Distinct LTP induction mechanisms: contribution of NMDA receptors and voltage-dependent calcium channels.

1. Our results indicate that there are two distinct components of long-term potentiation (LTP) induced by the K+ channel blocker tetraethylammonium chloride (TEA) at synapses of hippocampal CA1 pyramidal neurons. Preincubation of hippocampal slices in the N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5 phosphonovalerate (D,L-APV, 50 microM), reduced the magnitude of TEA LTP. In addition, the L-type voltage-dependent Ca2+ channel (VDCC) antagonist nifedipine (10 microM) attenuated TEA LTP. Only the combined application of D,L-APV plus nifedipine blocked the induction of TEA LTP. 2. Occlusion experiments demonstrated that saturation of VDCC-dependent TEA LTP did not reduce or occlude NMDA-receptor-dependent TEA LTP. These results indicate that the mechanisms underlying VDCC and NMDA receptor components of TEA LTP are different and do not share a common saturable mechanism. 3. TEA LTP was strictly dependent on NMDA receptor activity in slices with CA3-CA1 connections severed (isolated CA1 slices). In contrast to results obtained in slices with intact CA3-CA1 connections, the NMDA receptor antagonists APV (50 microM) or MK-801 dizocilpine (10 microM) completely blocked TEA LTP in isolated CA1. Consistent with this observation, the properties of TEA LTP in isolated CA1 were very similar to other types of NMDA-receptor-dependent plasticity such as tetanus-induced LTP; TEA LTP required presynaptic stimulation, displayed pathway specificity, and was occluded by tetanus-induced LTP.(ABSTRACT TRUNCATED AT 250 WORDS)