Plasticity of rat motoneuron rhythmic firing properties with varying levels of afferent and descending inputs

• Published in: Journal of neurophysiology 2012-01, Vol.107 (1), p.265-272
• Main Authors: MacDonell, C W, Button, D C, Beaumont, E, Cormery, B, Gardiner, P F
• Format: Article
• Language: English
• Subjects: Action Potentials - physiology; Afferent Pathways - physiology; Animals; Biological Clocks - physiology; Cellular Biology; Efferent Pathways - physiology; Female; Life Sciences; Motor Activity - physiology; Motor Neurons - physiology; Neuronal Plasticity - physiology; Physical Conditioning, Animal - methods; Rats; Rats, Sprague-Dawley
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.00122.2011

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.