Strychnine-like action of the convulsant barbiturate, CHEB

Calcium-dependent actions of the convulsant barbiturate, CHEB, on transmitter release at the rat neuromuscular junction

1. The effect of convulsant barbiturates on spontaneous and evoked acetylcholine release was studied at the rat neuromuscular junction in vitro. 2. The convulsant barbiturates (+)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(+)-DMBB], 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB), 5-ethyl-5-(3-methylbut-2'-enyl) barbituric acid (3M2B) and 5-ethyl-5-(1,3-dimethylbut-1'-enyl) barbituric acid (1,3M1B) all produced a concentration-dependent increase in miniature end-plate potential (MEPP) frequency. 3. With CHEB (100 microM) this increase in MEPP frequency was found to be dependent on the [Ca2+]o. CHEB in 0.5 mM [Ca2+]o did not alter MEPP amplitude, but in 1.3 and 2.5 mM [Ca2+]o CHEB significantly reduced the amplitude. 4. At a [Ca2+]o of 0.5 mM, CHEB produced an increase in both EPP amplitude and quantal content, while at 1.3 mM [Ca2+]o CHEB did not alter EPP amplitude or quantal content. 5. The plot of log quantal content vs log [Ca2+]o showed a parallel shift to the left in the presence of 100 microM CHEB. This change occurred without any alteration in the maximum quantal content. This suggests that the enhancement of transmitter release may be mediated by an effect on calcium fluxes in the pre-junctional nerve terminal.

Depolarizing actions of convulsant barbiturates on isolated rat dorsal root ganglion cells

The actions of convulsant barbiturates were studied on dorsal root ganglion (DRG) cells in vitro using intracellular recording techniques. Only the convulsant barbiturates (+)-DMBB and CHEB produced a concentration-dependent depression in the responses to gamma-aminobutyric acid (GABA). All convulsant barbiturates were found to produce a direct depolarization of the DRG cell membrane which was accompanied by a decrease in the input resistance of the cell and a reduction in the orthodromic action potential. A sub-population of DRG cells were found to be refractory to these actions but there was no relationship between the cell type (A beta, A delta and C) and ability to respond to the convulsant barbiturates.

Differing actions of convulsant and nonconvulsant barbiturates: an electrophysiological study in the isolated spinal cord of the rat

The effects of various pairs of convulsant and nonconvulsant barbiturates on mono- and polysynaptic activity were studied in the isolated spinal cord of the immature rat, using extracellular recording. The convulsant barbiturates, 5-ethyl-5-(3-methylbut-2'-enyl) barbituric acid (3M2B), 5-ethyl-5-(1,3-dimethylbut-1'-enyl) barbituric acid (1,3M1B) and (+)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(+) DMBB] all increased the monosynaptic reflex at concentrations between 5 and 50 microM with no change in polysynaptic activity. When the concentration was raised to between 100 and 300 microM, however, the convulsants all reduced the monosynaptic reflex, thus producing a biphasic dose-response relationship. The nonconvulsant barbiturates phenobarbital, 5-ethyl-5-(3-methylbut-1'-enyl) barbituric acid (3M1B), amylobarbital (3MB) and (-)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(-)DMBB] produced only a decrease in mono- and polysynaptic reflexes. At concentrations which enhanced the monosynaptic reflex, the responses of motoneurones to glycine and eledoisin-related peptide (an analogue of substance P) were reduced by (+)DMBB, while 1,3M1B and 3M2B had no significant effects upon any of the neurotransmitters tested. At concentrations which depressed the monosynaptic reflex, the convulsants all reduced the response to glycine whereas the nonconvulsant barbiturates all increased the response to GABA. With the exception of phenobarbital, both convulsant and nonconvulsant barbiturates produced a direct depolarisation of the presynaptic terminal membrane, with only the convulsants producing a depolarisation of the membrane of the motoneurone. Using another convulsant barbiturate, 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB), this direct depolarising action was found to be calcium-dependent.