Infrared Laser Stimulation of Cerebral Cortex Cells - Aspects of Heating and Cellular Responses

Sammanfattning: The research of functional stimulation of neural tissue is of great interest within the field of clinical neuroscience to further develop new neural prosthetics. A technique which has gained increased interest during the last couple of years is the stimulation of nervous tissue using infrared laser light. Successful results have been reported, such as stimulation of cells in both the central nervous system, and in the peripheral nervous system, and even cardiomyocytes. So far, the details about the stimulation mechanism have been a question of debate as the mechanism is somewhat hard to explain. The mechanism is believed to have a photo-thermal origin, where the light from the laser is absorbed by water, thus increasing the temperature inside and around the target cell. Despite the mechanism questions, the technique holds several promising features compared to traditional electrical stimulation. Examples of advantages are that it is contact free, no penetration is needed, it has high spatial resolution and no toxic electrochemical byproducts are produced during stimulation. However, since the laser pulses locally increase the temperature of the tissue, there is a risk of heat induced damage. Therefore, the effect of increased temperatures must be investigated thoroughly. One method of examining the changes in temperature during stimulation is to model the heating.This thesis is based on the work from four papers with the main aim to investigate and describe the response of heating, caused by laser pulses, on central nervous system cells. In paper one, a model of the heating during pulsed laser stimulation is established and used to describe the dynamic temperature changes occurring during functional stimulation of cerebral cortex cells. The model was used in all four papers. Furthermore, single cell responses, as action potentials, as well as network responses, as activity inhibition, were observed. In paper two, the response of rat astrocytes exposed to laser induced hyperthermia was investigated. Cellular migration was observed and the migration limit was used to calculate the kinetic parameters for the cells, i.e., the reaction activation energy, Ea (321.4 kJ?mol-1), and the frequency factor, Ac (9.47 x 1048 s-1). Furthermore, a damage signal ratio (DSR) for calculating a threshold for cellular damage was defined, and calculated to six percent. In paper three, the response of hyperthermia to cerebral cortex cells was investigated, in the same way as in the second paper. Fluorescence staining of the metabolic activity was used to reveal the heat response, and by using the limit of the observed increased fluorescence the kinetic parameters, Ea (333.6 kJ?mol-1), and Ac (9.76 x 1050 s-1), were calculated. The DSR for the cells was calculated to five percent. In paper four, the behavior of action potentials triggered by laser stimulation was investigated. More specifically, the time delay from the start of a laser pulse to the detection of an action potential, delta-t, were investigated. Two different behaviors for the initial action potentials were observed: fast decreasing delta-t and slow decreasing delta-t. The results show the dynamic behavior of action potential responses to infrared light.The work of this thesis show the dynamic changes of the temperature during optical stimulation, using an infrared laser working at 1,550 nanometers. It also shows how the changes cause astrocytes to migrate for pulses several seconds long, and neurons to fire action potentials for pulses in the millisecond range. Furthermore, a damage signal ratio was defined and calculated for the cell systems.