Smith JC, Abdala AP, Koizumi H, Rybak IA, Paton JF. Spatial and functional architecture of the mammalian brain stem respiratory network: a hierarchy of three oscillatory mechanisms. J Neurophysiol 98: 3370 -3387, 2007. First published October 3, 2007; doi:10.1152/jn.00985.2007. Mammalian central pattern generators (CPGs) producing rhythmic movements exhibit extremely robust and flexible behavior. Network architectures that enable these features are not well understood. Here we studied organization of the brain stem respiratory CPG. By sequential rostral to caudal transections through the pontine-medullary respiratory network within an in situ perfused rat brain stem-spinal cord preparation, we showed that network dynamics reorganized and new rhythmogenic mechanisms emerged. The normal three-phase respiratory rhythm transformed to a two-phase and then to a one-phase rhythm as the network was reduced. Expression of the three-phase rhythm required the presence of the pons, generation of the two-phase rhythm depended on the integrity of Bötzinger and pre-Bötzinger complexes and interactions between them, and the one-phase rhythm was generated within the pre-Bötzinger complex. Transformation from the three-phase to a two-phase pattern also occurred in intact preparations when chloridemediated synaptic inhibition was reduced. In contrast to the threephase and two-phase rhythms, the one-phase rhythm was abolished by blockade of persistent sodium current (I NaP ). A model of the respiratory network was developed to reproduce and explain these observations. The model incorporated interacting populations of respiratory neurons within spatially organized brain stem compartments. Our simulations reproduced the respiratory patterns recorded from intact and sequentially reduced preparations. Our results suggest that the three-phase and two-phase rhythms involve inhibitory network interactions, whereas the one-phase rhythm depends on I NaP . We conclude that the respiratory network has rhythmogenic capabilities at multiple levels of network organization, allowing expression of motor patterns specific for various physiological and pathophysiological respiratory behaviors. I N T R O D U C T I O NThe structural and functional organizations of various central pattern generators (CPGs) producing rhythmic movements have been studied for decades in an attempt to understand the neural basis of motor behavior. In contrast to CPGs in several invertebrates and lower vertebrates (Grillner 2006;Marder and Calabrese 1996;Selverston and Ayers 2006), the spatial and functional architectures of CPG circuits in the mammalian CNS, such as those generating breathing movements, have not been well defined. Breathing in mammals is a dynamically mutable motor behavior that not only performs a vital homeostatic function but is also integrated with many other physiological functions including suckling, swallowing, sniffing, chewing, and vocalization. With such functional diversity, respiratory CPG circuits must have a robust, yet highly flexible organ...

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.

We studied respiratory neural activity generated during expiration. Motoneuronal activity was recorded simultaneously from abdominal (AbN), phrenic (PN), hypoglossal (HN) and central vagus nerves from neonatal and juvenile rats in situ. During eupnoeic activity, low-amplitude post-inspiratory (post-I) discharge was only present in AbN motor outflow. Expression of AbN late-expiratory (late-E) activity, preceding PN bursts, occurred during hypercapnia. Biphasic expiratory (biphasic-E) activity with pre-inspiratory (pre-I) and post-I discharges occurred only during eucapnic anoxia or hypercapnic anoxia. Late-E activity generated during hypercapnia (7-10% CO 2 ) was abolished with pontine transections or chemical suppression of retrotrapezoid nucleus/ventrolateral parafacial (RTN/vlPF). AbN late-E activity during hypercapnia is coupled with augmented pre-I discharge in HN, truncated PN burst, and was quiescent during inspiration. Our data suggest that the pons provides a necessary excitatory drive to an additional neural oscillatory mechanism that is only activated under conditions of high respiratory drive to generate late-E activity destined for AbN motoneurones. This mechanism may arise from neurons located in the RTN/vlPF or the latter may relay late-E activity generated elsewhere. We hypothesize that this oscillatory mechanism is not a necessary component of the respiratory central pattern generator but constitutes a defensive mechanism activated under critical metabolic conditions to provide forced expiration and reduced upper airway resistance simultaneously. Possible interactions of this oscillator with components of the brainstem respiratory network are discussed.

Breathing movements in mammals are driven by rhythmic neural activity generated within spatially and functionally organized brainstem neural circuits comprising the respiratory central pattern generator (CPG). This rhythmic activity provides homeostatic regulation of gases in blood and tissues and integrates breathing with other motor acts. We review new insights into the spatial–functional organization of key neural microcircuits of this CPG from recent multidisciplinary experimental and computational studies. The emerging view is that the microcircuit organization within the CPG allows the generation of multiple rhythmic breathing patterns and adaptive switching between them, depending on physiological or pathophysiological conditions. These insights open the possibility for site- and mechanism-specific interventions to treat various disorders of the neural control of breathing.

Sympathetic nerve activity (SNA) is elevated in established hypertension. We tested the hypothesis that SNA is elevated in neonate and juvenile spontaneously hypertensive (SH) rats prior to the development of hypertension, and that this may be due to augmented respiratory-sympathetic coupling. Using the working heart-brainstem preparation, perfusion pressure, phrenic nerve activity and thoracic (T8) SNA were recorded in male SH rats and normotensive Wistar-Kyoto (WKY) rats at three ages: neonates (postnatal day 9-16), 3 weeks old and 5 weeks old. Perfusion pressure was higher in SH rats at all ages reflecting higher vascular resistance. The amplitude of respiratory-related bursts of SNA was greater in SH rats at all ages (P < 0.05). This was reflected in larger Traube-Hering pressure waves in SH rats (1.4 +/- 0.8 versus 9.8 +/- 1.5 mmHg WKY versus SH rat, 5 weeks old, n = 5 per group, P < 0.01). Recovery from hypocapnic-induced apnoea and reinstatement of Traube-Hering waves produced a significantly greater increase in perfusion pressure in SH rats (P < 0.05). Differences in respiratory-sympathetic coupling in the SH rat were not secondary to changes in central or peripheral chemoreflex sensitivity, nor were they related to altered arterial baroreflex function. We have shown that increased SNA is already present in SH rats in early postnatal life as revealed by augmented respiratory modulation of SNA. This is reflected in an increased magnitude of Traube-Hering waves resulting in elevated perfusion pressure in the SH rat. We suggest that the amplified respiratory-related bursts of SNA seen in the neonate and juvenile SH rat may be causal in the development of their hypertension.

