Transient mitogen-activated protein kinase activation is confined to a narrow temporal window required for the induction of two-trial long-term memory in Aplysia

Abstract

Although it is commonly appreciated that spaced training is superior to massed training in memory formation, the molecular mechanisms underlying this feature of memory are largely unknown. We previously described the selective benefit of multiple spaced (vs massed) training trials in the induction of long-term memory (LTM) for sensitization in Aplysia californica. We now report that LTM can be induced with only two spaced training trials [tail shocks (TSs)] when the second TS is administered 45 min after the first. In contrast, spacing intervals of 15 and 60 min are ineffective. This surprisingly narrow permissive training window for two-trial LTM is accompanied by an equally narrow window of transient mitogen-activated protein kinase (MAPK) activation, a necessary signaling molecule for LTM induction, at 45 min after a single TS. Thus, the transient recruitment of MAPK following a single TS may provide a narrow molecular window for two-trial LTM formation.

Figure 1.

A single spaced TS before or after massed training induces LTM. A, B, Four spaced TSs (ISI of 15 min) induce 24 and 48 h LTM (A), whereas massed TSs (ISI of 1 s) do not (B). C, D, Three massed TSs contribute to LTM induction when a single spaced TS is administered either 45 min after (C) or 45 min before (D) the massed TSs. Training patterns are illustrated above their respective datasets. In this and subsequent figures, data are expressed as mean ± SEM.

Figure 2.

A, B, A narrow temporal window for LTM induction. Two TSs (ISI of 45 min) induce LTM comparable to that induced by four TSs (ISI of 15 min) (A), whereas two TSs spaced 15 or 60 min apart do not (B).

Figure 3.

Schematic summary of permissive and nonpermissive training patterns for LTM induction. Green horizontal bars represent permissive training patterns for LTM induction, and red horizontal bars represent nonpermissive patterns.

Figure 4.

MAPK is transiently activated in tail SNs 45 min following a single TS. A, Representative immunoblots of membranes probed for both P-MAPK and T-MAPK fractions in tail SNs from trained animals. std, A common CNS standard used to normalize MAPK activation from different samples. B, Histograms reflect mean ± SEM normalized MAPK activation ratios from tail SNs isolated (1) during nonpermissive intervals for LTM induction (15 and 60 min after a single TS; red), (2) during a permissive training interval (45 min; green), or (3) from untrained controls (white).