Synaptic consolidation: an approach to long-term learning

Abstract

Synaptic plasticity is thought to be the basis of learning and memory, but it is mostly studied on the timescale of mere minutes. This review discusses synaptic consolidation, a process that enables synapses to retain their strength for a much longer time (days to years), instead of returning to their original value. The process involves specific plasticity-related proteins, and depends on the dopamine D1/D5 receptors. Here, we review the research on synaptic consolidation, describing electrophysiology experiments, recent modeling work, as well as behavioral correlates.

Keywords: Behavior; Electrophysiology; Model; Review; Synaptic consolidation; Synaptic plasticity; Synaptic tagging.

Fig. 1

Schematic of synaptic tagging experiments. a Synaptic weight change resulting of a presynaptic extra-cellular stimulation of a weak tetanus (21 pulses at 100 Hz), b strong tetanus (3 times 100 pulses at 100 Hz), c weak tetanus in the S1 pathway (dashed line) and strong tetanus in the S2 pathway (solid line), d strong tetanus in the S2 pathway followed 1 h later by a weak tetanus in the S1 pathway. Plasticity-related-protein synthesis inhibitor is applied during a period that starts 25 min before the induction of the strong tetanus, and lasts until 1 h after the weak tetanus induction (horizontal bar). e. Same as (d) but the inhibitor is applied during a period that starts 25 min after the strong tetanus, and lasts until 1 h after the weak tetanus. See Frey and Morris (1997), Sajikumar and Frey (2004a) for details

Fig. 2

Models of synaptic consolidation. a–c TagTriC model by Clopath et al. (2008). The synaptic weight is a sum of the early-weight and the late-weight. The three phases of the model. a Tag: The early-weight can be in the neutral (N), low (L) or high (H) state. If in the H or L state, a tag is set (flag). b Trigger: If the number of tags exceed a threshold as of function of dopamine (DA), plasticity-related proteins are synthesized (squares). c Consolidation: If the synapse is tagged and plasticity-related proteins are available, the late-weight (circle) goes to the elevated stable state if the early-weight is in H or the low stable state if the early-weight is in L. E stands for the energy landscape of the late-weight, showing the two stable states unless the tag and the plasticity-related proteins are presents, then E only has one stable state. d–f Consolidation Model by Barrett et al. (2009). Each synapse has 2 possible states with 3 meta-states each, so 6 states in total. d The synapse can be in the early-state without tag, e early-state with the tag, or f consolidated state, each for both a low weight or a high weight. See original papers for details

Fig. 3

Behavioral tagging. a Inhibitory avoidance training, followed by a couple of hours of memory. b Inhibitory avoidance training paired with novel experience followed by more than 24 h of memory. See Moncada and Viola (2007) for details