Characterization of a novel tonic γ-aminobutyric acid_A receptor-mediated inhibition in magnocellular neurosecretory neurons and its modulation by glia

In addition to mediating conventional quantal synaptic transmission (also known as phasic inhibition), γ-aminobutyric acid_A (GABA _A) receptors have been recently shown to underlie a slower, persistent form of inhibition (tonic inhibition). Using patch-clamp electrophysiology and immunohistochemistry, we addressed here whether a GABA_A receptor-mediated tonic inhibition is present in supraoptic nucleus (SON) neurosecretory neurons; identified key modulatory mechanisms, including the role of glia; and determined its functional role in controlling SON neuronal excitability. Besides blocking GABA_A-mediated inhibitory postsynaptic currents, the GABA_A receptor blockers bicuculline and picrotoxin caused an outward shift in the holding current (I_tonic), both in oxytocin and vasopressin neurons. Conversely, the high-affinity antagonist gabazine selectively blocked inhibitory postsynaptic currents. Under basal conditions, I_tonic was independent on the degree of synaptic activity but was strongly modulated by the activity GABA transporters (GATs), mostly the GAT3 isoform, found here to be localized in SON glial cells/processes. Extracellular activation of GABAergic afferents evoked a small gabazine-insensitive, bicuculline-sensitive current, which was enhanced by GAT blockade. These results suggest that I_tonic may be activated by spillover of GABA during conditions of strong and/or synchronous synaptic activity. Blockade of I_tonic increased input resistance, induced membrane depolarization and firing activity, and enhanced the input-output function of SON neurons. In summary, our results indicate that GABA_A receptors, possibly of different molecular configuration and subcellular distribution, mediate synaptic and tonic inhibition in SON neurons. The latter inhibitory modality plays a major role in modulating SON neuronal excitability, and its efficacy is modulated by the activity of glial GATs.