BDNF increases release probability and the size of a rapidly recycling vesicle pool within rat hippocampal excitatory synapses

Abstract

Exerting its actions pre-, post-and peri-synaptically, brain-derived neurotrophic factor (BDNF) is one of the most potent modulators of hippocampal synaptic function. Here, we examined the effects of BDNF on a rapidly recycling pool (RRP) of vesicles within excitatory synapses. First, we estimated vesicular release in hippocampal cultures by performing FM4-64 imaging in terminals impinging on enhanced green fluorescent protein (eGFP)-labelled dendritic spines -a hallmark of excitatory synapses. Consistent with a modulation of the RRP, BDNF increased the evoked destaining rate of FM4-64 only during the initial phase of field stimulation. Multiphoton microscopy in acute hippocampal slices confirmed these observations by selectively imaging the RRP, which was loaded with FM1-43 by hyperosmotic shock. Slices exposed to BDNF showed an increase in the evoked and spontaneous rates of FM1-43 destaining from terminals in CA1 stratum radiatum, mostly representing excitatory terminals of Schaffer collaterals. Variance-mean analysis of evoked EPSCs in CA1 pyramidal neurons further confirmed that release probability is increased in BDNF-treated slices, without changes in the number of independent release sites or average postsynaptic quantal amplitude. Because BDNF was absent during dye loading, imaging, destaining and whole-cell recordings, these results demonstrate that BDNF induces a long-lasting enhancement in the probability of transmitter release at hippocampal excitatory synapses by modulating the RRP. Since the endogenous BDNF scavenger TrkB-IgG prevented the enhancement of FM1-43 destaining rate caused by induction of long-term potentiation in acute hippocampal slices, the modulation of a rapidly recycling vesicle pool may underlie the role of BDNF in hippocampal long-term synaptic plasticity.

Coordinate action of pre- and postsynaptic brain-derived neurotrophic factor is required for AMPAR trafficking and acquisition of in vitro classical conditioning

“…Additional findings that high-frequency stimulation or more natural theta patterns of afferent activity upregulates BDNF expression further corroborate its role in synaptic plasticity (Gooney and Lynch, 2001;Balkowiec and Katz, 2002). Mechanisms underlying the function of BDNF in plasticity include presynaptic (Gottschalk et al, 1998;Pozzo-Miller et al, 1999;Jovanovic et al, 2000;Xu et al, 2000;Zakharenko et al, 2003;Tyler et al, 2006) or postsynaptic actions Lin et al, 1998;Di Luca et al, 2001;Kovalchuk et al, 2002) as well as coordinate pre-and postsynaptic processes (Alder et al, 2005;Gartner et al, 2006). Despite a great deal of progress on the function of BDNF in LTP, investigation of its role in associative learning has only more recently begun to be examined.…”

BDNF-induced local protein synthesis and synaptic plasticity

“…Studies performed in cultured hippocampal neurons showed that BDNF increases the frequency of miniature excitatory postsynaptic currents, further supporting a presynaptic effect of the neurotrophin (Lessmann and Heumann, 1998;Li et al, 1998;Schinder et al, 2000;Tyler and Pozzo-Miller, 2001). The rapidly recycling pool of synaptic vesicles is targeted by a BDNF-dependent mechanism in the hippocampal CA1 region, and this modulation was shown to be necessary for the enhancement of exocytosis caused by induction of LTP (Tyler et al, 2006). The recycling of synaptic vesicles induced by BDNF requires the presence myosin VI, a minus end-directed actin-based motor, and the adaptor protein GIPC1 [PDZ (postsynaptic density-95/Discs large/zona occludens-1) domain-containing adaptor protein, type 1], which form a complex that can engage TrkB receptors (Yano et al, 2006).…”

Ultrastructural readout of functional synaptic vesicle pools in hippocampal slices based on FM dye labeling and photoconversion

“…1a-f). The principal dye-labelling step is based on an established approach adopted in a number of previous studies 7,13,33,[38][39][40][41][42] and we outline key strategies to validate its success. Subsequently, detailed methodology for rapid microwave-enhanced fixation 43 and calibrated photoconversion of the target region are provided.…”

“…While vesicles appear morphologically equivalent they can be sub-divided into pools on the basis of their functional behaviour, including a recycling pool (see 2,3 , readilyreleasable pool 4 , spontaneous pool 5,6 and superpool 7,8 . Understanding the properties of these pools has become increasingly important with the realization that they are potentially critical substrates in setting synaptic strength 9,10 and represent modifiable targets on which forms of plasticity 9,[11][12][13][14] or disease-like conditions [15][16][17] might act to modulate or disrupt information flow.…”