Verapamil (5 microM), nifedipine (10 microM), and ryanodine (10 microM) potentiated rhythmic activity of newly formed synapses, while apamin produced no effect on this potentiation. Ryanodine (1 microM) suppressed synaptic activity, and this effect can be prevented with nifedipine. It was hypothesized that in newly formed synapses Ca2+ entry through L-type channels triggers the release of stored Ca2+, which inhibits secretion of the neurotransmitter.