It is widely appreciated that memory processing engages a wide range of molecular signaling cascades in neurons, but how these cascades are temporally and spatially integrated is not well understood. To explore this important question, we used
Aplysia californica as a model system. We simultaneously examined the timing and subcellular location of two signaling molecules, MAPK (ERK1/2) and protein kinase A (PKA), both of which are critical for the formation of enduring memory for sensitization. We also explored their interaction during the formation of enduring synaptic facilitation, a cellular correlate of memory, at tail sensory-to-motor neuron synapses. We find that repeated tail nerve shock (TNS, an analog of sensitizing training) immediately and persistently activates MAPK in both sensory neuron somata and synaptic neuropil. In contrast, we observe immediate PKA activation only in the synaptic neuropil. It is followed by PKA activation in both compartments 1 h after TNS. Interestingly, blocking MAPK activation during, but not after, TNS impairs PKA activation in synaptic neuropil without affecting the delayed PKA activation in sensory neuron somata. Finally, by applying inhibitors restricted to the synaptic compartment, we show that synaptic MAPK activation during TNS is required for the induction of intermediate-term synaptic facilitation, which leads to the persistent synaptic PKA activation required to maintain this facilitation. Collectively, our results elucidate how MAPK and PKA signaling cascades are spatiotemporally integrated in a single neuron to support synaptic plasticity underlying memory formation.During signal transduction, single molecules often generate different cellular effects, depending on their temporal dynamics, spatial distribution, and interacting partners (
1). In considering the wide range of molecules implicated in memory processing, the question of how multiple signaling cascades are integrated in time and space to contribute to memory formation and its underlying synaptic plasticity remains a fundamental issue.We have begun to explore this general question in
Aplysia californica, a model system well suited for mechanistic analyses of simple forms of learning. We focused on two molecules, MAPK (ERK1/2) and protein kinase A (PKA), both known to be engaged in many forms of memory and synaptic plasticity (
2–
4). Recent studies, however, suggest the timing, cellular location, and cross-talk between these kinases are critical in determining their ultimate effects (
5–
10). Thus, in addition to knowing that MAPK and PKA are required, it also is important to understand their spatiotemporal dynamics and their interactions during memory formation.
Aplysia provides unique advantages for analyzing these questions. In
Aplysia, memory for sensitization induced by tail shock (TS) is supported in large measure by synaptic facilitation at identified tail sensory-to-motor neuron (SN-MN) synapses (
11). As an analog of behavioral training, tail nerve shock (TNS) also induces synaptic facilitation (
12–
14). A single TNS induces short-term facilitation (STF) lasting <30 min, whereas repeated spaced TNS induces intermediate-term (ITF) and long-term facilitation (LTF) lasting hours and days, respectively. TS/TNS triggers the release of serotonin (5-HT) around SN soma and SN-MN synapses, which activates a series of signaling cascades, including MAPK and cAMP/PKA (
11,
12). MAPK activation is required for the formation of ITF and LTF, but not for STF, whereas cAMP/PKA is required for all three (
15–
18). Finally, although signaling in the synaptic compartment is critical for all forms of synaptic facilitation, it has not yet been established that MAPK and PKA can indeed be activated and exert their function locally at the SN-MN synapse. Nor is it known how they interact with each other during synaptic facilitation.In the present paper, we simultaneously examined MAPK and PKA activation in two subcellular compartments (SN soma and synaptic neuropil) at two time points (immediately and 1 h) after TNS. We found that MAPK was activated immediately and persistently in both compartments after repeated TNS. In contrast, although immediate and persistent PKA activation by repeated TNS also occurred in synaptic neuropil, we observed only delayed PKA activation in SN soma. Interestingly, MAPK activation during, but not after, TNS was essential for synaptic, but not somatic, PKA activation. Synaptic integration of these two signaling cascades in turn led to ITF. These results provide unique insights into both the spatial and temporal features of these two critical molecular cascades, and suggest a model of how they interact to regulate synaptic plasticity underlying memory formation.
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