Increased sympathetic outflow in juvenile rats submitted to chronic intermittent hypoxia correlates with enhanced expiratory activity

Abstract

Chronic intermittent hypoxia (CIH) in rats produces changes in the central regulation of cardiovascular and respiratory systems by unknown mechanisms. We hypothesized that CIH (6% O(2) for 40 s, every 9 min, 8 h day(-1)) for 10 days alters the central respiratory modulation of sympathetic activity. After CIH, awake rats (n = 14) exhibited higher levels of mean arterial pressure than controls (101 +/- 3 versus 89 +/- 3 mmHg, n = 15, P < 0.01). Recordings of phrenic, thoracic sympathetic, cervical vagus and abdominal nerves were performed in the in situ working heart-brainstem preparations of control and CIH juvenile rats. The data obtained in CIH rats revealed that: (i) abdominal (Abd) nerves exhibited an additional burst discharge in late expiration; (ii) thoracic sympathetic nerve activity (tSNA) was greater during late expiration than in controls (52 +/- 5 versus 40 +/- 3%; n = 11, P < 0.05; values expressed according to the maximal activity observed during inspiration and the noise level recorded at the end of each experiment), which was not dependent on peripheral chemoreceptors; (iii) the additional late expiratory activity in the Abd nerve correlated with the increased tSNA; (iv) the enhanced late expiratory activity in the Abd nerve unique to CIH rats was accompanied by reduced post-inspiratory activity in cervical vagus nerve compared to controls. The data indicate that CIH rats present an altered pattern of central sympathetic-respiratory coupling, with increased tSNA that correlates with enhanced late expiratory discharge in the Abd nerve. Thus, CIH alters the coupling between the central respiratory generator and sympathetic networks that may contribute to the induced hypertension in this experimental model.

Figure 1. Cardiovascular parameters measured in awake juvenile control and chronic intermittent hypoxia-treated (CIH) rats

Diastolic (DAP, A), systolic (SAP, B) and mean arterial pressure (MAP, C), and pulse pressure (PP, D) of control (n = 15) and CIH-treated rats (n = 14). * Different from control group, P < 0.05.

Figure 2. Raw and integrated (∫) activities of motor nerves recorded simultaneously from cervical vagus (cVN), abdominal (Abd) and phrenic nerve (PND) from one representative working heart–brainstem preparation (WHBP) in the control and CIH groups

The shaded grey area in the recordings indicates the late expiratory phase of the respiratory cycle and the arrows in the cVN indicate the beginning of post-inspiratory activity. Note the emergence of late-expiratory bursts in the Abd of the CIH rat (*).

Figure 3. Analysis of cervical vagus nerve activity (cVN) in control and CIH-treated rats

Upper panel: phrenic-triggered average of integrated cervical vagus nerve (∫cVN), obtained from 10 phrenic cycles in one representative WHBP of control and CIH groups. The early post-inspiratory activity (early PI) of cVN observed in CIH rats was reduced in relation to controls, but there was no change in late-PI and inspiratory activities. Lower panel: mean values of the area under the curve (AUC) of the different components of cVN in control (n = 10) and CIH groups (n = 10). The values were normalized by the total cVN area (equivalent to the sum of the areas of the three cVN components). * Different from control group, P < 0.05.

Figure 5. Simultaneous recordings of raw and integrated (∫) abdominal (Abd), thoracic sympathetic (tSNA) and phrenic nerve activities (PND) in a representative control and CIH-treated WHBP (CIH)

The shaded grey area in the recordings represents the late expiratory phase of the respiratory cycle. Note the emergence of late expiratory discharges in both the Abd and tSNA (arrowed) of CIH-treated rats.

