A non-injurious assay to study mechanisms of transmission of noxious stimulus

尽管疼痛仍然是患者寻求医疗保健的最常见原因，但疼痛的传递和感知机制尚不完全清楚。为了开发治疗疼痛的新药物和新技术，必须了解感知有害和无害刺激的神经生理机制。因此，需要临床相关的疼痛动物模型来进一步了解疼痛传递机制。目前可用的研究疼痛传递的模型使用热、压力或化学刺激来施加疼痛。这些模型导致了该领域的许多发现，但也有局限性。一些限制包括：热和压力可能与动物受伤有关，刺激可以非特异性地刺激几种类型的神经纤维（A -压力，A - δ尖锐痛，c -慢灼痛），这些刺激可能与习惯学习有关。为了研究有害刺激的传递机制，我们开发了一种新的非伤害性伤害感受实验，优先研究有害刺激在活体系统中通过神经纤维的传递。我们使用一种神经刺激器，以不同的频率和强度向皮肤传递电刺激，产生短暂的不适，但不会造成伤害。每一种使用的电频率优先刺激一种特定类型的疼痛纤维，并允许测量每种类型的疼痛纤维的发声阈值。在模型中，我们将发声定义为对电刺激的疼痛回避行为，并将其作为终点来测量每个疼痛纤维的电流发声阈值。不像目前使用的疼痛模型，可能会产生一些组织损伤，以诱导疼痛，该试验是无损伤的，优先刺激每一种类型的疼痛纤维。因此，它可能成为研究疼痛机制的一个有价值的工具。

Although pain is still the most common reason why patients seek health care, the mechanisms of transmission and perception of pain are incompletely understood. In order to develop new drugs and techniques to treat pain, it is imperative to undrestand the neurophysiologic mechanisms by which noxious and non-noxious stimuli are perceived. Therefore, clinically relevant animal models of pain are needed to further our understanding of the mechanisms of pain transmission. Currently available models to study transmission of pain use heat, pressure, or chemical stimulation to inflict pain. These models have led to many discoveries in the field but have limitations. Some of the limitations include the fact that heat and pressure can be associated with injury to animals, the stimuli can nonspecifically stimulate several types of nerve fibers (A beta-pressure, A delta-sharp pain, C-slow burning pain), and these stimuli can be associated with habituation learning. In order to study the mechanisms of the transmission of noxious stimuli, we developed a novel and non-injurious nociception assay to preferentially study transmission of noxious stimulus via nerve fibers in a live system. We use a neuro-stimulator that delivers electrical stimuli to the skin at different frequencies and intensities and produces transient discomfort without producing injury. Each of the electrical frequencies used preferentially stimulate a specific type of pain fiber and allows for the measure of vocalization threshold for each type of pain fiber. In the model, we define vocalization as the pain avoiding behavior to the electrical stimulus and use it as the end point to measure current vocalization threshold for each of the pain fibers. Unlike currently used pain models that may produce some tissue-injury in order to induce pain, this assay is non-injurious and preferentially stimulates each type of pain fibers. Therefore it might become a valuable tool for the study of the mechanisms of pain. We completed pilot studies for the development of this assay in mice, developed normative values for the response to each frequency, and have fully developed software application to control the neuro-stimulator, and automated a large portion of the protocol. In addition, we are implementing upgrades to enable automated data collection and storage. We are now using the model to study the impact of aging, differences in gender as well as the role of nitric oxide synthases in the transmission of noxious stimuli in specific nociceptive nerve fibers. We are further developing the model to evaluate pharmacologic interventions such as morphine with systemic and local analgesics, different strains and species of animals, and different pain models.