Relative contribution of endogenous neurotrophins in hippocampal long-term potentiation

Abstract

Recent evidence has shown that brain-derived neurotrophic factor (BDNF) is involved in hippocampal long-term potentiation (LTP). Because the reagents used in acute experiments react not only with BDNF but also with neurotrophin-4/5 (NT4/5) and neurotrophin-3 (NT3), we examined the involvement of these neurotrophins in LTP using two highly specific, function-blocking monoclonal antibodies against BDNF and NT3, as well as a TrkB-IgG fusion protein. Our results show that NT3 antibodies did not have any effects on LTP. However, both TrkB-IgG fusion proteins and BDNF antibody similarly reduced LTP, suggesting that only BDNF but no other ligands of the TrkB-receptor are likely to be involved in LTP induction. The reduction in LTP depended on the inducing stimuli and was only observed with theta-burst stimulation (TBS) but not with tetanic stimulation. We further observed that LTP was only reduced if BDNF was blocked before and during TBS stimulation, and BDNF antibodies did not affect early or late stages of LTP if they were applied 10, 30, or 60 min after TBS stimulation. These results point toward a specific and unique role of endogenous BDNF but not of other neurotrophins in the process of TBS-induced hippocampal LTP. Additionally, they suggest that endogenous BDNF is required for a limited time period only shortly before or around LTP induction but not during the whole process of LTP.

Fig. 1.

Basal synaptic transmission at the Schaffer collateral–CA1 pathway of hippocampal slices is not affected by treatment with TrkB-IgG fusion protein or BDNF antibody. The slope of field EPSPs was plotted against stimulus strength (in microamperes).A, TrkB-IgG fusion protein (triangles,dotted line; n = 7 slices, 4 mice) and controls (diamonds, solid line;n = 8 slices, 4 mice) (p> 0.2). B, BDNF antibody (triangles,dotted line; n = 8 slices, 4 mice) and controls (diamonds, solid line;n = 7 slices, 4 mice) (p> 0.3).

Fig. 2.

Short-term plasticity measured by PPF remains normal and unaffected at the Schaffer collateral–CA1 pathway of hippocampal slices after treatment with TrkB-IgG fusion protein or BDNF antibody. Plot depicting the percent ratio of responses to the second pulse relative to the first one. Pairs of pulses were delivered at interpulse intervals of 30, 40, 60, and 100 msec. A, TrkB-IgG fusion protein-treated slices (triangles,dotted line; n = 7 slices, 4 mice) versus control slices (diamonds, solid line; n = 8 slices, 4 mice). B, BDNF antibody-treated slices (triangles, dotted line; n = 8 slices, 4 mice) versus control slices (diamonds, solid line;n = 7 slices, 4 mice).

Fig. 3.

Effects of blocking all endogenous TrkB ligands on LTP. Pretreatment with TrkB-IgG fusion protein for 1 hr reduces LTP in a stimulus-dependent manner; only LTP induced by TBS at the Schaffer collateral–CA1 synapses of hippocampal slices is affected but not LTP induced by tetanic stimulation. Field EPSP slopes were plotted as percent of baseline (mean ± SEM). Straight black line represents the presence of TrkB-IgG fusion protein.First data point after LTP induction represents PTP.A, Tetanus-induced LTP and the effects of TrkB-IgG fusion protein (TrkB-IgG, n = 6 slices, 4 mice; control, n = 6 slices, 4 mice). B, TBS-induced LTP and the effects of TrkB-IgG fusion protein (TrkB-IgG,n = 7 slices, 4 mice; control,n = 5 slices, 4 mice).

Fig. 4.

Effect of selectively blocking endogenous BDNF on LTP. Pretreatment with BDNF antibody for 1 hr significantly reduces LTP induced by TBS but not by tetanic stimulation. A, Effects of BDNF antibody on tetanus-induced LTP (BDNF Ab,n = 7 slices, 4 mice; control,n = 7 slices, 4 mice). B, Effects of BDNF antibody on TBS-induced LTP (BDNF Ab, n = 6 slices, 4 mice; control, n = 7 slices, 4 mice).

Fig. 5.

Effect of blocking endogenous NT3 on LTP. NT3 antibody does not affect LTP induced by either tetanus or TBS. Pretreatment time for slices was again 1 hr before recordings were started. A, Effects of NT3 antibody on tetanus-induced LTP (NT3 Ab, n = 14 slices, 10 mice; control,n = 15 slices, 9 mice). B, Effects of NT3 antibody on TBS-induced LTP (NT3 Ab, n = 13 slices, 10 mice; control, n = 11 slices, 8 mice).

Fig. 6.

Effect on LTP is dependent on the time when endogenous BDNF is blocked by the antibodies. A, BDNF antibodies do not reduce LTP significantly when applied 10 min after TBS. B, Applications 30 or 60 min after TBS also do not affect LTP and its maintenance. Field EPSP slopes were plotted as percent of baseline. Bars indicate level of LTP at the end of each recording averaged over a 5 min period. Late phase LTP (3 hr after TBS) was measured for 30 and 60 min treatment groups (10 min group: BDNF Ab, n = 9 slices, 7 mice; control,n = 8 slices, 7 mice; 30 min group: BDNF Ab,n = 6 slices, 6 mice; control,n = 4 slices, 4 mice; 60 min group: BDNF Ab,n = 5 slices, 5 mice; control,n = 5 slices, 5 mice).