Ultraviolet irradiation of the eye and Fos-positive neurons induced in trigeminal brainstem after intravitreal or ocular surface transient receptor potential vanilloid 1 activation

Enhancement of ERK phosphorylation and photic responses in Vc/C1 neurons of a migraine model

Although it is well known that migraine pain is enhanced by photic stimulation of the eye, the mechanisms underlying this response are not yet understood. Noxious stimulation to the dura is known to activate trigeminal spinal subnucleus caudalis and upper cervical spinal cord (Vc/C1) neurons, causing migraine pain. Intense photic stimulation to the eye is also known to activate certain Vc/C1 neurons, thus increasing migraine pain. In this study, we hypothesized that Vc/C1 neurons receiving noxious dural input would be further activated by intense photic stimulation, resulting in the enhancement of migraine pain. However, mechanisms underlying the interactions between dural and photic sensory information in Vc/C1 neurons is unknown. To evaluate the above hypothesis, we studied phosphorylated extracellular signal-regulated kinase (pERK) -immunoreactive (IR) cells in Vc/C1 in dural mustard oil (DMO)-administrated rats. The change in neuronal excitability of Vc/C1 nociceptive neurons receiving input from the dura in DMO rats was examined and tested if those neurons were modulated by intense flush light stimulation. There were many pERK-IR cells in the lateral portion of Vc/C1 after MO administration to the dura. Flashlight presentation to the eye in DMO rats caused an enhancement of ERK phosphorylation in Vc/C1 neurons and pERK-IR cells were significantly suppressed after intracisternal administration of MEK1 inhibitor PD98059. Dura-light sensitive (DL) neurons were recorded in the lateral portion of Vc/C1 and photic responses of DL neurons were significantly enhanced following dural MO administration. These findings indicate that DL Vc/C1 neurons in DMO rats intensified their responses to intense photic stimulation and that ERK phosphorylation in Vc/C1 neurons receiving noxious dural input increased with intense photic stimulation, suggesting that Vc/C1 nociceptive neurons are involved in the enhancement of dural nociception associated with intense light stimulation.

Estrogens Exacerbate Nociceptive Pain via Up-Regulation of TRPV1 and ANO1 in Trigeminal Primary Neurons of Female Rats

Several trigeminal pain disorders show sex differences, and high levels of estrogens may underlie these differences. The interaction between transient receptor potential vanilloid 1 (TRPV1) and anoctamin 1 (ANO1) plays an important role in peripheral nociception. However, whether TRPV1 and ANO1 are involved in estrogen-modulated trigeminal pain sensitivity is unclear. In this study, we examined estradiol (E2) modulation of nociception through behavioral and immunohistological experiments after application of capsaicin (Cap), a selective TRPV1 agonist, onto the ocular surface in ovariectomized rats treated with high-dose E2 (HE) or low-dose E2 (LE) for 2 days. In addition, we used real-time PCR to study the effects of E2 on the expression levels of TRPV1 and ANO1 mRNA in trigeminal ganglia. In the behavioral experiment, the HE group showed significant potentiation of Cap-evoked nocifensive behavior compared with the LE group. Immunohistochemistry showed that Cap evoked a significantly greater number of cells that were immunoreactive for c-Fos, a marker of nociceptive activation, in the trigeminal subnucleus caudalis/upper cervical cord in the HE group than in the LE group. The number of c-Fos-immunoreactive cells in the ventral trigeminal interpolaris/caudalis were similar in the 2 groups. Real-time PCR showed that the levels of TRPV1 and ANO1 mRNA in the HE group were significantly higher than levels in the LE group. Thus, high levels of estrogens may be a risk factor for Cap-evoked nociceptive pain, and estrogen-dependent increases in TRPV1 and ANO1 are likely involved in modulating the nociceptive response in the trigeminal area.

Ocular inflammation induces trigeminal pain, peripheral and central neuroinflammatory mechanisms

