U-92032

Thalamic relay neurons express high levels of T-type Ca2+ channels, which support the generation of robust burst discharges. This intrinsically mediated form of phasic spike firing is thought to be critical in the generation of slow (3–4 Hz) synchronous oscillatory activity of absence epilepsy. Recordings made from brain slices or whole animals have shown that slow synchronous absence-like activity can be abolished when Ca2+-dependent burst firing in relay neurons is interrupted by the pharmacological or genetic inactivation of T-channels. Because succinimide drugs act as incomplete and nonspecific antagonists, we tested whether the novel T-channel antagonist U-92032 could provide stronger support for a role of T-channels in slow oscillatory activity. Ca2+-dependent rebound (LTS) bursts were recorded using whole cell current clamp in relay cells of the ventral basal complex (VB) from thalamic slices of adult rats. We used LTS kinetics to measure the availability of T-channels in VB cells after TTX. U-92032 (1 and 10 μM) reduced the maximum rate of depolarization of the isolated LTS by 51% and 90%, respectively, compared with the 35% reduction due to 2 mM methylphenylsuccinimide (MPS), the active metabolite of the antiabsence drug methsuximide. U-92032 (1 and 10 μM) also suppressed evoked, slow oscillations in thalamic slices with a time course similar for observed intracellular effects. Unlike MPS, we observed no substantial effects of short-term U-92032 applications (≤2 h) on the generation of action potentials in VB cells. Our findings show U-92032 is a more potent, effective, and specific T-channel antagonist than previously studied succinimide antiabsence drugs and that it dramatically reduces epileptiform synchronous activity. This suggests that U-92032 or other specific T-channel antagonists may provide effective drug treatments for absence epilepsy.

Calcium conductances have been found in neonatal inferior colliculus neurons, however the biophysical and pharmacological profiles of the underlying calcium currents have not yet been characterized. In this study, we examined which types of voltage-activated calcium currents comprise the whole-cell inward current of neonatal inferior colliculus neurons (10-22microm in diameter). On the basis of their voltage-dependence and pharmacological sensitivities, three major components of barium currents were identified. A low threshold voltage-activated current that activated around -70mV, a mid threshold voltage-activated current that activated near -50mV, and a high threshold voltage-activated current that activated around -40mV. Low and mid threshold voltage-activated currents were present in 33% and 41% of the recordings, respectively, whereas high threshold voltage-activated currents were recorded in all inferior colliculus neurons tested. Nickel chloride (50microM) and U-92032 (1microM), which both block low threshold voltage-activated currents, reduced the amplitude of low threshold voltage-activated peak currents at a test potential of -60mV by 72% and 10%, respectively. In addition, 50microM nickel chloride and 1microM U-92032 reduced the amplitude of mid threshold voltage-activated peak currents measured at -20mV by 55% and 21%, respectively. Further pharmacological analysis indicated the presence of multiple types of high threshold voltage-activated currents in neonatal inferior colliculus neurons. The dihydropyridine nimodipine (1microM), a selective L-type current antagonist, reduced the amplitude of high threshold voltage-activated peak currents by 25%. In addition, FPL 64176 (1microM), a non-dihydropyridine L-type current agonist caused a dramatic 534% increase in the amplitude of the slow sustained component of the tail current measured at -40mV. These data indicate that inferior colliculus neurons express L-type channels. omega-Conotoxin GVIA (1microM), a selective blocker of N-type current, inhibited high threshold voltage-activated peak currents by 28% indicating the presence of N-type channels. omega-Agatoxin IVA (300nM), a potent P/Q-type antagonist, reduced high threshold voltage-activated peak currents by 27%, suggesting that inferior colliculus neurons express P/Q-type channels. Concomitant application of nimodipine (1microM), omega-conotoxin GVIA (1microM) and omega-agatoxin IVA (300nM) onto inferior colliculus neurons decreased the control high threshold voltage-activated peak currents only by 62%.Thus, inferior colliculus neurons may express at least one more type of calcium current in addition to low and mid threshold voltage-activated currents and L-type, N-type and P/Q-type high threshold currents.