Age-related Changes In Phrenic Motoneuron Excitability In The Rat

Abstract

PURPOSE: The functional excitability of motoneuron is determined by the amount of central drive and intrinsic membrane properties, which are related to motoneuron size. To investigate age-related changes in motoneuron excitability, we examined the number of serotonergic terminals and intrinsic electrophysiological excitability of phrenic motoneuron in the rat. METHODS: Using an intracellular recording technique, the electrophysiological properties of motoneurons were measured in young adult (less than 1-year, n=15) and old (2-year, n=15) rats. Under anesthesia and artificial respiration, rheobase (nA) and input resistance (M Ohm) were measured in the motoneurons. In another set of animals (6 rats from each age group), the phrenic motoneurons were retrogradely labeled with cholera toxin subunit B (CTB), and then the spinal cord was processed with immunohistochemical staining (1st antibody to CTB and 2nd antibody with Cy3, 1st antibody to serotonin and 2nd antibody with FITC). Three-dimensional morphological properties of soma and number of serotonergic terminals on the labeled phrenic motoneuron were measured by confocal imaging techniques. RESULTS: Compared to that in young adult rats, the motoneuron from old rats showed a tendency toward a decrease in rheobase and increase in input resistance. Furthermore, there was significantly smaller somal volume, smaller initial diameter of the primary dendrite, and smaller number of serotonergic terminal in the motoneuron from old rats compared to those in motoneuron from young rats. CONCLUSIONS: These results suggest that unstable action potential generation due to aging is attributable to decreases in the amount of serotonergic terminals on the phrenic motoneuron, and that the decrease in motoneuron size may be a compensatory adaptation for the decreased excitability inputs.