# Brainstem dysfunction induced by laser-induced shock wave results in hippocampal CA3 neuronal injury in mice

## TL;DR

A new mouse model shows that brainstem injury from a laser-induced shock wave causes hypoxemia and selective damage to hippocampal CA3 neurons.

## Contribution

A novel model using laser-induced shock waves to replicate brainstem dysfunction and study its secondary hypoxic effects on the hippocampus.

## Key findings

- LISW caused transient respiratory arrest and hypoxemia, leading to increased reactive astrocytes in hippocampal CA1 and CA3 regions.
- CA3 pyramidal neurons showed time-dependent necrosis and early apoptosis, indicating selective vulnerability to hypoxemia.
- No significant microglial activation was observed in CA1 or CA3 regions following LISW-induced injury.

## Abstract

The brainstem is anatomically vulnerable to trauma, and dysfunction often results in respiratory arrest and subsequent hypoxemia. Although animal models of traumatic brainstem injury have provided insights into local pathology, little is known about secondary effects, such as hypoxic brain injury, in other brain regions. Although a wide variety of traumatic brain injury models have been developed, few have been designed to replicate systemic hypoxemia as a primary insult. In this study, we reproduced brainstem dysfunction using laser-induced shock wave (LISW) and investigated their effect on hippocampal CA1 and CA3 pyramidal neurons, both of which are highly vulnerable to hypoxic injury. We applied LISWs to the upper neck region of mice, which caused immediate and transient respiratory arrest. Consequently, oxygen saturation rapidly declined and severe hypoxemia persisted for several minutes, reflecting transient brainstem dysfunction. Glial fibrillary acidic protein (GFAP) immunostaining revealed a significant increase in reactive astrocytes in both CA1 and CA3 regions on day 3. Iba-1 (ionized calcium-binding adapter molecule 1) immunostaining revealed no significant difference in the number of amoeboid microglia between the CA1 and CA3 regions. Cresyl violet staining revealed a time-dependent increase in the number of necrotic pyramidal neurons in the CA3 region, particularly on days 7 and 28. Terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining demonstrated a significant increase in apoptotic cells in the CA3 region on day 1, indicating early-phase activation of apoptosis-related pathways prior to the emergence of delayed neuronal degeneration. These findings indicate that LISW-induced hypoxemia secondary to brainstem dysfunction selectively damages the CA3 pyramidal neurons. This model may provide a useful platform for studying brainstem dysfunction–induced hypoxemia and assessing potential therapeutic strategies.