Calcium current block by (-)-pentobarbital, phenobarbital, and CHEB but not (+)-pentobarbital in acutely isolated hippocampal CA1 neurons: Comparison with effects on GABA-activated Cl- current

Abstract

Block of a voltage-activated Ca2+ channel current by phenobarbital (PHB), 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB), and the optical R(-)- and S(+)-enantiomers of pentobarbital (PB) was examined in freshly dissociated adult guinea pig hippocampal CA1 neurons; the effects of the barbiturates on GABA-activated Cl- current were also characterized in the same preparation. (-)-PB, PHB, and CHEB produced a reversible, concentration-dependent block of the peak Ca2+ channel current (3 mM Ba2+ as the charge carrier) evoked by depolarization from -80 to -10 mV (IC50 values, 3.5, 72, and 118 μM, respectively). In contrast, (+)-PB was nearly inactive at concentrations up to 1 mM. The inhibitory action of PHB was decreased at acid pH, indicating that the dissociated (anionic) form of the molecule is the active species. Block by (-)-PB was voltage dependent with the fractional block increasing at positive membrane potentials; calculations according to the method of Woodhull indicated that the (-)-PB blocking site senses ~40% of the transmembrane electric field. The time course and voltage dependence of activation of the Ca2+ channel current were unaffected by (- )-PB, PHB, and CHEB. The rate of inactivation was enhanced by (-)-PB and CHEB, with the major effect being acceleration of the slow phase of the biexponential decay of the current. GABA-activated Cl- current was potently enhanced by (-)-PB and PHB (EC50 values, 3.4 and 12 μM), whereas (+)-PB was only weakly active. At concentrations of (-)-PB > 100 μM and PHB > 200- 300 μM, Cl- current responses were activated even in the absence of GABA. These results demonstrate that in CA1 hippocampal neurons, PB causes a stereoselective block of a voltage-activated Ca2+ current; PHB is also effective, but at higher concentrations. For (-)-PB, the effect on Ca2+ channel current occurred at similar concentrations as potentiation of GABA responses. In contrast, PHB was more potent as a GABA enhancer than as blocker of Ca2+ current, but the maximal potentiation of GABA responses was 40% of that obtained with (-)-PB. Consequently, the anticonvulsant action of PHB at clinically relevant concentrations may relate to modest enhancement of GABA responses and partial blockade of Ca2+ current, whereas the sedative effects that occur at higher concentrations could reflect stronger Ca2+ current blockade. The powerful sedative-hypnotic action of (-)-PB may reflect greater maximal enhancement of GABA responses in conjunction with strong inhibition of Ca2+ current. The convulsant action of CHEB is unlikely to be related to its effects on the Ca2+ current.