Induction of long-term potentiation and depression is reflected by corresponding changes in secretion of endogenous brain-derived neurotrophic factor

Abstract

Neurotrophins play an important role in modulating activity-dependent neuronal plasticity. In particular, threshold levels of brain-derived neurotrophic factor (BDNF) are required to induce long-term potentiation (LTP) in acute hippocampal slices. Conversely, the administration of exogenous BDNF prevents the induction of long-term depression (LTD) in the visual cortex. A long-standing missing link in the analysis of this modulatory role of BDNF was the determination of the time-course of endogenous BDNF secretion in the same organotypic preparation in which LTP and LTD are elicited. Here, we fulfilled this requirement in slices of perirhinal cortex. Classical theta-burst stimulation patterns evoking LTP lasting >180 min elicited a large increase in BDNF secretion that persisted 5-12 min beyond the stimulation period. Weaker theta-burst stimulation patterns leading only to the initial phase of LTP ( approximately 35 min) were accompanied by a smaller increase in BDNF secretion lasting <1 min. Sequestration of BDNF by TrkB-IgG receptor bodies prevented LTP. Low-frequency stimulations leading to LTD were accompanied by reductions in BDNF secretion that never lasted beyond the duration of the stimulation.

Fig. 1.

BDNF expression in PRh cortex. (A) Schematic representation of the slice preparation that we used, including the PRh cortex, lateral entorhinal cortex, and hippocampus (white background), in which BDNF expression was revealed by immunohistochemistry using a polyclonal anti-BDNF Ab. (B) Double-labeling experiments show that, in a representative area of layer II/III, BDNF immunoreactivity (green) predominantly correlates with that of the neuronal nuclear marker NeuN (red) (BDNF/NeuN). Note that BDNF immunoreactivity shows discontinuous patchy features. BDNF immunoreactivity is not detectable in astrocytes (red), as shown in a representative area of layer I (BDNF/glial fibrillary acidic protein). (Scale bar indicates 50 μm.)

Fig. 2.

Parallel analysis of FPs in PRh cortex and endogenous BDNF secretion in the perfusion medium during basal synaptic stimulation. (A) Individual slices were placed in a submersion-recording chamber and perfused at 3 ml/min. Electrophysiological recordings were carried out by an extracellular microelectrode (Rec) placed in layers II/III of PRh cortex (see Fig. 1 A). Constant-current square pulses applied to horizontal afferent fibers using a concentric bipolar electrode (Stim) evoked negative FPs of which the amplitude did not change during the 100 min of recording (n = 6). (B) BDNF concentrations in the slice-perfusion medium were assayed by ELISA in 1-min fractions collected during the experiments described in A. Note that BDNF concentrations decreased progressively during the experiments (to 92 ± 4% after 20 min and to 86 ± 3% after 100 min). The 100% BDNF concentrations in the individual experiments were as follows: 135, 24, 34, 68, 24, and 19 pg/ml. (C) Electrophysiological recordings, as described in A, were performed in the absence (0-10 min) and in the presence (10-100 min) of the sodium channel blocker TTX (1 μM) added (arrow) to the perfusing medium. Note that TTX treatment abolishes the FPs (n = 6). (D) BDNF secretion during the experiments reported in C. BDNF secretion decreased initially (to 87 ± 4% after 20 min, i.e., 10 min after TTX application), reaching similar values as under control conditions after 100 min (to 84 ± 3%). The 100% BDNF concentrations in the individual experiments were as follows: 79, 35, 19, 56, 22, and 47 pg/ml.

Fig. 3.

Parallel analysis of FPs in PRh cortex and endogenous BDNF secretion during basal stimulation and LTP-inducing TBS. (A) LTP was induced by 100-Hz TBS. This stimulation induced an increase in FP amplitude that persisted for >180 min (n = 6). (B) Time course of BDNF secretion during the experiments shown in A. BDNF secretion rapidly increased after 100-Hz TBS, and the high levels persisted for 5-12 min before declining to basal values. The 100% BDNF concentrations in the individual experiments were as follows: 166, 16, 18, 73, 25, and 39 pg/ml. (C) LTP induction by 100-Hz TBS was prevented by the presence of TrkB-IgG receptor bodies (1 μg/ml) (n = 5).

Fig. 4.

Parallel analysis of FPs in PRh cortex and endogenous BDNF secretion during basal and weak TBSs. (A) A 100-Hz TBS (two sets) consisting of two instead of four sets of stimulations, induced only the initial phase of LTP (n = 8). (B) Time course of BDNF secretion during the experiments reported in A.A 100-Hz TBS (two sets) caused a small increase in BDNF secretion. The 100% BDNF concentrations in the individual experiments were as follows: 37, 29, 79, 122, 189, 20, 48, and 21 pg/ml. (C) The 25-Hz TBS failed to induce LTP. The 25-Hz TBS differs from 100-Hz TBS only in that the frequency was delivered at 25 Hz instead of 100 Hz. In our experiments, this reduced frequency always failed to induce LTP (n = 6). (D) Time course of BDNF secretion during the experiments reported in C. The 25-Hz TBS caused a small increase in BDNF secretion that was restricted to a single (1-min) collected fraction. The 100% BDNF concentrations in the individual experiments were as follows: 61, 19, 22, 82, 224, and 41 pg/ml.

Fig. 5.

Parallel analysis of FPs in PRh cortex and endogenous BDNF secretion in the perfusion medium, during basal and LTD-inducing stimulations. (A) LTD induced by a LFS of 3,000 stimuli at 5 Hz. LTD persisted for >180 min (n = 6). (B) Time course of BDNF secretion during the experiments shown in A. LFS immediately induced a reduction in BDNF secretion. Maximum reduction in BDNF secretion occurred within the first min of LFS and lasted for several minutes before returning to basal levels. The 100% BDNF concentrations in the individual experiments were as follows: 73, 39, 95, 45, 224, and 73 pg/ml. (C) LFS consisting of 900 stimuli at 1 Hz induced a LTD lasting ≈40 min (n = 6). (D) Time course of BDNF secretion during the experiments shown in C. The 100% BDNF concentrations in the individual experiments were as follows: 51, 69, 221, 172, 124, and 91 pg/ml.