A complete transection of the spinal cord at a low thoracic level induces a paraplegic syndrome that is accompanied by a loss of spinal cord serotonin content. Former experimental data suggest that the central pattern generator for locomotion, located in the lumbar segments of the spinal cord, might be able to generate rhythmic motor outputs (similar to automatic walking under certain circumstances) involving exteroceptive stimulations and activation of serotonergic receptors. In the present study, we investigated the effects of a chronic treatment using a serotonergic agonist, delivered continuously to the sublesionned spinal cord, and its effect on motor function recovery. The data obtained from behavioural, kinematic and electromyographic measurements suggest that the chronic stimulation of 5-HT2 type receptors allows motor function recovery. Behavioural measurements show a clear improvement in motor performances when compared to spinal animals (confirmed by kinematic observations): alternating steps and foot placement is recovered in these animals. However, electromyographic data demonstrate that the pattern of activation of the muscles is only restored partially.

After thoracic spinal cord transection, a paraplegic syndrome occurs. Previous data showed that an acute administration of a 5-HT2 agonist (quipazine) could promote motor function recovery in spinal rats. However, continuous subdural perfusion of quipazine via an osmotic pump over 1 month proved to be more effective. The present study was designed to investigate the possible involvement of 5-HT1A receptors in such recovery. Motor performances and locomotor parameters were analysed in spinal animals receiving daily, for 1 month, a dose of the 5-HT1A agonist 8-OHDPAT. The results were compared to those obtained in spinal rats receiving either a placebo or quipazine in the same conditions. Using daily injections instead of continuous perfusion of either receptor agonist to spinal animals allowed characterization of short- and long-term consequences of pharmacological stimulation of 5-HT1A and 5-HT2 receptors on motor function recovery. Our data demonstrate that daily injections of a 5-HT1A agonist induce long-term, cumulative, positive effects on motor function recovery, as assessed by the improvement in the walking parameters observed before the 'day-test' injection. This might involve use-dependent processes depending on a chronic and/or repetitive stimulation of the spinal network for locomotion in relation to 5-HT receptor activation. A further improvement in the motor parameters, transiently observed following the injection, suggests a more direct action of 5-HT1A and 5-HT2 receptor activation on spinal neurons involved in motor pattern generation.

No abstract

The biogenic amine serotonin has been described in the literature as a powerful modulator of the spinal central pattern generator for locomotion. In the present study, we tested whether administration of serotonin or its agonist quipazine could restore motor activity in a model of paraplegia. One to three weeks after a complete transection of the spinal cord at a low thoracic level, rats were given either intrathecal injections of serotonin (5 mM, 15 microL) or intraperitoneal injections of quipazine (400-600 microg/kg). Both treatments allowed recovery of locomotor activity on a treadmill in response to tail pinching. As compared with the activity elicited before treatment, the locomotor activity produced by spinal animals was characterised by longer locomotor sequences with a larger number of successive steps, better body support, better interlimb coordination, and a higher amplitude of electromyographic bursts. These results suggest that serotonergic drugs could be used for the recovery of motor functions after lesions of the spinal cord.

The tachykinin substance P (SP) is present in the ventral and medial area of the lumbar spinal cord. Its localisation suggests that it could modulate the spinal network for locomotion. We have investigated its effects on motor outputs by applying SP, in vitro, to the lumbosacral segments of an isolated spinal cord of new-born rats. SP was applied to the lumbosacral spinal cord either on a quiescent preparation or during episodes of fictive locomotion induced by N-methyl-D,L-aspartate. When applied on quiescent preparations, SP induced a slow rhythmic activity (period >30 s). During fictive locomotion, SP increased both the locomotor frequency and the duration of the bursts of cyclic activity. Furthermore, SP stabilised the locomotor rhythm. These results demonstrate that SP is able to modulate both the "clock" and the pattern generator for locomotion.

