Anesthetics fragment hippocampal network activity, alter spine dynamics and affect memory consolidation

Abstract

20General anesthesia is characterized by reversible loss of consciousness accompanied by 21 transient amnesia. Yet, long-term memory impairment is an undesirable side-effect. How 22 different types of general anesthetics (GAs) affect the hippocampus, a brain region central to 23 memory formation and consolidation, is poorly understood. Using extracellular recordings, 24 chronic 2-photon imaging and behavioral analysis, we monitor the effects of isoflurane (Iso), 25 medetomidine/midazolam/fentanyl (MMF), and ketamine/xylazine (Keta/Xyl) on network 26 activity and structural spine dynamics in the hippocampal CA1 area of adult mice. GAs robustly 27 reduced spiking activity, decorrelated cellular ensembles, albeit with distinct activity signatures, 28 and altered spine dynamics. Iso anesthesia most closely resembled wakefulness, and network 29 alterations recovered more readily than with Keta/Xyl and MMF. Correspondingly, memory 30 consolidation was impaired after exposure to Keta/Xyl and MMF, but not Iso. Thus, different 31 anesthetics distinctly alter hippocampal network dynamics, synaptic connectivity, and memory 32 consolidation, with implications for GA strategy appraisal in animal research and clinical 33 settings. 34 35 KEYWORDS 36 General anesthesia, isoflurane, ketamine, fentanyl, hippocampus, population dynamics, 37 network activity, spine turnover, episodic memory 38 39 Here, using behavioral analysis, extracellular recordings and chronic 2-photon calcium 86 imaging, we systematically assessed how memory performance, CA1 network dynamics and 87 synaptic structure are affected by three commonly used combinations of GAs: isoflurane (Iso), 88 midazolam/medetomidine/fentanyl (MMF), and ketamine in combination with xylazine 89 (Keta/Xyl). All three GAs strongly reduced overall neuronal spiking and, opposite to what has 90 been found in the neocortex (Goltstein et al., 2015; Greenberg et al., 2008; Wenzel et al., 91 2019), decorrelated network activity, leading to a fragmented network state. However, the 92 induced patterns of activity were highly distinct between the three different conditions and 93 recovered to the pre-anesthetic status with disparate rates. Testing the effect of repeated 94 anesthesia on spine dynamics revealed that Keta/Xyl, the condition which most strongly 95 affected calcium activity, significantly reduced spine turnover, leading to an overall 96 (over)stabilization of hippocampal synapses. In contrast, Iso and MMF mildly increased spine 97 turnover. Finally, we show that the two anesthetic conditions which induce longer-lasting 98 network alterations, Keta/Xyl and MMF, negatively influenced hippocampus-dependent 99 memory consolidation. Thus, different anesthetics, despite reaching a similar physiological 100 state, strongly differ in their effects on synaptic stability, hippocampal network activity, and 101 memory consolidation. 102 103 RESULTS 104 Iso, Keta/Xyl and MMF induce distinct patterns of network activity 105Iso, Keta/Xyl and MMF have distinct molecular targets and modes of ac...

In VivoCalcium Imaging of CA3 Pyramidal Neuron Populations in Adult Mouse Hippocampus

“…The distribution of mean ΔF/F values was significantly shifted to higher values during wakefulness compared to anesthesia (Fig. 3B; 23 ± 24% vs. 11 ± 10%, mean ± s.d., p<0.001, two-sided Wilcoxon rank sum test), in qualitative agreement with recent findings in CA1 (Yang et al, 2020). The distribution of mean ΔF/F values for the awake condition showed a pronounced tail of large events, with a substantial fraction reaching >100% mean F/F.…”

Developmental increase of inhibition drives decorrelation of neural activity

“…The STTC, a metric that tracks correlations between spike trains and is robust to changes in firing rate, was calculated as previously described 51,93 (Figure 3A): where P A is defined as the proportion of spikes in spike train A that falls within ±Δt of a spike from spike train B. T A is defined as the proportion of time that occurs within (is “tiled” by) ±Δt from the spikes of spike train A. The same applies for P B and T B .…”

Spine dynamics of PSD-95-deficient neurons in the visual cortex link silent synapses to structural cortical plasticity

“…Imaging dendritic spines in awake animals is important because acute exposure to various anesthetics has been documented to increase spine formation in both cortical and hippocampal neurons in young rats ( 39 , 40 ) and decrease the elimination of filopodia-type dendritic protrusions in vivo in 1-mo-old mice ( 57 ). More importantly, a recent study has systematically investigated the effects of anesthesia on dendritic spine dynamics of CA1 pyramidal neurons after repeated exposure ( 41 ), as it is necessary for chronic imaging studies. Isoflurane, the anesthetic used in most in vivo imaging studies, was shown to increase spine turnover and spine density over five imaging sessions with 4-d intervals ( 41 ).…”

“…More importantly, a recent study has systematically investigated the effects of anesthesia on dendritic spine dynamics of CA1 pyramidal neurons after repeated exposure ( 41 ), as it is necessary for chronic imaging studies. Isoflurane, the anesthetic used in most in vivo imaging studies, was shown to increase spine turnover and spine density over five imaging sessions with 4-d intervals ( 41 ). Thus, in order to observe spine dynamics under physiological conditions, imaging in awake mice, as in the present study, is essential.…”

“…Here, we performed chronic two-photon imaging of dendrites of L2/3 pyramidal neurons in binocular V1 of PSD-95 KO (and KD) and WT mice, tracking the same dendritic spines longitudinally before, during, and after a 4-d period of MD. As previous studies have reported anesthesia effects on spine dynamics ( 39 – 41 ), we performed our experiments in awake mice, thoroughly trained for head fixation under the two-photon microscope. Our chronic spine imaging experiments revealed that in adult PSD-95 KO and KD mice, a brief MD indeed increased spine elimination about twofold, while adult WT mice did not display experience-dependent changes in spine elimination or spine formation.…”