Action of cyclic AMP on intracellular calcium concentration and bursting activity

In order to clarify its role in the provocation of seizure activity, the effects of cyclic AMP were examined on the intracellular calcium concentration of pentylenetetrazol (PTZ)-sensitive neurons as well as of PTZ-non-sensitive neurons of the Japanese land snail, Euhadra peliomphala. Extracellular application of isobutylmethylxanthine (IBMX) and dibutyryl cyclic AMP showed bursting activity-like firing in the PTZ-sensitive neurons. When 5′-guanylylimidodiphosphate (GNP-PNP) was injected into PTZ-sensitive neurons in the extracellular presence of IBMX and dibutyryl cyclic AMP, bursting activity followed by long-lasting hyperpolarization occurred. Intracellular injection of cyclic AMP into PTZ-sensitive neurons caused hyperpolarization coincident with an increase in intracellular calcium concentration. This increase in intracellular calcium concentration was the same under conditions in which the calcium influx was inhibited by the substitution of extracellular calcium chloride by cobalt chloride. In PTZ-non-sensitive neurons, cyclic AMP-induced bursting activity was not observed. These results suggest that an increase in cyclic AMP provoked bursting activity via an increase in intracellular calcium concentration.