Mild Blast Injury Produces Acute Changes in Basal Intracellular Calcium Levels and Activity Patterns in Mouse Hippocampal Neurons

Abstract

Mild traumatic brain injury (mTBI) represents a serious public health concern. Although much is understood about long-term changes in cell signaling and anatomical pathologies associated with mTBI, little is known about acute changes in neuronal function. Using large scale Ca imaging in vivo, we characterized the intracellular Ca dynamics in thousands of individual hippocampal neurons using a repetitive mild blast injury model in which blasts were directed onto the cranium of unanesthetized mice on two consecutive days. Immediately following each blast event, neurons exhibited two types of changes in Ca dynamics at different time scales. One was a reduction in slow Ca dynamics that corresponded to shifts in basal intracellular Ca levels at a time scale of minutes, suggesting a disruption of biochemical signaling. The second was a reduction in the rates of fast transient Ca fluctuations at the sub-second time scale, which are known to be closely linked to neural activity. Interestingly, the blast-induced changes in basal Ca levels were independent of the changes in the rates of fast Ca transients, suggesting that blasts had heterogeneous effects on different cell populations. Both types of changes recovered after ∼1 h. Together, our results demonstrate that mTBI induced acute, heterogeneous changes in neuronal function, altering intracellular Ca dynamics across different time scales, which may contribute to the initiation of longer-term pathologies.

Rapid Changes in Synaptic Strength After Mild Traumatic Brain Injury

“…The moderate synaptic changes seen at 48 h post-injury were unexpected given the behavioral and neuroinflammatory effects we observed. TBI responses can evolve rapidly in vivo during the hours after injury, including successive periods of reduced and heightened excitability (Kharatishvili et al, 2006; Ding et al, 2011; Hansen et al, 2018). Compensatory effects can also contribute to recovery as early as 48 h (Griesemer and Mautes, 2007; Greer et al, 2012), perhaps accounting for the apparent stability in synaptic input.…”

“…EEG recordings revealed hyperactivation and brief seizure activity in rat parietal cortex 1–2 min after cortical compression injury, followed by a post-ictal depression lasting ≥2 h (Nilsson et al, 1994). In contrast, in vivo Ca 2+ imaging showed a decrease in hippocampal activity within seconds of blast injury in mice that recovered after ∼60 min (Hansen et al, 2018). Similarly, multi-unit firing in rat neocortex was initially suppressed at 5–15 min after cortical compression, followed by subsequent hyper-responsiveness appearing approximately 2 h after injury (Kharatishvili et al, 2006; Ding et al, 2011).…”

Population imaging of neural activity in awake behaving mice

“…Hippocampus and striatum imaging window implantation.-Hippocampal and striatal window surgeries were performed similar to those described previously 11,29 . For each imaging window, a virus/drug infusion cannula (26G, PlasticsOne Inc., C135GS-4/ SPC) was attached to a stainless steel imaging cannula (OD: 3.17mm, ID: 2.36mm, 1 or 2mm height, AmazonSupply, B004TUE45E).…”

“…Cryoprotected brains were frozen in OCT in a dry ice bath and sliced (coronal) to 50μm thickness using a cryostat. Glial and microglial antibody staining were performed with anti-GFAP 29 (1:250, Clone N206/A8, Neuromab) and anti-Iba1 31 (1:500, 019-19741, Wako Chemicals) primary antibodies, followed by Alexa Fluor 568 (1:1000, Goat anti-Mouse IgG (H+L) Cross-Adsorbed Secondary Antibody, A11004, InVitrogen) and 633 secondary antibodies (1:1000, Goat anti-Rabbit IgG (H+L) Cross-Adsorbed Secondary Antibody, A21070, InVitrogen). All antibodies were used according to the protocols that have been validated by suppliers.…”

Distinct Neuronal Populations Contribute to Trace Conditioning and Extinction Learning in the Hippocampal CA1

“…In order to address these questions, we performed calcium imaging of individual CA1 neurons in mice over multiple days during the course of trace eye-blink conditioning. Calcium imaging allows us to measure hundreds to thousands of neurons simultaneously with single-cell resolution, across multiple trials and multiple days of learning 33,34 . Once conditioning was achieved, mice underwent a final conditioning session followed by extinction training, enabling us to track the same neuron population during both learning paradigms.…”