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NEUROSCIENCE |
-motoneurones
1 Spinal Cord Research Center, Department of Physiology, University of Manitoba, Canada
The purpose of this study was to describe the frequency–current (f–I) relationships of hindlimb 
-motoneurones (MNs) in both anaesthetized and decerebrate rats in situ. Sprague–Dawley rats (250–350 g) were anaesthetized with ketamine and xylazine (KX) or subjected to a precollicular decerebration prior to recording electrophysiological properties from sciatic nerve MNs. Motoneurones from KX-anaesthetized rats had a significantly (P < 0.01) hyperpolarized resting membrane potential and voltage threshold (Vth), increased rheobase current, and a trend (P
= 0.06) for a smaller after-hyperpolarization (AHP) amplitude compared to MNs from decerebrate rats. In response to 5 s ramp current injections, MNs could be categorized into four f–I relationship types: (1) linear; (2) adapting; (3) linear + sustained; and (4) late acceleration. Types 3 and 4 demonstrated self-sustained firing owing to activation of persistent inward current (PIC). We estimated the PIC amplitude by subtracting the current at spike derecruitment from the current at spike recruitment. Neither estimated PIC nor f–I slopes differed between fast and slow MNs (slow MNs exhibited AHP half-decay times > 20 ms) or between MNs from KX-anaesthetized and decerebrate rats. Motoneurones from KX-anaesthetized rats had significantly (P < 0.02) hyperpolarized ramp Vth values and smaller and shorter AHP amplitudes and decay times compared to MNs from decerebrate rats. Pentobarbitone decreased the estimated PIC amplitude and almost converted the f–I relationship from type 3 to type 1. In summary, MNs of animals subjected to KX anaesthesia required more current for spike initiation and rhythmic discharge but retained large PICs and self-sustained firing. The KX-anaesthestized preparation enables direct recording of PICs in MNs from intact animals.
(Received 10 February 2006;
accepted after revision 6 April 2006;
first published online 13 April 2006)
Corresponding author P. Gardiner: Spinal Cord Research Center, Department of Physiology, University of Manitoba, 730 William Avenue, 436 BMSB, Winnipeg, Manitoba, Canada R3E 3J7. Email: gardine2{at}cc.umanitoba.ca
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