Memory formation is highly sensitive to specific patterns of training, but the cellular and molecular mechanisms underlying pattern sensitivity are not well understood. We explored this general question by using Aplysia californica as a model system. We examined the regulation of MAPK (ERK1/2) activation by small G proteins in the CNS by using different patterns of analog stimuli that mimic different patterns of behavioral training for memory induction. We first cloned and characterized the Aplysia homologs of the small G proteins, Ras and Rap1 (ApRas and ApRap, respectively). We next examined changes in ApRas and ApRap activity that accompany MAPK activation. Last, by delivering recombinant ApRas and ApRap into the CNS, we directly manipulated their activity and examined the resultant MAPK activation. We found that MAPK activation induced by analog training depends on the combined activity of ApRas and ApRap, rather than the individual activity of either one alone. Also, ApRas and ApRap have a complex role in MAPK activation: they can act as activators or inhibitors, depending on the specific pattern of the training. The patternsensitive regulation of MAPK by interactive ApRas and ApRap activity that we have identified could contribute to the molecular routing of different downstream effects of spatially localized MAPK required for the induction of specific pattern-sensitive forms of synaptic facilitation and memory.

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. D uring 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 crosstalk 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)(...