Age-related enhancement of a protein synthesis-dependent late phase of LTP induced by low frequency paired-pulse stimulation in hippocampus

Abstract

Protein synthesis-dependent late phase of LTP (L-LTP) is typically induced by repeated high-frequency stimulation (HFS). This form of L-LTP is reduced in the aged animal and is positively correlated with age-related memory loss. Here we report a novel form of protein synthesis-dependent late phase of LTP in the CA1 region of hippocampus induced by a brief 1-Hz paired-pulse stimulation (PP-1 Hz, 1 min). In contrast to L-LTP induced by HFS, the late phase of PP-1 Hz LTP does not exist in young adult animals. Rather, it emerges and becomes enhanced in an age-related way. Thus, in 1.5- to 2-mo-old mice, a brief PP-1 Hz stimulation induces only a short lasting LTP, decaying to baseline in about 90 min. By contrast, PP-1 Hz stimulation induces an enduring and protein synthesis dependent LTP in 12- to 18-mo-old mice. The PP-1 Hz-induced L-LTP is dependent on NMDA receptor activation, requires voltage-dependent calcium channels, and is modulated by dopamine D1/D5 receptors. Because memory ability declines with aging, the age-related enhancement of L-LTP induced by PP-1 Hz stimulation indicates that this form of L-LTP appears to be inversely correlated with memory ability.

Figure 1.

A brief PP-1 Hz stimulation induces LTP (in 6- to 8-wk-old mice). (A) Fifteen minutes of PP-1 Hz stimulation (900 pairs) induced LTD. (B) Ten minutes of PP-1 Hz stimulation (600 pairs) induced a weak synaptic depression. (C) Three minutes of PP-1 Hz stimulation (180 pairs) induced a weak synaptic potentiation. (D) A brief PP-1 Hz stimulation (1 min, 60 pairs) induced LTP. (E) A summary of synaptic strength 15 min after PP-1 Hz stimulation (1 min–15 min). (F) Representative EPSPs waveforms before, during, and after PP-1 Hz stimulation (the smaller EPSP is before PP-1 Hz). Calibration: 2 mV, 10 ms.

Figure 2.

Age-related enhancement of late-phase LTP induced by PP-1 Hz stimulation. (A) Brief PP-1 Hz stimulation (1 min) induced E-LTP only in 6- to 8-wk-old mice. (B) PP-1 Hz stimulation (1 min) induced a weak L-LTP in 3-mo-old mice. (C) PP-1 Hz stimulation (1 min) induced a substantial L-LTP in 6- to 7-mo-old mice. (D) PP-1 Hz stimulation (1 min) induced L-LTP in 12-mo-old mice. (E) PP-1 Hz stimulation (1 min) induced L-LTP in 18-mo-old mice. (F) A histogram showing the magnitude of L-LTP (2.5 h after PP-1 Hz stimulation) in mice of different ages. (G) A histogram showing the magnitude of E-LTP (30 min after PP-1 Hz stimulation) in mice of different ages. (H) Input–output curves in 3-mo-old and 12- to 18-mo-old mice. Insets in A, B, C, D, and E show sample traces of fEPSPs 10 min before and 2.5 h after PP-1 Hz stimulation. Calibration: 3 mV, 5 ms.

Figure 3.

Protein synthesis inhibitors block L-LTP induced by PP-1 Hz stimulation. (A) L-LTP induced by PP-1 Hz stimulation lasts up to 5 h. Insets are sample traces at 1 h, 3 h, and 5 h after PP-1 Hz stimulation. Calibration: 3mV, 5 ms. (B) Emetine (100 μM) blocked L-LTP induced by PP-1 Hz stimulation. (○) Control; (●) Emetine. The addition of emetine did not alter the baseline EPSPs (▵, n = 5). (C) Ramapycin (0.5 μM) depressed L-LTP induced by PP-1 Hz stimulation. (○) Control; (●) Rapamycin. The addition of rapamycin did not alter the baseline EPSPs (▵, n = 5). Insets in B and C show sample traces of EPSPs 10 min before and 2.5 h after PP-1 Hz stimulation. Calibration: 2 mV, 5 ms.

Figure 4.

L-LTP induced by PP-1 Hz stimulation is dependent on NMDA receptor activation. (A) NMDA receptor antagonist D-APV (50 μM) completely blocked the L-LTP induced by PP-1 Hz stimulation. (○) Control; (●) D-APV. (B) mGluR antagonist LY341495 did not alter the expression of L-LTP induced by PP-1 Hz stimulation. (○) Control; (●) LY341495. (C) VDCC antagonist Nifedipine selectively blocked the late phase of LTP induced by PP-1 Hz stimulation. (○) Control; (●) Nifedipine. (D) Nifedipine did not block the E-LTP induced by PP-1 Hz stimulation in 6- to 8-wk-old mice. Insets in B, C, and D, show sample traces of EPSPs 10 min before and 2.5 h after (A, 90 min after) PP-1 Hz stimulation. Calibration: 2mV, 5ms.

Figure 5.

L-LTP induced by PP-1 Hz stimulation is mediated by D1/D5 receptors. (A) D1/D5 receptor antagonist SCH23390 selectively blocked the L-LTP induced by PP-1 Hz stimulation in 6- to 7-mo-old mice. (○) Control; (●) SCH. (B) SCH23390 did not block the E- LTP induced by PP-1 Hz stimulation in 6- to 8-wk-old mice. (C) D1/D5 agonist 6-bromo-APB did not facilitate the L-LTP induced by PP-1 Hz stimulation in 6- to 8-wk-old mice. (D) PKA inhibitor KT5720 blocked the L-LTP induced by PP-1 Hz stimulation in 6- to 7-mo-old old mice. (○) Control; (●) KT5720. Insets in A, B, C, and D are sample traces of EPSPs 10 min before and 2.5 h after PP-1 Hz stimulation. Calibration: 2mV, 5 ms.

Figure 6.

HFS L-LTP does not occlude L-LTP induced by PP-1 Hz stimulation. (○) L-LTP induced by PP-1 Hz stimulation without priming of HFS L-LTP. (●) PP-1 Hz L-LTP after L-LTP induced by four trains of 100 Hz tetanus (1 sec). Insets show sample traces of EPSP before and 2.5 h after PP-1 Hz stimulation in control and occlusion experiments. Calibration: 3 mV, 10 ms.