Sm. Todorovic et Cj. Lingle, PHARMACOLOGICAL PROPERTIES OF T-TYPE CA2-RAT SENSORY NEURONS - EFFECTS OF ANTICONVULSANT AND ANESTHETIC AGENTS( CURRENT IN ADULT), Journal of neurophysiology, 79(1), 1998, pp. 240-252
We have used the whole cell patch-clamp method to study pharmacologica
l properties of low-voltage-activated (LVA) Ca2+ current in freshly di
ssociated neurons from dorsal root ganglia of adult rats. Inward bariu
m current [in the presence of internal fluoride to reduce L-type high-
voltage-activated (HVA) and external 1 mu M omega-conotoxin GVIA to bl
ock N-type HVA current] was evoked from negative holding potentials of
-90 mV to lest potentials of -25 mV and showed complete inactivation
during 200-ms test pulses. Amiloride blocked similar to 90% of current
with half-maximal block (EC50) of 75 mu M and a Hill coefficient (n)
of 0.99. LVA current: was blocked completely by inorganic Ca2+ channel
blockers: lanthanum (EC50 = 0.53 mu M) > zinc (EC50 = 11.3 mu M) > ca
dmium (EC50 = 20 mu M) > nickel (EC50 = 51 mu M). The antiepileptics,
ethosuximide (EC50 = 23.7 mM, n = 1.4), phenytoin (EC50 = 7.3 mu M, n
= 1.3), alpha-methyl-alpha-phenylsuccinimide (EC50 = 170 mu M, n = 2.1
), and valproic acid (EC50 = 330 mu M, n = 1.9) maximally blocked simi
lar to 100, 60, 26, and 17% of T current, respectively. Another antiep
ileptic, carbamazepine (less than or equal to 100 mu M), and convulsan
ts such as pentylenetetrazole (1 mM) and tert-butyl-bicyclo [2.2.2] ph
osphorothionate (50 mu M) had no effect on T current. Barbiturates com
pletely blocked T current: thiopental (EC50 = 153 mu M, n = 1.2) > pen
tobarbital (EC50 = 334 mu M, n = 1.2) > methohexital (EC50 = 502 mu M,
n = 1.3) > phenobarbital (EC50 = 1.7 mM, n = 1.2). Blockade by thiope
ntal and pentobarbital did not show voltage or use dependence. General
anesthetics blocked T current completely and reversibly: propofol (EC
50 = 12.9 mu M, n = 1.3) > octanol (EC50 = 122 mu M, n = 1.2) > etomid
ate (EC50 = 205 mu M, n = 1.3) > isoflurane (EC50 = 303 mu M, n = 2.3)
> halothane (EC50 = 655 mu M, n = 2.0) > ketamine (EC50 = 2.5 mM, n =
1.1). Mibefradil, a novel Ca2+ channel blocker, blocked dorsal root g
anglion T current in a voltage- and use-dependent fashion with an EC50
of similar to 3 mu M (n = 1.3). When compared with results on other T
currents, these data indicate that significant differences exist amon
g different T currents in terms of pharmacological sensitivities. Furt
hermore, differences in pharmacological sensitivity of T currents amon
g peripheral neurons, CNS, and neuroendocrine cells may contribute to
the spectrum of effects of particular analgesic, anticonvulsant, and a
nesthetic drugs.