Epileptic Stimulus Increases Homer 1a Expression to Modulate Endocannabinoid Signaling in Cultured Hippocampal Neurons

Neuropharmacology. 2012 Nov;63(6):1140-9. doi: 10.1016/j.neuropharm.2012.07.014. Epub 2012 Jul 16.

Abstract

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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • 4-Aminopyridine
  • Animals
  • Bicuculline
  • Calcium / physiology
  • Carrier Proteins / biosynthesis*
  • Carrier Proteins / genetics
  • Cells, Cultured
  • Convulsants
  • DNA / genetics
  • Endocannabinoids / physiology*
  • Epilepsy / chemically induced
  • Epilepsy / genetics*
  • Excitatory Postsynaptic Potentials / drug effects
  • Hippocampus / cytology
  • Hippocampus / physiology*
  • Homer Scaffolding Proteins
  • Neuroimaging
  • Neurons / physiology*
  • Patch-Clamp Techniques
  • Potassium Channel Blockers
  • Pyridines / pharmacology
  • Rats
  • Real-Time Polymerase Chain Reaction
  • Signal Transduction / genetics*
  • Transfection

Substances

  • Carrier Proteins
  • Convulsants
  • Endocannabinoids
  • Homer Scaffolding Proteins
  • Potassium Channel Blockers
  • Pyridines
  • 6-methyl-2-(phenylethynyl)pyridine
  • DNA
  • 4-Aminopyridine
  • Calcium
  • Bicuculline