Intracellular calcium ion dynamics involved in long-term potentiation in hippocampal CA1 neurons in mice lacking the IP3 type 1 receptor

In the present study, mice lacking the type 1 inositol-1,4,5-trisphosphate receptor (IP3R) were used to study the role of type 1 IP3Rs in the induction of long-term potentiation (LTP) in hippocampal CA1 neurons. The magnitude of the LTP induced by high frequency stimulation (HFS) consisting of 20 pulses at 30 Hz in mice lacking type 1 IP3Rs was significantly larger than that in wild-type mice in terms of the field excitatory postsynaptic potential and population spike. By measuring changes in the intracellular Ca2+ concentration ([Ca2+]i) in CA1 pyramidal neurons using fluorometry, we found that the decay time of the transient increase in the [Ca2+]i evoked by the HFS in mutant mice was significantly longer than that in wild-type mice, whereas the [Ca2+]i at rest and the magnitude of the [Ca2+]i increases caused by the HFS were no different from those in wild-type mice. In slices from the mutant mice, paired-pulse stimulation (PPS) delivered at an interval of 10 ms resulted in significantly weaker paired-pulse inhibition (PPI) than in wild-type mice, suggesting that lack of type 1 IP3Rs reduces the PPI induced by PPS in the CA1 region. These results indicate that a lack of type 1 IP3Rs causes a slower decay of the transient [Ca2+]i in CA1 pyramidal neurons and attenuates the activity of inhibitory interneurons, resulting in enhancement of LTP induction.