Pharmacological properties of T-type Ca²⁺ current in adult rat sensory neurons: Effects of anticonvulsant and anesthetic agents

We have used the whole cell patch-clamp method to study pharmacological properties of low-voltage-activated (LVA) Ca²⁺ current in freshly dissociated neurons from dorsal root ganglia of adult rats. Inward barium current [in the presence of internal fluoride to reduce L-type high-voltage- activated (HVA) and external 1 μM ω-conotoxin GVIA to block N-type HVA current] was evoked from negative holding potentials of -90 mV to test potentials of -25 mV and showed complete inactivation during 200-ms test pulses. Amiloride blocked ~90% of current with half-maximal block (EC₅₀) of 75 μM and a Hill coefficient (n) of 0.99. LVA current was blocked completely by inorganic Ca²⁺ channel blockers: lanthanum (EC₅₀ = 0.53 μM) > zinc (EC₅₀ = 11.3 μM) > cadmium (EC₅₀ = 20 μM)> nickel (EC₅₀ = 51 μM). The antiepileptics, ethosuximide (EC₅₀ = 23.7 mM, n = 1.4), phenytoin (EC₅₀ = 7.3 μM, n = 1.3), α-methyl-α-phenylsuccinimide (EC₅₀ = 170 μM, n = 2.1), and valproic acid (EC₅₀ = 330 μM, n = 1.9) maximally blocked ~100, 60, 26, and 17% of T current, respectively. Another antiepileptic, carbamazepine (<100 μM), and convulsants such as pentylenetetrazole (1 mM) and tert-butyl-bicyclo [2.2.2] phosphorothionate (50 μM) had no effect on T current. Barbiturates completely blocked T current: thiopental (EC₅₀ = 153 μM, n = 1.2) > pentobarbital (EC₅₀ = 334 μM, n = 1.2) > methohexital (EC₅₀ = 502 μM, n = 1.3) > phenobarbital (EC₅₀ = 1.7 mM, n = 1.2). Blockade by thiopental and pentobarbital did not show voltage or use dependence. General anesthetics blocked T current completely and reversibly: propofol (EC₅₀ = 12.9 μM, n = 1.3) > octanol (EC₅₀ = 122 μM, n = 1.2) > etomidate (EC₅₀ = 205 μM, n = 1.3) > isoflurane (EC₅₀ = 303 μM, n = 2.3) > halothane (EC₅₀ = 655 μM, n = 2.0) > ketamine (EC₅₀ = 2.5 mM, n = 1.1). Mibefradil, a novel Ca²⁺ channel blocker, blocked dorsal root ganglion T current in a voltage- and use-dependent fashion with an EC₅₀ of ~3 μM (n = 1.3). When compared with results on other T currents, these data indicate that significant differences exist among different T currents in terms of pharmacological sensitivities. Furthermore, differences in pharmacological sensitivity of T currents among peripheral neurons, CNS, and neuroendocrine cells may contribute to the spectrum of effects of particular analgesic, anticonvulsant, and anesthetic drugs.