Wearable Microscopes Image Transmission of Pain Signals

Researchers at the Salk Institute for Biological Studies have developed wearable microscopes and an implantable microprism that together aim to help scientists uncover the role that the spinal cord plays in relaying pain signals at the cellular level. The researchers used the microscopes and microprisms to perform high-definition, real-time imaging of the spinal cord’s activity in freely moving mice. Using the technology, the researchers investigated regions of the spinal cord that were previously inaccessible.

The researchers developed two wearable microscopes, about 7- and 14-mm wide, respectively. They feature custom compound microlenses and allow for high-speed imaging at 45 fps. The microscopes also provide high-resolution, high-contrast (about 1.5 μm) imaging and multicolor imaging from the visible to the near-infrared.

Neurons in the spinal cord (blue), including neurons that send signals about pain (green), are captured using one of the new, wearable microscopes. Courtesy of the Salk Institute.

The microprisms, which have a 700-μm field of view, were implanted in the mice to image sensory and premotor regions of the spinal cord and measure long-term activity across spinal laminae.

According to researcher Erin Carey, the function of the microprism is to increase the depth of imaging so previously unreachable cells can be viewed. “It also allows cells at various depths to be imaged simultaneously and with minimal tissue disturbance,” she said.

The technology addresses several issues that limited previously introduced wearable microscopes — including constraints in working distance, resolution, contrast, and achromatic range. Still, researcher Pavel Shekhtmeyster said, the wearable microscopes are light enough to be carried by mice.

Earlier work by the team suggested that astrocytes — star-shaped nonneuronal glial cells — could be involved in pain processing. Using chronically implanted microprisms, the researchers imaged sensory and motor-evoked activity in unexplored regions of the mouse spinal cord. They found that squeezing the tails of the mice activated the astrocytes, sending coordinated signals across segments of the spinal cord. The researchers showed that highly localized, painful mechanical stimuli evoke widespread, coordinated astrocyte excitation across multiple spinal segments.

“Being able to visualize when and where pain signals occur and what cells participate in this process allows us to test and design therapeutic interventions,” researcher Daniela Duarte said. The newly developed wearable microscopes will further enhance the ability to study pain.

Salk researchers developed two wearable microscopes to image cellular activity in previously inaccessible regions of the spinal cord of moving mice in real time. Courtesy of the Salk Institute.

According to the researchers, repeated imaging through the spinal cord-implanted microprism is feasible over at least four weeks. This recording period could be long enough to support the study of prolonged structural and functional biological processes related to disease and treatment. Researchers could gain insight into the neural basis of sensations and movement in conditions such as chronic pain, itch, amyotrophic lateral sclerosis, and multiple sclerosis.

The team has already begun investigating how neuronal and nonneuronal activity in the spinal cord is altered in different pain conditions and how various treatments can control abnormal cell activity. “These new wearable microscopes allow us to see nerve activity related to sensations and movement in regions and at speeds inaccessible by other high-resolution technology,” professor Axel Nimmerjahn said.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-023-36959-2) and Nature Biotechnology (www.doi.org/10.1038/s41587-023-01700-3).