SHORT-TERM PLASTICITY IN HINDLIMB MOTONEURONS OF DECEREBRATE CATS

Cat hindlimb motoneurons possess noninactivating voltage-gated inward currents that can, under appropriate conditions, regeneratively produc e sustained increments in depolarization and firing of the cell (i.e., plateau potentials). Recent studies in turtle dorsal horn neurons and motoneurons indicate that facilitation of plateaus occurs with repeat ed plateau activation( decreased threshold and increased duration; thi s phenomenon is referred to as warm-up). The purpose of the present st udy was to study warm-up in cat motoneurons. Initially, cells were stu died by injecting a slow triangular current ramp intracellularly to de termine the threshold for activation of the plateau. In cells where th e sodium spikes were blocked with intracellular QX314, plateau activat ion was readily seen as a sudden jump in membrane potential, which was not directly reversed as the current was decreased(cf. hysteresis). W ith normal spiking, the plateau activation (the noninactivating inward current) was reflected by a steep and sustained jump in firing rate, which was not directly reversed as the current was decreased (hysteres is). Repetitive plateau activation significantly lowered the plateau a ctivation threshold in 83% of cells (by on average 5 mV and ii Hz with and without QX314, respectively). This interaction between successive plateaus (warm-up) occurred when tested with 3- to 6-s intervals; no interaction occurred at times >20 s. Plateaus initiated by synaptic ac tivation from muscle stretch were also facilitated by repetition. Repe ated slow muscle stretches that produced small phasic responses when a cell was hyperpolarized with intracellular current bias produced a la rger and more prolonged responses (plateau) when the bias was removed, and the amplitude and duration of this response grew with repetition. The effects of warm-up seen with intracellular recordings during musc le stretch could also be recorded extracellularly with gross electromy ographic (EMG) recordings. That is, the same repetitive stretch as abo ve produced a progressively larger and more prolonged EMG response. Wa rm-up may be a functionally important form of shortterm plasticity in motoneurons that secures efficient motor output once a threshold level is reached for a significant period. Finally, the finding that warm-u p can be readily observed with gross EMG recordings will be useful in future studies of plateaus in awake animals and humans.