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Whole-cell patch-clamp experiments were performed with neurons cultured from rat dorsal root ganglia (DRG). Two types of Na+ currents were identified on the basis of sensitivity to tetrodotoxin. One type was blocked by 0.1 nM tetrodotoxin, while the other type was insensitive to 10 microM tetrodotoxin. The peak amplitude of the tetrodotoxin-insensitive Na+ current gradually decreased after depolarization of the membrane. The steady-state value of the peak amplitude was attained several minutes after the change of holding potential. Such a slow inactivation was not observed in tetrodotoxin-sensitive Na+ current. The slow inactivation of the tetrodotoxin-insensitive Na+ current was kinetically distinct from the ordinary short-time "steady-state" inactivation. The voltage dependence of the slow inactivation could be described by a sigmoidal function, and its time course had a double-exponential process. A decrease of external pH partially antagonized the slow inactivation, probably through an increased diffusion potential across the membrane. However, the slow inactivation was not due to change in surface negative charges, since a shift of the kinetic parameters along the voltage axis was not observed during the slow inactivation. Due to the slow inactivation, the inactivation curves for the tetrodotoxin-insensitive Na+ current were shifted in the negative direction as the prepulse duration was increased. Consequently, the window current activated at potentials close to the resting membrane potential was markedly reduced. Thus, the slow inactivation may be involved in the long-term regulation of the excitability of sensory neurons.

Alcohol modulation of single-channel kinetics of GABA(A) receptor currents was studied with rat dorsal root ganglion neurons using the excised outside-out patch clamp technique. GABA (1 microM) alone or GABA (1 microM) plus ethanol (30-300 mM) or n-Octanol (30-300 microM) were applied by pressure ejection to evoke single-channel currents. The main single-channel conductance was not changed by either ethanol or n-Octanol at 25 pS. Both alcohols exerted the same effects on the single-channel kinetics, although n-Octanol was more potent than ethanol. The frequency of openings, the mean open time, the percentage of open time, the frequency of bursts, and the mean burst duration were all increased, but the mean closed time was decreased. These changes in channel kinetics account for the increase in whole-cell current amplitude caused by ethanol and n-Octanol.

1 K+ currents were recorded from neurones of the newborn rat cultured dorsal root ganglia, by a whole cell variation of the patch-clamp technique. 2 Chlorpromazine (CPZ), a neuroleptic, reversibly reduced the amplitude of the transient K+ current (referred to as 'IT' hereafter) with a dissociation constant (Kd) of 4.5 j4M. The inhibition of the delayed rectifier K+ current (IDR) was much less potent (Kd, 120 l4M). CPZ (100 tM) had no effect on the inward rectifier K+ current.3 The blocking action of CPZ on IT was about seven times more potent than that of 4-aminopyridine (4-AP) which had a Kd of 31 gM. The inhibition of IT followed one-to-one binding stoichiometry with both drugs. 4 The decay time course of IT was not affected by CPZ, whereas 4-AP markedly accelerated the decay phase of IT.5 The steady-state inactivation curve of IT was shifted in the negative direction (about 5 mV) by CPZ, whereas the curve was shifted in the positive direction (about 13 mV) by 4-AP. 6 The recovery from inactivation as measured by a conventional double pulse protocol was described by two exponential components in the control solution. CPZ markedly reduced the first component and slowed down the recovery from inactivation. In contrast, in the presence of 4-AP, the peak amplitude of IT was rather increased by a preceding IT possibly through voltage-dependent unbinding of 4-AP molecules.7 These results indicate that CPZ has a preferential blocking action on IT and the mechanism underlying this block is markedly different from the mechanism underlying the blocking action of 4-AP.

SUMMARY1. The action of baclofen on the voltage-gated Ca2+ current (ICa) was studied, using cultured neurones of the newborn rat dorsal root ganglia (DRG). Two major categories of ICa were identified: a small transient current activated positive to -60 mV (low voltage-activated ICa) and a large and slowly inactivating current activated positive to -30 mV (high voltage-activated Ica). H. TATEBAYASHI AND N. OGATA current size increased by the pre-pulse was similar in both the control and baclofencontaining solutions. 7. These results suggest that baclofen selectively blocks the inactivating component of the high voltage-activated ICa which forms a rapid rising phase of this current, thus slowing the activation phase of the total high voltage-activated ICa.

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Neurones of the neostriatum were freshly dispersed from the adult guinea pig brain. A fast, transient inward Na+ current (INa) was analysed using the whole-cell patch-clamp technique. Upon depolarizations, INa developed with a sigmoidal time course, which was described by m3 kinetics. INa showed an activation threshold of about -60 mV, a peak current at -30 to -20 mV, and a reversal of polarity at +60 mV. The steady-state activation (m infinity) curve for INa had a slope factor of about 9 mV for a change in the rate constant by a factor of e) in the range between -50 mV and -20 mV. Conversely, the backward rate constant (beta m) decreased as the membrane was depolarized (about 31 mV for an e-fold change). The steady-state inactivation (h infinity) curve was well expressed by the Boltzmann's equation with a half-inactivation potential of -62 mV and a slope parameter of 6 mV. The time course of INa decay followed a second-order process, whereas the recovery from inactivation was described as a first-order process. The tau h curve showed a bell-shaped configuration with a maximum value at -60 mV. The forward rate constants (alpha h) decreased as the membrane was depolarized (about 17 mV for an e-fold change) in the range between -50 mV and -20 mV. The backward rate constants (beta h) increased as the membrane was depolarized (about 10 mV for an e-fold change). There was a significant overlap between m infinity and h infinity curves, suggesting a steady influx of Na+ (window current).