Effects of a convulsant barbiturate on dorsal root ganglion cells and other components of the spinal reflex arc

Electrophysiological techniques were used to study the effects of the convulsant barbiturate, 5-(2-cyclohexylideneëthyl)-5-ethyl barbituric acid (CHEB), in unanesthetized spinal cats. Responses of motoneurons and dorsal root ganglion cells to CHEB were assessed by intracellular recording. Effects of CHEB on dorsal root reflexes and potentials and on peripheral nerve and its sensory terminals were also tested. CHEB produced maximal seizures in unparalyzed, spinal cats similar to those produced by pentylenetetrazol or electroshock but different from those produced by strychnine. No evidence of increased excitability in axons of peripheral nerve or in their sensory terminals was found. Spontaneous dorsal root reflexes were elicited by significantly lower doses of CHEB than produce seizure activity. Mononeurons were depolarized by approximately the same dose of CHEB that elicits strong seizure activity. Dorsal root ganglion cells were also depolarized but by slightly higher doses of CHEB. Depolarization of both motoneurons and dorsal root ganglion cells was accompanied by increased electrical excitability of the cell membranes and by spontaneous discharges. Depolarization of motoneurons was partially due to an increase in spontaneous synaptic potentials presumably elicited by an effect of the drug on spinal interneurons. On the other had, depolarization and increased excitability of dorsal root ganglion cells were entirely attributable to a direct effect of CHEB on the excitatory mechanism of cell membranes. Depolarization of ganglion cells were accompanied by a marked fall in passive membrane resistance indicating and increased permeability to one or more ions. Spontaneous discharge of ganglion cells was limited to the period of rapid depolarization of the cell membrane. It is suggest that convulsant barbiturates produce many, and perhaps all, of their excitatory effects through a direct action on cell membranes, possibly involving and increase in sodium permeability.