TIME-DEPENDENT INCREASE IN PROTEIN-PHOSPHORYLATION FOLLOWING ONE-TRIAL ENHANCEMENT IN HERMISSENDA

One-trial conditioning of the nudibranch mollusk Hermissenda produces short- and long-term changes in excitability (enhancement) of identifi ed sensory neurons. To investigate the biochemical mechanisms underlyi ng this example of plasticity, we have examined changes in protein pho sphorylation at different times following the in vitro conditioning tr ial. Changes in the incorporation of (PO4)-P-32 into proteins were det ermined using two-dimensional polyacrylamide gel electrophoresis, auto radiography, and densitometry. Conditioning resulted in increases in l evels of several phosphoproteins, five of which, ranging in apparent m olecular mass from 22 to 55 kDa, were chosen for analysis. The increas ed phosphorylation of the 46- and 55-kDa phosphoproteins detected 2 h postconditioning was significantly greater than the level of phosphory lation detected in an unpaired control group, indicating that long-ter m enhancement is pairing specific. Statistically significant increases in phosphorylation as compared with the control group that received o nly light were detected immediately after conditioning (5 min) for the 55-, 46-, and 22-kDa phosphoproteins, at 1 h for the 55- and 46-kDa p hosphoproteins, and at 2 h for the 55-, 46-, and 22-kDa phosphoprotein s. The 46- and 55-kDa phosphoproteins are putative structural proteins , and the 22-kDa phosphoprotein is proposed to be a protein kinase C s ubstrate previously identified in Hermissenda following multitrial cla ssical conditioning. Time-dependent increases in protein phosphorylati on may contribute to the induction and maintenance of different memory stages expressed in sensory neurons after one-trial conditioning.