Brain tissue slice thickness monitored by ion-profile measurement.

The thickness of a brain tissue slice preparation governs the amount of time required for substances to diffuse from the bathing solution to preparation. Slice thickness may increase during the experiment, e.g., in cases of hypoxia where osmotic pressure within the tissue changes, enabling water to enter the preparation. With increasing slice thickness diffusion paths from the bath to central layers of the preparation increase possibly resulting in an insufficient O2 supply to central layers. Therefore, the actual slice thickness should be monitored during the experiment especially in cases where osmolarity is changed or during hypoxia. This paper describes a simple method to monitor the actual slice thickness using ion profiles measured by ion selective micro-electrodes driven at a constant rate of approximately 10 microns/s (sample rate ca. 10/s). The method is based on steep changes in the concentration gradients at the upper and lower surfaces of the preparation induced by simple diffusion in the presence of concentration gradients between the non-tortuous bath and the tortuous tissue. The thickness of the preparation is derived from the location of the steep gradient changes as reflected by the registered profile.