Pharmacological activation of locomotor patterns in larval and adult frog spinal cords

The effects of amino acids, catecholamines, and their agonists shown to elicit locomotor activity in several vertebrate species were examined in spinal animals and isolated nervous systems of developing tadpoles (Rana catesbiana) and adult frogs (R. catesbiana and pipiens). Elicited activity was correlated in spinal animals by video and electromyographic analysis, and in in vitro spinal cords by recordings of tail and hindlimb motor activity. Of the agents tested, only N-methyl-DL-aspartate (NMA), an amino acid agonist, was effective in eliciting motor activity in spinal animals. In isolated nervous systems, both NMA and D-glutamate added to the bath activated locomotor activity. NMA injected i.p. into tadpoles with high spinal cord transections elicited coordinated swimming motor activity in axial and hindlimb muscles that was roughly typical for the stage of development of the animal. In late stage tadpoles (st. XX), NMA also elicited wiping and alternating or synchronous (i.e. kicking or jumping) hindlimb movements. Addition of NMA or glutamate to a bath containing an in vitro tadpole spinal cord preparation elicited ventral root motor activity characteristic of swimming, but without a rostrocaudal phase lag. Rhythmic activity thought to underlie stepping and kicking was seen in lateral ventral rootlets innervating the hindlimbs. In adult frogs with high spinal cord transections, injection of NMA elicited a general sequence of spontaneous hindlimb motor functions: reflex wiping, stepping, and kicking or jumping. Isolated frog spinal cords were not responsive to bath applied NMA, under the present conditions. The activation by amino acids or their agonists of different motor functions in both larval and adult frogs, as well as in higher and lower vertebrates, suggests a general significance of amino acid-activated receptors in the neural networks controlling locomotor function.