Following hours to days in hypoxia, a time dependent increase in ventilation occurs, termed ventilatory acclimatization to hypoxia (VAH). Hallmarks of VAH include an increased slope of the isocapnic hypoxic ventilatory response (HVR) and hyperventilation that persists upon return to normoxia (Powell et al., 1998). VAH involves changes in the sensitivity of the carotid body to hypoxia (Nielsen et al., 1988) and the central nervous system to carotid body afferent input (Dwinell & Powell, 1999). My dissertation tests the hypothesis that plasticity occurs in the Nucleus Tractus Solitarius (NTS), which receives the first synapse from the carotid body afferents (Housley et al., 1987) and also contains CO₂-sensitive cells (Dean et al., 1989). To determine if plasticity in the NTS contributes to VAH, we used two approaches: stimulating CO₂-sensitive cells in the NTS and lesioning a specific type of neuron in the NTS and measuring ventilatory responses to CO₂ and O₂ before and after acclimatization to chronic hypoxia. Stimulation of the NTS central chemoreceptor cells by focal acidosis increased phrenic nerve activity in anesthetized rats, as reported by other investigators (Coates et al., 1993). The response in chronically hypoxic animals was significantly less than in the normoxic controls in contrast to the increase we predicted to explain the persistent hyperventilation in normoxia with VAH. However, this decreased responsiveness of NTS CO₂-sensitive cells in chronic hypoxia is consistent with cellular data suggesting an increase in the number of cells inhibited by hypercapnia after chronic hypoxia (Nichols et al., 2009). Our lesion studies targeted neurokinin 1 receptor positive cells (NK1R) in the NTS because similar lesions in other central chemosensitive sites have been shown to reduce the hypercapnic ventilatory response (Nattie & Li, 2002b; Nattie et al., 2004). We measured the ventilatory responses to hypercapnia and hypoxia in normoxic and chronically hypoxic rats following the lesion of NK1R cells in the NTS. No effects of the lesion were seen in normoxic rats. However, ventilation in room air and hypoxia was significantly greater in lesioned animals after chronic hypoxia. This suggests that NK1R cells in the NTS attenuate the arterial chemoreflex after chronic hypoxia. There was also a non-significant trend for ventilation to increase during hypercapnia in lesioned animals after chronic hypoxia. These studies have established plasticity in the NTS during chronic hypoxia but the exact mechanisms and their contribution to VAH remain to be determined.

The voltage-gated sodium channel (Na_V), Na_V1.1, is well-studied in the central nervous system; conversely, its contribution to peripheral sensory neuron function is more enigmatic. Here, we identify a new role for Na_V1.1 in mammalian proprioception. RNAscope analysis and in vitro patch-clamp recordings in genetically identified mouse proprioceptors show ubiquitous channel expression and significant contributions to intrinsic excitability. Notably, genetic deletion of Na_V1.1 in sensory neurons caused profound and visible motor coordination deficits in conditional knockout mice of both sexes, similar to conditional Piezo2-knockout animals, suggesting that this channel is a major contributor to sensory proprioceptive transmission. Ex vivo muscle afferent recordings from conditional knockout mice found that loss of Na_V1.1 leads to inconsistent and unreliable proprioceptor firing characterized by action potential failures during static muscle stretch; conversely, afferent responses to dynamic vibrations were unaffected. This suggests that while a combination of Piezo2 and other Na_V isoforms is sufficient to elicit activity in response to transient stimuli, Na_V1.1 is required for transmission of receptor potentials generated during sustained muscle stretch. Impressively, recordings from afferents of heterozygous conditional knockout animals were similarly impaired, and heterozygous conditional knockout mice also exhibited motor behavioral deficits. Thus, Na_V1.1 haploinsufficiency in sensory neurons impairs both proprioceptor function and motor behaviors. Importantly, human patients harboring Na_V1.1 loss-of-function mutations often present with motor delays and ataxia; therefore, our data suggest that sensory neuron dysfunction contributes to the clinical manifestations of neurological disorders in which Na_V1.1 function is compromised. Collectively, we present the first evidence that Na_V1.1 is essential for mammalian proprioceptive signaling and behaviors.