Learning-induced lateralized activation of the MAPK/ERK cascade in identified neurons of the food-aversion network in the mollusk Helix lucorum.

The MAPK/ERK pathway plays an important role in the regulation of gene expression during memory formation both in vertebrates and invertebrates. In the mollusk Helix lucorum, serotonin induces activation of MAPK/ERK in the central nervous system (CNS) upon food aversion learning. Such learning depends on a neuronal network in which specialized neurons play distinct roles so that they may exhibit different activation levels of the MAPK/ERK pathway. Here we performed a comparative analysis of MAPK/ERK activation in single neurons of the food-aversion network, focusing both on command neurons, which mediate withdrawal behavior and process information pertaining to the unconditioned stimulus, and on neurons of the procerebrum, the mollusk's olfactory center, which process information from the conditioned stimulus. By means of Western blots designed to detect micro amounts of proteins, we determined MAPK/ERK activation in these neurons and found that after food aversion learning phospho-ERK levels increased significantly in RPa(2/3) command neurons of the right parietal ganglia and in the procerebrum. Such an increase was prevented by injection of PD98095, an inhibitor of the ERK upstream kinase (MEK-1). In contrast, no activation of MAPK/ERK was detected in similar conditions in the corresponding neurons of the left parietal ganglia LPa(2/3). This asymmetry was verified after serotonin application to the CNS in order to mimic learning. Our results thus show that learning involves synchronous and asymmetric serotonin-dependent MAPK/ERK activation. Such an asymmetry may reflect lateralization of memory processes in the mollusk brain.