Effects of Age and Early Life Ozone Exposure on the Developmental Dynamics of Afferent Airway Neurons in Postnatal Rats

The environmental irritant ozone (O3) plays a role in airway inflammation and is known to cause airway hyperresponisiveness especially in susceptible populations such as asthmatics and young children. O3 exposure increases nerve growth fact (NGF) mRNA and protein levels in the airways, and alters the expression of substance P (SP) in nodose and jugular afferent airway neurons and in sensory nerve fibers in the airways. The growth and development of vagal afferent neurons is in part regulated by NGF, and because these sensory neurons continue to develop throughout postnatal life O3 exposures during this period may adversely affect their development. The objectives of these studies were to 1) characterize the normal postnatal development of vagal ganglia sensory neurons, tracheal epithelial innervation, and NGF levels in lung lavage and 2) investigate changes in nodose and jugular ganglia neuron number and SP content following acute O3 exposure in early postnatal life.;Initial studies determined that NGF levels in bronchoalveolar lavage fluid (BALF) and tracheal epithelial nerve fiber density (NFD) changed in an age-related manner, and both were maximally increased on postnatal day (PD) 10. Preliminary studies conducted to determine an efficient and accurate method for quantifying vagal sensory neurons used retrograde tracing techniques to identify afferent airway neurons located in the nodose and jugular ganglia. A novel quantification technique combining neuronal isolation, immunocytochemistry and flow cytometry was developed. Age-related changes were also noted in afferent airway neurons, which corresponded to the changes measured in NFD and NGF levels. These findings indicated that the early postnatal development of airway sensory neurons and innervation occurs in a rapid manner and may be influenced by NGF.;In the next set of studies, rats were exposed to O3 (2ppm for 3 hrs) on PD5 to evaluate possible changes in neuronal development and SP content. A significant decrease in the total number of nodose and jugular ganglia neurons was noted 16 days after the O3 exposure (PD21), and the number of SP-containing airway neurons was markedly increased on PD28 (23 days after O3) compared with filtered air (FA) controls. No differences in the total number of airway neurons between O3 and FA exposed groups were noted. These findings reveal that early life O 3 exposure can significantly alter vagal sensory neuron development.;In conclusion, sensory neuron development is significantly altered by exposure to O3 in early postnatal life, and increases in the number of SP containing airway neurons so far removed from the exposure, may provide insight into why early life O3 exposure enhances airway responses to subsequent irritant exposures later in life

Early postnatal ozone exposure alters rat nodose and jugular sensory neuron development

Sensory neurons originating in nodose and jugular ganglia that innervate airway epithelium (airway neurons) play a role in inflammation observed following exposure to inhaled environmental irritants such as ozone (O3). Airway neurons can mediate airway inflammation through the release of the neuropeptide substance P (SP). While susceptibility to airway irritants is increased in early life, the developmental dynamics of afferent airway neurons are not well characterized. The hypothesis of this study was that airway neuron number might increase with increasing age, and that an acute, early postnatal O3 exposure might increase both the number of sensory airway neurons as well as the number SP-containing airway neurons. Studies using Fischer 344 rat pups were conducted to determine if age or acute O3 exposure might alter airway neuron number. Airway neurons in nodose and jugular ganglia were retrogradely labeled, removed, dissociated, and counted by means of a novel technique employing flow cytometry. In Study 1, neuron counts were conducted on postnatal days (PD) 6, 10, 15, 21, and 28. Numbers of total and airway neurons increased significantly between PD6 and PD10, then generally stabilized. In Study 2, animals were exposed to O3 (2 ppm) or filtered air (FA) on PD5 and neurons were counted on PD10, 15, 21, and 28. O3-exposed animals displayed significantly less total neurons on PD21 than FA controls. This study shows that age-related changes in neuron number occur, and that an acute, early postnatal O3 exposure significantly alters sensory neuron development