One of the most perplexing aspects of our studies involves the mechanisms by which ultrasound can stimulate neuronal activity. Ultrasound can indeed be used to produce thermal fluctuations in brain tissue and have therapeutic effects (Tyler et al, 2010), such as circuit ablation shown by others. We have found however that stimulation of brain circuits with ultrasound can be achieved without heating the tissue (Tufail et al, 2011) and thus focus our mechanistic investigations on the mechanical bioefffects of ultrasound, which are observed at intensities < 500 mW/cm2. Here, we have formulated a continuum mechanics hypothesis of ultrasonic neuromodulation where ultrasound produces consequences on brain fluids (lipid bilayers and cerebrospinal fluid) to influence the resting membrane potentials of neurons as depicted above (Tyler, 2011). Data from some of our biophysical investigations on how low-intensity ultrasound can influence fluid mechanics in biological systems are shown below.
The image shown at right depicts the influence of an ultrasound waveform on the lipid bilayers of CHO cells, which we use to screen various biophysical mechanisms of action due to our ability to easily regulate the protein ion channels they express. The data was obtained by performing confocal line scans of CHO cells labeled with a lipid stain and the membrane permeant calcium indicator Fluo-4 AM. One can clearly see the mechanical energy of low-intensity ultrasound produces robust effects on lipid bilayer cell membranes (a non-Newtonian fluid).
Using brain slices prepared from thy-1-GFP mice, we have made observations in neurons similar to those shown. Combining optical imaging of cellular structure in response to mechanical energy imparted by ultrasound enables us to study some aspects of the mechanisms underlying the ability of pulsed ultrasound to stimulate brain circuits.
Several other bioeffects of ultrasound on fluid behavior shown in the model above may act to influence neuronal excitability. For example, acoustic streaming, eddying, or microjet formation could produce mechanically-mediated deplorization of neurons in response to ultrasound. To understand how various acoustic impedance mismatches in brain tissue may lead to fluid dynamics, we conduct optical imaging of microbubbles while we transmit ultrasound into various fluid conditions. The movie below illustrates an example of one such experiment where acoustic streams can be observed forming in a fluorescent media around microbubbles in response to ultrasound.
Using methods similar to those just described above, we are studying both stable and inertial cavitation of microbubbles in response to ultrasound as shown in the movie below.
Finally, through collaborations with Dr. Kenichiro Koshiyama we are using Nonequilibrium Molecular Dynamics Simulations to study the effects of shock wave impulses and ultrasound on lipid bilayer fluidity. The movie below illustrates three picoseconds of data generated using MDS models, where a shock wave alters lipid bilayer properties. The model includes phospholipid bilayers, water molecules, sodium (blue) and chloride (green) ions. You can see how an instataneous pressure wave influences these molecules in the movie below. Combining the biophysical approaches described above represents a rather powerful set of approaches for gaining insight into the mechanisms of action by which pulsed ultrasound may influence neuronal excitability through mechanical (nonthermal) actions.
Reference: W.J. Tyler, 2011 Ultrasound for Neuromodulation? A Continuum Mechanics Hypothesis. The Neuroscientist. PDF