Ontogeny of retrograde labeled chemoafferent neurons in the newborn rat nodose-petrosal ganglion complex: An ex vivo preparation

Hypoxic chemosensitivity of the peripheral arterial chemoreceptors in the carotid body is developmentally regulated. Essential neural elements of the chemotransducing unit in the carotid body consist of the Type I cell that depolarizes and releases neurotransmitters in response to hypoxemia and the chemoafferent fibers which form synapses with Type I cells, contain postsynaptic receptors and have cell bodies in the petrosal ganglion. While many properties of the Type I cells have been characterized during postnatal development, less is known about the effect of development on the number and properties of the chemoafferents since localization of the cell bodies of chemoafferents are intermingled with the cell bodies of other sensory neurons that innervate the upper airway. Here, we describe a novel ex vivo preparation that we have developed to retrogradely label cell bodies of chemoafferents in the petrosal ganglion with rhodamine dextran. With this technique, in newborn rats, we show that there is a three-fold increase in retrogradely labeled neurons in the nodose-petrosal ganglion complex from postnatal day (PND) 3-7 with a three-fold decrease by PND 14 (P < 0.001, ANOVA). Furthermore, greater than 85% of these retrogradely labeled neurons co-express TH mRNA in all age groups. This novel ex vivo technique circumvents many of the technical difficulties encountered with retrogradely labeling chemoafferents in small newborn animals in vivo, and provides a method to identify and characterize essential neural components of the chemotranductive unit of the peripheral arterial chemoreceptors.