Training Alone, Not the Tripeptide RGD, Modulates Calexcitin in Hermissenda

In the nudibranch mollusc Hermissenda crassicornis, the intensity of immunostaining for the Ca2+/GTP-binding protein calexcitin (1) correlates positively with the degree of learning obtained and the level of memory expressed after Pavlovian conditioning (2, 3). When inhibitors of transcription (actinomycin-D; Act-D) and translation (anisomycin; ANI) are applied after training, they affect the animal's ability to recall the learned behavior. Epstein et al. (4) have recently described the time windows for these effects. They noted that there are two phases of sensitivity to the inhibitors: an early phase, immediately after training (0-13 min), and a later phase (70-160 min for Act-D, and 70-220 min for ANI). Subsequently, Epstein et al. (5) reported that long-term memory also has two distinct biological configurations: long-term memory (LTM), which lasts about 24 h, and consolidated long-term memory (CLTM), which persists up to 6 days. The amount of memory retained by conditioned animals appears to depend, in part, on how much consolidation has been completed; and the transition from LTM to CLTM is sensitive to a cell adhesion molecule (CAM) inhibitor, the tripeptide arginyl-glycyl-aspartate (RGD). One working hypothesis predicts that increased calexcitin levels are needed to establish LTM, and that RGD-sensitive CAMs are subsequently involved in the transition from LTM to CLTM. What was unknown was the possible relationship between calexcitin levels and the effects of RGD during this critical transition period. Thus, as part of a continuing study to describe and define the memory stages demonstrated in Hermissenda (6, 4, 5), we undertook an immunocytochemical study to investigate possible correlations between calexcitin levels and the observed effects of RGD