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Key points: In newborn rats, L‐DOPA increases the occurrence of air‐stepping activity without affecting movement characteristics.L‐DOPA administration increases the spinal content of dopamine in a dose‐dependent manner.Injection of 5‐HTP increases the spinal serotonin content but does not trigger air‐stepping.5‐HTP counteracts the pro‐locomotor action of L‐DOPA.Less dopamine and serotonin are synthesized when L‐DOPA and 5‐HTP are administered as a cocktail. The catecholamine precursor, L‐3,4‐dihydroxyphenylalanine (L‐DOPA), is a well‐established pharmacological agent for promoting locomotor action in vertebrates, including triggering air‐stepping activities in the neonatal rat. Serotonin is also a well‐known neuromodulator of the rodent spinal locomotor networks. Here, using kinematic analysis, we compared locomotor‐related activities expressed by newborn rats in response to varying doses of L‐DOPA and the serotonin precursor 5‐hydroxytryptophan (5‐HTP) administered separately or in combination. L‐DOPA alone triggered episodes of air‐stepping in a dose‐dependent manner (25–100 mg/kg), notably determining the duration of locomotor episodes, but without affecting step cycle frequency or amplitude. In contrast, 5‐HTP (25–150 mg/kg) was ineffective in instigating air‐stepping, but altered episode durations of L‐DOPA‐induced air‐stepping, and decreased locomotor cycle frequency. High performance liquid chromatography revealed that L‐DOPA, which was undetectable in control conditions, accumulated in a dose‐dependent manner in the lumbar spinal cord 30 min after its administration. This was paralleled by an increase in dopamine levels, whereas the spinal content of noradrenaline and serotonin remained unaffected. In the same way, the spinal levels of serotonin increased in parallel with the dose of 5‐HTP without affecting the levels of dopamine and noradrenaline. When both precursors are administrated, they counteract each other for the production of serotonin and dopamine. Our data thus indicate for the first time that both L‐DOPA and 5‐HTP exert opposing neuromodulatory actions on air‐stepping behaviour in the developing rat, and we speculate that competition for the production of dopamine and serotonin occurs when they are administered as a cocktail. [ABSTRACT FROM AUTHOR]

Key points: Stimulation of hindlimb afferent fibres can both stabilize and increase the activity of fore‐ and hindlimb motoneurons during fictive locomotion.The increase in motoneuron activity is at least partially due to the production of doublets of action potentials in a subpopulation of motoneurons.These results were obtained using an in vitro brainstem/spinal cord preparation of neonatal rat. Quadrupedal locomotion relies on a dynamic coordination between central pattern generators (CPGs) located in the cervical and lumbar spinal cord, and controlling the fore‐ and hindlimbs, respectively. It is assumed that this CPG interaction is achieved through separate closed‐loop processes involving propriospinal and sensory pathways. However, the functional consequences of a concomitant involvement of these different influences on the degree of coordination between the fore‐ and hindlimb CPGs is still largely unknown. Using an in vitro brainstem/spinal cord preparation of neonatal rat, we found that rhythmic, bilaterally alternating stimulation of hindlimb sensory input pathways elicited coordinated hindlimb and forelimb CPG activity. During pharmacologically induced fictive locomotion, lumbar dorsal root (DR) stimulation entrained and stabilized an ongoing cervico‐lumbar locomotor‐like rhythm and increased the amplitude of both lumbar and cervical ventral root bursting. The increase in cervical burst amplitudes was correlated with the occurrence of doublet action potential firing in a subpopulation of motoneurons, enabling the latter to transition between low and high frequency discharge according to the intensity of DR stimulation. Moreover, our data revealed that propriospinal and sensory pathways act synergistically to strengthen cervico‐lumbar interactions. Indeed, split‐bath experiments showed that fully coordinated cervico‐lumbar fictive locomotion was induced by combining pharmacological stimulation of either the lumbar or cervical CPGs with lumbar DR stimulation. This study thus highlights the powerful interactions between sensory and propriospinal pathways which serve to ensure the coupling of the fore‐ and hindlimb CPGs for effective quadrupedal locomotion. [ABSTRACT FROM AUTHOR]