Differential Role of Mitogen-Activated Protein Kinase in Three Distinct Phases of Memory for Sensitization in Aplysia

MAP kinase activity is required for intermediate-term facilitation of tail SN–tail MN synapses. Representative traces (A) and summary data (B) of EPSP amplitudes showing that ITF was induced by five tail nerve shocks in preparations treated with the inactive analog of MEK inhibitor (U0124), but ITF was completely blocked in preparations treated with the MEK inhibitor (U0126). U0126 or U0124 was applied 45 min before tail nerve shock and was present during the course of nerve shocks and throughout the testing period. Data are presented as mean ± SEM (percentage of change from baseline; 30 min, n = 4 and 5, U0126 and U0124, respectively; 45 min, n = 4 per group). The dashed lines indicate baseline EPSP amplitudes. *p < 0.05.

MAP kinase activity is required for intermediate-term memory for sensitization. A, Schematic diagram of the reduced preparation used for behavioral experiments. In this preparation, pharmacological agents can be selectively applied to the ring ganglia (shaded area). Shocks were administered laterally on the tail at a different site from where the test stimuli were applied (both sites are shown by arrows). B, U0126 (Active) or the inactive analog U0124 (Inactive) was applied to the ring ganglia 30 min before training and was present during training and throughout the testing period. Training was performed using five spaced shocks to the tail (5×S). NS, Control preparations not given any shock. Data are expressed as mean ± SEM duration of T-SW normalized to baseline (Active-5×S, n = 6; Inactive 5×S,n = 6; Active-NS, n = 3). In this and subsequent behavioral figures, horizontal dashed line denotes baseline T-SW, and vertical dashed line denotes training.

MAP kinase activity is required for the induction but not expression of intermediate-term memory. A, MAPK activity is required for the induction of ITM. The MEK inhibitor U0126 (Active; n = 4) or its inactive analog U0124 (Inactive; n = 4) was applied to the ring ganglia 30 min before training and was present during training but washed off immediately after training. Training was performed using five spaced shocks to the tail (5×S). Data are expressed as mean ± SEM duration of T-SW normalized to baseline. B, The MEK inhibitor blocks MAPK activation when applied 30 min after five pulses of 5-HT. Pleural-pedal ganglia were treated with five pulses of 5-HT. U0126 or U0124 was applied 30 min after the last pulse of 5-HT and incubated for 60 min, and MAPK activation was examined in the SNs using phospho-MAPK and phospho-independent MAPK antibodies. Sample blots with phospho-MAPK and total MAPK antibodies are shown. C, MAPK activity is not required for the expression of ITM. U0126 (Active;n = 6) or U0124 (Inactive; n = 5) was applied to the ring ganglia after the 30 min posttraining test and was present throughout testing. Training was performed using five spaced shocks to the tail (5×S). Data are expressed as mean ± SEM duration of T-SW normalized to baseline.

MAP kinase activity is not required for short-term memory for sensitization. U0126 (Active; n = 6) or the inactive analog U0124 (Inactive; n = 5) was applied 70 min before training and was present throughout testing. Training was performed using a single shock to the tail (1×S). Data are expressed as mean ± SEM duration of T-SW normalized to baseline. Inset, A single pulse of 5-HT does not activate MAPK. Pleural-pedal ganglia were treated with one pulse of 5-HT, SN clusters were excised, and MAPK activation was examined using phospho-MAPK and phospho-independent MAPK antibodies. Sample blots with phospho-MAPK and total MAPK antibodies are shown.