The requirement of BDNF for hippocampal synaptic plasticity is experience-dependent

Abstract

Brain-derived neurotrophic factor (BDNF) supports neuronal survival, growth, and differentiation and has been implicated in forms of hippocampus-dependent learning. In vitro, a specific role in hippocampal synaptic plasticity has been described, although not all experience-dependent forms of synaptic plasticity critically depend on BDNF. Synaptic plasticity is likely to enable long-term synaptic information storage and memory, and the induction of persistent (>24 h) forms, such as long-term potentiation (LTP) and long-term depression (LTD) is tightly associated with learning specific aspects of a spatial representation. Whether BDNF is required for persistent (>24 h) forms of LTP and LTD, and how it contributes to synaptic plasticity in the freely behaving rodent has never been explored. We examined LTP, LTD, and related forms of learning in the CA1 region of freely dependent mice that have a partial knockdown of BDNF (BDNF(+/-) ). We show that whereas early-LTD (<90min) requires BDNF, short-term depression (<45 min) does not. Furthermore, BDNF is required for LTP that is induced by mild, but not strong short afferent stimulation protocols. Object-place learning triggers LTD in the CA1 region of mice. We observed that object-place memory was impaired and the object-place exploration failed to induce LTD in BDNF(+/-) mice. Furthermore, spatial reference memory, that is believed to be enabled by LTP, was also impaired. Taken together, these data indicate that BDNF is required for specific, but not all, forms of hippocampal-dependent information storage and memory. Thus, very robust forms of synaptic plasticity may circumvent the need for BDNF, rather it may play a specific role in the optimization of weaker forms of plasticity. The finding that both learning-facilitated LTD and spatial reference memory are both impaired in BDNF(+/-) mice, suggests moreover, that it is critically required for the physiological encoding of hippocampus-dependent memory. © 2015 The Authors Hippocampus Published by Wiley Periodicals, Inc.

Keywords: CA1; LTD; LTP; enrichment; object recognition.

Figure 1

Early long‐term depression, but not short‐term depression is impaired in freely behaving BDNF^+/− mice. A: Low frequency stimulation at 1Hz (900 pulses, given as stimulus‐pairs, 25ms apart) elicits short‐term depression (STD) that lasts for ca. 45 min in both wild‐type and BDNF^+/− mice. Line‐breaks indicate a change in time‐scale. LFS was given at the time‐point indicated by the arrow. B: Low frequency stimulation at 1Hz (900 pulses, given as stimulus‐pairs, 50ms apart) elicits early (E)‐LTD in WT mice that lasts for at least 90 min. E‐LTP is significantly impaired in BDNF^+/− mice. C: Analogs represent fEPSPs that were recorded (1) pre‐LFS, (2) t = 5 min post‐LFS and (3) t = 24 h post‐LFS in WT (white circle, left traces) and BDNF^+/− mice (black square, right traces) that received LFS given in stimulus‐pairs at intervals of 25ms. Vertical scale bar corresponds to 2 mV and horizontal scale bar corresponds to 8 ms. D: Analogs represent fEPSPs that were recorded (1) pre‐LFS, (2) t = 5 min post‐LFS, and (3) t = 24 h post‐LFS in WT (white circle, left traces) and BDNF^+/− mice (black square, right traces) that received LFS given in stimulus‐pairs at intervals of 50ms. Vertical scale bar corresponds to 2 mV and horizontal scale bar corresponds to 8 ms.

