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

The rostral end of the ventral respiratory group (VRG) contains neurons tha t are intensely neurokinin-l receptor (NK1R) immunoreactive (ir). It has be en theorized that some of these cells might be critical to respiratory rhyt hmogenesis (Cray 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 wa s found between Bregma -12.3 and -13.0 mm. This region overlaps with the pr e-Botzinger complex (pre-BotC) as it was found to contain many pre-inspirat ory neurons, few E2-expiratory neurons, and no I-incremental neurons. VRG N K1R-ir neurons contain neither tyrosine hydroxylase (TH) nor choline acetyl transferase (ChAT) immunoreactivity, although dual-labeled neurons were fo und 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-BotC region (6 % of somatic profiles). GlyT2 mRNA was commonly found i n the pre-BotC region but rarely within NK1R-ir neurons (1.3 %). Up to 40% of VRG NK1R-ir neurons were retrogradely labeled by Fluoro-Gold (FG) inject ed in the contralateral pre-BotC region. Some NK1R-ir VRG neurons located c audal to Bregma -12.6 mm were retrogradely labeled by FG injected in the sp inal 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-BotC. Within this li mited region of the VRG, NK1R-ir neurons are neither cholinergic nor catech olaminergic, and very few are gamma -aminobutyric acid (GABA)ergic or glyci nergic. The data suggest that most NK1R-ir neurons of the pre-BotC region a re excitatory. Furthermore, the more rostral NR1R-ir cells are propriomedul lary, whereas some of the caudal ones project to the spinal cord. (C) 2001 Wiley-Liss, Inc.