Bidirectional regulation of hippocampal long-term synaptic plasticity and its influence on opposing forms of memory

Abstract

Reference memory characterizes the long-term storage of information acquired through numerous trials. In contrast, working memory represents the short-term acquisition of trial-unique information. A number of studies in the rodent hippocampus have focused on the contribution of long-term synaptic potentiation (LTP) to long-term reference memory. In contrast, little is known about the synaptic plasticity correlates of hippocampal-based components of working memory. Here, we described a mouse with selective expression of a dominant-negative mutant of the regulatory subunit of protein kinase A (PKA) only in two regions of the hippocampus, the dentate gyrus and area CA1. This mouse showed a deficit in several forms of LTP in both hippocampal subregions and a lowered threshold for the consolidation of long-term synaptic depression (LTD). When trained with one trial per day in a water maze task, mutant mice displayed a deficit in consolidation of long-term memory. In contrast, these mice proved to be more flexible after a transfer test and also showed a delay-dependent increased performance in working memory, when repetitive information (proactive interference) was presented. We suggest that through its bidirectional control over synaptic plasticity PKA can regulate opposing forms of memory. The defect in L-LTP disrupts long-term memory consolidation. The persistence of LTD may allow acquisition of new information by restricting the body of previously stored information and suppressing interference.

Figure 1.

R(AB) transgene expression in the mouse brain. A, Top, Schematic representation of the 5′ end grp gene promoter sequences in front of the translation initiation site ATG, which were used to generate grp-R(AB) transgene shown in the bottom. The grp-R(AB) transgene includes 12 kb of the grp promoter, three copies of the neuron-restrictive silencer element (NRSE), an SV40-derived hybrid exon/intron splicing sequences (closed boxes) and a 5′ untranslated leader, R(AB) cDNA, and an SV40-derived 3′ exon/intron splicing sequences and a polyadenylation signal. B, RNA in situ hybridization shows expression of the R(AB) transgene in the brain. Coronal brain sections from grp-R(AB) transgenic mice were hybridized with a digoxigenin-labeled RNA probe specific to the 3′ end SV40 sequences present in the transgene. Transgene expression is limited to CA1, dentate gyrus (DG) and cortex on the brain sections corresponding to the area ranging from the dorsal to ventral hippocampus. PFC, Medial prefrontal cortex.

Figure 2.

Forskolin-induced phosphorylation of CREB is reduced in the CA1/DG-R(AB) transgenic mice in the CA1 area of the hippocampus. A, Scheme of injection sites of forskolin (FSK) in the CA1 area. B, Representative sections illustrating P-CREB immunoreactivity in the dorsal hippocampus of naive mice (B1 and B2) and mice injected with FSK (B3 and B4) for control mice (B1 and B2) and mutant mice (B3 and B4). C, Bar graphs represent means (±SEM) of the number of pCREB-positive neurons/mm2 in the CA1 for naive mice (n = 5 for both genotypes) and 1 h after bilateral intrahippocampal injections of FSK (0.3 μg in 0.2 μl) (mutant mice, n = 8; control mice, n = 7). *Significantly different (p < 0.05).

Figure 3.

Deficit in the late phase of LTP correlates with R(AB) expression. A–C, Four trains of HFS (1 s train at 100 Hz, 5 min intertrain interval) produced the long-lasting L-LTP in control mice that was abolished only in SC-CA1 (A) and PP-DG (B), but not CP-CA3 (C), pathways in mutant mice (n = 5/5 for each experiment). D–F, Similarly, four trains of theta bursts stimulation (TBS, 5 min intertrain interval) produced L-LTP in control mice that was also abolished in SC-CA1 (D) and PP-DG (E), but not CP-CA3 (F), pathways in mutant mice (n = 5/5 for each experiment). The early phase of LTP is unaffected in mutant mice G, H, The transient early phase of LTP (E-LTP) induced by one train of HFS (1 s train at 100 Hz) was unaffected in the SC-CA1 (G) or the PP-DG (H) pathways in mutant mice (n = 5/5 for each experiment). I, J, Whereas one train of TBS produced normal LTP in the SC-CA1 pathway (I), a deficient LTP was observed in the PP-DG pathway (J) (n = 5/5 for each experiment).

Figure 4.

Enhanced LTD in DG/CA1-R(AB) transgenic mice. A, B, Transient LTD induced by 15 min of 1 Hz stimulation was transformed into a long-lasting LTD in mutant mice in the SC-CA1 (A) and in the PP-DG (B) pathways (n = 7/7 for each experiment). Insets, Effect of the PKA inhibitor KT5720 (1microM) on the expression of LTD in the SC-CA1 (A) and the PP-DG (B) pathways. C, D, Synaptic potentiation induced in response to one train of TBS was successfully depotentiated by 5 Hz stimulation for 3 min in both the SC-CA1 (C) and in the PP-DG (D) pathways in each group of mice (n = 5/5 for each experiment). Metaplastic shift in the PP-DG pathway. E, F, LTP induced by 90 s of 10 Hz stimulation (900 pulses) produces a long-lasting and a transient form of LTP in the SC-CA1 (E) and in the PP-DG (F) pathways respectively in control mice. Transgene expression differentially affected each pathway in mutant mice (n = 5/5 for each experiment). G, H, Three minutes of 5 Hz stimulation (900 pulses) produced no long-term plasticity change in control mice in either the SC-CA1 (G) or the PP-DG (H) pathways. However, a modest LTD was observed in the PP-DG pathway, but not in the SC-CA1 pathway, of mutant mice in response to this stimulation (n = 5/5 for each experiment). Frequency–response curves (1, 5, and 10 Hz) from SC-CA1 (I) and PP-DG (J) pathways. Mutant mice show a mild but significant metaplastic shift toward the LTD portion of the curve at the PP-DG but not the SC-CA1 pathway.

