In humans, the SNPs in the NPSR1
gene have been associated with asthma phenotypes, including AHR. We examined Npsr1
-deficient mice using two models of asthma. Consistent with a previous study [1
], we found that there were no differences in the lung inflammation and respiration in response to allergen challenge between WT and Npsr1
-deficient mice, suggesting that NPSR1 is not directly involved in experimental asthma development. Further, our PCR results showed that Npsr1
has a very low level of expression in the basal or allergen-challenged lung, supporting that the direct role of NPS/NPSR1 in the lung may be limited. Our results are in contrast to the study from Laitinen et al. (16
) which suggest that NPSR1
is implicated in human asthma. One of possible explanations is that there may be species differences in biological function of NPSR1 because our study is based in mouse.
Studies from Xu et al. [32
] showed that Nps
were mainly expressed in the brain stem, limbic system, and/or cerebral cortex. These regions are involved in regulating respiration, suggesting that NPS and NPSR1 have a role in this process. Unrestrained whole-body plethysmography (Buxco system), a noninvasive method, quantifies pulmonary physiological functions, including respiratory frequency, tidal volume, and Penh, in conscious mice repeatedly (12
). However, many researchers have criticized that Penh does not accurately measure airway resistance (2
). Although the unrestrained whole-body plethysmography is a useful tool to screen for the presence of lung diseases, it is not suitable to discriminate different mechanisms by which altered patterns of breathing come about in the patients with respiratory diseases,(29
). The flexiVent system, an invasive method, allows specific analysis of pulmonary mechanics. In our study, we used these two methods to identify whether ICV NPS had a direct mechanical effect on the airway or changed the respiratory pattern through the CNS. Measured by whole-body plethysmography, mice treated with ICV NPS showed increased respiratory frequency, decreased Penh and no change in tidal volume following methacholine exposure compared with aCSF-treated mice. However, ICV NPS did not change airway mechanics in response to methacholine as measured by flexiVent, further supporting that the ICV NPS-induced decrease in Penh in response to methacholine is attributable to increased minute ventilation by ICV NPS.
has been associated with panic disorders in Japanese males [24
]. Besides recurrent spontaneous anxiety attacks, panic disorders are dominated by respiratory symptoms such as hyperventilation, a feeling of being smothered, and shortness of breath, and respiratory dysregulation has been identified as a biological marker for panic disorder [9
]. Hypersensitivity of brainstem nuclei regulating respiratory activity has been proposed in patients with panic disorder [7
]. Our study demonstrated that ICV NPS could increase respiratory frequency in an NPSR1-dependent manner at baseline or in response to methacholine challenge. In addition, our earlier study showed that ICV NPS induced hyperlocomotion, increased exploration, and plasma corticosterone release in WT mice, not in Npsr1
-deficient mice [34
]. All these physiological alterations triggered by ICV administration of NPS are part of typical fight-or-flight response, an adaptive response to danger. It is suggested that respiratory change triggered by ICV NPS is a psychophysiological activity in response to stress. As such, we speculate that psychological triggers NPS/NPSR1 mediate changes in respiration and anxiety, possibly modulating asthma symptoms. Further studies are needed to examine the expression of NPS and NPSR1 in the brain in a model of stress. Experimental and clinical evidence has emerged that there is an association of respiration changes with psychological stress and emotional diseases. Experimental exposure to emotional stimuli has been shown to increase respiratory resistance in asthma [27
]. Chronic stress can exacerbate the symptoms of asthmatic patients [3
]. It will be interesting to define what the role of NPS/NPSR1 is in the exacerbation of asthma by chronic stress in the future as we have shown that NPS/NPSR1 signaling regulates stress and anxiety responses [34
Our study is limited by testing a dose of ICV NPS (1 nmol) that is indeed a pharmacological dose because our previous paper showed that 1 nmol of ICV NPS had the best effect on anxiety, locomotion and exploration compared with 0.1 and 10 nmol of ICV NPS [34
]. In addition, the identified effect of NPS on respiratory frequency was conducted using whole-body plethysmography, and the confinement chamber may have anxiety-promoting effects. However, it is notable that the effect of NPS on respiration is NPSR1-dependent, providing evidence that the triggered pathway is specific to NPS/NPSR1.
It has not escaped our attention that ICV NPS induced increase in respiratory frequency and decrease in tidal volume resembles the respiratory panting behavior observed during hyperthermia [8
] and mechanical associated high frequency ventilation that is clinically used to treat lung injury [15
]. It will be interesting to determine whether natural panting responses may be NPS/NPSR1 dependent. And our results have implications for potentially understanding the responses to mechanical associated high frequency ventilation.
Taken together, our studies have demonstrated that the direct role of NPSR1 in the development of experimental asthma is limited and the effect of NPS/NPSR1 on respiratory changes is likely mediated via a CNS-mediated pathway, providing a pathway that connects respiratory and stress responses.