Activity-dependent glycine release in the CA1 hippocampal region
1 Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, Germany
2 Research School of Chemistry
3 Research School of Chemistry, The Australian National University, Canberra, Australia
In the hippocampus, plasticity-inducing stimuli have been shown to elevate extracellular glycine levels. High-frequency stimulation (HFS) of the CA3-CA1 Schaffer collaterals results in a transient increase in extracellular glycine in the CA1 stratum radiatum. However, the molecular mechanisms underlying this activity-dependent glycine release remain unclear. To investigate these mechanisms, we used an optical glycine indicator and two-photon excitation fluorescence microscopy to monitor extracellular glycine levels in acute hippocampal slices.
First, we examined the dependence of HFS-induced glycine release on glutamatergic synaptic transmission. Our findings indicate that the activity of glutamate receptors of AMPA and NMDA receptors is required for activity-dependent increases of extracellular glycine. Next, we also investigated the role of glycine transporters (GlyT). We hypothesized that astrocytic uptake of synaptically released glutamate, which leads to depolarization and an intracellular Na+ increase in astrocytes, could support GlyT reversal and glycine release. Indeed, increasing intracellular Na+ by local application of D-aspartate resulted in elevated extracellular glycine levels. Furthermore, glutamate transporter inhibition abolished the glycine increase during HFS.
Additionally, we used pharmacology to explore the role of GlyT1 in activity-dependent glycine release. Inhibition of GlyT1 by n-ethyl glycine, and by NFPS, led to an increase in basal glycine levels, but no further increase was observed during HFS. Thus, GlyT1 appears to reduce extracellular glycine concentration in the absence of activity but reverses its transport direction to release glycine during increased neuronal activity; a process that can be mimicked by glutamate transporter activation. Our results suggest that cellular depolarization through glutamate receptors and glutamate uptake jointly drive activity-dependent glycine release.