Block of Cloned Human T-Type Calcium Channels by Succinimide Antiepileptic Drugs

Abstract

Inhibition of T-type Ca(2+) channels has been proposed to play a role in the therapeutic action of succinimide antiepileptic drugs. Despite the widespread acceptance of this hypothesis, recent studies using rat and cat neurons have failed to confirm inhibition of T-type currents at therapeutically relevant concentrations. The present study re-examines this issue using the three cloned human channels that constitute the T-type family: alpha 1G, alpha 1H, and alpha 1I. The cloned cDNAs were stably transfected and expressed into mammalian cells, leading to the appearance of typical T-type currents. The results demonstrate that both ethosuximide and the active metabolite of methsuximide, alpha-methyl-alpha-phenylsuccinimide (MPS), block human T-type channels in a state-dependent manner, with higher affinity for inactivated channels. In contrast, succinimide analogs that are not anticonvulsive were relatively poor blockers. The apparent affinity of MPS for inactivated states of the three channels was estimated using two independent measures: K(I) for alpha 1G and alpha 1I was 0.3 to 0.5 mM and for alpha 1H was 0.6 to 1.2 mM. T-type channels display current at the end of long pulses (persistent current), and this current was especially sensitive to block (ethosuximide IC(50) = 0.6 mM). These drugs also reduced both the size of the T-type window current region and the currents elicited by a mock low threshold spike. We conclude that succinimide antiepileptic drugs are capable of blocking human T-type channels at therapeutically relevant concentrations.

Molecular Targets for Antiepileptic Drug Development

“…T-type Ca 2ϩ channels are believed to be the targets of antiabsence agents such as ethosuximide, which weakly block native and recom- binant T-type Ca 2ϩ channel currents. [72][73][74] As expected from this pharmacological observation, mice that lack ␣ 1G T-type Ca 2ϩ channels are resistant to absence seizures. 75,76 The three recessive mutations in Cacna1a (Ca v 2.1) that produce absence-like syndromes in tottering, leaner and rocker mice all impair channel function, reducing P/Q-type Ca 2ϩ currents.…”

“…73,154 Nevertheless, the effect on T-type currents is undoubtedly real, because it has been demonstrated in expressed human receptors 74 ; it is probably large enough to block slow oscillatory firing. Microinfusion of ethosuximide into the thalamus is, however, less effective at blocking spikeand-wave discharges than infusion into the somatosensory cortex, suggesting that molecular targets relating to the initiation of spikes in the cortex (e.g., Na ϩ channels, HCN channels) may be more important than thalamic T-type Ca 2ϩ channels.…”

Molecular Physiology of Low-Voltage-Activated T-type Calcium Channels

“…Ethosuximide and MPS are capable of blocking all three recombinant T-type Ca 2ϩ channels (137). As observed with mibefradil, block is enhanced up to 10-fold by holding the membrane at potentials that partially inactivate T-type channels.…”

Molecular Targets of Cannabidiol in Neurological Disorders

“…It is also notable that CBD's effects at Ttype channels produces a hyperpolarizing shift of the activation potential, while changes in steady-state activation and activity, even at strongly hyperpolarizing potentials, suggest the ability to induce a block when the channel is in either the open or closed state. Interestingly, while ethosuximide induces a similar hyperpolarizing shift, it is only able to block channels when in the open state [143].…”