In the spontaneously hypertensive (SH) rat, hyperoxic inactivation of the carotid body (CB) produces a rapid and pronounced fall in both arterial pressure and renal sympathetic nerve activity (RSA). Here we show that CB de-afferentation through carotid sinus nerve denervation (CSD) reduces the overactive sympathetic activity in SH rats, providing an effective antihypertensive treatment. We demonstrate that CSD lowers RSA chronically and that this is accompanied by a depressor response in SH but not normotensive rats. The drop in blood pressure is not dependent on renal nerve integrity but mechanistically accompanied by a resetting of the RSA-baroreflex function curve, sensitization of the cardiac baroreflex, changes in renal excretory function and reduced T-lymphocyte infiltration. We further show that combined with renal denervation, CSD remains effective, producing a summative response indicative of an independent mechanism. Our findings indicate that CB de-afferentation is an effective means for robust and sustained sympathoinhibition, which could translate to patients with neurogenic hypertension.

Key points• Peripheral chemoreflex sensitivity is enhanced in hypertension yet the role of these receptors in the development and maintenance of high blood pressure remains unknown.• Carotid chemoreceptors were denervated in both young and adult spontaneously hypertensive rats (SHRs) by sectioning the carotid sinus nerves bilaterally while recording arterial blood pressure chronically using radio telemetry.• Carotid sinus denervation (CSD) in the young animals prevented arterial pressure from reaching the hypertensive levels observed in sham-operated animals whereas in adult SHRs arterial pressure fell by ∼20 mmHg.• After CSD there was a decrease in sympathetic activity, measured indirectly using power spectral analysis and hexamethonium, and an improvement in baroreceptor reflex gain.• Carotid bodies are active in the SHR and contribute to both the development and maintenance of hypertension; whether carotid body ablation is a useful anti-hypertensive intervention in drug-resistant hypertensive patients remains to be resolved. AbstractThe peripheral chemoreflex is known to be enhanced in individuals with hypertension. In pre-hypertensive (PH) and adult spontaneously hypertensive rats (SHRs) carotid body type I (glomus) cells exhibit hypersensitivity to chemosensory stimuli and elevated sympathoexcitatory responses to peripheral chemoreceptor stimulation. Herein, we eliminated carotid body inputs in both PH-SHRs and SHRs to test the hypothesis that heightened peripheral chemoreceptor activity contributes to both the development and maintenance of hypertension. The carotid sinus nerves were surgically denervated under general anaesthesia in 4-and 12-week-old SHRs. Control groups comprised sham-operated SHRs and aged-matched sham-operated and carotid sinus nerve denervated Wistar rats. Arterial blood pressure was recorded chronically in conscious, freely moving animals. Successful carotid sinus nerve denervation (CSD) was confirmed by testing respiratory responses to hypoxia (10% O 2 ) or cardiovascular responses to I.V. injection of sodium cyanide. In the SHR, CSD reduced both the development of hypertension and its maintenance (P < 0.05) and was associated with a reduction in sympathetic vasomotor tone (as revealed by frequency domain analysis and reduced arterial pressure responses to administration of hexamethonium; P < 0.05 vs. sham-operated SHR) and an improvement in baroreflex sensitivity. No effect on blood pressure was observed in sham-operated SHRs or Wistar rats. In conclusion, carotid sinus nerve inputs from the carotid body are, in part, responsible for elevated sympathetic tone and critical for the genesis of hypertension in the developing SHR and its maintenance in later life.

Neural circuits controlling breathing in mammals are organized within serially arrayed and functionally interacting brainstem compartments extending from the pons to the lower medulla. The core circuit components that constitute the neural machinery for generating respiratory rhythm and shaping inspiratory and expiratory motor patterns are distributed among three adjacent structural compartments in the ventrolateral medulla: the Bö tzinger complex (Bö tC), pre-Bö tzinger complex (pre-Bö tC) and rostral ventral respiratory group (rVRG). The respiratory rhythm and inspiratoryexpiratory patterns emerge from dynamic interactions between: (i) excitatory neuron populations in the pre-Bö tC and rVRG active during inspiration that form inspiratory motor output; (ii) inhibitory neuron populations in the pre-Bö tC that provide inspiratory inhibition within the network; and (iii) inhibitory populations in the Bö tC active during expiration that generate expiratory inhibition. Network interactions within these compartments along with intrinsic rhythmogenic properties of pre-Bö tC neurons form a hierarchy of multiple oscillatory mechanisms. The functional expression of these mechanisms is controlled by multiple drives from more rostral brainstem components, including the retrotrapezoid nucleus and pons, which regulate the dynamic behaviour of the core circuitry. The emerging view is that the brainstem respiratory network has rhythmogenic capabilities at multiple hierarchical levels, which allows flexible, state-dependent expression of different rhythmogenic mechanisms under different physiological and metabolic conditions and enables a wide repertoire of respiratory behaviours.