Determining the role of the paratrigeminal nucleus (Pa5) in airway defence
AffiliationAnatomy and Neuroscience
Document TypePhD thesis
Access StatusOpen Access
© 2019 Dr Alexandria Driessen
Respiratory sensations conveyed by airway sensory nerve fibres are poorly understood, yet they contribute significantly to morbidity in pulmonary disease. Our viral tracing studies identified a previously unknown airway sensory circuit in the brain projecting through an obscure medullary site known as the paratrigeminal nucleus (Pa5). The Pa5 is located in the dorsal lateral medulla and is commonly defined as a collection of interstitial cells in the dorsal tip of the spinal trigeminal tract. The Pa5 has been previously implicated in baroreceptor function and somatosensory processing. Astonishingly, very little investigations have been made to understand its role in airway afferent processing. We therefore investigated the anatomical connectivity of the Pa5, its involvement in initiating and controlling respiratory reflexes as well as in more complex respiratory behaviours including evoked cough. Conventional neuroanatomical tracing was conducted to determine the input and output connectivity of the Pa5. Firstly, retrograde tracing confirmed, in the guinea pig, that vagal afferents from the jugular versus the nodose ganglia project specifically into the brainstem primary afferent termination sites. That is the nodose vagal ganglia project almost exclusively to the nucleus of the solitary tract (nTS), while the jugular vagal ganglia project predominately to the Pa5. Furthermore, anterograde tracing was employed from the guinea pig Pa5, showing that this nucleus has extensive projections throughout the cardiorespiratory column, including the nTS, reticular nuclei, pre-bötzinger nucleus and in particular the parabrachial and Kölliker-Fuse nuclei in the pons. We have shown functional significance of these projections by implicating the Pa5 in the initiation and control of laryngeal-evoked respiratory reflexes. Initially we found that laryngeal-evoked respiratory slowing was significantly attenuated by blocking the Pa5 using the GABAA agonist muscimol (stimulus evoked change in breaths/min pre-muscimol was 40.9±3.5 breaths/min pre-muscimol vs. 21.4±2.9 post-muscimol, p=0.001), while this pharmacological modification in the nTS had no effect on the laryngeal-evoked respiratory slowing. Importantly, this novel finding showed that the Pa5 has a role in respiratory reflexes that is independent of the nTS. Furthermore, injection of glutamate receptor antagonists into the Pa5 were able to prevent respiratory slowing evoked by laryngeal stimulation (change in respiratory rate at the maximum response 40.3±4.1 breaths/min vs. 17.3±3.1 breaths/min; p=0.0002), while peptide receptor antagonists resulted in significantly smaller effects. Intriguingly, while injection of glutamate into the Pa5 resulted in respiratory slowing and in some cases apnoea, mimicking the laryngeal-evoked reflex, the direct release of neuropeptides by capsaicin microinjection into the Pa5 resulted in the paradoxical increase in respiratory rate. This unique response could be blocked by Substance P receptor antagonists but not by glutamate receptor antagonists, suggestive of two types of Pa5 postsynaptic neurons. Additional anatomical characterisation of the Pa5 revealed the existence of two phenotypically distinct subtypes of Pa5 neurons. That is, one expressing the neurokinin 1 receptor and the other expressing calbindin. Furthermore, dual retrograde tracing from nuclei important for respiratory rhythm and modulation and were shown to receive direct projections from the Pa5 (i.e. the ventrolateral medulla and pontine nuclei) revealed that less than 7% of Pa5 neurons were dual labelled and therefore revealed that Pa5 neurons project differentially throughout the cardiorespiratory column. Given the complex processing capability of the Pa5, it seemed plausible that this nociceptive region may be important in complex respiratory behaviours, such as evoked cough and the perception of airway irritations. We developed an assay in which we can study the conscious perception of an airway irritation using behaviours (enhanced grooming, moving and chewing), in addition to the standard assessment of evoked cough. Conscious unrestrained guinea pigs were aerosolised bradykinin (a jugular and nodose C-fibre stimulant) and adenosine 5’-triphosphate (ATP, a nodose selective stimulant). While bradykinin evoked both dose dependent increases in cough (8.9±2.7 at 1mg/ml and 25.7±3.7 at 3mg/ml) and behaviours (20.7±45.8 behaviour duration during saline vs., 350.4±46.7 total behaviour duration, p=0.04), ATP only evoked responses at the highest dose (22.7±5.6 coughs and 81.3 behaviour duration during saline vs. 404±109.5 total behaviour duration). These distinct cough and behavioural profiles are suggestive that jugular and nodose airway afferents differentially process respiratory sensations. This was further supported by showing that targeted toxin lesions (substance P-Saporin, 10ng/100nl) of the Pa5 resulted in decreased bradykinin evoked cough compared to controsl (i.e. 7.2±2.8 coughs control vs. 0±0 coughs lesion at 1mg/ml and 19.2±4.1 coughs control vs. 10.4±3 coughs lesion at 3mg/ml, p=0.02 and p=0.009 respectively). Alternately, lesioning NK1 receptor expressing neurons in the Pa5 had no effect on ATP evoked cough, in turn implicating NK1 receptor expressing Pa5 neurons specifically process jugular C-fibre evoked cough. Together these data are the first to reveal a complexity of airway afferent processing in the Pa5. Better understanding this putative airway somatosensory system may help identify therapeutic targets to alleviate respiratory discomfort in disease.
Keywordsrespiratory neurobiology; airway sensory nervous system; vagal sensory; jugular vagal ganglia; paratrigeminal; airway defence; cough
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