Figure 4. Respiratory–sympathetic coupling assessed in control and CIH-treated rats

Upper panels: phrenic-triggered average of thoracic sympathetic nerve activity (tSNA) obtained from 10 phrenic cycles in a representative control (left) and CIH-treated rat (right). Peak sympathetic activity was normalized to 100% and the noise level to 0%. Note the elevated tSNA in Late E in CIH-treated rats (arrows). Middle panels: mean tSNA during late expiratory (Late E), inspiratory (I), post-inspiratory (PI) and mid-expiratory (Mid E) parts of the respiratory cycle in control (n = 11) and CIH (n = 9) rats with intact peripheral chemoreceptors. * Different from control, P < 0.05. Lower panels: mean tSNA relative to the respiratory cycle in CIH (n = 5) and control rats (n = 3) with denervated peripheral chemoreceptors. * Different from control, P = 0.01.

Figure 6. Cross-correlation analyses among the activities of thoracic sympathetic (tSNA, upper panel), phrenic (PND, middle panel) and abdominal (Abd, reference activity) nerves of a representative control and CIH WHBP

PND and tSNA were triggered from the main peak in Abd. In control rats, the main Abd discharge occurred during the post-inspiratory period but in rats treated with CIH this occurred late in the expiratory phase. Note the switch in correlation of Abd to tSNA from post-inspiration in control rats to late expiration in CIH-treated rats.

Figure 7. Mean correlation coefficient (r) between the activities of thoracic sympathetic and abdominal nerves during post-inspiratory (PI) and late expiratory (Late E) phases in control (n = 5) and CIH (n = 5) groups

Note the switch in correlation between Abd and tSNA from PI to Late E phase in CIH rats.

Figure 8. Schematic drawings of the coupling between central respiratory and sympathetic activity in control and CIH rats

In control rats (left panel), expiratory neurons (post-inspiratory, post-I, and inhibitory (i) augmenting expiratory neurons, aug-E), mainly located in Bötzinger complex (BötC; Sun et al. 1998; Smith et al. 2007), and inspiratory neurons (insp) located in pre-Bötzinger complex (pre-BötC) and rostral ventral respiratory group (rVRG) are reciprocally connected by inhibitory pathways coordinating the respiratory rhythm (Richter, 1996; Tian et al. 1999; Richter & Spyer, 2001; Shen et al. 2003; Smith et al. 2007). The different types of inspiratory neurons recorded in pre-BötC and rVRG were classified together as ‘insp’ for clarity. The grey boxes represent the main tonic excitatory sources to BötC (intrinsic (in) BötC, Smith et al. 2007; retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG), Janczewski & Feldman, 2006; and pons, Smith et al. 2007) and pre-BötC (in pre-BötC (intrinsic, in) and pons, Johnson et al. 1994; Smith et al. 2007). The sympathetic–respiratory coupling observed in thoracic sympathetic activity in the WHBP of control rats is driven potentially by projections from insp and post-I, modulating the activity of premotor sympathetic neurons (symp) located in rostral ventrolateral medulla (RVLM) directly (Haselton & Guyenet, 1989) or via inhibitory neurons (GABA) located in caudal ventrolateral medulla (CVLM, Mandel & Schreihofer, 2006). In CIH rats (right panel), the excitability of aug-E neurons is probably increased after CIH exposure (indicated by the increased abdominal nerve activity during Late E), which, in turn, may depress the activity of the post-I neurons and consequently reduce the post-inspiratory activity in cervical vagus nerve (as reported herein). In addition, we hypothesize that the excitability of another population of excitatory (e) aug-E neurons is also increased in CIH rats and it may be the source of the neuronal activity that is driving excitatory inputs to symp and sympathetic preganglionic neurons of the intermediolateral column of spinal cord (IML; Sun et al. 1997; Tian et al. 1999), leading to the enhanced sympathetic activity observed during late expiration. This e aug-E population is speculative (?) and needs to be confirmed as a driver of sympathetic overactivity of CIH rats. No changes in inspiratory activity were observed in CIH possibly because of the reciprocal, and presumed equivalent, changes in aug-E and post-I inhibitory activity on inspiratory neurons.