Phenobarbital: concentration-dependent biphasic effect on aplysia burst-firing neurons

The effects of sodium phenobarbital (PNB) were tested on bursting pacemaker neurons in Aplysia. At low concentrations, PNB increased the slow oscillations of membrane potential that underline the bursting response. At higher concentrations the oscillations were suppressed and blocked. Under voltage-clamp conditions it was found that the increased slow-wave activity with low concentrations of PNB was due to an increase in the slow inward current that is carried primarily by Ca(2+) ions. The results suggest that PNB has a concentration dependent biphasic effect on Ca(2+) conductance mechanisms and illustrate the need for caution when comparing the effects of PNB between a variety of preparations.