Concentration dependent dual effect of the endozepine ODN on neuronal spiking activity

Abstract

Endozepines, known as the endogenous ligands of benzodiazepine-binding sites, include the diazepam binding inhibitor (DBI) and its processing products, the triakontatetraneuropeptide (TTN) and the octadecaneuropeptide (ODN). Despite indisputable evidences of the binding of ODN on GABAAR-BZ-binding sites, their action on this receptor lacks compelling electrophysiological observations, some studies reporting that ODN acts as a negative allosteric modulator (NAM) of GABAAR while others suggest the opposite (positive allosteric modulation, PAM effect). All these studies were carried out in vitro with various neuronal cell types. To further elucidate the role of ODN on neuronal excitability, we tested its effect in vivo in the cortex of the anesthetized mouse. Spontaneous neuronal spikes were recorded by the mean of an extracellular pipette in the vicinity of which ODN was micro-infused, either at high dose (10-5M) or low dose (10-11M). ODN at high dose induced a significant increase of neuronal spiking. This effect could be antagonized by the GABAAR-BZ-binding sites blocker flumazenil. In sharp contrast, at low concentration, ODN reduced neuronal spiking in a magnitude similar to GABA itself. Interestingly, this decrease of neuronal activity by low dose of ODN was not flumazenil dependent suggesting that this effect is mediated by another receptor. Finally, we show that astrocytes in culture, known to be stimulated by picomolar dose of ODN via a GPCR, increased their export of GABA when stimulated by low dose of ODN. Our results confirm the versatility of ODN in the control of GABA transmission, but suggest that its PAM-like effect is, at least in part, mediated via an astrocytic non-GABAAR ODN receptor.