Optimizing The Animal Model Of Acute Pain Based On Laser-evoked Brain Responses

Pain, both acute and chronic, is a significant health problem, with dramatic costs for both patient wellbeing and the society. Animal models of pain have been crucial in understanding fundamental mechanisms of pain and identify new analgesic targets. However, it is not easy to translate basic findings in animals into effective clinical analgesics. Two important issues we must concern: First, the sensitivity of sensory systems varies greatly across species. Correct translation of experimental results from animals to humans relies upon a careful consideration of the different sensitivity of sensory systems across species. Overlooking such differences could lead to incorrect interpretations of experimental data, and generate important misconceptions. While many researchers do not put a high value on the sensitivity differences now. Second, only a few basic science advances have been effectively translated to the clinical setting, most basic research findings of analgesic only succeed in laboratory, not in ward.Overlooking the first issue may result in severe mistake. Here, we provide a vivid example. Consistent with human studies, it is commonly reported that laser-evoked ERPs in freely-moving rodents also show different components at latencies compatible with the conduction velocity of Aδ-fibres(“Aδ-ERPs”) and C-fibres(“C-ERPs”). However, the interpretation of the responses recorded in those studies ignores two important facts. First, when delivered on the skin of humans and animals, laser pulses not only induce a steep temperature increase that activates nociceptors, but also generate a broadband ultrasound through a thermo-elastic mechanism. This laser-generated ultrasound is heard by rats, but not by humans. Second, as revealed by microneurographical recordings from single peripheral axons, conduction velocity assessment of peripheral afferents, and nocifensive behaviors, Aδ-fibres are virtually never activated by heat in rodents. Thus, laser-induced heat is detected by Aδ-fibres in humans, but not in rats.Considering these two striking differences in sensory sensitivity of both auditory and nociceptive systems between rodents and humans, it is surprising that plenty of studies keep ascribing the early part of laser-evoked murine brain responses to the activation of pain-related cutaneous Aδ-nociceptors(“Aδ-ERPs”). Instead, a physiologically more viable hypothesis is that such “Aδ-ERPs” reflect the activation of the auditory system by laser-generated ultrasounds.The second issue request researchers to optimize the current animal models of pain, and improve the translation of bench to bedside. Murine models of pain often consists in recording(1) threshold responses(e.g., the tail-flick reflex) elicited by(2) non-nociceptive specific input in(3) anaesthetized animals. The direct cortical recording of laser-evoked potentials(LEPs) in freely-moving rodents avoids these three important pitfalls, and has thus the potential of improving such translation.To demonstrate the first issue, in 5 experiments of Study 1, we tested this hypothesis by comprehensively characterizing and comparing the physiological properties of the electrocortical responses elicited by laser stimulation in rodents and humans. That the so-called "Aδ-ERPs" in rodents reflect the activation of the auditory system is supported by four important pieces of evidence. First, the "Aδ-ERP" latency was independent of stimulation site. This observation is in contrast with the notion that the stimulation of proximal and distal sites should elicit cortical responses at significantly different latencies if the somatosensory volleys were transmitted via peripheral Aδ afferents with a relatively slow conduction velocity of ~15 m/s(Shaw, Chen, Tsao, & Yen, 1999). Second, early "Aδ-ERPs" were completely abolished by ongoing white noise – an observation in contrast with several human studies reporting clear Aδ-related brain responses recorded using an equivalent amount of ongoing white noise. Third, laser pulses delivered on the cage at a distance of ~5-10 cm from the animal elicited a brain response virtually identical to the "Aδ-ERPs". Fourth, in both hairy and glabrous skin, laser pulses elicited identical "Aδ-ERPs", which were both abolished by ongoing white noise. This finding ruled out the possibility that the absence of "Aδ-ERPs" in rodents was due to the difficulty of activating Aδ nociceptors in the glabrous skin, as previously suggested.In Study 2, our study shows that removing the effect of auditory induced activity by ongoing white noise, laser-evoked cortical responses in freely-moving rodents reflect the activation of C-fibre afferent pathways well. We have characterized the stimulus-response functions of C-LEPs in freely-moving rats and we propose optimal ECoG montages to isolate LEP peaks(N1, N2, and P2 waves) that reflect functionally-distinct neural activities. All the basic properties of the rat C-LEP response match well with what is observed in human LEP recordings. This consistency demonstrates that recording LEPs in freely-moving rats is a valid model to translate experimental animal results into human physiology and pathophysiology.In summary, study1 provides compelling evidence that laser-generated ultrasounds are detected by the rat auditory system, and evoke a responsethathas been so far mistakenly interpreted as reflecting the Aδ-somatosensory input in tens of studies. As a result, we have to reconsider these relevant studies previously and should attach great importance to across-species variability in sensory sensitivity in future study. In study2, we provide a full description of the ECoG responses elicited by the selective laser stimulation of nociceptive afferents in freely-moving rats, by using ongoing white noise to abolish the laser-evoked ultrasound response.These functional properties of rat LEPs are similar with those of the LEPs recorded in healthy human participants. This similarity indicates that recording LEPs in freely-moving rats is a valid model to translate experimental animal results into human physiology and pathophysiology. Finally, we have built an optimized animal model of pain.