Blood Pressure Regulation by the Rostral Ventrolateral Medulla in Conscious Rats: Effects of Hypoxia, Hypercapnia, Baroreceptor Denervation, and Anesthesia

C1 neurons: a nodal point for stress?

NEW FINDINGS

What is the topic of this review? The C1 neurons (C1) innervate sympathetic and parasympathetic preganglionic neurons plus numerous brain nuclei implicated in stress, arousal and autonomic regulations. We consider here the contribution of C1 to stress-induced responses. What advances does it highlight? C1 activation is required for blood pressure stability during hypoxia and mild hemorrhage which exemplifies their homeostatic function. During restraint stress, C1 activate the splenic anti-inflammatory pathway resulting in tissue protection against ischemic injury. This effect, along with glucose release and, possibly, arousal are examples of adaptive non-homeostatic responses to stress that are also mediated by C1. The C1 cells are catecholaminergic and glutamatergic neurons located in the rostral ventrolateral medulla. Collectively, these neurons innervate sympathetic and parasympathetic preganglionic neurons, the hypothalamic paraventricular nucleus and countless brain structures involved in autonomic regulation, arousal and stress. Optogenetic inhibition of rostral C1 neurons has little effect on blood pressure (BP) at rest in conscious rats but produces large reductions in BP when the animals are anaesthetized or exposed to hypoxia. Optogenetic C1 stimulation increases BP and produces arousal from non-rapid eye movement sleep. C1 cell stimulation mimics the effect of restraint stress to attenuate kidney injury caused by renal ischaemia-reperfusion. These effects are mediated by the sympathetic nervous system through the spleen and eliminated by silencing the C1 neurons. These few examples illustrate that, depending on the nature of the stress, the C1 cells mediate adaptive responses of a homeostatic or allostatic nature.

Impaired chemosensory control of breathing after depletion of bulbospinal catecholaminergic neurons in rats

Bulbospinal catecholaminergic neurons located in the rostral aspect of the ventrolateral medulla (C1 neurons) or within the ventrolateral pons (A5 neurons) are involved in the regulation of blood pressure and sympathetic outflow. A stimulus that commonly activates the C1 or A5 neurons is hypoxia, which is also involved in breathing activation. Although pharmacological and optogenetic evidence suggests that catecholaminergic neurons also regulate breathing, a specific contribution of the bulbospinal neurons to respiratory control has not been demonstrated. Therefore, in the present study, we evaluated whether the loss of bulbospinal catecholaminergic C1 and A5 cells affects cardiorespiratory control during resting, hypoxic (8% O₂), and hypercapnic (7% CO₂) conditions in unanesthetized rats. Thoracic spinal cord (T4-T8) injections of the immunotoxin anti-dopamine β-hydroxylase-saporin (anti-DβH-SAP-2.4 ng/100 nl) and the retrograde tracer Fluor-Gold or ventrolateral pontine injections of 6-OHDA were performed in adult male Wistar rats (250-280 g, N = 7-9/group). Anti-DβH-SAP or 6-OHDA eliminated most bulbospinal C1 and A5 neurons or A5 neurons, respectively. Serotonergic neurons and astrocytes were spared. Depletion of the bulbospinal catecholaminergic cells did not change cardiorespiratory variables under resting condition, but it did affect the response to hypoxia and hypercapnia. Specifically, the increase in the ventilation, the number of sighs, and the tachycardia were reduced, but the MAP increased during hypoxia in anti-DβH-SAP-treated rats. Our data reveal that the bulbospinal catecholaminergic neurons (A5 and C1) facilitate the ventilatory reflex to hypoxia and hypercapnia.