Effects of localized intraspinal injections of a noradrenergic blocker on locomotion of high decerebrate cats.

Previous studies demonstrated that neuronal networks located in midlumbar segments (L3-L4) are critical for the expression of locomotion in cats following complete spinalization. In the present study the importance of several thoracolumbar segments (T8-L7) for the generation of spontaneous hindlimb locomotion in decerebrate cats was evaluated. Experiments were performed in high decerebrate cats (n = 18) walking spontaneously. Yohimbine, an alpha2-noradrenergic antagonist, was microinjected intraspinally in various thoracolumbar segments. Locomotor performance was evaluated with kinematics and electromyographic (EMG) recordings before and after each injection. When and if spontaneous locomotion (SL) was abolished, skin or perineal stimuli (exteroceptive stimuli) were used to trigger locomotion (exteroceptive-induced locomotion [EL]). Yohimbine injections at L3 or L4 completely inhibited SL and EL. In contrast, injections at T8 did not interfere with SL or EL. Injections at T10, T11, T12, L5, L6, and L7 inhibited SL but EL could still be evoked. Injections at T13, L1, and L2 had similar effects except that the quality of locomotion evoked by exteroceptive stimulation declined. Combined injections at T13, L1, and L2 abolished SL and EL, in contrast to injections restricted to the same individual segments. Simultaneous injections at L5, L6, and L7 also abolished SL but EL could still be induced. These results suggest that noradrenergic mechanisms in L3-L4 segments are involved in the expression of locomotion in decerebrate cats, whereas antagonizing noradrenergic inputs in individual rostral or caudal segments may alter the expression and overall quality of the locomotor pattern without abolishing locomotion.