Figure 2

Spatial object recognition (SOR) memory results in persistent synaptic plasticity that is altered in BDNF^+/− knockdown mice. A: Test‐pulse stimulation evoked fEPSPs that were stable over a 25h recording period in WT mice. Novel exposure of WT mice to objects in a spatial configuration triggers LTD in the CA1 region. Re‐exposure to the same objects in the same spatial locations fails to trigger LTD. A new spatial configuration of the familiar objects (re‐configuration) results in de novo LTD. B: Bar chart on left: Measurement of object exploration times in WT mice revealed a significant habituation to the objects during object re‐exposure. Exploration levels during object re‐configuration are equivalent to those recorded during novel object exploration. (*P < 0.05). Scatter plot on right: Plot shows the individual exploration times for the WT mice. Regardless of the initial level of exploration of the novel objects, all animals displayed less interest in the objects during the re‐exposure test and increased exploration when the same objects were spatially reconfigured during the “re‐configuration” test. C: Analogs represent fEPSPs that were recorded (1) pre‐object exposure, (2) t = 2 h post‐ object exposure, and (3) t = 24 h post‐ object exposure in WT mice during (i) novel object exposure, (ii) re‐exposure to the objects, and (iii) positional reconfiguration of the same objects. Vertical scale bar corresponds to 2 mV and horizontal scale bar corresponds to 8 ms. D: Test‐pulse stimulation evoked fEPSPs that were stable over a 25h recording period in BDNF^+/−. mice. Novel exposure of BDNF^+/− mice to objects in a spatial configuration fails to trigger LTD in the CA1 region. Re‐exposure to the same objects in the same spatial locations also fails to trigger LTD. A new spatial configuration of the familiar objects (re‐configuration) results in an initial synaptic depression that recovers to levels seen in controls. E: Bar chart on left: Measurement of object explorations times in BDNF^+/− mice reveals a tendency toward reduced object exploration during object re‐exposure that is not statistically significant from exploration levels during novel exploration. Exploration levels during object reconfiguration are equivalent to those recorded during novel object exploration and re‐exposure (*P < 0.05). Scatter plot on right: Plot shows the individual exploration times for the BDNF^+/− mice. Only two of the animals show a clear decline in object exploration during object re‐exposure (compared to novel exposure) that is followed by an increase in exploration times during the object “re‐configuration” test. One animal, that showed an initial low level of exploration, during novel object exploration shows a very subtle decrease, followed by a subtle increase in exploration times during the re‐exposure and re‐configuration tests. The remaining three animals exhibit a complete absence of learning. F: Analogs represent fEPSPs that were recorded (1) pre‐object exposure, (2) t = 2 h post‐ object exposure and (3) t = 24 h post‐ object exposure in BDNF^+/− mice during (i) novel object exposure, (ii) re‐exposure to the objects, and (iii) positional reconfiguration of the same objects. Vertical scale bar corresponds to 2 mV and horizontal scale bar corresponds to 8 ms.

Figure 3

The afferent stimulation protocol determines the requirement of BDNF for LTP in vivo. A: High‐frequency stimulation (HFS) (100 Hz (2 trains of 50 stimuli given at 5 min intervals) elicits robust LTP in freely behaving WT and BDNF^+/− mice. Line‐breaks indicate a change in time‐scale. HFS was given at the time‐point indicated by the arrow. B: Theta‐burst stimulation (3 trains 10s apart) elicits robust LTP in WT animals. BDNF^+/− mice exhibit a significant impairment in the magnitude of LTP. C: Theta‐burst stimulation (1 train) elicits LTP in WT animals that lasts for at least 4h. BDNF^+/− mice exhibit a significant impairment of LTP. D: Analogs represent fEPSPs that were recorded (1) pre‐HFS, (2) t = 5 min post‐HFS and (3) t = 4 h post‐HFS, (4) t = 24 h post‐HFS in WT (white circle, left traces) and BDNF^+/− mice (black square, right traces) that received HFS given at 100Hz (2 trains of 50 stimuli, 5 min intertrain interval). Vertical scale bar corresponds to 2 mV and horizontal scale bar corresponds to 8 ms. E: Analogs represent fEPSPs that were recorded (1) pre‐TBS (3 trains), (2) t = 5 min post‐ TBS (3 trains), and (3) t = 4 h post‐TBS (3 trains), (4) t = 24 h post‐TBS (3 trains) in WT (white circle, left traces), and BDNF^+/− mice (black square, right traces) that received TBS with three trains of 10 bursts. Vertical scale bar corresponds to 2 mV and horizontal scale bar corresponds to 8 ms. F: Analogs represent fEPSPs that were recorded (1) pre‐TBS (1 train), (2) t = 5 min post‐ TBS (1 train), and (3) t = 4 h post‐ TBS (1 train), (4) t = 24 h post‐TBS (1 train) in WT (white circle, left traces), and BDNF^+/− mice (black square, right traces) that received TBS with 1 train of 10 bursts. Vertical scale bar corresponds to 2 mV and horizontal scale bar corresponds to 8 ms.

Figure 4

Spatial reference memory requires BDNF. A: During the training phase for the spatial reference memory task, that took place on 4 consecutive days, learning performance (as determined by the time taken to find the food reward) was equivalent in WT and BDNF^+/− mice. B: On day 4, a probe test was conducted, whereby the distance travelled by the mice to the precise location of the (now absent food reward) was measured. BDNF^+/− mice were significantly impaired in their memory of the reward location. C: The average velocity of the mice was assessed (cm/s). No differences were identified between WT and BDNF^+/− mice.