Expression of MAP/ERK kinase cascade corresponds to the ability to develop food aversion in terrestrial snail at different stages of ontogenesis.

The mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) cascade plays an important role in gene expression regulation during memory formation in both vertebrates and invertebrates. MAPK/ERK regulates gene expression through phosphorylation of transcription factors binding to the regulatory elements SRE and CRE of target genes. Previously we reported that juvenile snails Helix lucorum differ from adult animals in a spectrum of transcription factors binding to DNA regulatory elements SRE and AP-1. In this study we analyzed the expression and activation of MAPK/ERK in CNS of H. lucorum during formation of the conditioned avoidance reflex at different stages of postnatal ontogenesis. Under conditions of learning, juvenile snails (aged 2-3 months) possessing immature mechanisms of avoidance reflex plasticity showed dramatically low level of phosphorylation and, correspondingly, low activation of MAPK/ERK in comparison to adult animals. Beside this, the MAPK/ERK cascade was not activated after 10 and 60 min after learning in juvenile snails in contrast to adults, while basal expression level of this kinase was similar in juveniles and adults. Low activation of MAPK/ERK cascade can cause a deficiency in phosphorylation of downstream transcription factors binding to SRE and thereby influence the expression of early response genes (particularly, of the family AP-1) and late response genes necessary for cellular and synaptic plasticity. These observations suggest that the MAPK/ERK regulatory cascade plays an essential role in the formation of conditioned avoidance reflexes in Helix. Low activation of this cascade might be one of the reasons for deficiency of long-term memory formation during avoidance learning in juvenile animals.