Regional differences in serotonergic input to canine parasternal intercostal motoneurons

There is a mediolateral gradient in activation of the parasternal intercost al (PI) muscle during inspiration. In the present study, we tested the hypo theses that serotonergic [5-hydroxytryptamine (5-HT)] input from descending central drive and/or intrinsic size-related properties of PI motoneurons l eads to the differential activation of PI muscles. In dogs, PI motoneurons innervating the medial and lateral regions of the PI muscles at the T-3-T-5 interspaces were retrogradely labeled by intramuscular injection of choler a toxin B subunit. After a 10-day survival period, PI motoneurons and 5-HT terminals were visualized by using immunohistochemistry and confocal imagin g. There were no differences in motoneuron morphology among motoneurons inn ervating the medial and lateral regions of the PI muscle. However, the numb er of 5-HT terminals and the 5-HT terminal density (normalized for surface area) were greater in motoneurons innervating the medial region of the PI m uscle compared with the lateral region. These results suggest that differen ces in distribution of 5-HT input may contribute to regional differences in PI muscle activation during inspiration and that differences in PI motoneu ron recruitment do not relate to size.