Long-term depression-associated signaling is required for an in vitro model of NMDA receptor-dependent synapse pruning

• Published in: Neurobiology of learning and memory 2017-02, Vol.138, p.39-53
• Main Authors: Henson, Maile A, Tucker, Charles J, Zhao, Meilan, Dudek, Serena M
• Format: Article
• Language: English
• Subjects: Animals; Cells, Cultured; Cerebral Cortex - cytology; Cerebral Cortex - drug effects; Cerebral Cortex - metabolism; Dendritic Spines - drug effects; Dendritic Spines - metabolism; Hippocampus - cytology; Hippocampus - drug effects; Hippocampus - metabolism; Long-Term Synaptic Depression - drug effects; Long-Term Synaptic Depression - physiology; N-Methylaspartate - pharmacology; Neuronal Plasticity - drug effects; Neuronal Plasticity - physiology; Neurons - cytology; Neurons - drug effects; Neurons - metabolism; Patch-Clamp Techniques; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate - metabolism; Synapses - drug effects; Synapses - metabolism
• ISSN: 1074-7427; 1095-9564
• DOI: 10.1016/j.nlm.2016.10.013

Activity-dependent pruning of synaptic contacts plays a critical role in shaping neuronal circuitry in response to the environment during postnatal brain development. Although there is compelling evidence that shrinkage of dendritic spines coincides with synaptic long-term depression (LTD), and that LTD is accompanied by synapse loss, whether NMDA receptor (NMDAR)-dependent LTD is a required step in the progression toward synapse pruning is still unknown. Using repeated applications of NMDA to induce LTD in dissociated rat neuronal cultures, we found that synapse density, as measured by colocalization of fluorescent markers for pre- and postsynaptic structures, was decreased irrespective of the presynaptic marker used, post-treatment recovery time, and the dendritic location of synapses. Consistent with previous studies, we found that synapse loss could occur without apparent net spine loss or cell death. Furthermore, synapse loss was unlikely to require direct contact with microglia, as the number of these cells was minimal in our culture preparations. Supporting a model by which NMDAR-LTD is required for synapse loss, the effect of NMDA on fluorescence colocalization was prevented by phosphatase and caspase inhibitors. In addition, gene transcription and protein translation also appeared to be required for loss of putative synapses. These data support the idea that NMDAR-dependent LTD is a required step in synapse pruning and contribute to our understanding of the basic mechanisms of this developmental process.