Interactive contributions of intracellular calcium and protein phosphatases to massed-trials learning deficits in Hermisssenda

Using Hermissenda as subjects, massed-trials training deficits were examine d. Associative pairings of light and rotation induced a progressively great er conditioned foot contraction in response to light as the intertrial inte rval (ITI) was extended (up to 8 min). In contrast, a short ITI (30 s) prod uced no evidence of learning. In a corresponding in vitro conditioning expe riment that mimicked training of the intact animal, facilitation of neurona l excitability in the animal's B photoreceptors paralleled the results obta ined in vivo. Imaging of intracellular Ca2+ using Fura-2 indicated that Ca2 + levels remained elevated during short ITIs. This Ca2+ accumulation appear s to induce activation of protein phosphatases because normal facilitation of the B photoreceptors was induced with a short ITI if training occurred i n the presence of a phosphatase inhibitor. These results suggest that intra cellular Ca2+ and protein phosphatases contribute interactively to the kine tics of memory formation and provide evidence that an accumulation of intra cellular Ca2+ across training trials may impede memory formation.