The rat retrotrapezoid nucleus (RTN) contains pH‐sensitive neurons that are putative central chemoreceptors. Here, we examined whether these neurons respond to peripheral chemoreceptor stimulation and whether the input is direct from the solitary tract nucleus (NTS) or indirect via the respiratory network. A dense neuronal projection from commissural NTS (commNTS) to RTN was revealed using the anterograde tracer biotinylated dextran amine (BDA). Within RTN, 51% of BDA‐labelled axonal varicosities contained detectable levels of vesicular glutamate transporter‐2 (VGLUT2) but only 5% contained glutamic acid decarboxylase‐67 (GAD67). Awake rats were exposed to hypoxia (n= 6) or normoxia (n= 5) 1 week after injection of the retrograde tracer cholera toxin B (CTB) into RTN. Hypoxia‐activated neurons were identified by the presence of Fos‐immunoreactive nuclei. CommNTS neurons immunoreactive for both Fos and CTB were found only in hypoxia‐treated rats. VGLUT2 mRNA was detected in 92 ± 13% of these neurons whereas only 12 ± 9% contained GAD67 mRNA. In urethane–chloralose‐anaesthetized rats, bilateral inhibition of the RTN with muscimol eliminated the phrenic nerve discharge (PND) at rest, during hyperoxic hypercapnia (10% CO₂), and during peripheral chemoreceptor stimulation (hypoxia and/or i.v. sodium cyanide, NaCN). RTN CO₂‐activated neurons were recorded extracellularly in anaesthetized intact or vagotomized rats. These neurons were strongly activated by hypoxia (10–15% O₂; 30 s) or by NaCN. Hypoxia and NaCN were ineffective in rats with carotid chemoreceptor denervation. Bilateral injection of muscimol into the ventral respiratory column 1.5 mm caudal to RTN eliminated PND and the respiratory modulation of RTN neurons. Muscimol did not change the threshold and sensitivity of RTN neurons to hyperoxic hypercapnia nor their activation by peripheral chemoreceptor stimulation. In conclusion, RTN neurons respond to brain P presumably via their intrinsic chemosensitivity and to carotid chemoreceptor activation via a direct glutamatergic pathway from commNTS that bypasses the respiratory network. RTN neurons probably contribute a portion of the chemical drive to breathe.

It is well known that breathing introduces rhythmical oscillations in the heart rate and arterial pressure levels. Sympathetic oscillations coupled to the respiratory activity have been suggested as an important homeostatic mechanism optimizing tissue perfusion and blood gas uptake/delivery. This respiratory-sympathetic coupling is strengthened in conditions of blood gas challenges (hypoxia and hypercapnia) as a result of the synchronized activation of brainstem respiratory and sympathetic neurons, culminating with the emergence of entrained cardiovascular and respiratory reflex responses. Studies have proposed that the ventrolateral region of the medulla oblongata is a major site of synaptic interaction between respiratory and sympathetic neurons. However, other brainstem regions also play a relevant role in the patterning of respiratory and sympathetic motor outputs. Recent findings suggest that the neurons of the nucleus of the solitary tract (NTS), in the dorsal medulla, are essential for the processing and coordination of respiratory and sympathetic responses to hypoxia. The NTS is the first synaptic station of the cardiorespiratory afferent inputs, including peripheral chemoreceptors, baroreceptors and pulmonary stretch receptors. The synaptic profile of the NTS neurons receiving the excitatory drive from afferent inputs is complex and involves distinct neurotransmitters, including glutamate, ATP and acetylcholine. In the present review we discuss the role of the NTS circuitry in coordinating sympathetic and respiratory reflex responses. We also analyze the neuroplasticity of NTS neurons and their contribution for the development of cardiorespiratory dysfunctions, as observed in neurogenic hypertension, obstructive sleep apnea and metabolic disorders.

— — Brain pathways controlling arterial pressure are distributed throughout the neuraxis and are organized in topographically selective networks. In this brief review, we will focus on the medulla oblongata. The nucleus tractus solitarius (NTS) is the primary site of cardiorespiratory reflex integration. It is well accepted that lesions or other perturbations in the NTS can result in elevations of arterial pressure (AP), with many of the associated features so commonly found in humans. However, recent studies have shown 2 distinct subpopulations of neurons within the NTS that can influence AP in opposite ways. Commissural NTS neurons located on the midline may contribute to maintenance of hypertension in spontaneously hypertensive rats (SHR), because small lesions in this area result in a very significant reduction in AP. Also involved in this blood pressure regulation network are 2 distinct regions of the ventrolateral medulla: caudal (CVLM) and rostral (RVLM). Neurons in CVLM are thought to receive baroreceptor input and to relay rostrally to control the activity of the RVLM. Projections from CVLM to RVLM are inhibitory, and a lack of their activity may contribute to development of hypertension. The RVLM is critical to the tonic and reflexive regulation of AP. In different experimental models of hypertension, RVLM neurons receive significantly more excitatory inputs. This results in enhanced sympathetic neuronal activity, which is essential for the development and maintenance of the hypertension.

New Findings: What is the central question of this study? This study was designed to investigate whether the Phox2b‐expressing neurons in the retrotrapezoid nucleus are important to breathing and chemoreflexes in conscious rats. What is the main finding and its importance? The high rate of destruction of a defined cell population (Phox2b⁺TH⁻) of the retrotrapezoid nucleus by the toxin suggests that the specialized connectivity of retrotrapezoid nucleus neurons, their glutamatergic nature and their relatively high sensitivity to CO₂ are determinant factors in explaining their large contribution to the central and peripheral chemoreflexes. Chemoreception is the classic mechanism by which the brain regulates breathing in response to changes in tissue CO₂/H⁺. A brainstem region called the retrotrapezoid nucleus (RTN) contains a population of Phox2b‐expressing glutamatergic neurons that appear to function as important chemoreceptors. In the present study, we ask whether the destruction of a type of pH‐sensitive interneuron that expresses the transcription factor Phox2b and is non‐catecholaminergic (Phox2b⁺TH⁻) could affect breathing in conscious adult rats. The injection of substance P (1 nmol in a volume of 50 nl) into the RTN increased respiratory frequency, tidal volume, minute ventilation and mean arterial pressure. Bilateral injections of the toxin substance P conjugated with saporin (SSP–SAP) into the RTN destroyed Phox2b⁺TH⁻ neurons but spared facial motoneurons, catecholaminergic and serotonergic neurons and the ventral respiratory column caudal to the facial motor nucleus. Bilateral inhibition of RTN neurons with SSP–SAP (0.6 ng in 30 nl) reduced resting ventilation and the increase in ventilation produced by hypercapnia (7% CO₂) in conscious rats with or without peripheral chemoreceptors. In anaesthetized rats with bilateral lesions of around 90% of the Phox2b⁺TH⁻ neurons, acute activation of the Bötzinger complex, the pre‐Bötzinger complex or the rostral ventral respiratory group with NMDA (5 pmol in 50 nl) elicited normal cardiorespiratory output. In conclusion, the destruction of the Phox2b⁺TH⁻ neurons is a plausible cause of the respiratory deficits observed after injection of SSP–SAP into the RTN. Our results also suggest that RTN neurons activate facilitatory mechanisms important to the control of breathing in resting or hypercapnic conditions in conscious adult rats.