Invisible infrared light could soon activate heart, ear cells

A discovery by University of Utah scientists might someday improve cochlear implants for the deaf and lead to devices to restore vision, maintain balance and treat movement disorders like Parkinson's.

The scientists used invisible infrared light to make rat heart cells contract and toadfish inner-ear cells send signals to the brain.

Infrared light can be felt as heat, raising the possibility the heart and ear cells were activated by heat rather than the infrared radiation itself.

Richard Rabbitt, a professor of bioengineering and senior author of the heart-cell and inner-ear-cell studies and colleagues exposed the cells to infrared light in the laboratory.

The low-power infrared light pulses in the study were generated by a diode - 'the same thing that's in a laser pointer, just a different wavelength', said Rabbitt.

The heart cells in the study were newborn rat heart muscle cells called cardiomyocytes, which make the heart pump. The inner-ear cells are hair cells, and came from the inner-ear organ that senses motion of the head. The hair cells came from oyster toadfish, which are well-establish models for comparison with human inner ears and the sense of balance.

Inner-ear hair cells 'convert the mechanical vibration from sound, gravity or motion into the signal that goes to the brain' via adjacent nerve cells, said Rabbitt.

Using infrared radiation, "we were stimulating the hair cells, and they dumped neurotransmitter onto the neurons that sent signals to the brain," he added.

He believes the inner-ear hair cells are activated by infrared radiation because "they are full of mitochondria, which are a primary target of this wavelength."

The infrared radiation affects the flow of calcium ions in and out of mitochondria - something shown by the companion study in neonatal rat heart cells.

That is important because for "excitable" nerve and muscle cells, "calcium is like the trigger for making these cells contract or release neurotransmitter," said Rabbitt.

The heart cell study found that an infrared pulse lasting a mere one-5,000th of a second made mitochondria rapidly suck up calcium ions within a cell, then slowly release them back into the cell - a cycle that makes the cell contract.

"Calcium does that normally," said Rabbitt.

"But it's normally controlled by the cell, not by us. So the infrared radiation gives us a tool to control the cell. In the case of the [inner-ear] neurons, you are controlling signals going to the brain. In the case of the heart, you are pacing contraction."

Rabbitt believes the research - including a related study of the cochlea last year - could lead to better cochlear implants that would use optical rather than electrical signals.

Nerve cells that send sound signals from the ears to the brain can fire more than 300 times per second, so ideally, a cochlear implant using infrared light would be able to perform as well. In the Utah experiments, the researchers were able to apply laser pulses to hair cells to make adjacent nerve cells fire up to 100 times per second. For a cochlear implant, the nerve cells would be activated within infrared light instead of the hair cells.

Rabbitt also sees potential for optical implants to treat balance disorders.

"When we get old, we shuffle and walk carefully, not because our muscles don't work but because we have trouble with balance," he said.

"This technology has potential for restoring balance by restoring the signals that the healthy ear sends to the brain about how your body is moving in space."

Optical stimulation also might provide artificial vision in people with retinitis pigmentosa or other loss of retinal cells - the eye cells that detect light and color - but who still have the next level of cells, known as ganglia, Rabbitt says.

The study has been published in The Journal of Physiology.