A large sustained Ca2+ elevation occurs in unstimulated spines during the LTP pairing protocol but does not change synaptic strength

Synapses in the CA1 region of the hippocampus undergo bidirectional synaptic modification in response to different patterns of activity. Postsynaptic Ca2+ elevation can trigger either synaptic strengthening or weakening, depending on the properties of the local Ca2+ signal. During the pairing protocol for long-term potentiation (LTP) induction, the cell is depolarized under voltage-clamp and is given low-frequency synaptic stimulation. As an initial step toward understanding the Ca2+ dynamics during this process, we used confocal microscopy to study the Ca2+ signals in spines evoked by the depolarization itself. This depolarization activates voltage-dependent Ca2+ channels (VDCC), but whether these channels inactivate rapidly or remain functional throughout the long depolarizations used in the pairing protocol remains unknown. Cells were depolarized to 0 mV for 2-3 min. This depolarization led to a large initial elevation of Ca2+ in spines that never decayed back to resting levels. The maintained signal was close to the Kd of the low-affinity (5 microM) Ca2+ dye, Magnesium Green. We attempted to determine the functional role of this elevation, using the Ca2+-channel blocker D-890. The addition of D-890 in the internal solution produced a nearly complete abolition of the Ca2+ elevation during depolarization. Under these conditions, the NMDA conductance was normal, but LTP was almost completely blocked. This might suggest the importance of VDCC in LTP; however, we found that high concentrations of D-890 can directly inhibit calmodulin protein kinase II (CaMKII), an enzyme required for LTP induction. Thus, whereas D-890 is a useful tool for blocking postsynaptic VDCC, it cannot be used to study the contribution of these channels to plasticity. We conclude that the activation of VDCC produces a large and persistent elevation of Ca2+ in all spines, but does not produce either LTP or long-term depression (LTD) in the absence of synaptic stimulation. The possible reasons for this are discussed.