Discussion
The primary findings in the present research are as follows: (1) ALFF
signals in the supplemental somatosensory area, secondary auditory
cortex, agranular insular cortex, temporal association areas, and
ectorhinal cortex brain areas were enhanced in the DE group. (2) Insular
cortex showed upregulated ALFF and played a dominant role in chronic
corneal pain. (3) Cyclotraxin-B injection in insular cortex markedly
blocked chronic corneal pain by inhibiting the upregulation of
inflammatory cytokines.
Recent research suggested that some dry eye patients described different
responses to the same treatment, implying different pathophysiological
mechanisms between patients with and without chronic corneal pain9, 10. It is well known that
persistent peripheral ocular nerve damage and inflammation might cause
structures and functional changes in the CNS11.
However, the evidence of the central sensitization mechanism underlying
chronic corneal pain is still scarce.
In this study, we applied ALFF by rs-fMRI to map the functional
brain associated with chronic corneal pain and distinguish spontaneous
cerebral neuro-activities. ALFF has been used to evaluate functional
abnormalities of brain in some pain-related diseases such as discogenic
low-back and leg pain, migraine, phantom limb pain, and trigeminal
neuralgia12-14. However, there’s only few rs-fMRI
alterations research about acute corneal pain in DED. Yan. H. et al.
revealed ReHo values alterations in the limbic-cortical circuits in
patients with DED15. Zhi-Ming Pan et al. showed higher
ALFF values in parahippocampal gyri and caudate in acute eye pain
patients 16. In our present study, the results
demonstrated increase ALFF in S2, GU, VISC, AUDp, AuV, AlD, AlP, TeA,
PERI, Ect, OLF, HIP, RHP, CTXsp, STR brain areas. We detected increased
ALFF in S2. This is different from the previous case reported which
uncovered noxious acute stimulation of the cornea could produce
somatotopic activation in primary somatosensory
cortex17. This may be due to the difference in acute
and chronic noxious stimulation to peripheral corneal nerves.
ALFF analysis also revealed some brain areas involved in anxiety. Many
studies demonstrated that DE patients experienced increased anxiety,
chronic pain can aggravate emotional disorders such as anxiety and
depression18. In current study, ALFF showed
significant changes in temporal areas in DE mice. The temporal cortex
has been reported involved in anxiety and could record the physical
impact of negative emotions 19,20. Moreover, we also
noticed an increase ALFF in the primary auditory area, which contributes
to anxiety disorders21.
C-Fos expression was also used to verify the ALFF changed brain areas
and find abnormal neuronal activity brain areas. C-Fos expression has
previously been found to have significant upregulation in frontal
cortex, the third ventricle, fewer in hippocampal area and the spinal
nucleus of the trigeminal nerve in the medulla after menthol-stimulated
rabbits, cornea22,23. In line with
the rs-fMRI results, the upregulated mount of c-Fos verified the
abnormal neuronal activity mapping especially in S2, AID, AIP, AuV, TeA,
Ect in chronic corneal pain mice.
Then, we found a significant correlation between ALFF values of the
insular cortex and eye blinking times after chemical application. This
result is in line with the documented role of the insular cortex in
trigeminal neuropathic pain, which showed insular cortex representation
of dynamic mechanical allodynia24. The insular cortex
receives afferents from sensory thalamic nuclei involved in widely
different functions, such as pain perception and speech production, to
the processing of social emotions25.
Experimental results in the present study showed mRNA expression of
pro-inflammatory cytokines, including IL-6, IL-1β, and TNF-α, in
addition to BDNF, increased, reflecting an underlying over-active state
related to altered neuron function. The present result is also
consistent with previous research that pro-inflammatory cytokines,
neuronal and microglial markers were upregulated in the trigeminal
brainstem sensory complex of DED animals4. BDNF,
crucial in synaptic transmission and neuronal plasticity, has been
defined as a neurotransmitter and neuromodulator26,
and the BDNF-TrkB signaling pathway could mediate inflammatory
responses27. BDNF could affect central sensitization
by activation of NMDA receptor28. Remarkably, it has
been reported that BDNF enhances the excitability of the small-diameter
TRG neurons projecting onto the Vi/Vc after trigeminal inflammatory
hyperalgesia29. So, we speculated an essential role of
BDNF-derived neuroinflammation changes in the central mechanism of
chronic corneal pain. The local inhibition of TrkB in the insular cortex
of DE mice suppressed chronic pain and increased inflammation factors
significantly.
The present research is the first attempt to apply ALFF in chronic
corneal pain, but a few issues need to be declared. Our study mainly
concentrated on one major brain area, other brain regions associated
with chronic corneal pain have not been further confirmed. Besides
Trk-B, the different type of BDNF receptors, such as the p75
neurotrophin receptor, and the signaling mechanism involved may also be
established in the future.