Rolipram, a type IV-specific phosphodiesterase inhibitor, facilitates the establishment of long-lasting long-term potentiation and improves memory

Abstract

In an attempt to improve behavioral memory, we devised a strategy to amplify the signal-to-noise ratio of the cAMP pathway, which plays a central role in hippocampal synaptic plasticity and behavioral memory. Multiple high-frequency trains of electrical stimulation induce long-lasting long-term potentiation, a form of synaptic strengthening in hippocampus that is greater in both magnitude and persistence than the short-lasting long-term potentiation generated by a single tetanic train. Studies using pharmacological inhibitors and genetic manipulations have shown that this difference in response depends on the activity of cAMP-dependent protein kinase A. Genetic studies have also indicated that protein kinase A and one of its target transcription factors, cAMP response element binding protein, are important in memory in vivo. These findings suggested that amplification of signals through the cAMP pathway might lower the threshold for generating long-lasting long-term potentiation and increase behavioral memory. We therefore examined the biochemical, physiological, and behavioral effects in mice of partial inhibition of a hippocampal cAMP phosphodiesterase. Concentrations of a type IV-specific phosphodiesterase inhibitor, rolipram, which had no significant effect on basal cAMP concentration, increased the cAMP response of hippocampal slices to stimulation with forskolin and induced persistent long-term potentiation in CA1 after a single tetanic train. In both young and aged mice, rolipram treatment before training increased long- but not short-term retention in freezing to context, a hippocampus-dependent memory task.

Figure 1

Effects of rolipram on cAMP metabolism of hippocampal slices. Low concentrations, 0.03 μM and 0.3 μM rolipram, had no significant effect on basal cAMP concentrations (open bars). However, the increase in cAMP concentration after 15 min of treatment with 5 μM forskolin (hatched bars) was significantly greater with 0.3 μM rolipram than with vehicle (^†P < 0.01). At 3.0 μM rolipram, the forskolin-stimulated increases of cAMP were also significantly amplified (^†P < 0.003 compared with control), but unstimulated basal cAMP concentrations were also significantly elevated (∗P < 0.001).

Figure 2

Effects of rolipram on synaptic transmission and LTP. Rolipram, 0.1 μM or 3.0 μM, had no significant effect on test fEPSPs generated at the CA3–CA1 synapse of C57\Bl6 hippocampal slices (A). However, when LTP was induced by a single tetanus of 100 Hz × 1 s at the same stimulus intensity in the presence of 0.1 μM or 0.3 μM rolipram, the resultant LTP was both larger and longer-lasting than in the absence of PDE inhibitor. Rolipram (0.1 μM) had no effect on the size or duration of LTP when perfused immediately after the stimulus (B).

Figure 3

Effects of rolipram on memory. Pretreatment with 0.1 μmol/kg rolipram had no effect on immediate freezing in the training chamber or on freezing to context when mice were returned to the training chamber after 1 hr. However, when mice were tested 24 hr later, rolipram increased freezing significantly over vehicle treatment alone (**P < 0.01). Injection with 3.0 μmol/kg rolipram before treatment had no effect on freezing to context after 24 hr. Pretreatment with 0.1 μmol/kg rolipram increased freezing to context in 18-month-old mice 24 hr after training (*P < 0.05).

Figure 4

Rolipram effects on hippocampus-independent behaviors. (A) Nociception: Injection of mice with 0.1 μmol/kg rolipram had no effect on nociception measured by average amperage to induce flinching, jumping, or running or crying by footshock. (B) Open field: Injection of mice with 0.1 μmol/kg or 3.0 μmol/kg rolipram affected open-field behavior only on the day of injection. Mice injected with 3.0 μmol/kg essentially lay motionless on the floor of the cage throughout the testing period on day one. Mice injected with 0.1 μmol/kg showed a modest decrease in activity from 20 to 60 min after injection but were identical to vehicle-injected controls by 70 min after injection. After 24 hr, there was no residual effect of either rolipram dose on open-field behavior. (C) Cued water maze: Injection of 0.1 μmol/kg 30 min before the first day of training on the visible platform water maze had no significant effect on first day behavior, but delayed acquisition of the task so that rolipram-treated animals required an extra day to match the behavior of vehicle-treated controls.