of the mechanisms to reduce HVA current. However, activation and inactivation of the LVA currents were not affected by trichlorothane, so inhibition of the LVA currents may have other mechanisms. Calcium currents are thought to be involved in the control of neuronal excitability and neurotransmitter release. The inhibitory effect of trichloroethane on calcium currents may be involved in trichloroethane-induced sensory discomfort.

Capsazepine block of voltage-activated calcium channels in adult rat dorsal root ganglion neurones in culture.

1. We have found that capsazepine, a competitive antagonist at the vanilloid (capsaicin) receptor, blocks voltage-activated calcium currents in sensory neurones. 2. The block of calcium current was slow to develop with a half time of about one minute at 100 microM and lasted for the duration of the experiment. The rate of block of calcium current was strongly concentration-dependent. 3. The EC50 for the blocking effect at 0 mV was 7.7 +/- 1.4 microM after 6 min exposure to capsazepine. The EC50 at equilibrium was estimated to be 1.4 +/- 0.2 microM. 4. The block of calcium current showed some voltage-dependence but there was no indication of any selectivity of action for a calcium channel subtype. The characteristics of the blocking action of capsazepine on the residual current of cells which were pretreated with either omega-conotoxin or nimodipine were similar to control. 5. The data suggest that capsazepine, in addition to its competitive antagonism of vanilloid receptors, has a non-specific blocking action on voltage-activated calcium channels which should be taken into account when interpreting the effects of this substance on intact preparations in vitro or in vivo.

Interaction of convergent pathways that inhibit N-type calcium currents in sensory neurons

Norepinephrine and GABA inhibit omega-conotoxin GVIA-sensitive (N-type) calcium current in embryonic sensory neurons by separate pathways. We have investigated the mechanisms that limit the modulation of N current by varying the level of activation for a single pathway or simultaneously activating multiple pathways. Calcium currents were measured with tight-seal, whole-cell recording methods. Simultaneous application of the two transmitters at saturating concentrations produced a larger inhibition of the current than either transmitter by itself, but the maximal inhibition was not linearly additive. Maximal, direct activation of GTP-binding proteins by intracellular application of guanosine 5'-(3-O-thio)-triphosphate (GTP gamma S) resulted in a similar limit to the inhibition; furthermore, GTP gamma S did not enhance the maximal inhibition produced by co-application of transmitters. Interventions downstream in the modulatory pathway (e.g. direct activation of protein kinase C or inhibition of protein phosphatases) were also unable to alter the maximal limit for inhibition. These results suggest that transmitter-mediated inhibition is not limited by receptor number, levels of G-protein or protein kinase C activation, or degree of phosphorylation; rather, the extent of inhibition may be limited by the structural properties of the N channels themselves.

Different action of ethosuximide on low- and high-threshold calcium currents in rat sensory neurons

The effects of ethosuximide on calcium channels were studied on dorsal root ganglion neurons from one-day-old rats using the patch-clamp technique. Bath application of ethosuximide induced dose-dependent and reversible suppression of calcium currents without affecting their time-course. Substantial differences between the effects of ethosuximide on the low-threshold and high-threshold (T- and L-) currents were observed. Ethosuximide reduced the T-current with greater potency than the L-current (Kd for T-current is 7 μM vs 15 μM for L-current). This relative specificity of its action still remained if applied at concentrations up to 1 mM. These data support the hypothesis according to which the anti-epileptic action of ethosuximide is related to reduction of the low-threshold calcium currents in sensory neurons.