Role of P2X4 receptors in synaptic strengthening in mouse CA1 hippocampal neurons

P2X4 receptors are calcium‐permeable cation channels gated by extracellular ATP. They are found close to subsynaptic sites on hippocampal CA1 neurons. We compared features of synaptic strengthening between wild‐type and P2X4 knockout mice (21–26 days old). Potentiation evoked by a tetanic presynaptic stimulus (100 Hz, 1 s) paired with postsynaptic depolarization was less in P2X4 −/− mice than in wild‐type mice (230 vs. 50% potentiation). Paired‐pulse ratios and the amplitude and frequency of spontaneous excitatory postsynaptic currents (EPSCs) were not different between wild‐type and knockout mice. Prior hyperpolarization (ten 3 s pulses to −120 mV at 0.17 Hz) potentiated the amplitude of spontaneous EPSCs in wild‐type mice, but not in P2X4 −/− mice; this potentiation was not affected by nifedipine, but was abolished by 10 m m 1,2‐bis(o‐aminophenoxy)ethane‐ N , N , N ′, N ′‐tetra‐acetic acid (BAPTA) in the recording pipette. The amplitude of N ‐methyl‐ d ‐aspartate EPSCs (in 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione, 10 or 30 μ m , at −100 mV) facilitated during 20 min recording in magnesium‐free solution. In wild‐type mice, this facilitation of the N ‐methyl‐ d ‐aspartate EPSC was reduced by about 50% by intracellular BAPTA (10 m m ), ifenprodil (3 μ m ) or 4‐(4‐fluorophenyl)‐2‐(4‐methylsulphinylphenyl)‐5‐(4‐pyridyl)1H‐imidazole (5 μ m ). In P2X4 −/− mice, the facilitation was much less, and was unaffected by intracellular BAPTA, ifenprodil (3 μ m ) or mitogen‐activated protein (MAP) kinase inhibitor 4‐(4‐fluorophenyl)‐2‐(4‐methylsulphinylphenyl)‐5‐(4‐pyridyl)1H‐imidazole (5 μ m ). This suggests that the absence of P2X4 receptors limits the incorporation of NR2B subunits into synaptic N ‐methyl‐ d ‐aspartate receptors.