Profound Desensitization by Ambient GABA Limits Activation of δ-Containing GABA_AReceptors during Spillover

Abstract

High-affinity extrasynaptic GABA A receptors (GABA A Rs) are a prominent feature of cerebellar granule neurons and thalamic relay neurons. In both cell types, the presence of synaptic glomeruli would be expected to promote activation of these GABA A Rs, contributing to phasic spillover-mediated currents and tonic inhibition. However, the precise role of different receptor subtypes in these two phenomena is unclear. To address this question, we made recordings from neurons in acute brain slices from mice, and from tsA201 cells expressing recombinant GABA A Rs. We found that ␦ subunit-containing GABA A Rs of both cerebellar granule neurons and thalamic relay neurons of the lateral geniculate nucleus contributed to tonic conductance caused by ambient GABA but not to spillover-mediated currents. In the presence of a low "ambient" GABA concentration, recombinant "extrasynaptic" ␦ subunit-containing GABA A Rs exhibited profound desensitization, rendering them insensitive to brief synaptic-or spillover-like GABA transients. Together, our results demonstrate that phasic spillover and tonic inhibition reflect the activation of distinct receptor populations.

Control of cerebellar granule cell output by sensory-evoked Golgi cell inhibition

“…Our findings provide, to our knowledge, the first direct characterization of the temporal dynamics of sensory-evoked inhibition in granule cells in Crus II. We demonstrate that brief sensory stimuli evoke short, high-frequency bursts of phasic IPSCs, which are superimposed on a slow sustained outward current, consistent with synchronous direct and spillover input from multiple Golgi cells (6,21,26,28,29,(43)(44)(45)(46). Although fast stimulus-locked inhibition of granule cells has thus far been difficult to detect in vivo (37), our voltage-clamp recordings revealed sensory-evoked phasic and spillover inhibition in all granule cells, consistent with the view that Golgi cell axons strongly influence many hundreds of granule cells across the granular layer (15,47).…”

“…1E), reflecting GABA spillover (21). Because extrasynaptic δ-subunit-containing GABA A receptors are insensitive to brief synaptic or spillover GABA transients (26), slow currents are unlikely to reflect changes in the tonic inhibitory conductance (27). In the majority of granule cells, the onset latencies for phasic and spillover events appeared similar, suggesting that both modes of inhibition are driven by the same mossy fiber input pathways.…”

The population firing rate in the presence of GABAergic tonic inhibition in single neurons and application to general anaesthesia

“…This indicates a specific role of these receptors. It is interesting to note that a recent study shows that d-sub unit receptors are not activated by spill-over (Bright et al 2011). This finding reduces the importance of synaptic spill-over to explain tonic inhibition.…”

A study of subunit selectivity, mechanism and site of action of the delta selective compound 2 (DS2) at human recombinant and rodent native GABAA receptors

“…GABAA receptors incorporating the d-subunit are primarily found in peri-or extrasynaptic locations where they are subject to activation by low ambient GABA concentrations (Belelli et al, 2009). Indeed, d-GABAA receptors are highly sensitive to GABA and exhibit little desensitization (Nusser et al, 1998;Farrant and Nusser, 2005;Houston et al, 2009;Bright et al, 2011). GABA 'spill-over' from the synapse reaching a concentration in the low mM range (Mody, 2005) is believed to contribute to the tonic activation of extrasynaptic receptors.…”