Instantaneous polarized light imaging reveals activity dependent structural changes of dendrites in mouse hippocampal slices

Intrinsic Optical Signal (IOS) imaging has been widely used to map patterns of brain activity in vivo in a label-free manner. Traditional IOS refers to changes in light transmission, absorption, and scattering, which have been correlated with neuronal swelling and volume changes in the observed tissue. Here we use polarized light for IOS imaging to monitor structural changes of cellular and sub-cellular architectures of neurons due to their synaptic activity in isolated brain slices. In order to reveal fast spatio-temporal changes of birefringence associated with neuronal activity, we developed the instantaneous PolScope. The instantaneous PolScope records changes in transmission, birefringence, and slow axis orientation in tissue at high spatial and temporal resolution using a single camera exposure. These capabilities enabled us to correlate polarization-sensitive IOS with traditional IOS on the same preparations. We detected reproducible spatio-temporal changes in both IOSs at the stratum radiatum in mouse hippocampal slices evoked by Schaffer collateral stimulation in the CA1 area. Upon stimulation, changes in traditional IOS signals were broadly similar across the area, while birefringence imaging revealed local variations not seen in traditional IOS. Locations with high resting birefringence produced larger stimulation-evoked birefringence changes than those with low resting birefringence. Local application of glutamate to the synaptic region in CA1 induced increase in both transmittance and birefringence signals. Blocking synaptic transmission with CNQX and D-APV (inhibitors of AMPA-and NMDA-type ionotropic glutamate receptors, respectively) reduced the peak amplitude of the optical signals. Changes in both IOSs were enhanced by an inhibitor of the membranous glutamate transporter, DL-TBOA. Our results indicate that birefringence imaging can monitor structural alterations of dendrites subjected to excitatory synaptic transmission also associated with neuronal activity in the brain.