Two forms of long-term potentiation in area CA1 activate different signal transduction cascades.

1. The effects of protein kinase inhibitors on N-methyl-D-aspartate (NMDA)-receptor-mediated, voltage-dependent calcium channel (VDCC)-mediated, and 100-Hz long-term potentiation (LTP) were studied in area CA1 of rat hippocampal slices. 2. A 25-Hz tetanus induced a quickly developing potentiation that was blocked by the NMDA antagonist D,L-2-amino-5-phosphonovaleric acid (APV) and was not affected by the L-type VDCC inhibitor nifedipine, suggesting that it was mediated by NMDA receptors (NMDA-LTP). 3. Application of a 200-Hz tetanus in APV induced a slowly developing NMDA-receptor-independent potentiation that was blocked by nifedipine and thus named VDCC-LTP. NMDA- and VDCC-LTP reached comparable magnitudes despite their different induction parameters and developmental kinetics. 4. Bath perfusion of the broad-spectrum serine/threonine kinase inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) blocked NMDA-LTP but not VDCC-LTP, whereas the tyrosine kinase inhibitors genistein and lavendustin A blocked VDCC-LTP but not NMDA-LTP. These results suggest a differential involvement of H-7-sensitive serine/threonine kinases and tyrosine kinases in the two forms of LTP. 5. Tetanization of 200 Hz in control media resulted in a compound potentiation twice as large as NMDA- or VDCC-LTP, implying that the two forms of LTP did not facilitate or reduce each other's expression. The often-used 100-Hz tetanus (1 s twice) induced a potentiation that was comparable in size with the 200-Hz compound LTP. Nifedipine, genistein, and lavendustin A reduced the 100-Hz LTP by approximately 50%, suggesting that this LTP is also a compound potentiation consisting of NMDA- and VDCC-mediated components and their corresponding signal transduction pathways.