Ocular surface diseases are among the most frequent ocular pathologies, with prevalence ranging from 20% of the general population. In addition, ocular pain following corneal injury is frequently observed in clinic. The aim of the study was to characterize the peripheral and central neuroinflammatory process in the trigeminal pathways in response to cornea alteration induced by chronic topical instillations of 0.2% benzalkonium chloride (BAC) in male C57BL/6J mice. In vitro BAC induced neurotoxicity and increases neuronal (FOS, ATF3) and pro-inflammatory (IL-6) markers in primary mouse trigeminal ganglion culture. BAC-treated mice exhibited 7days after the treatment reduced aqueous tear production and increased inflammatory cell infiltration in the cornea. Hypertonic saline-evoked eye wipe behavior was enhanced in BAC-treated animals that exhibited increased FOS, ATF3 and Iba1 immunoreactivity in the trigeminal ganglion. Ocular inflammation is associated with a significant increase in IL-6 and TNF-α mRNA expression in the trigeminal ganglion. We reported a strong increase in FOS and Iba1 positive cells in particular in the sensory trigeminal complex at the ipsilateral interpolaris/caudalis (Vi/Vc) transition and Vc/upper cervical cord (Vc/C1) regions. In addition, activated microglial cells were tightly wrapped around activated FOS neurons in both regions and phosphorylated p38 mitogen-activated protein kinase was markedly enhanced specifically in microglial cells during ocular inflammation. Similar data were obtained in the facial motor nucleus. These neuroanatomical data correlated with the increase in mRNA expression of pro-inflammatory (TNF-α, IL-6, CCL2) and neuronal (FOS and ATF3) markers. Interestingly, the suppression of corneal inflammation 10days following the end of BAC treatment resulted in a marked attenuation of peripheral and central changes observed in pathological conditions. This study provides the first demonstration that corneal inflammation induces activation of neurons and microglial p38 MAPK pathway within sensory trigeminal complex. These results suggest that this altered activity in intracellular signaling caused by ocular inflammation might play a priming role in the central sensitization of ocular related brainstem circuits, which represents a significant factor in ocular pain development.

Evidence for TRPA1 involvement in central neural mechanisms in a rat model of dry eye

Dry eye (DE) disease is commonly associated with ocular surface inflammation, an unstable tear film and symptoms of irritation. However, little is known about the role of central neural mechanisms in DE. This study used a model for persistent aqueous tear deficiency, exorbital gland removal, to assess the effects of mustard oil (MO), a transient receptor potential ankyrin (TRPA1) agonist, on eyeblink and eyewipe behavior and Fos-like immunoreactivity (Fos-LI) in the trigeminal brainstem of male rats. Spontaneous tear secretion was reduced by about 50% and spontaneous eyeblinks were increased more than 100% in DE rats compared to sham rats. MO (0.02-0.2%) caused dose-related increases in eyeblink and forelimb eyewipe behavior in DE and sham rats. Exorbital gland removal alone was sufficient to increase Fos-LI at the ventrolateral pole of trigeminal interpolaris/caudalis (Vi/Vc) transition region, but not at more caudal regions of the trigeminal brainstem. Under barbiturate anesthesia ocular surface application of MO (2-20%) produced Fos-LI in the Vi/Vc transition, in the mid-portions of Vc and in the trigeminal caudalis/upper cervical spinal cord (Vc/C1) region that was significantly greater in DE rats than in sham controls. MO caused an increase in Fos-LI ipsilaterally in superficial laminae at the mid-Vc and Vc/C1 regions in a dose-dependent manner. Smaller, but significant, increases in Fos-LI also were seen in the contralateral Vc/C1 region in DE rats. TRPA1 protein levels in trigeminal ganglia from DE rats ipsilateral and contralateral to gland removal were similar. Persistent tear reduction enhanced the behavioral and trigeminal brainstem neural responses to ocular surface stimulation by MO. These results suggested that TRPA1 mechanisms play a significant role in the sensitization of ocular-responsive trigeminal brainstem neurons in this model for tear deficient DE.

Trigeminal pathways for hypertonic saline- and light-evoked corneal reflexes

Cornea-evoked eyeblinks maintain tear film integrity on the ocular surface in response to dryness and protect the eye from real or potential damage. Eyelid movement following electrical stimulation has been well studied in humans and animals; however, the central neural pathways that mediate protective eyeblinks following natural nociceptive signals are less certain. The aim of this study was to assess the role of the trigeminal subnucleus interpolaris/caudalis (Vi/Vc) transition and subnucleus caudalis/upper cervical cord (Vc/C1) junction regions on orbicularis oculi electromyographic (OOemg) activity evoked by ocular surface application of hypertonic saline or exposure to bright light in urethane anesthetized male rats. The Vi/Vc and Vc/C1 regions are the main sites of termination for trigeminal afferent nerves that supply the ocular surface, while hypertonic saline (saline=0.15-5M) and bright light (light=5k-20klux) selectively activate ocular surface and intraocular trigeminal nerves, respectively, and excite second-order neurons at the Vi/Vc and Vc/C1 regions. Integrated OOemg activity, ipsilateral to the applied stimulus, increased with greater stimulus intensities for both modalities. Lidocaine applied to the ocular surface inhibited OOemg responses to hypertonic saline, but did not alter the response to light. Lidocaine injected into the trigeminal ganglion blocked completely the OOemg responses to hypertonic saline and light indicating a trigeminal afferent origin. Synaptic blockade by cobalt chloride of the Vi/Vc or Vc/C1 region greatly reduced OOemg responses to hypertonic saline and bright light. These data indicate that OOemg activity evoked by natural stimuli known to cause irritation or discomfort in humans depends on a relay in both the Vi/Vc transition and Vc/C1 junction regions.

