Abstract
The effects of the specific p42/44 mitogen-activated protein (MAP) kinase cascade inhibitor, PD98059, were investigated on three types of long-term potentiation (LTP) in the medial perforant path of the rat dentate gyrus in vitro: LTP induced by 1) high-frequency stimulation (HFS-LTP), 2) application for 10 min of the K+ channel blocker, tetraethylammonium chloride (TEA-LTP), and 3) application of the metabotropic glutamate receptor (mGluR) agonist (S)-dihydrophenylglycine (S-DHPG) for 2 min (DHPG-LTP). Bath perfusion of PD98059 (50 microM) for 1 h inhibited HFS-LTP (111 +/- 5%, mean +/- SE, at 90 min posttetanus in test slices compared with 144 +/- 5% in control slices; n = 6-7). Concentrations of 10 and 20 microM PD98059 had no effect on HFS-LTP (n = 6). PD98059 (50 microM) had no effect on the isolated N-methyl--aspartate excitatory postsynaptic potential (NMDA-EPSP) or on the maintenance phase of HFS-LTP. PD98059 (50 microM) did not affect paired-pulse depression (PPD; interstimulus intervals of 10 and 100 ms) of synaptic transmission as is typically observed in the medial perforant path of the dentate gyrus. Bath application of (S)-DHPG (40 microM) for 2 min gave rise to a potentiation of the EPSPs slope (148 +/- 4% at 1 h post-DHPG wash out; n = 5). Pretreatment of slices with PD98059 (50 microM) inhibited the DHPG-LTP (98 +/- 3% at 1 h post-DHPG wash out; n = 5). The TEA-LTP (125 +/- 4% at 1 h post-TEA wash out; n = 6) was found to be both -2-amino-5-phosphonopentanoic acid (-AP5; 100 microM) and nifedipine (20 microM) independent. However, the T type voltage-dependent calcium-channel blocker, NiCl2 (50 microM), completely inhibited the observed potentiation. The mGluR receptor antagonist alpha-methyl-4-carboxy-phenyl glycine (MCPG; 100 microM) and PD98059 (50 microM) caused a complete block of the TEA-LTP. These data show for the first time an involvement of the p42/44 MAP kinase in the induction and expression of both an NMDA-dependent and two forms of NMDA-independent LTP in the dentate gyrus.
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