Perrier, J.-F. | Search bioRxiv

bioRxiv2024

Glutamate receptor-dependent cytosolic acidification in hippocampal neurons involves passive flux of protons from the extracellular space

Glutamate receptor-dependent cytosolic acidification can be induced in hippocampal neurons by pharmacological or seizure-like stimulation. This acidification is thought to arise from Ca2+ and metabolism-related processes, however, the exact underlying mechanism as well as its functional role remains uncertain. To reassess the mechanism of cytosolic acidification in excitatory hippocampal neurons and address the physiological relevance of the phenomenon, we combined pH/Ca2+ biosensors to study activity-induced pH dynamics in hippocampal neurons. First, we addressed cytosolic acidification in relation to LTP at hippocampal CA3-CA1 synapses. Using hippocampal slices from adult rats of both sexes, we show that LTP-inducing stimulation at the Schaffer collaterals evokes transient cytosolic acidification in hippocampal CA1 neurons. This highlights neuronal pH shifts as a trait of general hippocampal neurotransmission rather than a marker of excitotoxicity, possibly serving as a secondary messenger. Moreover, using dissociated hippocampal neurons from rat embryos, we show that glutamate receptor agonists typically induce larger cytosolic acid shifts compared to simple depolarization or spontaneous activity, suggesting that glutamate receptor-mediated acidification involves several separate mechanisms; pyruvate-dependent dampening of neuronal acidification may reflect a direct inhibition of NMDA receptors rather than reduced glycolytic activity, questioning the previously reported involvement of metabolism in cytosolic acidification; and whereas acid shifts induced by simple depolarization show exclusive dependence on cytosolic Ca2+, AMPA-induced acidification depends both on cytosolic Ca2+ and on an inward electrochemical driving force for protons. These results suggest that glutamate receptor-induced cytosolic acidification relies both on cytosolic Ca2+ and on a passive proton influx, possibly mediated by the receptor itself. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=166 HEIGHT=200 SRC="FIGDIR/small/624027v1_ufig1.gif" ALT="Figure 1"> View larger version (48K): org.highwire.dtl.DTLVardef@1393cfcorg.highwire.dtl.DTLVardef@d2dc9corg.highwire.dtl.DTLVardef@191b24corg.highwire.dtl.DTLVardef@50b2f3_HPS_FORMAT_FIGEXP M_FIG C_FIG Significance statementAlthough several studies report that hippocampal pyramidal neurons show significant cytosolic acidification in response to activation by drugs, epilepsy and stroke, the molecular mechanism and functional impact of the phenomenon remains uncertain. Using live imaging of both Ca and H dynamics, we demonstrate that the induction of LTP at hippocampal synapses is associated with cytosolic acidification in the postsynaptic neurons, suggesting that the cytosolic acid shifts may be a general trait of neurotransmission and related plasticity. We further revisit the mechanistic relation between the two ion systems. Our results suggest that glutamate receptor-induced cytosolic acidification involves two distinct mechanisms, one related to cytosolic Ca and another involving a passive H influx, possibly mediated by the receptor itself.

neuroscience↗

bioRxiv2023

Microcircuit failure in STXBP1 encephalopathy leads to hyperexcitability

De novo mutations in Stxbp1 are among the most prevalent causes of neurodevelopmental disorders, and lead to haploinsufficiency, cortical hyperexcitability, epilepsy and other symptoms. Given that Munc18-1, the protein encoded by Stxbp1, is essential for both excitatory and inhibitory synaptic transmission, it is currently not understood why mutations cause hyperexcitability. We discovered that overall inhibition in canonical feedforward microcircuits is defective in a validated mouse model for Stxbp1 haploinsufficiency. However, unexpectedly, we found that inhibitory synapses were largely unaffected. Instead, excitatory synapses failed to recruit inhibitory interneurons. Modelling experiments confirmed that defects in the recruitment of inhibitory neurons in microcircuits cause hyperexcitation. Ampakines, compounds that enhance excitatory synapses, restored interneuron recruitment and prevented hyperexcitability. These findings identify deficits in excitatory synapses in microcircuits as a key underlying mechanism for cortical hyperexcitability in Stxbp1 disorder and identify compounds enhancing excitation as a direction for therapy design. Highlights- Neocortical microcircuits fail in Stxbp1 haploinsufficiency mouse models (Stxbp1hap) - Microcircuit impairments leads to cortical hyperexcitability due to a lack of inhibition. - Inhibitory synapses are not severely affected in Stxbp1hap, instead, excitatory synapses fail to recruit interneurons. - AMPAkines rescue microcircuit failure in Stxbp1hap

neuroscience↗

bioRxiv2019

Expression of c-Fos and Arc in hippocampal region CA1 marks neurons that exhibit learning-related activity changes

Immediate early genes (IEGs) are transcribed in response to neural activity and necessary for many forms of plasticity. However, the dynamics of their expression during learning, as well as their relationship to neural activity, remain unclear. Here we used two-photon imaging in transgenic mice that express a GFP-tagged variant of Arc or c-Fos and a red-shifted calcium indicator to measure learning-related changes in IEG expression levels and neural activity in hippocampal region CA1 as mice learned to perform a two-alternative forced choice task. Neural activity levels correlated positively with IEG expression levels in vivo. In addition, we found that with learning, a subset of neurons in CA1 increased their responses to the reward-predicting cue, and IEG expression levels early in learning were selectively increased in neurons that would exhibit the strongest learning-related changes. Our findings are consistent with an interpretation of IEG expression levels as markers for experience dependent plasticity.

neuroscience↗