Computational design of enhanced learning protocols

Abstract

Learning and memory are influenced by the temporal pattern of training stimuli. However, the mechanisms that determine the effectiveness of a particular training protocol are not well understood. We tested the hypothesis that the efficacy of a protocol is determined in part by interactions among biochemical cascades that underlie learning and memory. Previous findings suggest that the protein kinase A (PKA) and extracellular signal-regulated kinase (ERK) cascades are necessary to induce long-term synaptic facilitation (LTF) in Aplysia, a neuronal correlate of memory. We developed a computational model of the PKA and ERK cascades and used it to identify a training protocol that maximized PKA and ERK interactions. In vitro studies confirmed that the protocol enhanced LTF. Moreover, the protocol enhanced the levels of phosphorylation of the transcription factor CREB1. Behavioral training confirmed that long-term memory also was enhanced by the protocol. These results illustrate the feasibility of using computational models to design training protocols that improve memory.

Figure 1. Computational model of PKA and ERK pathways

(a) Schematic of the model. 5-HT activates PKA via cAMP and activates ERK via Raf-MEK. PKA and ERK interact, at least in part, via phosphorylation of transcription factors, to induce LTF. The variable inducer represents the PKA/ERK interaction. (b–c) Simulated time courses of activated PKA (PKA_C, red traces), activated ERK (ERK^pp, yellow traces), and inducer (violet traces) in response to five, 5-min pulses of 5-HT (green traces). The Standard protocol (b) represents the protocol generally used in studies of LTF in vitro. The Enhanced protocol (c) produced the largest peak in the concentration of inducer. The patterns of 5-HT pulses are illustrated in each panel. The Standard protocol had uniform ISIs of 20 min, whereas the Enhanced protocol had non-uniform ISIs of 10, 10, 5 and 30 min. Units of concentration of all variables are µM.

Figure 2. Enhanced protocol increased the magnitude and duration of LTF

Two 5-HT protocols (Enhanced, red traces; Standard, orange traces) were tested (protocol parameters as in Fig. 1b–c). In addition, a vehicle (Control) protocol (black traces) was tested in which sensorimotor co-cultures were not exposed to 5-HT. (a) A single EPSP was elicited immediately prior to 5-HT treatment (Pretest). Individual EPSPs were also elicited 1 d, 2 d, and 5 d after treatment (post-tests). The dashed lines indicate the amplitude of the Pretest EPSP. (b) In each sensorimotor pair, the EPSPs that were measured at 1, 2, and 5 days were normalized to the Pretest EPSP. A value of 1 (dashed line) represented no change in EPSP amplitude. The Enhanced protocol produced greater and longer-lasting LTF. In this figure and subsequent figures, summary data are represented as means ± s.e.m.

Figure 3. Enhanced protocol increased the levels of phosphorylated CREB1

(a) Immunofluorescence staining of phosphorylated CREB1 in sensory neuron cultures. One group of cultures received the Standard protocol (five, 5-min pulses of 5-HT with uniform ISIs of 20 min), a second group the Enhanced protocol (five, 5-min pulses of 5-HT with ISIs of 10, 10, 5, and 30 min) and a third group of cultures served as vehicle controls (five, 5-min pulses of vehicle with uniform ISIs of 20 min). Nuclear staining of phosphorylated CREB1 was measured in the cell bodies of sensory neurons immediately and 18 h after 5-HT treatment. Scale bar, 20 µm. (b) Summary data from the Standard (orange bars, S) and Enhanced (red bars, E) protocols. The Enhanced protocol induced significantly greater levels of phosphorylated CREB1 at both time points.

Figure 4. Enhanced protocol induced LTS that persisted for at least 5 d

For these behavioral experiments, one group of animals was trained using the Standard protocol (orange bars, S) and a second group was trained using the Enhanced protocol (red bars, E). Training stimuli were applied to only one side of each animal (i.e., the ipsilateral side). The contralateral side of each animal served as a control (black bars, C). Both the Standard and Enhanced protocols induced LTS, as indicated by the significant increase in responses (i.e., the duration of the tail-elicited siphon withdrawal) 1 d after training as compared to control. At 1 d, the magnitude of LTS was not significantly different between the Standard and Enhanced protocols (q₂ = 0.76). However, only the Enhanced protocol induced LTS that persisted for at least 5 d. For the Enhanced protocol, the levels of LTS at 1 d and 5 d were not significantly different (q₂ = 2.63).