Medioventral medulla-induced locomotion

L-dopa-induced air-stepping in decerebrate developing rats

Developing rats were anesthetized and a precollicular brainstem transection performed. Administration of L-3,4-dihydroxyphenylalanine (L-DOPA, 100 mg/kg, s.c.) induced locomotion in groups of animals 0-3, 6-8, 14-16 and 20-22 days of age. At early stages in development (0-3 days), continuous, long-lasting air-stepping was induced consisting of 4-limb walking with stronger alternation in the forelimbs compared to the hindlimbs. At 6-8 days of age, continuous air-stepping was characterized by better agreement between forelimb and hindlimb step cycles. By 14-16 days of age, the hindlimbs showed stronger and faster stepping than the forelimbs. The first evidence of consistent galloping was observed in this age group. In some cases, the forelimbs were held extended and only the hindlimbs walked or galloped. At 20-22 days, walking and galloping became episodic and the hindlimbs typically galloped while the forelimbs alternated. Each age group was tested for overground locomotion. Only in the 20-22 day group was weight-bearing overground stepping observed. At earlier ages, limb alternation was evident but of insufficient strength to lift the body off the ground. In general, the duration of the effect of the same dose of L-DOPA decreased with age. Animals in each group which received a mid-thoracic spinal cord transection showed forelimb alternation but not hindlimb stepping. These results indicate that L-DOPA induces stepping by activating brainstem and/or spinal centers via an as yet unknown mechanism. The pattern of the development of gait (forelimb to hindlimb gradient) at various ages was similar to that observed in intact developing rats.

Control of locomotion in vitro: II. Chemical stimulation

Previous studies have described the presence of alternating activity induced in left and right ventral roots of the neonate rat in vitro brainstem-spinal cord preparation, following application of certain neuroactive substances to the bathing solution. The present findings show the presence of chemically induced, adult-like coordinated airstepping demonstrated by electromyographic recordings in the hindlimb-attached in vitro brainstem-spinal cord preparation. Analysis of muscular activity demonstrated alternation between antagonists of one limb and between agonists of different limbs, as well as a proximodistal delay in agonists active at different joints of the same limb. Neuroactive agents were applied independently to either the brainstem or spinal cord bath. The substances surveyed in the present studies included some of those used previously, as well as additional compounds: bicuculline and picrotoxin (gamma-aminobutyric acid-ergic antagonists), N-methyl-D-aspartic acid (excitatory amino acid agonist), substance P, acetylcholine, carbachol (cholinergic agonist), and serotonin. Application of these substances to the brainstem bath produced rhythmic airstepping. Application of dopamine, aspartate, glutamate, and N-methyl-D-aspartic acid to the spinal cord bath also produced rhythmic airstepping, while application of acetylcholine produced tonic, long-lasting co-contractions. These findings reveal the presence of several neurochemical systems in the central nervous system that can be activated at birth to induce coordinated airstepping in the neonate rat in vitro brainstem-spinal cord preparation.

Control of locomotion in vitro: I. Deafferentation

We previously described the ability to induce adult-like, coordinated airstepping following electrical stimulation of the brainstem in the hindlimb-attached, in vitro brainstem-spinal cord preparation. These findings suggest the presence at birth of supraspinal systems capable of activating and modulating spinal locomotor mechanisms, which presumably also are present at birth. The current study employed the hindlimb-attached in vitro brainstem-spinal cord preparation from 0- to 4-day-old rats maintained in oxygenated artificial cerebrospinal fluid. After the control threshold-frequency relationship for eliciting airstepping was established, the dorsal roots to the attached limbs were severed and the procedure was repeated. No changes in electrical threshold or major differences in the elicited locomotor pattern were observed after deafferentation, although the amplitude of the electromyograms decreased. The mean frequency of alternation at threshold before deafferentation was similar to that after deafferentation. However, the maximum mean frequency induced by suprathreshold stimulation was significantly higher after deafferentation than that before deafferentation. These results suggest that (1) the supraspinal modulation of spinal locomotor mechanisms is not entirely dependent on afferent input; (2) intrinsic spinal locomotor mechanisms are present in the spinal cord at birth; and (3) afferent input may limit the maximum frequency of alternation of the limbs early in development.