Exp Brain Res (1994) 102:3444 9 Springer-Verlag 1994 R. M. Brownstone 9 J.-P. Gossard 9 H. Hultborn Voltage-dependent excitation of motoneurones from spinal locomotor centres in the cat Received: 11 January 1994 / Accepted: 14 June 1994 Abstract Lumbar motoneurones were recorded intra- cellularly during fictive locomotion induced by stimula- tion of the mesencephalic locomotor region in decere- brate cats. After blocking the action potentials using intracellular QX-314, and by using a discontinuous cur- rent clamp, it is shown that the excitatory component of the locomotor drive potentials behaves in a voltage-de- pendent manner, such that its amplitude increases with depolarisation. As the input to motoneurones during locomotion is comprised of alternating excitation and inhibition, it was desirable to examine the excitatory input in relative isolation. This was accomplished in spinalised decerebrate cats treated with nialamide and L-dihydroxy-phenylalanine (L-DOPA) by studying the excitatory post-synaptic potentials (EPSPs) evoked from the "flexor reflex afferents" (FRA) and extensor Ib afferents, both of which are likely to be mediated via the locomotor network. As expected, these EPSPs also demonstrate a voltage-dependent increase in amplitude. In addition, the input to motoneurones from the net- work for scratching, which is thought to share interneu- rones with the locomotor network, also results in voltage-dependent excitation. The possible underlying mechanisms of NMDA-mediated excitation and plateau potentials are discussed: both may contribute to the observed effect. It is suggested that this nonlinear increase in excitation contributes to the mechanisms in- volved in the production of the high rates of repetitive R. M. Brownstone ([~) Department of Physiology, Faculty of Medicine, University of Manitoba, 770 Bannatyne Ave, Winnipeg, Manitoba, Canada R3E 0W3 J.-P. Gossard 1 . H. Hultborn Department of Medical Physiology, Panum Institute, Copenhagen University, Blegdamsvej 3C, DK-2000 Copenhagen N, Denmark Present address: i Department of Physiology, Faculty of Medicine, University of Montr6al, C.P. 6128, Succ. A, Montr6al, Qu6bec, Canada H3C 3J7 firing of motoneurones typically seen during locomo- tion, thus ensuring appropriate muscle contraction. Key words Fictive locomotion - Scratch Flexor reflex afferents 9 Group Ib 9 Plateau potentials NMDA receptor 9 Cat Introduction The spinal network for the generation of locomotor ac- tivity can be activated in the paralysed decerebrate cat to produce locomotion by either pharmacologic means or by stimulation of the mesencephalic locomotor re- gion (MLR). It has been shown that during treadmill or "fictive" locomotion (central locomotor activity during neuromuscular blockade), the firing frequency of lum- bar motoneurones is not linearly related to the synaptic excitation they receive. When recruited, they tend to fire repetitively at relatively high rates (30 50 Hz; Severin et al. 1967; Zajac and Young 1980; Hoffer et al. 1987; Brownstone et al. 1992). This rate is rather unaffected either by increasing the locomotor drive by increasing brain stem stimulation (Severin et al. 1967; Zajac and Young 1980) or by injecting depolarising current intra- cellularly (Brownstone et al. 1992). Conversely, the in- jection of low amplitude hyperpolarising current com- pletely eliminates the repetitive firing (Brownstone et al. 1992). This very nonlinear input-output relation may be a result of voltage-dependent activation of motoneu- tones during fictive locomotion through a transmitter- regulated regenerative intrinsic property of the mo- toneurone such as that seen with plateau potentials (Hounsgaard et al. 1988), through a receptor-mediated process such as that seen with the activation of N- methyl D-aspartate (NMDA) receptors in other cell types (Nowak et al. 1984), or perhaps through a combi- nation of these processes. The current study was undertaken to examine the hypothesis that the nonlinear excitation of motoneu- rones during fictive locomotion is at least partly a man-