Plasticity in the nucleus tractus solitarius and its influence on lung and airway reflexes

Abstract

The nucleus tractus solitarius (NTS) is the first central nervous system (CNS) site for synaptic contact of the primary afferent fibers from the lungs and airways. The signal processing at these synapses will determine the output of the sensory information from the lungs and airways to all downstream synapses in the reflex pathways. The second-order NTS neurons bring to bear their own intrinsic and synaptic properties to temporally and spatially integrate the sensory information with inputs from local networks, higher brain regions, and circulating mediators, to orchestrate a coherent reflex output. There is growing evidence that NTS neurons share the rich repertoire of forms of plasticity demonstrated throughout the CNS. This review focuses on existing evidence for plasticity in the NTS, potential targets for plasticity in the NTS, and the impact of this plasticity on lung and airway reflexes.

Influence of Vagotomy on Monosynaptic Transmission at Second-Order Nucleus Tractus Solitarius Synapses

“…Previous studies have demonstrated that NTS neurons and their synapses can undergo homeostatic plasticity (reviewed by Bonham et al 2006). We therefore anticipated that vagotomy, by eliminating the propagation of action potentials from the visceral organs to monosynaptic NTS neurons, would cause a homeostatic, compensatory increase in synaptic efficacy at this first synapse.…”

“…Indeed vagal NTS synapses and autonomic reflexes have been shown to undergo homeostatic plasticity in response to a number of different presynaptic insults (see review by Bonham et al 2006). In accordance with homeostatic compensation, we therefore expected an electrical stimulus to the ST to elicit an increase in the synaptic efficacy of monosynaptic NTS neurons following vagotomy.…”

“…Similarly, NTS neurons from ozoneexposed primates not only have a more depolarized membrane potential, but electrical stimulation of primary afferents in the tractus solitarius (ST) elicits smaller excitatory postsynaptic responses when compared with controls (Chen et al 2001). Each of these studies revealed that NTS neurons and their primary afferent synapses can scale their electrophysiological properties to compensate for increased or decreased activity, a form of homeostatic plasticity (Bonham et al 2006;Turrigiano and Nelson 2000).…”

“…Thus second-order NTS neurons do not merely relay vagal activity to higher brain centers. Instead they incorporate afferent information with activity in local circuits and upstream efferent signals to control autonomic homeostasis, the maintenance of equilibrium between the sympathetic and parasympathetic systems (Andresen and Kunze 1994;Bonham et al 2006;Travagli et al 2006). Consequently, alterations to the information coming from vagal afferents could change synaptic communication to NTS neurons, modifying upstream circuits and throwing off autonomic homeostatic balance.…”

“…Similarly, NTS neurons from ozoneexposed primates not only have a more depolarized membrane potential, but electrical stimulation of primary afferents in the tractus solitarius (ST) elicits smaller excitatory postsynaptic responses when compared with controls (Chen et al 2001). Each of these studies revealed that NTS neurons and their primary afferent synapses can scale their electrophysiological properties to compensate for increased or decreased activity, a form of homeostatic plasticity (Bonham et al 2006; Turrigiano and Nelson 2000).In addition to pathophysiological disorders that cause insult to the peripheral sensory nerve terminals of the vagus, there are instances when the vagus nerve itself becomes injured. Heart and lung transplant, as well as treatments for gastrointestinal disorders, result in either full or partial section of the vagus nerve (vagotomy).…”