SUMMARY1. The role of serotonin (5-HT) and excitatory amino-acids (EAAs) in the activation of the neural networks (i.e. the central pattern generators) that organize locomotion in mammals was investigated in an isolated brainstem-spinal cord preparation from the newborn rat.2. The neuroactive substances were bath applied and the activity of fictive locomotion was recorded in the ventral roots.3. Serotonin initiated an alternating pattern of right and left action potential bursts. The period of this rhythm was dose dependent, i.e. it decreased from around 10 s at 2 x 10-5 M to 5 s at 10-4 M. The effects of serotonin were blocked by a 5-HT1 antagonist (propanolol) and by 5-HT2 antagonists (ketanserin, cyproheptadine, mianserin). 5-HT3 antagonists were ineffective. The effects of methoxytryptamine, a non-selective 5-HT agonist, mimicked the 5-HT effects.4. The endogenous EAAs, glutamate and aspartate, also triggered an alternating rhythmic pattern. Their effects were blocked by 2-amino-5-phosphonovaleric acid (AP-5; a N-methyl-D-aspartate (NMDA) receptor blocker) and 6,7-dinitro-quinoxaline-2,3-dione (a non-NMDA receptor blocker).5. Several EAA agonists (N-methyl-D,L-aspartate (NMA) and kainate) initiated rhythmic activity. The period of the induced rhythm (from 3 to 1 s) was similar with both of these substances but in a range of concentrations which was ten times lower in the case of kainate

How is load sensed by receptors, and how is this sensory information used to guide locomotion? Many insights in this domain have evolved from comparative studies since it has been realized that basic principles concerning load sensing and regulation can be found in a wide variety of animals, both vertebrate and invertebrate. Feedback about load is not only derived from specific load receptors but also from other types of receptors that previously were thought to have other functions. In the central nervous system of many species, a convergence is found between specific and nonspecific load receptors. Furthermore, feedback from load receptors onto central circuits involved in the generation of rhythmic locomotor output is commonly found. During the stance phase, afferent activity from various load detectors can activate the extensor part in such circuits, thereby providing reinforcing force feedback. At the same time, the flexion is suppressed. The functional role of this arrangement is that activity in antigravity muscles is promoted while the onset of the next flexion is delayed as long as the limb is loaded. This type of reinforcing force feedback is present during gait but absent in the immoble resting animal.

An in vitro preparation of newborn rat isolated brainstem/spinal cord was used in order to locate the spinal network responsible in mammals for producing patterned locomotor activity. The spinal cord was partitioned by building Vaseline walls at various lumbar levels. When a mixture of serotonin and N-methyl-D,L-aspartate was bath applied to the upper lumbar cord (L1/L2 segments), rhythmic locomotor-like activity was induced and recorded in all the lumbar segments (from L1 to L5). Conversely, when the mixture of transmitters was bath applied to the lower lumbar cord, only tonic activity was induced in the lower lumbar segments. Intracellular recordings performed on motoneurons revealed that during elicited L1/L2 locomotor-like activity, they received a rhythmic synaptic drive that was often below the threshold for spiking, because the excitability of the neurons was too low. When the L1/L2 segments were isolated, their burst production capacities remained. The network located at the L1/L2 level was found to be responsible not only for generating the rhythm but also for organizing its alternating pattern. We demonstrated that the rhythmic synaptic drive that the motoneurons receive during locomotor-like activity comes directly from the L1/L2 network and that there is no relay at the segmental level. We conclude from our study that the network that organizes locomotion in the newborn rat is not segmentally distributed but is restricted to a specific part of the cord. This finding has important consequences, since it means that it is now feasible to study the activity of the rhythmic spinal network independently from that of the motoneurons.

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