BDNF Activates Postsynaptic TrkB Receptors to Induce Endocannabinoid Release and Inhibit Presynaptic Calcium Influx at a Calyx-Type Synapse

Brain-derived neurotropic factor (BDNF) has been shown to play critical roles in neural development, plasticity, and neurodegenerative diseases. The main function of BDNF in the brain is widely accepted to be synaptic regulation. However, how BDNF modulates synaptic transmission, especially the underlying signaling cascades between presynaptic and postsynaptic neurons, remains controversial. In the present study, we investigated the actions of BDNF at rat calyx-type synapses of either sex by measuring the excitatory postsynaptic current (EPSC) and presynaptic calcium current and capacitance changes. We found that BDNF inhibits the EPSC, presynaptic calcium influx, and exocytosis/endocytosis via activation of the presynaptic cannabinoid Type 1 receptors (CB1Rs). Inhibition of the CB1Rs abolished the BDNF-induced presynaptic inhibition, whereas CB1R agonist mimicked the effect of BDNF. Exploring the underlying signaling cascade, we found that BDNF specifically activates the postsynaptic TrkB receptors, inducing the release of endocannabinoids via the PLCγ/DGL pathway and retrogradely activating presynaptic CB1Rs. We also reported the involvement of AC/PKA in modulating vesicle endocytosis, which may account for the BDNF-induced calcium-dependent and -independent regulation of endocytosis. Thus, our study provides new insights into the BDNF/endocannabinoid-associated modulation of neurotransmission in physiological and pathologic processes. SIGNIFICANCE STATEMENT BDNF plays critical roles in the modulation of synaptic strength. However, how BDNF regulates synaptic transmission and its underlying signaling cascade(s) remains elusive. By measuring EPSC and the presynaptic calcium current and capacitance changes at rat calyces, we found that BDNF inhibits synaptic transmission via BDNF-TrkB-eCB signaling pathway. Activation of postsynaptic TrkB receptors induces endocannabinoid release via the PLCγ/DGL pathway, retrogradely activating the presynaptic CB1Rs, inhibiting the AC/PKA, and suppressing calcium influx. Our findings provide a comprehensive understanding of BDNF/endocannabinoid-associated modulation of neuronal activities.