Khakh, B. | bioRxiv Science

bioRxiv2024

Tonic dendritic GABA release by substantia nigra dopaminergic neurons

Recent studies have implicated dopaminergic signaling within the substantia nigra pars reticulata (SNr) in the emergence of Parkinsonian motor deficits. To better understand the mechanisms underlying this dependence, the ability of dopamine to modulate SNr neuron activity was studied in ex vivo mouse brain slices. In addition to presynaptically inhibiting phasic GABA release, D2 dopamine receptor (D2R) signaling unexpectedly suppressed a tonic, GABAA receptor-mediated inhibition of SNr neuron spiking. Our studies demonstrated that this tonic modulation was largely mediated by GAT-1-dependent GABA release from ALDH1A1-expressing dopaminergic neurons. In contrast, the ability of D2R agonists to disinhibit SNr neurons depended upon D2R-expressing astrocytes and stimulation of GAT-3 uptake of GABA from the extracellular space. In addition to underscoring the importance of dendritically released dopamine in modulating synaptic transmission, our studies demonstrate that dopaminergic modulation of astrocytes plays a key role in modulating SNr circuits and motor behavior. TeaserSNc dopaminergic neurons and astrocytes regulate tonic GABAergic inhibition of SNr neurons.

neuroscience↗

bioRxiv2022

Proximity of astrocyte leaflets to the synapse determines memory strength

SummaryAstrocyte distal processes, known as leaflets or perisynaptic astrocyte processes (PAPs), fine-tune synaptic activity by clearing neurotransmitters and limiting extrasynaptic glutamate diffusion. While learning and memory depends on orchestrated synaptic activity of neuronal ensembles within the hippocampus, it is becoming increasingly evident that astrocytes residing in the environment of these synapses play a central role in shaping memories. However, how astroglial synaptic coverage contributes to mnemonic processing remains largely unknown. Here, we targeted astrocyte leaflet structure in vivo by depleting Ezrin, an integral leaflet-structural protein, in astrocytes of the adult hippocampal CA1 using a CRISPR-Cas9 genetic approach. This resulted in significantly smaller astrocyte territories and reduced astroglial synaptic coverage. In addition, using genetically encoded glutamate sensors and whole-cell patch-clamp recordings from pyramidal neurons, we found that Ezrin deletion and the resultant manipulation of leaflet structure boosted extrasynaptic glutamate diffusion and NMDA-receptor activation. Importantly, these cellular phenotypes translated to enhanced fear memory expression that was accompanied by increased activation of CA1 pyramidal neurons in the days after learning occurred. We show that Ezrin is critical for astrocyte morphology as well as for adult hippocampal synapse integrity and function. Our data show that astrocyte leaflet structure gates memory strength by regulating glutamate spillover in the vicinity of memory-related synaptic activity.

neuroscience↗