Artificial sleep-like up/down-states induce synaptic plasticity in cortical neurons from mouse brain slices

Abstract

During non-rapid eye movement (NREM) sleep, cortical neuron activity alternates between a depolarized (firing, up-state) and a hyperpolarized state (down-state) coinciding with delta electroencephalogram (EEG) slow-wave oscillation (SWO, 0. 5-4 Hz) in vivo. Recently, we have found that artificial sleep-like up/down-states can potentiate synaptic strength in layer V cortical neurons ex vivo. Using mouse coronal brain slices, whole cell voltage-clamp recordings were made from layer V cortical pyramidal neurons to record spontaneous excitatory synaptic currents (sEPSCs) and inhibitory synaptic currents (sIPSCs). Artificial sleep-like up/down-states (as SWOs, 0.5 Hz, 10 min, current clamp mode) were induced by injecting sinusoidal currents into layer V cortical neurons. Baseline pre-SWO recordings were recorded for 5 min and post-SWO recordings for at least 25-30 min. Compared to pre-SWO sEPSCs or sIPSCs, post-SWO sEPSCs or sIPSCs in layer V cortical neurons exhibited significantly larger amplitudes and a higher frequency for 30 min. This finding suggests that both sEPSCs and sIPSCs could be potentiated in layer V cortical neurons by the low-level activity of SWOs, and sEPSCs and sIPSCs maintained a balance in layer V cortical neurons during pre- and post-SWO periods. Overall, this study presents an ex vivo method to show SWO's ability to induce synaptic plasticity in layer V cortical neurons, which may underlie sleep-related synaptic potentiation for sleep-related memory consolidation in vivo.

Keywords: brain slices; homeostatic synaptic plasticity; sleep; up/down-state; whole-cell recordings.

Figure 1

Sinusoidal current injection to induce cortical neuron up/down-states as slow-wave oscillations (SWOs). (A) Shows the two consecutive up/down-states with action potentials riding at the peaks with an inset of differential interference contrast (DIC) layer V cortical neuron image, expanded from two consecutive up/down-states in (B) (with an arrow indicating). (B) Shows the 60-s episode of SWOs (0.5 Hz) with injected sinusoidal current waveforms to induce neuronal up/down-states as SWOs. Scale bars are indicated as labeled. The vertical lines with dual arrows in (A,B) indicate the injected sinusoidal current amplitudes for up- (to induce action potentials) and down-states (0 pA injection, unless stated in Sections “Method/Result”). The sinusoidal waves are not in the same scale in (A,B).

Figure 2

Artificial SWO induction potentiates spontaneous excitatory synaptic currents (sEPSCs) in layer V cortical neurons. (A) Shows one representative pre-SWO (top) and post-SWO sEPSC (below) in layer V cortical neurons and the middle panel shows one SWO. Individual sEPSC events (with arrow indicating) are expanded in a small temporal scale. Scale bars are indicated as labeled. (B) Shows the summary data of pre- (empty bar) and post-SWO (gray bar) sEPSC amplitudes from the same cortical neurons (dots connected with one line). (C) Shows the summary data of pre- (empty bar) and post-SWO (gray bar) sEPSC frequencies from the same cortical neurons (dots connected with one line). *Indicates a significant difference p < 0.05.

Figure 3

Artificial SWO induction potentiates spontaneous inhibitory synaptic currents (sIPSCs) in layer V cortical neurons. (A) One representative pre- (top) and post-SWO sIPSC (below) in layer V cortical neurons and the middle panel shows one SWO. Individual sIPSC events (with arrow indicating) are expanded in a small temporal scale. Scale bars are indicated as labeled. (B) Shows the summary data of pre- (empty bar) and post-SWO (gray bar) sIPSC amplitudes from the same cortical neurons (dots connected with one line). (C) Shows the summary data of pre- (empty bar) and post-SWO (gray bar) sIPSC frequencies from the same cortical neurons (dots connected with one line). *Indicates a significant difference p < 0.05.