Dose-Dependent Dual Effect of the Endozepine ODN on Neuronal Spiking Activity

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 evidence of the binding of ODN on GABA_AR-BZ-binding sites, their action on this receptor lacks compelling electrophysiological observations, with some studies reporting that ODN acts as a negative allosteric modulator (NAM) of GABA_AR 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 in neuronal excitability, we tested its effect in vivo in the cerebral cortex of the anesthetized mouse.

Spontaneous neuronal spikes were recorded by means of an extracellular pipette, in the vicinity of which ODN was micro-infused, either at a high dose (10^-5 M) or low dose (10^-11 M).

ODN at a high dose induced a significant increase in neuronal spiking. This effect could be antagonized by the GABA_AR-BZ-binding site blocker flumazenil. In sharp contrast, at low concentrations, ODN reduced neuronal spiking with a magnitude similar to GABA itself. Interestingly, this decrease in 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 doses 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-GABA_AR ODN receptor release of GABA.