Differential modulation of thermal preference after sensitization by optogenetic or pharmacological activation of heat-sensitive nociceptors

Abstract

Common approaches to studying mechanisms of chronic pain and sensory changes in pre-clinical animal models involve measurement of acute, reflexive withdrawal responses evoked by noxious stimuli. These methods typically do not capture more subtle changes in sensory processing nor report on the consequent behavioral changes. In addition, data collection and analysis protocols are often labour-intensive and require direct investigator interactions, potentially introducing bias. In this study, we develop and characterize a low-cost, easily assembled behavioral assay that yields self-reported temperature preference from mice that is responsive to peripheral sensitization. This system uses a partially automated and freely available analysis pipeline to streamline the data collection process and enable objective analysis. We found that after intraplantar administration of the TrpV1 agonist, capsaicin, mice preferred to stay in cooler temperatures than saline injected mice. We further observed that gabapentin, a non-opioid analgesic commonly prescribed to treat chronic pain, reversed this aversion to higher temperatures. In contrast, optogenetic activation of the central terminals of TrpV1+ primary afferents via in vivo spinal light delivery did not induce a similar change in thermal preference, indicating a possible role for peripheral nociceptor activity in the modulation of temperature preference. We conclude that this easily produced and robust sensory assay provides an alternative approach to investigate the contribution of central and peripheral mechanisms of sensory processing that does not rely on reflexive responses evoked by noxious stimuli.

Chronic pain mediated changes in the appetitive value of affective gentle touch in mice

“…We used a spinal implant to deliver light to the central projections of the primary afferents that originate from the hindlimb, a strategy that can reliably produce optogenetic responses without any motor or sensory deficits. 24 Preference for the initially non preferred (INP) arm could be increased by delivering blue light stimulation in that arm in MrgprB4-ChR2 male mice ( Fig. 1E ).…”

“…In experiments with stimulation through the epidural fiber stimulation was performed at a frequency of 10 Hz. The epidural fiber implant and spinal implant surgeries were performed as previously described 23,24 . Both central methods targeted the light source to the central projections of the primary afferents originating from the hindlimbs.…”

“…Depending on the needs of the experiment light delivery for optogenetic activation of the labelled primary afferents was achieved through either transdermal stimulation, an epidural implant 23 or a vertebral implant 24 . For the central methods of light delivery, the intensity and frequency of the 470 nm laser was controlled using a LED driver (Thor Labs) and a pulse width modulator (XY-LPWM; Protosupplies, USA) or an ANY-maze Optogenetic interface (Stoelting, Co, Wood Dale, IL).…”

“…In experiments with stimulation through the epidural fiber stimulation was performed at a frequency of 10 Hz. The epidural fiber implant and spinal implant surgeries were performed as previously described 23,24 .…”

Probing pain aversion in rats with the “Heat Escape Threshold” paradigm

“…For example, thermal gradient tracks can be used to examine the animal’s preference to specific temperature ranges. 28,45 However, the detection of thermal hyperalgesia in localized pain models (e.g., intraplantar carrageenan) is likely to be at least partially masked by paw guarding behaviour, which allows the animal to occupy warm regions of the gradient while avoiding direct contact of the affected paw with the heated surface. This problem would be especially pronounced in larger animals such as adult rats, where the distance between the lifted paw and the surface can be substantial.…”