No abstract

The retrotrapezoid nucleus (RTN) is one of several CNS nuclei that contribute, in various capacities (e.g. CO detection, neuronal modulation) to the central respiratory chemoreflex (CRC). Here we test how important the RTN is to PCO homeostasis and breathing during sleep or wake. RTN Nmb-positive neurons were killed with targeted microinjections of substance P-saporin conjugate in adult rats. Under normoxia, rats with large RTN lesions (92 ± 4% cell loss) had normal blood pressure and arterial pH but were hypoxic (-8 mmHg PaO ) and hypercapnic (+10 mmHg ). In resting conditions, minute volume (V ) was normal but breathing frequency (f ) was elevated and tidal volume (V ) reduced. Resting O consumption and CO production were normal. The hypercapnic ventilatory reflex in 65% FiO had an inverse exponential relationship with the number of surviving RTN neurons and was decreased by up to 92%. The hypoxic ventilatory reflex (HVR; FiO 21-10%) persisted after RTN lesions, hypoxia-induced sighing was normal and hypoxia-induced hypotension was reduced. In rats with RTN lesions, breathing was lowest during slow-wave sleep, especially under hyperoxia, but apnoeas and sleep-disordered breathing were not observed. In conclusion, near complete RTN destruction in rats virtually eliminates the CRC but the HVR persists and sighing and the state dependence of breathing are unchanged. Under normoxia, RTN lesions cause no change in V but alveolar ventilation is reduced by at least 21%, probably because of increased physiological dead volume. RTN lesions do not cause sleep apnoea during slow-wave sleep, even under hyperoxia.

Neurogenic hypertension is associated with excessive sympathetic nerve activity to the kidneys and portions of the cardiovascular system. Here we examine the brain regions that cause heightened sympathetic nerve activity in animal models of neurogenic hypertension, and we discuss the triggers responsible for the changes in neuronal activity within these regions. We highlight the limitations of the evidence and, whenever possible, we briefly address the pertinence of the findings to human hypertension. The arterial baroreflex reduces arterial blood pressure variability and contributes to the arterial blood pressure set point. This set point can also be elevated by a newly described cerebral blood flow-dependent and astrocyte-mediated sympathetic reflex. Both reflexes converge on the presympathetic neurons of the rostral medulla oblongata, and both are plausible causes of neurogenic hypertension. Sensory afferent dysfunction (reduced baroreceptor activity, increased renal, or carotid body afferent) contributes to many forms of neurogenic hypertension. Neurogenic hypertension can also result from activation of brain nuclei or sensory afferents by excess circulating hormones (leptin, insulin, Ang II [angiotensin II]) or sodium. Leptin raises blood vessel sympathetic nerve activity by activating the carotid bodies and subsets of arcuate neurons. Ang II works in the lamina terminalis and probably throughout the brain stem and hypothalamus. Sodium is sensed primarily in the lamina terminalis. Regardless of its cause, the excess sympathetic nerve activity is mediated to some extent by activation of presympathetic neurons located in the rostral ventrolateral medulla or the paraventricular nucleus of the hypothalamus. Increased activity of the orexinergic neurons also contributes to hypertension in selected models.

