Spontaneous swallowing occurs during autoresuscitation in the in situ brainstem preparation of rat
Introduction
Swallowing clears the upper airway of foreign material e.g. food, excess saliva, regurgitated gastric contents in infants, by directing such material into the oesophagus and away from the lower airways. Swallowing is tightly coordinated with respiration to maximise protection against aspiration, including its preferential initiation in the postinspiratory/expiratory phase in rats and humans (Bautista and Dutschmann, 2014, Hårdemark-Cedborg et al., 2009, Martin-Harris et al., 2003, Saito et al., 2002, Sun et al., 2011). Some evidence exists that the coordination of reflex swallowing with breathing is mediated via brainstem nuclei, including the pontine Kölliker-Fuse nucleus (KF) (Bonis et al., 2011, Sun et al., 2011). The post-inspiratory phase of respiration is gated by the KF. Recently, we and others (Bautista and Dutschmann, 2014, Bonis et al., 2013, Bonis et al., 2011) have reported an increased incidence of spontaneous swallowing subsequent to inactivation of the dorsolateral pontine region, including the KF.
A poorly understood example of swallowing/breathing interaction is the presence of swallowing during autoresuscitation from respiratory arrest. Severe hypoxia (e.g. during cardiac arrest) leads to hypoxic coma, which describes a general depression of the central nervous system resulting in apnea and apparent lifelessness. Spontaneous recovery from this state involves the onset of gasping and eventual return to normal breathing (‘autoresuscitation’) if oxygen is made available to the lungs (Gershan et al., 1990, Gunteroth and Kawabori, 1975). Patients in both hypoxic coma and the gasping stage were initially thought areflexic. However, some studies in cats and mice demonstrate the preservation of upper airway protective reflexes in these states (Khurana and Thach, 1996, Tomori et al., 1991, Tomori et al., 1993). The presence of the swallowing reflex may even facilitate the autoresuscitation process (Khurana and Thach, 1996, Tomori et al., 2013). Specifically, all mice that exhibited reflexive swallowing in response to oral administration of water were able to successfully autoresuscitate from hypoxic coma (Khurana and Thach, 1996). On the other hand, all non-swallowing mice failed to autoresuscitate. Similarly, in humans, the presence of swallowing is used as an early predictor of survival and/or recovery of consciousness following cardio-pulmonary resuscitation (Delooz and Lewi, 1989, Jørgensen, 1997).
In the current study, we investigated swallowing/breathing interaction during autoresuscitation from respiratory arrest in the in situ perfused brainstem preparation of juvenile rats. In obtaining the preparation, respiratory arrest is induced when the animal is exsanguinated and rapidly cooled. Autoresuscitation occurs following reperfusion with oxygenated and warmed (31 °C) artificial cerebrospinal fluid (Paton, 1996). We hypothesised that spontaneous swallowing would be frequently observed prior to the resumption of respiratory activity during autoresuscitation. In our experience, the pattern of the respiratory activity that recovers is apneustic, i.e. exhibiting prolonged inspiration and absent post-inspiration. Furthermore, we demonstrated earlier that chemical inhibition of KF results in an increase in spontaneous swallowing in addition to apneusis (Bautista and Dutschmann, 2014). Therefore, we also hypothesised that during autoresuscitation, frequent spontaneous swallowing would accompany the transient apneustic respiratory pattern. Lastly, we hypothesised that spontaneous swallowing facilitates recovery of the 3 phase respiratory pattern (defined as ‘successful autoresuscitation’). Here we define the observation of spontaneous swallowing without respiratory activity during autoresuscitation as ‘stage 1 autoresuscitation’. The presence of rhythmic respiratory activity that is transiently non-eupneic was termed ‘stage 2 autoresuscitation’. To test our hypotheses, swallowing and breathing activities were monitored following reperfusion of the in situ preparation and were compared to inhibition of the KF.
Section snippets
Methods
All experimental procedures were performed in accordance with the Australian code of practice for the care and use of animals for scientific purposes. Approval for the study was obtained from the animal ethics committee of Florey Institutes of Neuroscience and Mental Health (AEC 12-084).
All chemicals were purchased from Sigma–Aldrich, Australia unless otherwise stated.
Swallowing activity during stage 1 autoresuscitation in situ
VNA was recorded immediately after reperfusion in n = 6 preparations. In all preparations, frequent spontaneous fictive swallowing activity was seen in VNA (5 ± 2 swallows/min; hereafter referred to as swallowing) before respiratory-related activity was observed in PNA (Fig. 1). The spindle-shaped VNA pattern and duration (300–500 ms) of these early swallowing activities were virtually identical to swallows triggered by oral water injection in the fully re-oxygenated and re-warmed perfused brainstem
Discussion
An intact swallowing reflex is thought to facilitate autoresuscitation from hypoxic coma (Khurana and Thach, 1996). Here we show that autoresuscitation from hypoxic and hypothermic conditions in situ follows a distinct sequence that we define for the first time. Stage 1 autoresuscitation was characterised by the lack of rhythmic respiratory activity but the notable presence of spontaneous swallowing. The ensuing stage 2 autoresuscitation was characterised by the recovery of rhythmic but
Conclusions
We conclude that although swallowing ability recovers before respiration, spontaneous swallowing does not appear to directly promote complete recovery of respiratory network i.e. successful autoresuscitation. The sequence of recovery is an epiphenomenon possibly owing to the different locations of the swallowing and respiratory CPGs. Alternatively, it is tempting to speculate that the order of autoresuscitation of brainstem CPGs resembles ontogeny and phylogeny. It appears that the activity of
Acknowledgements
The authors’ work is funded by a start-up fund from the Florey Institute of Neuroscience and Mental Health. M. Dutschmann is supported by an ARC Future Fellowship (FT120100953). We also acknowledge the support of the Victorian Government through the Operational Infrastructure Scheme.
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