Barbiturates block divalent cation action potentials in leech nociceptive cells.

Phenobarbital (PNB), pentobarbital (PTB) and methohexital (MTX) decreased the maximum rate of depolarization Vmax and duration of divalent cation action potentials elicited in leech nociceptive neurons in Na+-free solutions containing the K+-channel blocker TEA, without significantly affecting resting membrane potential or conductance. The block of the divalent cation action potentials was reversible and dose-dependent, ED50 for inhibition of Vmax being 560 microM for MTX, 800 microM for PTB and 3000 microM for PNB. This order of potency correlated well with the ratio of unchanged/charged form of the drugs at physiological pH suggesting that in leech, as in other preparations, the non-ionized form was the active one. In Na+-containing Ringer, the 3 barbiturates depolarized and decreased membrane resistance in the lateral nociceptive cells, but not the medial nociceptive cells. These results provide additional information regarding the newly described pharmacological differences among closely related neurons. These membrane actions may be related to some of the excitatory properties described for other barbiturates in invertebrate and mammalian preparations.