Neuronal Activity and Brain-Derived Neurotrophic Factor Regulate the Density of Inhibitory Synapses in Organotypic Slice Cultures of Postnatal Hippocampus

Abstract

Hippocampal interneurons inhibit pyramidal neurons through the release of the neurotransmitter GABA. Given the importance of this inhibition for the proper functioning of the hippocampus, the development of inhibitory synapses must be tightly regulated. In this study, the possibility that neuronal activity and neurotrophins regulate the density of GABAergic inhibitory synapses was investigated in organotypic slice cultures taken from postnatal day 7 rats.In hippocampal slices cultured for 13 d in the presence of the GABA A receptor antagonist bicuculline, the density of glutamic acid decarboxylase (GAD) 65-immunoreactive terminals was increased in the CA1 area when compared with control slices. Treatment with the glutamate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione decreased the density of GAD65-immunoreactive terminals in the stratum oriens of CA1. These treatments had parallel effects on the density of GABAimmunoreactive processes. Electron microscopic analysis after postembedding immunogold labeling with antibodies against GABA indicated that bicuculline treatment increased the density of inhibitory but not excitatory synapses. Application of exogenous BDNF partly mimicked the stimulatory effect of bicuculline on GAD65-immunoreactive terminals. Finally, antibodies against BDNF, but not antibodies against nerve growth factor, decrease the density of GAD65-immunoreactive terminals in bicucullinetreated slices.Thus, neuronal activity regulates the density of inhibitory synapses made by postnatal hippocampal interneurons, and BDNF could mediate part of this regulation. This regulation of the density of inhibitory synapses could represent a feedback mechanism aimed at maintaining an appropriate level of activity in the developing hippocampal networks.

Neurotrophin signaling among neurons and glia during formation of tripartite synapses

“…Far less is known about how TrkB signaling modulates the formation of presynaptic inhibitory terminals. Presynaptic activation of TrkB increases glutamic acid decarboxylase (GAD) expression in interneurons, and enhances presynaptic GABA release and uptake at inhibitory terminals (Vicario-Abejon et al, 1998;Marty et al, 2000;Yamada et al, 2002). Consistent with these findings, increased miniature inhibitory postsynaptic current (mIPSC) frequency and enhanced quantal content of GABA containing vesicles have been observed in hippocampal and cerebellar neurons following treatment with BDNF (Bao et al, 1999;Bolton et al, 2000).…”

“…BDNF promotion of inhibitory axon outgrowth and synapse formation is well characterized in hippocampus, cortex and, in particular, cerebellum in vitro and in vivo (Huang et al, 1999;Marty et al, 2000;Seil and Drake-Baumann, 2000;Yamada et al, 2002). Accordingly, fewer GABAergic synapses are observed in cerebellar slices following scavenging of endogenous BDNF (Seil and DrakeBaumann, 2000) and in conditional TrkB deletion mutants (Rico et al, 2002).…”

Excitatory synaptic activity is associated with a rapid structural plasticity of inhibitory synapses on hippocampal CA1 pyramidal cells

“…Under conditions of chronic activity blockade in neuronal and organotypic cultures from hippocampus, the amount of inhibition and density of GABAergic terminals were reported to decrease (Marty et al, 2000;Hartman et al, 2006). In contrast, chronic activity or repetitive activation such as occurs with kindling has been shown to lead to a strengthening of inhibitory synaptic inputs, an enlargement of apposition zones and an increase in the number of GABA A receptors at inhibitory synapses, detected both with light microscopy and EM (Nusser et al, 1998;Marty et al, 2004).…”

Estrogen and brain-derived neurotrophic factor (BDNF) in hippocampus: Complexity of steroid hormone-growth factor interactions in the adult CNS

“…This appears to be dependent on activity as well as BDNF, because GABA A receptor blockade of cultured neurons induces NPY expression by a trk-dependent mechanism, but the opposite occurs if AMPA receptors are blocked [108]. In addition, cultures from BDNF knockout mice do not show upregulation of NPY protein [109].…”