Background & Aims

Trigeminal neuropathic pain is emotionally distressing and disabling, with current treatments largely ineffective. Preclinical models of neuropathic pain show that non-neural changes are important for the development of the pain, although these data are derived almost exclusively from post-mortem histological/immunohistochemical analyses. It has been assumed that cephalic nerve injuries evoke identical molecular, cellular, and sex dependent neuroimmune changes to those following extra-cephalic injury. We sought to compare the behavioural consequences of trigeminal nerve injury with: (a) temporal patterns of recruitment of macrophages at the site of injury and the corresponding ganglion; and (b) the degree of demyelination at the injury site, and to visualise in-vivo the recruitment of macrophages and glia during the development of the pain using PET imaging of the translocator protein 18 kDa (TSPO) with [18F]PBR06, combined with magnetic resonance imaging.

Methods

We characterised the temporal development of stimulation-evoked ‘pain’ behaviours in male (n=31) and female (n=42) Sprague-Dawley rats over a 28-day period following chronic constriction injury of the infraorbital nerve (ION-CCI). The development of ‘pain’ behaviours in the ION-CCI rats was compared to uninjured controls (male = 36, female = 30); and rats that had received a sham-surgical procedure (male = 27, female = 36). Post-mortem, we determined: (i) the temporal patterns of demyelination at the injury site, and (ii) the accumulation of CD-68 immunoreactive macrophages at both the site of injury and in the trigeminal ganglia. We also determined the in vivo TSPO binding along the trigeminal pathway in male rats (n = 79), prior to, and up to 28 days after ION-CCI, compared to sham-injured and naïve counterparts.

Results

Our data demonstrated that testing only for sensory withdrawal thresholds, as is done in extra-cephalic neuropathic pain models does not provide access to a range of injury-specific nociceptive responses, reflective of the experience of trigeminal neuropathic pain. We also identified sex-specific neuroimmune changes along the trigeminal pathway and significant increases in TSPO signal in ION-CCI rats in the trigeminal ganglion, spinal trigeminal nucleus and paratrigeminal nucleus 2-14 days following nerve injury which returned to control levels by day 28. Post-mortem, the cellular locations of TSPO binding were confirmed using immunofluorescence as, (a) macrophages that accumulated in the trigeminal ganglion, and (b) reactive microglia in the brainstem trigeminal complex based on immunoreactivity for the Peripheral-Type Benzodiazepine Receptor, an ex vivo marker of TSPO binding.

Conclusions

Consistent with an emerging view that neuropathic pain arising from cephalic regions does not have the same qualities as pain arising from extra-cephalic regions, our data demonstrate that simply testing mechanical thresholds to determine allodynia and hyperalgesia in cephalic regions does not reveal regionally specific behavioural outcomes, equivalent to those described for extra-cephalic sensory testing. Trigeminal neuroimmune changes evoked by nerve injury are not the same as those identified in models of extra-cephalic neuropathy. Specifically, the timing, magnitude, and pattern of ION-CCI evoked macrophage activity differs between sexes. Further, these findings show, at least in males, that macrophage and glial changes in the periphery and brain during the development of chronic pain can be identified in vivo, providing a platform for translation into human patients.

References

Zimmermann M. Ethical guidelines for investigations of experimental pain in conscious animals. Pain 1983; 16(2): 109-10.

Presenting Author

James Kang

Poster Authors

James Kang

PhD

University of Sydney

Lead Author

Gaelle Emvalomenos

BBiomedEng

The University of Sydney

Lead Author

OIivia Davanzo

BSc

Lead Author

Sabrina Salberg

Monash University

Lead Author

Crystal Li

MSc

Lead Author

Bianca Jupp

PhD

Lead Author

Mathew Long

PhD

Lead Author

Mohammad Haskali

PhD

Lead Author

Sunil Kellapatha

PhD

Lead Author

Hyunsol Lim

BSc

Lead Author

Richelle Mychasiuk

Monash University

Lead Author

Luke Henderson

PhD

The University of Sydney

Lead Author

Kevin Keay

PhD

The University of Sydney

Lead Author

Topics

  • Pain Imaging