Introduction: Chronic Hypoxia (CH) is a key feature and a major driver of pathology in Chronic Obstructive Pulmonary Disease (COPD). Emerging evidence suggests that carotid body hyperactivity in CH has an important role in causing cardiovascular pathology. The carotid body inputs signals to the brainstem through chemoafferent fibres running in the carotid sinus nerve (CSN), a branch of the glossopharyngeal nerve. The traditional view is that a rise in mean chemoafferent frequency is the sole signal for reflex activation. However, no study has assessed a potential alteration in the pattern of chemoafferent firing, which may code for selective reflex activation and cardiovascular-respiratory adjustment.

 

Aims and Objectives: This study compared the baseline firing patterns of single carotid body chemoafferent fibres recorded from normoxic (N) and chronically hypoxic (CH) animals.

 

Method: Extracellular ex vivo recordings of chemoafferent activity were made from carotid bodies isolated from adult male Wistar rats (150-350g), following exposure to either 10 days of ambient air (N) or F_iO₂ 12% (CH). Carotid bodies were isolated under terminal non-recovery anaesthesia (isoflurane 3-5% in O₂ 1.5 L min^-1), death by cervical dislocation and confirmed by exsanguination. Baseline activity was measured at a superfusate PO₂ of 300 mmHg, PCO₂ of 40 mmHg, and was captured for at least 100 seconds. Chemoafferent patterning was monitored by evaluating the variability of interspike intervals (ISI) and Shannon Entropy (the ability to predict the next ISI). Data presented as mean ± SEM. Statistical significance was taken as p<0.05; unpaired Student’s t-test.

 

Results: A total of 80 single chemoafferent fibres were analysed (N, n=36) and (CH, n=44), and total spike counts were 3309 for N and 9463 for CH. The CH group exhibited a significant decrease in SD ISI (N: 3741 ± 401 vs. CH: 1071 ± 99 ms, p<0.0001) and co-efficient of variation (SD/Mean ISI) (N: 1.18 ± 0.029 vs CH: 1.06 ± 0.014 AU, p<0.0001). Poincaré plot analysis of ISI, illustrated a decrease in both short-term (SD1) (N: 5385 ± 603 vs. CH: 1517 ± 147 ms; p<0.0001) and long-term (SD2) (N= 7089 ± 782 vs. CH= 2168 ± 199 ms; p<0.0001) variability, and a decrease in overall ellipse area, consistent with a more regular firing pattern. The total Shannon Entropy was also decreased in the CH group (N: 1.572 ± 0.03 vs. CH: 1.390 ± 0.01 bits, p<0.0001), indicative of a more predictable spike pattern.

 

Conclusions: In addition to increasing the mean frequency, CH exposure fundamentally reshapes the baseline carotid body chemoafferent discharge pattern. Key changes included a reduction in the variability and spread of ISIs, and an increase in predictability. This alteration in pattern may modify central signal integration thereby contributing to hypersensitive cardiovascular-respiratory reflex activation associated with CH.