Non-Injurious Pain Model Using Mice
Patients cite pain as the most common reason for seeking health care, and some medical organizations include pain as a fifth vital sign along with blood pressure, heart rate, temperature, and respirations. While the understanding of the neurophysiologic mechanisms by which noxious and non-noxious stimuli are perceived has substantially improved, the mechanisms of pain are not entirely understood. Despite progress in the development of new drugs and treatments, humans still suffer a great deal of pain while in the hospital, with many suffering from debilitating chronic pain throughout their lives.
Using humans in pain research has many limitations, including the lack of objective measures of pain and the inability to control environment factors. Our goal was to establish a model of non-injurious pain in mice which would overcome these limitations. Mouse models allow an investigator to study specific pain mechanisms by using transgenic animals, controlled environmental conditions, and larger sample sizes. As a key component of this model, we used an FDA-approved neuro-stimulator that is currently used in humans (adult and children) for the diagnosis of pain syndromes and neurologic diseases. The neuro-stimulator delivers electrical stimuli to the skin at different frequencies and intensities producing transient discomfort without injury. During a pilot study, we demonstrated that mice vocalize at repeatable thresholds of stimulus, providing an objective measure of pain. As currently used pain models for mice may produce some tissue-injury to induce pain, the development of a non-injury producing animal model adds an invaluable tool for the study of the mechanisms of pain and the understanding of the transmission of pain.
SPIS engineers integrated the commercially available neuro-stimulator device with a microphone, laptop, remote handheld control, and custom software application. The custom software application automates the delivery of the electrical stimulus per pre-defined protocols, reducing investigator fatigue relative to manually implementing lengthy, repetitive stimulus protocols. In addition, the application analyzes real-time audio signals from the microphone to automatically detect a mouse vocalization in presence of noise caused by the movement of the mouse and nearby environment noise sources. The detection of a vocalization causes immediate cessation of the stimulus, preventing unnecessary discomfort to the mouse. The remote handheld device allows the investigator to manually halt the stimulus in the case of a missed vocalization, or abort the experiment due to other animal safety concerns or general experiment difficulties.
Components of the non-injurious nociception assay in mice used to study age-related changes in pain stimulus threshold to electrical stimuli. The investigator conducts the entire experiment through a custom software application. The software application controls the neuro-stimulator (Neurometer, Neurotron, Inc., Baltimore, MD) to generate the electrical stimulus, analyzes the audio from the microphone to detect vocalizations, and interfaces with a remote handheld control to allow the user to override the system.
The mouse holder gently restrains the mouse with the tail secured and accessible, enabling the investigator to attach the neuro-stimulator leads. To achieve reliable microphone pick-up of the vocalizations, the microphone is placed close to the expected location of the mouse head. Since the mouse is only gently restrained, the mouse may move its head closer to the tail area and chew on the holder, generating audible noise.
The custom software application allows the investigator to design a protocol (not shown) which defines the following: specific mice to use, days of the week to perform the study, length of the study, and specific stimuli (i.e., order of the stimulus frequencies, and corresponding range of intensities) for each day. During an experiment, the application provides feedback on the progress of the protocol, automates the correct stimuli per the protocol, provides investigator override funtions, and saves the results (specific intensities at which a vocalization occurred) of the protocol.
Software Licensing: Neurotron, Inc