5-(2-Cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB): correlation of hypnotic and convulsant properties with alterations of synaptosomal 45Ca2+ influx

CHEB, a convulsant barbiturate, evokes calcium-dependent spontaneous glutamate release from rat cerebrocortical synaptosomes

CHEB [5-(2-cyclohexylidene-ethyl)-5-ethyl barbituric acid] is a potent convulsant barbiturate that causes direct neuronal excitation by an unknown mechanism. We have analyzed the effects of CHEB on the release of endogenous glutamate from rat cerebrocortical synaptosomes using an on-line enzyme-coupled fluorimetric assay. CHEB evoked spontaneous Ca(2+)-dependent glutamate release with an EC50 = 14.2 microM and an Emax = 3.2 mumol/min/mg. The non-convulsant barbiturates pentobarbital and phenobarbital evoked significantly less glutamate release at high concentrations. CHEB (30 microM) increased intrasynaptosomal [Ca2+] by 58 +/- 4 nM (p < 0.01; n = 4) above baseline compared to an increase of 5 +/- 4 nM (NS; n = 4) produced by pentobarbital (30 microM). CHEB-evoked glutamate release was inhibited by pentobarbital, phenobarbital, EGTA, CoCl2/CdCl2 and flunarizine, but not by local anesthetics, tetrodotoxin, nitrendipine or omega-conotoxin GVIA. These results demonstrate that CHEB acts as a potent and effective secretogogue for glutamate by a pre-synaptic mechanism that does not require activation of Na+ channels or of L-type or N-type Ca2+ channels. Stimulation of spontaneous glutamate release may contribute to the convulsant properties of CHEB.

Acute, chronic and differential effects of several anesthetic barbiturates on glutamate receptor activation in neuronal culture

The acute and chronic effects of several anesthetic barbiturates, in therapeutic concentrations, on the excitatory amino acid (EAA)-induced elevation of intracellular calcium levels ([Ca2+]i) were examined in neuronal tissue culture. The ultrashort-acting barbiturate, thiamylal, was effective in blocking elevations of [Ca2+]i induced by kainate, N-methyl-D-aspartate (NMDA), and quisqualate or by membrane depolarization with 40 mM KCl. The structurally similar barbiturate, secobarbital which differs from thiamylal only by having an oxygen in place of a sulfur, was able to block elevations induced by the above EAAs but was less effective than thiamylal and did not significantly reduce [Ca2+]i that resulted from membrane depolarization with KCl. Pentobarbital, while differing from secobarbital by only a methyl group, was without effect on either the NMDA- or 40 mM KCl-induced elevations of [Ca2+]i. By contrast, cyproheptadine, a compound that has been shown to block Ca2+ channels, has a different profile from the above barbiturates in that cyproheptadine is more effective in blocking elevation of [Ca2+]i induced by membrane depolarization with KCl while the barbiturates are more effective in reducing [Ca2+]i induced by EAAs. An anticonvulsant barbiturate, phenobarbital, did not reduced elevations of [Ca2+]i induced by any EAA tested or by membrane depolarization with KCl. When cells were treated chronically with thiamylal for 4 days, 2-6 h after the abrupt drug withdrawal there was a hyperresponsiveness to the elevations of [Ca2+]i induced by both kainate and NMDA but not by quisqualate. A similar hyperresponsiveness was not seen after the chronic treatment with phenobarbital.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of the sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase by pentobarbital

The dependence of the (Ca2+ + Mg2+)-ATPase activity of sarcoplasmic reticulum vesicles upon the concentration of pentobarbital shows a biphasic pattern. Concentrations of pentobarbital ranging from 2 to 8 mM produce a slight stimulation, approximately 20-30%, of the ATPase activity of sarcoplasmic reticulum vesicles made leaky to Ca2+, whereas pentobarbital concentrations above 10 mM strongly inhibit the activity. The purified ATPase shows a higher sensitivity to pentobarbital, namely 3-4-fold shift towards lower values of the K0.5 value of inhibition by this drug. These effects of pentobarbital are observed over a wide range of ATP concentrations. In addition, this drug shifts the Ca2+ dependence of the (Ca2+ + Mg2+)-ATPase activity towards higher values of free Ca2+ concentrations and increases several-fold the passive permeability to Ca2+ of the sarcoplasmic reticulum membranes. At the concentrations of pentobarbital that inhibit this enzyme in the sarcoplasmic reticulum membrane, pentobarbital does not significantly alter the order parameter of these membranes as monitored with diphenylhexatriene, whereas the temperature of denaturation of the (Ca2+ + Mg2+)-ATPase is decreased by 4-5 C degrees, thus, indicating that the conformation of the ATPase is altered. The effects of pentobarbital on the intensity of the fluorescence of fluorescein-labeled (Ca2+ + Mg2+)-ATPase in sarcoplasmic reticulum also support the hypothesis of a conformational change in the enzyme induced by millimolar concentrations of this drug. It is concluded that the inhibition of the sarcoplasmic reticulum ATPase by pentobarbital is a consequence of its binding to hydrophobic binding sites in this enzyme.

Effects of 1,1,1-trichloroethane on synaptosomal calcium accumulation in mouse brain

Addition of 1,1,1-trichloroethane (TCE), in vitro to synaptosomes isolated from the mouse cerebellum and cerebral cortex inhibited the slow phase of K+-stimulated 45Ca2+ influx and the net 45Ca2+ influx (delta k). In the brain stem, however, TCE increased the fast and slow phases of calcium uptake under depolarizing conditions and also delta k. The non-depolarized calcium influx was not altered by TCE added in vitro. Two hours after injection of TCE (2.4 g/kg) the calcium accumulation in the presence of high K+ was lowered in the cerebellar synaptosomes, while it was increased in brain stem synaptosomes. TCE administered in vivo did not alter the calcium influx into cerebrocortical synaptosomes nor did it affect the non-depolarization-induced 45Ca2+ influx. Thus, these data indicate that TCE may influence voltage-dependent calcium channels in mouse brain synaptosomes.