Functional characterisation of spontaneously active GABAA receptors in rat dentate gyrus granule cells
GABAA receptors (GABAARs) are the principal inhibitory neurotransmitter receptors in the adult mammalian central nervous system. GABAARs mediate two forms of inhibition: fast, phasic conductance; and slow, tonic conductance. Tonic conductance arises due to the persistent activation of GABAARs. This persistent activation can occur by GABA-dependent or GABAindependent mechanisms. Low concentrations of ambient GABA activate high affinity GABAARs located outside the synapse – at peri-/extra-synaptic sites – to generate GABA-dependent tonic conductance. In contrast, GABA-independent tonic conductance is generated by GABAARs that activate spontaneously, in the absence of GABA, due to constitutive receptor gating. Because spontaneously active GABAARs (s-GABAARs) do not require GABA to activate, they are resistant competitive antagonists, e.g. SR-95531, but can be inhibited by the channel-blockers, e.g. picrotoxin. s-GABAARs have been shown to produce GABA-independent tonic conductances in the hippocampus and the amygdala. However, despite the good evidence for the presence of sGABAARs, their function and pharmacology remain largely unknown. Here we show, for the first time, using both current- and voltage-clamp recording techniques, that the s-GABAAR-mediated tonic conductance exerts a powerful inhibitory effect in rat dentate gyrus granule cells. We find that at resting membrane potential, s-GABAARs generate a shunting conductance that decreases both the membrane resistance and the membrane time constant of the neuron. When the membrane potential is depolarised, s-GABAARs conduct hyperpolarising currents that exhibit outward-rectification; this means that their net inhibitory effect is greater when the neuron is close to firing threshold than when it is at rest. Consistent with this, we find that block of s-GABAARs shifts the neuron into a more excitable state, as evidenced by the increase in the gain of the input-output relationship and the decrease in the rheobase current and the hyperpolarisation of the action potential threshold. At the network level, s-GABAARs regulate the precision of signal transmission in the dentate gyrus: blocking sGABAARs widens the temporal window over which multiple excitatory inputs can be successfully summated to generate an action potential. Finally, we report that s-GABAAR tonic currents are resistant to pharmacological compounds that target extrasynaptic GABAARs (L-655,708 and DS2), but are augmented by the clinically used benzodiazepine site modulators, zolpidem and midazolam, and partially inhibited by the inverse agonist, DMCM. The sensitivity of s-GABAARs to these compounds suggests the involvement of the γ2-subunit.