Epileptiform stimulus increases Homer 1a expression to modulate synapse number and activity in hippocampal cultures

Neurons adapt to seizure activity structurally and functionally to attenuate hyperactive neural circuits. Homer proteins provide a scaffold in the postsynaptic density (PSD) by binding to ligands through an EVH1 domain and to other Homer proteins by a coiled-coil domain. The short Homer isoform 1a (H1a) has a ligand-binding domain but lacks a coiled-coil domain and thus acts in a dominant-negative manner to uncouple Homer scaffolds. Here, we show that treating rat hippocampal cultures with bicuculline and 4-aminopyridine (Bic+4-AP) evoked epileptiform activity and synchronized Ca²⁺ spiking, measured with whole cell current-clamp and fura-2-based digital imaging; Bic+4-AP increased H1a mRNA through the activation of metabotropic glutamate receptor 5 (mGluR5). Treatment with Bic+4-AP for 4 h attenuated burst firing and induced synapse loss. Synaptic changes were measured using a confocal imaging-based assay that quantified clusters of PSD-95 fused to green fluorescent protein. Treatment with an mGluR5 antagonist blocked H1a expression, synapse loss, and burst attenuation. Overexpression of H1a inhibited burst firing similar to Bic+4-AP treatment. Furthermore, knockdown of H1a using a short hairpin RNA (shRNA) strategy reduced synapse loss and burst attenuation induced by Bic+4-AP treatment. Thus an epileptiform stimulus applied to hippocampal neurons in culture induced burst firing and H1a expression through the activation of mGluR5; a 4-h exposure to this stimulus resulted in synapse loss and burst attenuation. These results suggest that H1a expression functions in a negative-feedback manner to reduce network excitability by regulating the number of synapses.