Collectively, the retrotrapezoid nucleus (RTN) and adjacent C1 neurons regulate breathing, circulation and the state of vigilance, but previous methods to manipulate the activity of these neurons have been insufficiently selective to parse out their relative roles. We hypothesize that RTN and C1 neurons regulate distinct aspects of breathing (e.g., frequency, amplitude, active expiration, sighing) and differ in their ability to produce arousal from sleep. Here we use optogenetics and a combination of viral vectors in adult male and female Th-Cre rats to transduce selectively RTN (Phox2b 1 /Nmb 1 ) or C1 neurons (Phox2b 1 /Th 1 ) with Channelrhodopsin-2. RTN photostimulation modestly increased the probability of arousal. RTN stimulation robustly increased breathing frequency and amplitude; it also triggered strong active expiration but not sighs. Consistent with these responses, RTN innervates the entire pontomedullary respiratory network, including expiratory premotor neurons in the caudal ventral respiratory group, but RTN has very limited projections to brainstem regions that regulate arousal (locus ceruleus, CGRP 1 parabrachial neurons). C1 neuron stimulation produced robust arousals and similar increases in breathing frequency and amplitude compared with RTN stimulation, but sighs were elicited and active expiration was absent. Unlike RTN, C1 neurons innervate the locus ceruleus, CGRP 1 processes within the parabrachial complex, and lack projections to caudal ventral respiratory group. In sum, stimulating C1 or RTN activates breathing robustly, but only RTN neuron stimulation produces active expiration, consistent with their role as central respiratory chemoreceptors. Conversely, C1 stimulation strongly stimulates ascending arousal systems and sighs, consistent with their postulated role in acute stress responses.

Arousal in response to asphyxia is a life‐saving reflex that helps restore normal breathing and blood gas homeostasis. The carotid bodies (CB) are essential to hypoxia‐induced arousal whereas CNS serotonergic neurons and the lateral parabrachial nucleus contribute to CO2‐induced arousal. Here we wished to test whether the retrotrapezoid nucleus (RTN) is also implicated in CO2‐induced arousal. Our hypothesis was that RTN contributes to arousal elicited by hypercapnia but not hypoxia. The rationale is as follows. RTN is an important pluricellular CO2 detector that mediates the bulk of the hypercapnic ventilatory reflex (HCVR). RTN is likely absent since birth in central congenital hypoventilation syndrome, CCHS, a developmental disease in which the HCVR and asphyxia‐induced arousal are both greatly reduced. To test our hypothesis, we studied four groups of adult Sprague‐Dawley rats. In one group, we nearly completely destroyed RTN (“RTN lesion” group; n=8) with microinjections of saporin conjugated with substance‐P (2.4 ng/injection). The cognate control group (“RTN control”; n=7) received saline. The CBs were ablated in a third rat cohort (“CBx rats”; n=7). The fourth group (“CB control rats”; n=7) underwent sham surgery. Four weeks later, rats were placed in a plethysmograph chamber where breathing frequency and amplitude, EEG and EMG were recorded. Using mass flow controllers, the gas mixture perfusing the chamber was intermittently changed for exactly 1 min from normoxia (21% O2/balance N2) back to normoxia (control for flow interruption), from normoxia to hypercapnia (15% CO2/21% O2/balance N2), or from normoxia to hypoxia (10% O2, balance N2). Sleep survival curves representing the cumulative probability for SWS to persist after onset of the gas change were obtained. Two‐way ANOVA for repeated measures was used for statistical analysis of these curves. Sleep architecture was assessed by measuring the proportion of time the rats spent in SWS, REM sleep or quiet waking and the number of sleep stage transitions. Based on these measurements, sleep was unaffected by CBx or RTN lesion. However, the probability of SWS to persist 20 sec after the beginning of stimulus was significantly higher in RTN lesion rats compared to control (73.6 ± 14% vs. 54.8 ± 18 % at 20 sec from the beginning of stimulus). The corresponding figures at the 30 s time‐point were 57.8 ± 12 vs. 9.6 ± 13% and, at 40 sec, 40.1 ± 11 vs. 0 %. By contrast, RTN lesion had no detectable effect on hypoxia‐induced arousal. The arousal deficits elicited by CBx were different. CBx‐rats exposed to hypoxia had a significantly probability of SWS to persist compared to CB Control rats (89.9 ± 9 vs. 67.1 ± 9 % at 20 sec; 81.5 ± 15 vs. 56.4 ± 14 % at 30 sec, and 70.8 ± 16 vs. 39.9 ± 11 at 40 sec after the beginning of stimulus). In summary, we confirm the importance of the CBs to hypoxia‐induced arousal and demonstrate that arousal to hypercapnia is selectively reduced after RTN lesion. Neither CBx nor RTN lesions eliminated the arousal elicited by hypoxia or h...