The present study was designed to compare the firing profiles exhibited by lumbar flexor or extensor motoneurons in response to injection of depolarizing/repolarizing currents. Motoneurons were recorded intracellularly in the in vitro brainstem-spinal cord of newborn rats (P4 -P7). They were synaptically isolated and identified by antidromic stimulations of the central stump of flexor or extensor muscle nerves: tibialis anterior (ankle flexor) and gastrocnemius medialis or lateralis (ankle extensors). Two protocols were applied to establish the four firing profiles previously described (type I-IV) (Bennett et al., 2001): (1) symmetric depolarizing/repolarizing ramps of current and (2) progressive steps of depolarizing currents followed by equivalent steps of repolarizing current. According to such profiles, this study clearly shows that flexor and extensor motoneurons are different. The whole population of flexor motoneurons solely exhibited the type II profile, characterized by a frequency-current (F-I) relationship with a clockwise hysteresis. In contrast, in addition to this type II profile, the other three profiles of repetitive firing (type I, III and IV) were observed in extensor motoneurons; a linear F-I relationship (type I profile), a self-sustained discharge pattern together with a linear F-I relationship (type III profile) and a self-sustained firing pattern together with an F-I relationship showing a counter-clockwise hysteresis (type IV profile). Thus, during the early postnatal development, a significant part of the population of extensor motoneurons, but not flexors, are able to produce selfsustained discharges known to involve the activation of persistent inward currents.

The biogenic amine serotonin has been described in the literature as a powerful modulator of the spinal central pattern generator for locomotion. In the present study, we tested whether administration of serotonin or its agonist quipazine could restore motor activity in a model of paraplegia. One to three weeks after a complete transection of the spinal cord at a low thoracic level, rats were given either intrathecal injections of serotonin (5 mM, 15 microL) or intraperitoneal injections of quipazine (400-600 microg/kg). Both treatments allowed recovery of locomotor activity on a treadmill in response to tail pinching. As compared with the activity elicited before treatment, the locomotor activity produced by spinal animals was characterised by longer locomotor sequences with a larger number of successive steps, better body support, better interlimb coordination, and a higher amplitude of electromyographic bursts. These results suggest that serotonergic drugs could be used for the recovery of motor functions after lesions of the spinal cord.

1. In lamprey, stretch receptor neurons (SRNs), also referred to as edge cells, are located along the lateral margin of the spinal cord. They sense the lateral movements occurring in each swim cycle during locomotion. The isolated lamprey spinal cord in vitro was used to investigate the activity of SRNs during fictive locomotion induced by bath-applied N-methyl-D-aspartate (NMDA). Intracellular recordings with potassium acetate filled electrodes showed that 63% of SRNs had a clear locomotor-related modulation of their membrane potential. 2. Of the modulated SRNs, two-thirds had periods of alternating excitation and inhibition occurring during the ipsilateral and the contralateral ventral root bursts, respectively. The phasic hyperpolarization could be reversed into a depolarizing phase after the injection of chloride ions into the cells; this revealed a chloride-dependent synaptic drive. The remaining modulated SRNs were inhibited phasically during ipsilateral motor activity. 3. Experiments with barriers partitioning the recording chamber with the spinal cord into three pools, allowed an inactivation of the locomotor networks within one pool by washing out NMDA from the pool in which the SRN was recorded. This resulted in a marked reduction, but not an abolishment, of the amplitude of the membrane potential oscillations. Both the excitatory and the inhibitory phases were reduced, resulting from removal of input from inhibitory and excitatory interneurons projecting from the adjacent pools. If the glycine receptor antagonist strychnine (1 microM) was applied in one pool, the phasic hyperpolarizing phase disappeared without affecting the excitatory phase. 4. Bath application of the gamma-aminobutyric acid (GABA)A receptor antagonist, bicuculline (50-100 microM) blocked the spontaneous large unitary inhibitory postsynaptic potentials, which occurred without a clear phasic pattern. Bicuculline had no significant effect on the peak to peak amplitude of the locomotor-related membrane potential oscillations. The inhibition in SRNs therefore has a dual origin: glycinergic interneurons provide phasic inhibition, while the GABA system can exert a tonic inhibition via GABAA receptors. 5. These data show that, in addition to the stretch-evoked excitation, which SRNs receive during each locomotor cycle, most of them also receive excitation from the central pattern generator network during the ipsilateral contraction, which may ensure a maintained high level of sensitivity to stretch during the shortening phase of the locomotor cycle. This arrangement is analogous to the efferent control of muscle spindles exerted by gamma-motoneurons in mammals, which as a rule are coactivated with alpha-motoneurons to the same muscle (alpha-gamma linkage).