## Abstract
Intranasal trigeminal sensations are important in everyday life of human beings, as they play a governing role in protecting the airways from harm. Trigeminal sensations arise from the binding of a ligand to various sub-types of transient receptor potential (TRP) channels located on mucosal branches of the trigeminal nerve. Which underlying neural networks are involved in the processing of various trigeminal inputs is still unknown. To target this unresolved question fourteen healthy human subjects were investigated by completing three functional magnetic resonance imaging (fMRI) scanning sessions during which three trigeminal substances, activating varying sub-types of chemoreceptors and evoking different sensations in the nose were presented: CO2, menthol and cinnamaldehyde. We identified similar functional networks responding to all stimuli: an olfactory network, a somatosensory network and an integrative network. The processing pathway of all three stimulants was represented by the same functional networks, although CO2 evokes painful but virtually odorless sensations, and the two other stimulants, menthol and cinnamaldehyde are perceived as mostly non painful with a clear olfactory percept. Therefore, our results suggest a common central processing pathway for trigeminal information regardless of the trigeminal chemoreceptor and sensation type.
## Discussion
In this study, we were able to show that distinct trigeminal chemoreceptors share the same processing pathway in the brain. Specifically, stimulation of three different TRP channels with relatively specific substances led to the activation of very similar brain networks. In all three stimuli, we observed three distinct networks, namely an olfactory network, a somatosensory network, and an integrative network.
Results of previous studies have shown that TRPV1 and TRPA1 are often expressed in the same sensory neurons [^16][^17]. In contrast, TRPM8 is exclusively expressed in different sensory neurons [^17]. Interestingly, Wang et al. [^33] have shown that CO2, which is known to activate TRPV1, also evokes responses in TRPA1. These findings may suggest that the neuronal processing networks evoked by CO2 and cinnamaldehyde show a higher degree of concordance compared to the spatial network pattern induced by menthol. However, the results of our study revealed highly consistent networks for all three substances, indicating a common processing pathway of trigeminal stimuli.
In the following paragraphs, all three determined functional networks will be discussed in relation to our data and the published literature in detail.
### The olfactory network
The olfactory network includes clusters in areas related to odor processing, such as the PIR, the entorhinal cortex, the parahippocampal areas, and the insula. These findings are in line with previously published functional imaging data (for review see [^34]). This network was determined for all investigated substances. It may be no surprise that menthol and cinnamaldehyde with their clear olfactory precepts evoked an olfactory network. It is, however, interesting to note that even CO2, which is a strong trigeminal stimulus (causing stinging or tingling sensations) with virtually no odor, activated this olfactory network—and to the same extent as the other two stimuli. The evaluation of the three stimuli revealed that the majority of raters were able to label menthol and cinnamaldehyde (cinnamon) correctly, whereas CO2 was labelled as ‘no specific odor’. This activation of olfactory areas evoked by CO2 result supports the close relationship of the olfactory and the trigeminal pathway, as suggested by previous studies. A recently published study that used electrophysiological single- and multi-unit recordings may shed light on the impact of olfactory brain areas on trigeminal stimulus processing [^42]. The neuronal activation pattern in the PIR of mice, evoked by CO2 stimulation, differed from the activation pattern induced by olfactory stimuli. This finding suggests that the PIR is responsible for the encoding of the stimulus modality (olfactory or trigeminal). In addition, CO2 stimulation led to a delay in PIR activation, indicating that trigeminal stimuli enter the PIR via a different route compared to olfactory stimuli. Unfortunately, fMRI is not appropriate for the examination of these temporal differences; however, it would be interesting to work on this question in future investigations. Thus, the PIR may be seen as a chemosensory processing area rather than a pure olfactory area.
### The somatosensory network
The second network we observed, covered parts of the so called “pain matrix” [^32], a network which is involved in processing a large variety of painful sensations. The core regions of this network are the primary and secondary somatosensory areas, the insula, and the anterior cingulate cortex. In our study we were able to determine a network involving the primary and secondary somatosensory areas as well as the insula. Even though two of the applied stimuli were not perceived as painful, core regions of the pain network were activated in all three substances. The trigeminal pathway processes a variety of sensory inputs, such as temperature, somatosensory or nociceptive stimuli [^49]. Hence, any sensory input which is processed via the trigeminal pathway is potentially harmful and painful and may therefore prepare and activate parts of the pain matrix.
In accordance with previously published findings investigating central processing of pain, we detected larger activation clusters in the right hemisphere contralateral to stimulation. One possible reason for increased activation in the right hemisphere has been described in the homeostatic model of awareness, which claims an asymmetry of emotional awareness. Here, both, positive emotions and sensory input are processed in the left anterior insula (AI), whereas negative emotions and sensations, such as pain, are processed predominantly in the right AI. However, our results argue against this interpretation, as we did not observe any significant differences in the neuronal activation pattern between the investigated stimuli; thus, even the non-painful stimuli (cinnamaldehyde and menthol) led to predominantly right-sided activations (see Fig. 1B).
### The integrative network
The detected network was lateralized to the left and included neuronal activation in the orbitofrontal gyrus, the inferior parietal lobule, the superior temporal gyrus, and a large cluster in the AI. These areas are involved in multisensory integration, particularly that involving chemosensory stimuli. In fact, the same brain areas were recently identified as constituting a task-independent network for the processing of mixed olfactory-trigeminal stimuli. However, that study reported a predominance of the right hemisphere, in contrast to our findings. We would like to remind the reader that we stimulated participants exclusively through their left nostril, whereas the earlier study used bilateral stimulation. However, our observation of an ipsilateral network is in accordance with the notion that olfactory information is processed predominantly, but not exclusively, ipsilaterally [^49].
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## Conclusion
Within the last several years, the intranasal trigeminal system has been investigated in more detail, resulting in a deeper insight into the transduction of potentially noxious stimuli via the trigeminal pathway. Although previous studies identified different subfamilies of TRP channels, serving as receptors that evoke trigeminal sensations, the neuronal basis and processing pathway of these substances was still unclear. The results of our study suggest a common processing pathway for trigeminal stimulation, as the investigated substances targeted various sub-types of TRP channels and evoked distinct sensations in the nose. Thus, we assume that, although the TRPM8 receptor is exclusively found in different sensory neurons from the TRPV1 and TRPA1 receptors, the neuronal activation is processed in the same neuronal network.
## Author Contributions
- Conceived and designed the experiments: EU JF VS.
- Performed the experiments: K. Kollndorfer K. Kowalczyk EH CAM JK ST VS.
- Analyzed the data: K. Kollndorfer JF VS.
- Wrote the paper: K. Kollndorfer JF VS.
### Academic Editor
David D. McKemy, University of South California, UNITED STATES
### License
© 2015 Kollndorfer et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
### Competing interests
The authors have declared that no competing interests exist.
[^16]: https://doi.org/10.1111/j.1460-9568.2009.06702.x
[^17]: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0121091
[^33]: https://doi.org/10.1523/JNEUROSCI.2715-10.2010
[^34]: https://doi.org/10.1016/j.neuroimage.2012.10.030
[^42]: https://doi.org/10.1523/JNEUROSCI.0422-13.2013
[^32]: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0121091
[^49]: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0121091