Morphology of pulmonary rapidly adapting receptor relay neurons in the rat

Abstract

The term rapidly adapting pulmonary stretch receptor (RAR) refers to one of the major pulmonary sensory receptors that responds to inflation and deflation of the lungs as well as to irritant stimuli with rapidly adapting irregular discharges. The functional role and central pathways are largely unknown. The aim of this study was to elucidate morphological characteristics of second-order neurons (RAR cells) activated by vagal afferent fibers originating from RARs. A mixture of horseradish peroxidase (HRP) and Neurobiotin was injected intracellularly into physiologically identified RAR cells in Nembutal-anesthetized, immobilized, and artificially ventilated Wister rats. Direct visualization of individual RAR cells (n = 12), including their somata, dendritic arborizations, and fine axonal branches with terminal boutons, was possible for the first time. Their somata were located in the commissural or medial subdivision of the nucleus of the solitary tract, caudal to the level of the area postrema. The RAR cells had, in addition to dendrites extending into the NTS area, one or two long dendrites extending laterally and/or ventrolaterally into the medullary reticular formation. The stem axons issuing from the RAR cells first coursed ventrolaterally toward the reticular formation in the vicinity of the ambiguus nucleus and then bifurcated into ascending and descending axons: three RAR cells possessed only ascending axons. Some of the ascending axons could be traced as far as the level of the facial nucleus and some of the descending axons beyond the spinomedullary junction. These ascending and/or descending axons gave off extensive axon collaterals distributing boutons within and in the vicinity of the ambiguus nucleus. These results, showing an anatomical substrate for the network implicated in RAR-evoked reflexes, provide useful clues for study of the RAR system.

Neurochemical and physiological correlates of a critical period of respiratory development in the rat

“…The expression of α2 subunit remains relatively constant during development (Liu and Wong-Riley, 2004). Similar trends are observed in the ventrolateral subnucleus of the solitary tract nucleus (NTS VL ), which receives input from the carotid body (Finley and Katz, 1992) and relays slowly-adapting pulmonary stretch receptor information via its pump cells (which are mainly GABAergic) to other regions of the NTS (such as the commissural nucleus) and to the ventrolateral medulla and pons (Miyazaki et al, 1999; Otake et al, 2001; Kubin et al, 2006). Whether these trends exist in the commissural and other subnuclei of NTS is unknown at this time.…”

Central Autonomic System

“…The GLP1 neurons contain leptin receptors constituting yet another mechanism for satiety signaling in this portion of the NTS (Scott et al, 2011;Kanoski et al, 2012;Zhao et al, 2012). Cardiovascular afferents terminate in the caudal half of the nucleus, in the dorsomedial, medial, parvicellular, and commissural subnuclei, as well as in the area postrema (Wallach and Loewy, 1980;Davies and Kalia, 1981;Ciriello, 1983;Seiders and Stuesse, 1984;Erickson and Millhorn, 1991;Chan et al, 2000) and respiratory afferents end mainly in the ventrolateral, intermediate, interstitial, and commissural subnuclei Richter, 1985, 1988;Haxhiu and Loewy, 1996;Otake et al, 2001;Ezure et al, 2002;Kubin et al, 2006;McGovern et al, 2012). The NTS also receives afferents from the superficial layers of the spinal and trigeminal dorsal horns (Menétrey and Basbaum, 1987).…”

Widespread depolarization during expiration: A source of respiratory drive?

“…This well-characterized reflex between the lung and brain stem is called the Hering-Breuer inflation reflex [27]. Once activated, SARs and RARs induce action potentials through projections that run along the vagus nerve and synapse on second order neurons in the caudal portion of the nucleus of the solitary tract (NTS) in the medullary region of the brainstem [49,50]. Current evidence points to SAR activation as a major link between the mechanical stimulus induced by breathing and the physiological responses induced by this stimulus during respiration.…”