Epileptic stimulus increases Homer 1a expression to modulate endocannabinoid signaling in cultured hippocampal neurons.
Endocannabinoid (eCB) signaling serves as an on-demand neuroprotective system. eCBs are produced postsynaptically in response to depolarization or activation of metabotropic glutamate receptors (mGluRs) and act on presynaptic cannabinoid receptor-1 to suppress synaptic transmission. Here, we examined the effects of epileptiform activity on these two forms of eCB signaling in hippocampal cultures. Treatment with bicuculline and 4-aminopyridine (Bic + 4-AP), which induced burst firing, inhibited metabotropic-induced suppression of excitation (MSE) and prolonged the duration of depolarization-induced suppression of excitation (DSE). The Homer family of proteins provides a scaffold for signaling molecules including mGluRs. It is known that seizures induce the expression of the short Homer isoform 1a (H1a) that acts in a dominant negative manner to uncouple Homer scaffolds. Bic + 4-AP treatment increased H1a mRNA. A group I mGluR antagonist blocked the Bic + 4-AP-evoked increase in burst firing, the increase in H1a expression, and the inhibition of MSE. Bic + 4-AP treatment reduced mGluR-mediated Ca(2+) mobilization from inositol trisphosphate-sensitive stores relative to untreated cells. Expression of H1a, but not a mutant form that cannot bind Homer ligands, mimicked Bic + 4-AP inhibition of MSE and mGluR-mediated Ca(2+) mobilization. In cells expressing shRNA targeted to Homer 1 mRNA, Bic + 4-AP did not affect mGluR-mediated Ca(2+) release. Furthermore, knockdown of H1a prevented the inhibition of MSE induced by Bic + 4-AP. Thus, an epileptic stimulus increased H1a expression, which subsequently uncoupled mGluR-mediated eCB production. These results indicate that seizure activity modulates eCB-mediated synaptic plasticity, suggesting a changing role for the eCB system following exposure to aberrant patterns of excitatory synaptic activity.
Department of Pharmacology, University of Minnesota Medical School, Minneapolis, MN 55455, USA.
Yan
Y
Li
Kelly A
KA
Krogh
Stanley A
SA
Thayer
D015761:4-Aminopyridine; D000818:Animals; D001640:Bicuculline; D002118:Calcium / Q000502:physiology; D002352:Carrier Proteins / Q000096:biosynthesis* / Q000235:genetics; D002478:Cells, Cultured; D003292:Convulsants; D004247:DNA / Q000235:genetics; D063388:Endocannabinoids / Q000502:physiology*; D004827:Epilepsy / Q000139:chemically induced / Q000235:genetics*; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D006624:Hippocampus / Q000166:cytology / Q000502:physiology*; D000072233:Homer Scaffolding Proteins; D059906:Neuroimaging; D009474:Neurons / Q000502:physiology*; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers; D011725:Pyridines / Q000494:pharmacology; D051381:Rats; D060888:Real-Time Polymerase Chain Reaction; D015398:Signal Transduction / Q000235:genetics*; D014162:Transfection
D002352:Carrier Proteins; D003292:Convulsants; D063388:Endocannabinoids; D000072233:Homer Scaffolding Proteins; D026902:Potassium Channel Blockers; D011725:Pyridines; C121465:6-methyl-2-(phenylethynyl)pyridine; D004247:DNA; D015761:4-Aminopyridine; D002118:Calcium; D001640:Bicuculline