Neurokinin‐1 receptor‐immunoreactive neurons of the ventral respiratory group in the rat

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Abstract
The rostral end of the ventral respiratory group (VRG) contains neurons that are intensely neurokinin‐1 receptor (NK1R) immunoreactive (ir). It has been theorized that some of these cells might be critical to respiratory rhythmogenesis (Gray et al. [1999] Science 286:1566–1568). In the present study we determined what major transmitter these NK1R‐ir cells make and whether they are bulbospinal or propriomedullary. NK1R‐ir neurons were found in the VRG between Bregma levels −11.7 and −13.6 mm. The highest concentration was found between Bregma −12.3 and −13.0 mm. This region overlaps with the pre‐Bötzinger complex (pre‐BötC) as it was found to contain many pre‐inspiratory neurons, few E2‐expiratory neurons, and no I‐incremental neurons. VRG NK1R‐ir neurons contain neither tyrosine hydroxylase (TH) nor choline acetyl‐transferase (ChAT) immunoreactivity, although dual‐labeled neurons were found elsewhere within the rostral medulla. GAD67 mRNA was commonly detected in the ventrolateral medulla (VLM) but rarely in the NK1R‐ir neurons of the pre‐BötC region (6 % of somatic profiles). GlyT2 mRNA was commonly found in the pre‐BötC region but rarely within NK1R‐ir neurons (1.3 %). Up to 40% of VRG NK1R‐ir neurons were retrogradely labeled by Fluoro‐Gold (FG) injected in the contralateral pre‐BötC region. Some NK1R‐ir VRG neurons located caudal to Bregma −12.6 mm were retrogradely labeled by FG injected in the spinal cord (C4–C5, T2–T4). In sum, NK1R immunoreactivity is present in many types of ventral medullary neurons. Within the VRG proper, NK1R‐ir neurons are concentrated in an area that overlaps with the pre‐BötC. Within this limited region of the VRG, NK1R‐ir neurons are neither cholinergic nor catecholaminergic, and very few are γ‐aminobutyric acid (GABA)ergic or glycinergic. The data suggest that most NK1R‐ir neurons of the pre‐BötC region are excitatory. Furthermore, the more rostral NK1R‐ir cells are propriomedullary, whereas some of the caudal ones project to the spinal cord. J. Comp. Neurol. 434:128–146, 2001.