Physiological evidence that pyramidal neurons lack functional water channels

Area of Science:

• Neuroscience
• Cell Biology
• Physiology

Background:

• Mammalian neuron swelling is clinically significant but poorly understood.
• Aquaporins are water channels, yet most central nervous system (CNS) neurons lack them, creating a paradox regarding osmotic regulation.
• Real-time measurement of neuronal volume is crucial for understanding brain swelling.

Purpose of the Study:

• To investigate the real-time volume changes of intact neurons in live brain slices under osmotic stress.
• To determine if CNS neurons, lacking aquaporins, regulate their volume in response to osmotic challenges.
• To elucidate the mechanisms of neuronal swelling during conditions like stroke and seizure.

Main Methods:

• Utilized 2-photon laser scanning microscopy (2PLSM) to monitor neuronal volume in real time within live brain slices.
• Measured the volume of pyramidal neurons in the cerebral cortex and cerebellar mossy fiber axon terminals.
• Assessed brain tissue volume changes using light transmittance and extracellular resistance measurements across a range of osmotic conditions (-80 to +80 mOsm).

Main Results:

• Pyramidal neurons (somata, dendrites, spines) and cerebellar axon terminals maintained stable volumes under acute osmotic stress (-40 to +80 mOsm).
• Brain tissue volume changes confirmed expected responses to osmotic stress, indicating water movement across non-neuronal compartments.
• Neurons swelled significantly only under oxygen/glucose deprivation or elevated extracellular potassium, suggesting non-aquaporin water channels open during prolonged depolarization.

Conclusions:

• Central nervous system neurons tenaciously maintain their volume, independent of aquaporins, preserving electrophysiological stability.
• Glial cells and endothelia, rich in aquaporins, likely drive osmotic brain swelling.
• Prolonged neuronal depolarization, as seen in stroke or seizure, triggers neuronal swelling via non-aquaporin pathways.