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The Journal of Neuroscience, November 16, 2005, 25(46):10664-10670; doi:10.1523/JNEUROSCI.2443-05.2005

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Cellular/Molecular
Mitogen-Activated Protein Kinase-Mediated Reinforcement of Hippocampal Early Long-Term Depression by the Type IV-Specific Phosphodiesterase Inhibitor Rolipram and Its Effect on Synaptic Tagging

Sheeja Navakkode, Sreedharan Sajikumar, and Julietta Uta Frey

Department of Neurophysiology, Leibniz Institute for Neurobiology, D-39118 Magdeburg, Germany

Rolipram, a selective inhibitor of cAMP-specific phosphodiesterase 4 (PDE4), has been shown to reinforce an early form of long-term potentiation (LTP) to a long-lasting LTP (late LTP). Furthermore, it was shown that the effects of rolipram-mediated reinforcement of LTP interacts with processes of synaptic tagging (Navakkode et al., 2004). Here we show in CA1 hippocampal slices from adult rats in vitro that rolipram also converted an early form of long-term depression (LTD) that normally decays within 2-3 h, to a long-lasting LTD (late LTD) if rolipram was applied during LTD-induction. Rolipram-reinforced LTD (RLTD) was NMDA receptor- and protein synthesis-dependent. Furthermore, it was dependent on the synergistic coactivation of dopaminergic D₁ and D₅ receptors. This let us speculate that RLTD resembles electrically induced, conventional CA1 late LTD, which is characterized by heterosynaptic processes and synaptic tagging. We therefore asked whether synaptic tagging occurs during RLTD. We found that early LTD in an S1 synaptic input was transformed into late LTD if early LTD was induced in a second independent S2 synaptic pathway during the inhibition of PDE by rolipram, supporting the interaction of processes of synaptic tagging during RLTD. Furthermore, application of PD 98059 (2'-amino-3'-methoxyflavone) or U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene), specific inhibitors of mitogen-activated protein kinases (MAPKs), prevented RLTD, suggesting a pivotal role of MAPK activation for RLTD. This MAPK activation was triggered during RLTD by the synergistic interaction of NMDA receptor- and D₁ and D₅ receptor-mediated Rap/B-Raf pathways, but not by the Ras/Raf-1 pathway in adult hippocampal CA1 neurons, as shown by the use of the pathway-specific inhibitors manumycin (Ras/Raf-1) and lethal toxin 82 (Rap/B-Raf).

Key words: hippocampus; long-term potentiation; neuromodulation; protein synthesis; reinforcement; long-term depression; memory formation

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Received June 15, 2005; revised August 10, 2005; accepted October 7, 2005.

This article has been cited by other articles:

				S. Sajikumar, S. Navakkode, V. Korz, and J. U. Frey Cognitive and emotional information processing: protein synthesis and gene expression J. Physiol., October 15, 2007; 584(2): 389 - 400. [Abstract] [Full Text] [PDF]

				S. Sajikumar, S. Navakkode, and J. U. Frey Identification of Compartment- and Process-Specific Molecules Required for "Synaptic Tagging" during Long-Term Potentiation and Long-Term Depression in Hippocampal CA1 J. Neurosci., May 9, 2007; 27(19): 5068 - 5080. [Abstract] [Full Text] [PDF]

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