Active cortical networks promote shunting fast synaptic inhibition in vivo

Sleep-wake-related changes in intracellular chloride regulate plasticity at glutamatergic cortical synapses

Wakefulness and sleep affect the brain's ability to exhibit plastic changes.1,2 For instance, the potentiation of cortical excitatory synaptic connections is associated with the active period, when animals are mainly awake.3,4,5,6,7 It is unclear, however, how changes in neuronal physiology that are associated with sleep-wake history, affect the mechanisms responsible for synaptic plasticity. Recently, it has been shown that sleep-wake history alters transmembrane chloride (Cl-) gradients in cortical pyramidal neurons via Cl- cotransporter activity, which shifts the reversal potential for gamma-aminobutyric acid (GABA) type A receptors (EGABAA) when assessed in vivo and in vitro.8,9 Hyperpolarizing EGABAA values are associated with recent sleep, whereas depolarizing EGABAA values are associated with recent waking. Here, we demonstrate that sleep-wake-history-related changes in EGABAA affect membrane potential dynamics and glutamatergic long-term potentiation (LTP) elicited by spiking activity in pyramidal neurons of the mouse cortex. Reducing the depolarized shift in EGABAA during the active period reduces the potentiation of cortical excitatory synapses onto layer 5 (L5) pyramidal neurons. Depolarized EGABAA values facilitate LTP induction by promoting residual membrane depolarization during synaptically evoked spiking. Changes in LTP induction associated with sleep-wake history can be reversed by switching the EGABAA-dependent effects, either by using direct current injection to counteract the effects upon residual membrane potential depolarization or by modulating cotransporters that regulate EGABAA. We conclude that EGABAA dynamics provide a functional link between changes in a neuron's physiology that are associated with sleep-wake history and the mechanisms responsible for the induction of glutamatergic synaptic plasticity.

All-optical reporting of inhibitory receptor driving force in the nervous system

Ionic driving forces provide the net electromotive force for ion movement across receptors, channels, and transporters, and are a fundamental property of all cells. In the nervous system, fast synaptic inhibition is mediated by chloride permeable GABAA and glycine receptors, and single-cell intracellular recordings have been the only method for estimating driving forces across these receptors (DFGABAA). Here we present a tool for quantifying inhibitory receptor driving force named ORCHID: all-Optical Reporting of CHloride Ion Driving force. We demonstrate ORCHID's ability to provide accurate, high-throughput measurements of resting and dynamic DFGABAA from genetically targeted cell types over multiple timescales. ORCHID confirms theoretical predictions about the biophysical mechanisms that establish DFGABAA, reveals differences in DFGABAA between neurons and astrocytes, and affords the first in vivo measurements of intact DFGABAA. This work extends our understanding of inhibitory synaptic transmission and demonstrates the potential for all-optical methods to assess ionic driving forces.

Optogenetic Determination of Dynamic and Cell-Type-Specific Inhibitory Reversal Potentials

The reversal potential refers to the membrane potential at which the net current flow through a channel reverses direction. The reversal potential is determined by transmembrane ion gradients and, in turn, determines how the channel's activity will affect the membrane potential. Traditional investigation into the reversal potential of inhibitory ligand-gated ion channels (EInh) has relied upon the activation of endogenous receptors, such as the GABA-A receptor (GABAAR). There are, however, challenges associated with activating endogenous receptors, including agonist delivery, isolating channel responses, and the effects of receptor saturation and desensitization. Here, we demonstrate the utility of using a light-gated anion channel, stGtACR2, to probe EInh in the rodent brain. Using mice of both sexes, we demonstrate that the properties of this optically activated channel make it a suitable proxy for studying GABAAR receptor-mediated inhibition. We validate this agonist-independent optogenetic strategy in vitro and in vivo and further show how it can accurately capture differences in EInh dynamics following manipulations of endogenous ion fluxes. This allows us to explore distinct resting EInh differences across genetically defined neuronal subpopulations. Using this approach to challenge ion homeostasis mechanisms in neurons, we uncover cell-specific EInh dynamics that are supported by the differential expression of endogenous ion handling mechanisms. Our findings therefore establish an effective optical strategy for revealing novel aspects of inhibitory reversal potentials and thereby expand the repertoire of optogenetics.

Physiological features of parvalbumin-expressing GABAergic interneurons contributing to high-frequency oscillations in the cerebral cortex

Parvalbumin-expressing (PV+) inhibitory interneurons drive gamma oscillations (30-80 Hz), which underlie higher cognitive functions. In this review, we discuss two groups/aspects of fundamental properties of PV+ interneurons. In the first group (dubbed Before Axon), we list properties representing optimal synaptic integration in PV+ interneurons designed to support fast oscillations. For example: [i] Information can neither enter nor leave the neocortex without the engagement of fast PV+ -mediated inhibition; [ii] Voltage responses in PV+ interneuron dendrites integrate linearly to reduce impact of the fluctuations in the afferent drive; and [iii] Reversed somatodendritic Rm gradient accelerates the time courses of synaptic potentials arriving at the soma. In the second group (dubbed After Axon), we list morphological and biophysical properties responsible for (a) short synaptic delays, and (b) efficient postsynaptic outcomes. For example: [i] Fast-spiking ability that allows PV+ interneurons to outpace other cortical neurons (pyramidal neurons). [ii] Myelinated axon (which is only found in the PV+ subclass of interneurons) to secure fast-spiking at the initial axon segment; and [iii] Inhibitory autapses - autoinhibition, which assures brief biphasic voltage transients and supports postinhibitory rebounds. Recent advent of scientific tools, such as viral strategies to target PV cells and the ability to monitor PV cells via in vivo imaging during behavior, will aid in defining the role of PV cells in the CNS. Given the link between PV+ interneurons and cognition, in the future, it would be useful to carry out physiological recordings in the PV+ cell type selectively and characterize if and how psychiatric and neurological diseases affect initiation and propagation of electrical signals in this cortical sub-circuit. Voltage imaging may allow fast recordings of electrical signals from many PV+ interneurons simultaneously.