title: Plasticity of rat motoneuron rhythmic firing properties with varying levels of afferent and descending inputs.
abstract: Hindlimb motoneuron excitability was compared among exercise-trained (E), sedentary (S), and spinal cord transected (T) Sprague-Dawley rats by examining the slope of the frequency-current (F/I) relationship with standard intracellular recording techniques in rats anesthetized with ketamine-xylazine. The T group included spinal transected and spinal isolated rats; the E animals were either spontaneously active (exercise wheel) or treadmill trained; and rats in the S group were housed in pairs. An analysis of motoneuron initial [1st interspike interval (ISI)], early (mean of 1st three ISIs), and steady-state (mean of last 3 ISIs) discharge rate slopes resulting from increasing and decreasing 500-ms injected square-wave depolarizing current pulses was used to describe rhythmic motoneuron properties. The steepest slope occurred in the S group (55.3 ± 22.2 Hz/nA), followed by the T group (35.5 ± 15.3 Hz/nA), while the flattest slope was found in the E group (25.4 ± 10.9 Hz/nA). The steepest steady-state slope occurred in the S group but was found to be similar between the T and E groups. Furthermore, a spike-frequency adaptation (SFA) index revealed a slower adaptation in motoneurons of the E animals only (∼40% lower). Finally, evidence for a secondary range of firing existed more frequently in the T group (41%) compared with the S (12%) and E (31%) groups. The lower F/I slope and lower SFA index of motoneurons for E rats may be a result of an increase in Na(+) conductance at the initial segment. The results show that motoneuronal rhythmic firing behavior is plastic, depending on the volume of daily activation and on intact descending pathways.
authors.affiliation: 404 Basic Medical Science Bldg., Spinal Cord Research Centre, Dept. of Physiology, Faculty of Medicine, Univ. of Manitoba, Winnipeg, MB, Canada R3E 3J7.
authors.forename: C W
authors.initials: CW
authors.lastname: MacDonell
authors.forename: D C
authors.initials: DC
authors.lastname: Button
authors.forename: E
authors.initials: E
authors.lastname: Beaumont
authors.forename: B
authors.initials: B
authors.lastname: Cormery
authors.forename: P F
authors.initials: PF
authors.lastname: Gardiner
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000344:Afferent Pathways / Q000502:physiology*; D000818:Animals; D001683:Biological Clocks / Q000502:physiology*; D004525:Efferent Pathways / Q000502:physiology*; D005260:Female; D009043:Motor Activity / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D009473:Neuronal Plasticity / Q000502:physiology*; D010805:Physical Conditioning, Animal / Q000379:methods; D051381:Rats; D017207:Rats, Sprague-Dawley