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

Extracellular GABA waves regulate coincidence detection in excitatory circuits

Acknowledgements This study was supported by the Wellcome Trust Principal Fellowship (212251_Z_18_Z), ERC Advanced Grant (323113), and European Commission NEUROTWIN grant (857562) to DAR; University of Edinburgh Chancellor's Fellowship to SS.Peer reviewedPublisher PD

Biphasic Modulation of NMDA Receptor Function by Metabotropic Glutamate Receptors

Peer reviewedPublisher PD

Inorganic polyphosphate regulates AMPA and NMDA receptors and protects against glutamate excitotoxicity via activation of P2Y receptors

Glutamate is one of the most important neurotransmitters in the process of signal transduction in the central nervous system. Excessive amounts of this neurotransmitter lead to glutamate excitotoxicity which is accountable for neuronal death in acute neurological disorders including stroke, trauma, and in neurodegenerative diseases. Inorganic polyphosphate (PolyP) plays multiple roles in the mammalian brain, including function as a calcium-dependent gliotransmitter mediating communication between astrocytes, while its role in the regulation of neuronal activity is unknown. Here we studied the effect of polyP on glutamate-induced calcium signal in primary rat neurons in both physiological and pathological conditions. We found that pre-incubation of primary neurons with polyP reduced glutamate- and AMPA- but not the NMDA-induced calcium signal. However, in rat hippocampal acute slices polyP reduced ion flux through NMDA and AMPA receptors in native neurons. The effect of polyP on glutamate and specifically on the AMPA receptors was dependent on the presence of P2Y1 but not of P2X receptor inhibitors and also could be mimicked by P2Y1 agonist 2MeSADP. Pre-incubation of cortical neurons with polyP significantly reduced the initial calcium peak as well as the number of neurons with delayed calcium deregulation in response to high concentrations of glutamate and resulted in protection of neurons against glutamate-induced cell death. As a result, activation of P2Y1 receptors by polyP reduced calcium signal acting through AMPA receptors, thus protecting neurons against glutamate excitotoxicity by reduction of the calcium overload and restoration of mitochondrial function.Significance StatementOne of the oldest polymers in the evolution of living matter is the inorganic polyphosphate. It is shown to play a role of gliotransmitter in the brain; however, the role of polyphosphate in neuronal signalling is not clear. Here we demonstrate that inorganic polyphosphate is able to reduce calcium signal, induced by physiological or high concentrations of glutamate. The effect of polyphosphate on glutamate-induced calcium signal in neurons is due to the effect of this polymer on the AMPA receptors.The effect of polyP on glutamate- and AMPA-induced calcium signal is dependent on P2Y receptor antagonist. The ability of polyphosphate to restrict glutamate-induced calcium signal lies in the basis of its protection of neurons against glutamate excitotoxicity