Calcium oscillations induced by gambierol in cerebellar granule cells

Abstract

Gambierol is a marine polyether ladder toxin derived from the dinoflagellate Gambierdiscus toxicus. To date, gambierol has been reported to act either as a partial agonist or as an antagonist of sodium channels or as a blocker of voltage-dependent potassium channels. In this work, we examined the cellular effect of gambierol on cytosolic calcium concentration, membrane potential and sodium and potassium membrane currents in primary cultures of cerebellar granule cells. We found that at concentrations ranging from 0.1 to 30 microM, gambierol-evoked [Ca(2+)]c oscillations that were dependent on the presence of extracellular calcium, irreversible and highly synchronous. Gambierol-evoked [Ca(2+)]c oscillations were completely eliminated by the NMDA receptor antagonist APV and by riluzole and delayed by CNQX. In addition, the K(+) channel blocker 4-aminopyridine (4-AP)-evoked cytosolic calcium oscillations in this neuronal system that were blocked by APV and delayed in the presence of CNQX. Electrophysiological recordings indicated that gambierol caused membrane potential oscillations, decreased inward sodium current amplitude and decreased also outward IA and IK current amplitude. The results presented here point to a common mechanism of action for gambierol and 4-AP and indicate that gambierol-induced oscillations in cerebellar neurons are most likely secondary to a blocking action of the toxin on voltage-dependent potassium channels and hyperpolarization of sodium current activation.

Gambierol Inhibition of Voltage-Gated Potassium Channels Augments Spontaneous Ca2+Oscillations in Cerebrocortical Neurons

“…Although this effect of gambierol parallels that observed in the this study in which a potentiation of spontaneous Ca 21 oscillations was observed, the response in cerebellar granule neurons required micromolar gambierol concentrations that were somewhat higher than that required for K v channel blockade (Ghiaroni et al, 2005;Cuypers et al, 2008;Kopljar et al, 2009). Whether measured at the singlecell (Alonso et al, 2010) or population level (this report), both studies observed that Ca 21 oscillations in the presence of gambierol were highly synchronous. Inhibition of K v channels with Ba 21 was previously reported to increase the frequency of Ca 21 oscillations in spontaneously active hypothalamic neurons (Costantin and Charles, 2001), and it is reasonable to infer that both the induction of Ca 21 oscillations in cerebellar granule neurons (Alonso et al, 2010) and the increase in the frequency of Ca 21 oscillations in cerebrocortical neurons reported herein derive from gambierol-induced inhibition of K v channels.…”

“…In cerebellar granule neurons, gambierol has been reported to induce Ca 21 oscillations, which was interpreted to be a consequence of potassium channel blockade (Alonso et al, 2010). Although this effect of gambierol parallels that observed in the this study in which a potentiation of spontaneous Ca 21 oscillations was observed, the response in cerebellar granule neurons required micromolar gambierol concentrations that were somewhat higher than that required for K v channel blockade (Ghiaroni et al, 2005;Cuypers et al, 2008;Kopljar et al, 2009).…”

“…Whether measured at the singlecell (Alonso et al, 2010) or population level (this report), both studies observed that Ca 21 oscillations in the presence of gambierol were highly synchronous. Inhibition of K v channels with Ba 21 was previously reported to increase the frequency of Ca 21 oscillations in spontaneously active hypothalamic neurons (Costantin and Charles, 2001), and it is reasonable to infer that both the induction of Ca 21 oscillations in cerebellar granule neurons (Alonso et al, 2010) and the increase in the frequency of Ca 21 oscillations in cerebrocortical neurons reported herein derive from gambierol-induced inhibition of K v channels. In oocyte recording experiments, gambierol inhibited K v 1.1-K v 1.5, but not K v 1.6, channel currents (Cuypers et al, 2008;Kopljar et al, 2009).…”

“…These results are consonant with a previous demonstration of the ability of 4-AP and TEA to produce Ca 21 oscillations in cortical neurons (Wang and Gruenstein, 1997), and suggest that K v 3.1 channel inhibition contributes to gambierol's potentiation of spontaneous Ca 21 oscillation frequency. In cerebellar granule neurons, both 4-AP and gambierol-induced Ca 21 oscillations involved glutamate release and NMDA receptor activation, suggesting a shared mechanism of action (Alonso et al, 2010).…”

“…The ability of gambierol to inhibit both brevetoxin-induced elevation of [Ca 21 ] i and neurotoxicity was attributed to gambierol acting as a functional antagonist of neurotoxin site 5 on the VGSC. Another study using gambierol concentrations ranging from 0.1 to 30 mM reported the production of Ca 21 oscillations in cerebellar granule neurons at the single-cell level (Alonso et al, 2010). Although this latter action of micromolar gambierol was proposed to be secondary to inhibition of voltage-gated potassium channels, the requirement for micromolar concentrations is not consistent with gambierol's nanomolar affinity for voltagegated potassium channels (Ghiaroni et al, 2005;Cuypers et al, 2008;Kopljar et al, 2009).…”