Capsaicin-responsive corneal afferents do not contain TRPV1 at their central terminals in trigeminal nucleus caudalis in rats

We examined the substrates for ocular nociception in adult male Sprague-Dawley rats. Capsaicin application to the ocular surface in awake rats evoked nocifensive responses and suppressed spontaneous grooming responses. Thus, peripheral capsaicin was able to activate the central pathways encoding ocular nociception. Our capsaicin stimulus evoked c-Fos expression in a select population of neurons within rostral trigeminal nucleus caudalis in anesthetized rats. These activated neurons also received direct contacts from corneal afferent fibers traced with cholera toxin B from the corneal surface. However, the central terminals of the corneal afferents that contacted capsaicin-activated trigeminal neurons did not contain TRPV1. To determine if TRPV1 expression had been altered by capsaicin stimulation, we examined TRPV1 content of corneal afferents in animals that did not receive capsaicin stimulation. These studies confirmed that while TRPV1 was present in 30% of CTb-labeled corneal afferent neurons within the trigeminal ganglion, TRPV1 was only detected in 2% of the central terminals of these corneal afferents within the trigeminal nucleus caudalis. Other TRP channels were also present in low proportions of central corneal afferent terminals in unstimulated animals (TRPM8, 2%; TRPA1, 10%). These findings indicate that a pathway from the cornea to rostral trigeminal nucleus caudalis is involved in corneal nociceptive transmission, but that central TRP channel expression is unrelated to the type of stimulus transduced by the peripheral nociceptive endings.

Trigeminal interpolaris/caudalis transition neurons mediate reflex lacrimation evoked by bright light in the rat

Abnormal sensitivity to bright light can cause discomfort or pain and evoke protective reflexes such as lacrimation. Although the trigeminal nerve is probably involved, the mechanism linking luminance to somatic sensory nerve activity remains uncertain. This study determined the effect of bright light on second-order ocular neurons at the ventral trigeminal interpolaris/caudalis transition (Vi/Vc) region, a major termination zone for trigeminal sensory fibers that innervate the eye. Most Vi/Vc neurons (80.9%) identified by responses to mechanical stimulation of the ocular surface also encoded bright light intensity. Light-evoked neural activity displayed a long latency to activation (> 10 s) and required transmission through the trigeminal root ganglion. Light-evoked neural activity was inhibited by intravitreal injection of phenylephrine or l-N(G) -nitro-arginine methyl ester (L-NAME), suggesting a mechanism coupled to vascular events within the eye. Laser Doppler flowmetry revealed rapid light-evoked increases in ocular blood flow that occurred prior to the increase in Vi/Vc neural activity. Synaptic blockade of the Vi/Vc region by cobalt chloride prevented light-evoked increases in tear volume, whereas blockade at the more caudal spinomedullary junction (Vc/C1) had no effect. In summary, Vi/Vc neurons encoded bright light intensity and were inhibited by drugs that alter blood flow to the eye. These results support the hypothesis that light-responsive neurons at the Vi/Vc transition region are critical for ocular-specific functions such as reflex lacrimation, whereas neurons at the caudal Vc/C1 junction region probably serve other aspects of ocular nociception.

The role of trigeminal interpolaris-caudalis transition zone in persistent orofacial pain

Previous studies have established the role of the medullary dorsal horn or the subnucleus caudalis of the spinal trigeminal complex, a homolog of the dorsal horn of the spinal cord, in trigeminal pain processing. In addition to the medullary dorsal horn, recent studies have pointed out increased excitability and sensitization of trigeminal interpolaris and caudalis transition zone (Vi/Vc) following deep orofacial injury, involving neuron-glia-cytokine interactions. The Vi/Vc transition zone accesses rostral brain regions that are important for descending pain modulation, and somatovisceral and somatoautonomic processing and plays a unique role in coordinating trigeminal nocifensive responses.