Figure 5.

NMDA receptor function in mutant mice. A, NMDA receptor component is increased in mutant mice. Top traces, Depolarization envelope during 1 s at 100 Hz 1 in normal ACSF solution in slices from control (black) and mutant (gray) mice. Middle traces, Same condition but in slices treated with the NMDA receptor blocker APV (100 μm). Bottom traces, Arithmetic subtraction between the ACSF (top) and APV (middle) traces reveals the NMDA receptor activation-dependent component of the depolarization envelope. Each trace corresponds to the mean of six independent observations. B, Analysis of the first 50 ms of 1 s tetanic stimulation (bottom). The amplitude of synaptic decrease during tetanus is similar in control (black) and mutant (gray) mice (top left); however, mutant mice show larger depolarization (cumulative area under each trace) compared with control mice (top right). Each trace corresponds to the mean of eight independent observations. C, NMDA-LTD is enhanced in mutant mice. Addition of NMDA (30 μm, 10 min) induces long-lasting synaptic depression with larger amplitude in mutant mice.

Figure 6.

Mutant mice showed normal acquisition and memory when trained with four trials per day in the water maze. Performance (path length: each point represents the average of 4 trials/d) of mutant and control (n = 15/15) mice in the water maze task. A, B, Visible platform nonspatial/hippocampal (A) and hidden platform spatial/hippocampal (B) versions of the task (no significant effect of genotype in both cases). C, Memory of the platform location was tested with a probe trial realized 1 week after training, on day 19. No significant difference in the amount spent in the target quadrant (TQ: where the platform was located during training) was found between groups (AQ: average of the two adjacent quadrants; OQ: opposite quadrant). Inset shows the amount of crossings in each virtual platform location in the corresponding quadrants. D–F, No difference was found in swim speed (D), percentage of time spent floating (E), or thigmotaxis (percentage of time spent in periphery; F), indicating that transgene expression did not lead to major behavioral deficit.

Figure 7.

Mutant mice showed increased forgetting and cognitive flexibility. A–C, Performance (A) and probe (B) trials performed at the end of training (probe 1) and 1 week after (probe 2) (C) of a second group of naive mice when trained with a one-trial-per-day regimen. Even though no difference was initially seen between groups during training (A), the statistical analysis realized on probe trial (B) showed that mutant mice were not able to locate as accurately as controls the target platform location (significant genotype effect: P TQ p = 0.042). A week later (C), they also show a total loss of memory for this position as they were not even able to preferentially swim in the target quadrant as controls did (TQ p = 0.001; P TQ p = 0.015) suggesting impaired ability to consolidate information in long-term memory. We thus observed a decrease in the time spent searching the TQ (percentage of time spent in TQ during probe 2 − percentage of time spent during probe 1) in mutant, but not control mice (genotype effect; p = 0.008), indicating forgetting of the platform location. D, Path length (same group of mice as in Fig. 6) during a transfer task (platform placed in the opposite quadrant) plotted against day (one trial per day); no significant difference between groups. E, F, Statistical analysis of the probe trials realized on day 9 (E) and a week later on day 16 (F) revealed that mutant mice were able to flexibly learn the new platform position more efficiently than controls [Significant genotype effect for probe (F) p = 0.017, and probe (F) p = 0.004] as they spent more time in the new TQ (TQ: new target quadrant; OQ: old target quadrant). Mutant mice were also crossing significantly more than controls in the platform target location on day 16 (p = 0.003) but not on day 9 (insets E and F).

Figure 8.

Mutant mice showed enhanced delay-dependent working memory when manipulating a limited number of items presented repetitively. A, Number of errors in a LI/HML working memory task (limited interference/high memory load). Inset shows an example for three trials (1 per day) composed of a sample phase (4 arms, randomly chosen every day, are opened; dark blue), followed by a delay phase (mouse restrained on the central platform for 5 s), followed by the test phase [all 8 arms are opened; number of errors (runs in an already visited/nonbaited arm) is scored]. No significant effect of genotype was observed. B, Score (same group of mice; n = 12/12) in a LML working memory task (LML low memory load; 1 arm to remember among 2). Inset shows an example for three trials (4 per day) composed of a sample phase (one arm and then another opposite arm, randomly chosen, are opened), followed by a delay phase (mouse restrained on the platform for 5 s (training), 1 or 2 min), followed by the test phase (two pairs of arms are opened; the same two pairs for every trials; the pairs of arms is composed by an arm already visited during the sample phase and an adjacent baited arm) in which the number of correct choices is scored (runs in an arm not visited during the sample phase). The bar graph represents the average score for 1 week of experiment at each delay. No difference was found between groups during training (5 s delay), but mutant mice showed enhanced performance when the delay increased to 1 and 2 min (p = 0.008 and 0.028 respectively) compared with control subjects. C, Score (new group: control n = 14; mutant n = 12) in a LI/LML working memory task. Only one pair (pseudorandomly determined; different pair for every trial among 8 possible pair arrangements) is now presented by trial; six trials/d; score plotted against blocks (one block = two daily sessions of 6 trials each = 12 trials). During both training (block B1 to B5; delay phase = 15 s) and testing (increased delay phase; 20–75 s), no difference was observed between groups. D, Same task as in C. However, the same pair of arms is now used for every trial (increased proactive interference; HI/LML protocol). Using this procedure, mutant mice showed enhanced performance for a delay of 55 s compared with controls (p = 0.027).