Low-frequency stimulation (LFS) is widely used to induce long-term depression (LTD) at rodent CA3-CA1 hippocampal synapses. The relationship between the efficacy of LFS induction and postnatal age remains controversial in the rat, and has yet to be studied in the mouse. The data presented here strongly suggest that LFS-induced LTD is highly age dependent at mouse hippocampal synapses.
Briefly, transverse hippocampal slices (400 mm) were prepared from humanely killed B6xCBA mice aged between postnatal days P6-P116. Slices were maintained at 28°C in an interface chamber and perfused with oxygenated artificial cerebrospinal fluid. Field excitatory postsynaptic potentials (fEPSPs) were recorded using microelectrodes filled with 1 M sodium acetate placed in the stratum radiatum of the CA1 field. Stimulation of CA3 Shaffer collaterals was performed with monopolar stainless-steel electrodes. Test shocks were applied at 30 s intervals, set to evoke either 50 % of the maximum response or a maximal sub-threshold population spike response. LFS consisted of 900 shocks at 1 Hz and LTD defined as a stable reduction (> 10 %) of the fEPSP slope 1 h post-conditioning. Data are expressed as the per cent change in fEPSP slope ± S.E.M. Statistical analysis was carried out using Dunnett’s multiple comparison test for significance with respect to adult slices (P100+).
LFS conditioning induced a highly significant, age-dependent LTD, apparent from postnatal day P6 but no longer significant by P18. Slices prepared from animals aged P6-9 exhibited LTD (-28.97 ± 5.99% n = 8, P < 0.01) in all experiments. By age P10-13, LTD (-19.8 ± 2.27% n = 39, P < 0.01) was induced in 77 % of experiments. Similarly, in slices from P14-17 LTD (-20.03 ± 4.85% n = 15 P < 0.01) was induced in 80 % of experiments. By P18-21 (-4.585 ± 4.22% n = 14) LTD is induced in 29 % of experiments but the group mean is no longer significant (P > 0.05). P22-28 (-1.812 ± 3.48% n = 15) and P29-49 (4.365 ± 5.99% n = 6) exhibited LTD in 13 and 17 % of experiments, respectively. P50-100 (9.39 ± 4.71% n = 7) and P100+ (10.942 ± 4.69% n = 9) failed to exhibit LTD. LFS-induced LTD was also homosynaptic (P12; n = 4) and reversibly blocked by the NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid (25 mM AP5) (-5.49 ± 4.08 %); following removal of AP5 a second period of LFS conditioning successfully induced LTD (-23.47 ± 5.56% n = 6; P < 0.05, Student’s unpaired t test; P12-15).
Our data demonstrate that, in mouse, LFS (900 shocks at 1 Hz) -induced LTD at CA3-CA1 synapses requires the activation of NMDA receptors and is highly age dependent.
The authors wish to thank S. Walters, D. Sadler and K. Evans for technical assistance.
All procedures accord with current UK legislation.