Pharmacological Properties of T-Type Ca Current in Adult Rat Sensory Neurons: Effects of Anticonvulsant and Anesthetic Agents

Todorovic, Slobodan M. and Christopher J. Lingle. Pharmacounderstanding of the diversity within those Ca channels actilogical properties of T-type Ca current in adult rat sensory neuvated at more negative potentials remains more limited. Lowrons: effects of anticonvulsant and anesthetic agents. J. Neurophysvoltage-activated (LVA) Ca current, also called T-type curiol. 79: 240–252, 1998. We have used the whole cell patch-clamp rent, has been observed in a variety of cell types including method to study pharmacological properties of low-voltage–actineurons (Carbone and Lux 1984), endocrine cells (Armstrong vated (LVA) Ca current in freshly dissociated neurons from and Matteson 1985; Herrington and Lingle 1992; Matteson and dorsal root ganglia of adult rats. Inward barium current [ in the Armstrong 1986), and cardiac muscle (Droogmans and Nilius presence of internal fluoride to reduce L-type high-voltage–acti1989; Hirano et al. 1989). T-type current appears to play a vated (HVA) and external 1 mM v-conotoxin GVIA to block critical role in the regulation of neuronal bursting and lowN-type HVA current] was evoked from negative holding potentials of 090 mV to test potentials of 025 mV and showed complete inactiamplitude voltage oscillations (Crunelli et al. 1989; Huguenard vation during 200-ms test pulses. Amiloride blockedÇ90% of current and Prince 1992; Llinas 1988; Manor et al. 1997). Furthermore, with half-maximal block (EC50) of 75 mM and a Hill coefficient (n) T-type current has been implicated in seizure susceptibility and of 0.99. LVA current was blocked completely by inorganic Ca initiation (Huguenard and Prince 1994; Tsakiridou et al. 1995). channel blockers: lanthanum (EC50 Å 0.53 mM)ú zinc (EC50 Å 11.3 Despite the critical role of some T currents in the regulation mM) ú cadmium (EC50 Å 20 mM)ú nickel (EC50 Å 51 mM). The of neuronal excitability, the extent to which T current among antiepileptics, ethosuximide (EC50 Å 23.7 mM, n Å 1.4), phenytoin different cells exhibits similarities and differences is not clear. (EC50 Å 7.3 mM, n Å 1.3), a-methyl-a-phenylsuccinimide (EC50 Å Available data suggest that considerable diversity not only ex170 mM, n Å 2.1), and valproic acid (EC50 Å 330 mM, n Å 1.9) ists among T currents in terms of fundamental kinetic properties maximally blockedÇ100, 60, 26, and 17% of T current, respectively. (e.g., Huguenard et al. 1993), but also in terms of pharmacoAnother antiepileptic, carbamazepine (°100 mM), and convulsants such as pentylenetetrazole (1 mM) and tert-butyl-bicyclo [2.2.2] logical sensitivities (Huguenard 1996). For example, although phosphorothionate (50 mM) had no effect on T current. Barbiturates the primary target of the clinically used anticonvulsant, ethocompletely blocked T current: thiopental (EC50 Å 153 mM, n Å suximide, has been proposed to involve T current blockade in 1.2) ú pentobarbital (EC50 Å 334 mM, n Å 1.2) ú methohexital thalamic neurons (Coulter et al. 1989a,b), T current in GH3 (EC50 Å 502 mM, n Å 1.3) ú phenobarbital (EC50 Å 1.7 mM, n Å cells is relatively resistant to blockade by ethosuximide (Her1.2). Blockade by thiopental and pentobarbital did not show voltage rington and Lingle 1992). or use dependence. General anesthetics blocked T current completely Although the T-type current in dorsal root ganglion (DRG) and reversibly: propofol (EC50 Å 12.9 mM, n Å 1.3) ú octanol neurons was one of the first T currents to be described (Carbone (EC50 Å 122 mM, n Å 1.2) ú etomidate (EC50 Å 205 mM, n Å and Lux 1984), the pharmacological description of this current 1.3) ú isoflurane (EC50 Å 303 mM, n Å 2.3) ú halothane (EC50 Å remains incomplete. Specifically, for many blockers of DRG 655 mM, nÅ 2.0)ú ketamine (EC50Å 2.5 mM, nÅ 1.1). Mibefradil, a novel Ca channel blocker, blocked dorsal root ganglion T current T current, concentration-response information is not available. in a voltageand use-dependent fashion with an EC50 of Ç3 mM Given the likelihood that different T currents may exhibit well(nÅ 1.3). When compared with results on other T currents, these data defined pharmacological differences, clear definition of those indicate that significant differences exist among different T currents in differences is of key importance in the development of agents terms of pharmacological sensitivities. Furthermore, differences in with selective clinical efficacies. Furthermore, better defined pharmacological sensitivity of T currents among peripheral neurons, pharmacological tools are essential to the understanding of the CNS, and neuroendocrine cells may contribute to the spectrum of roles of different T currents in nervous system function. As effects of particular analgesic, anticonvulsant, and anesthetic drugs. part of ongoing work aimed at illuminating similarities and differences among native T currents, here we have examined the pharmacological properties of T current in acutely dissociI N T R O D U C T I O N ated adult rat DRG neurons. Particular attention has been diBoth central and peripheral neurons possess multiple types rected toward defining complete concentration-response curves of voltage-gated calcium (Ca) channels (Bean 1989; Carto allow detailed comparison with similar data from other T bone and Lux 1984; De Waard et al. 1996; Hess 1990; Llinas currents. and Yarom 1981) that often are distinguished on the basis of M E T H O D S the membrane potentials at which they are activated. Although Acutely dissociated DRG neurons multiple subtypes of high-voltage-activated (HVA) Ca currents have been described in a variety of neurons (Fox et al. Male rats (100–250 g; Sprague-Dawley) were anesthetized with