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The classical GABA/glycine hyperpolarizing inhibition is not observed in the immature spinal cord. GABA A and glycine receptors are anions channels and the efficacy of inhibitory transmission in the spinal cord is largely determined by the gradient between intracellular and extracellular chloride concentrations. The concentration of intracellular chloride in neurons is mainly regulated by two cation–chloride cotransporters, the potassium–chloride cotransporter 2 (KCC2) and the sodium–potassium–chloride co-transporter 1 (NKCC1). In this study, we measured the reversal potential of IPSPs (E IPSP ) of lumbar motoneurons during the first postnatal week and we investigated the expression of KCC2 and NKCC1 in the ventral horn of the spinal cord from the embryonic day 17 to the postnatal day 20 in the rat. Our results suggest that the negative shift of E IPSP from above to below the resting membrane potential occurs during the first postnatal week when the expression of KCC2 increases significantly and the expression of NKCC1 decreases. KCC2 immunolabeling surrounded motoneurons, presumably in the plasma membrane and NKCC1 immunolabeling appeared outside this KCC2-labeled fine strip. Taken together, the present results indicate that maturation of chloride homeostasis is not completed at birth in the rat and that the upregulation of KCC2 plays a key role in the shift from depolarizing to hyperpolarizing IPSPs.

The in vitro brainstem-spinal cord preparation of newborn rats (0 to six-days-old) was used to investigate the development of pathways descending ventrally from the brainstem, which are important for the control of posture and locomotion. The ventral funiculus of the spinal cord was stimulated at the cervical (C1) level. Responses were recorded at the lumbar level from either motoneurons or ventral roots using intracellular microelectrodes or suction electrodes, respectively. Responses consisted of a pure excitation lasting 15 ms, followed by mixed excitatory/inhibitory responses. The inhibition was, at least partly, mediated by glycine. Excitatory amino acid transmission appears to be responsible for the excitation. The characteristics of the ventral funiculus-evoked postsynaptic potentials and ventral root potentials changed significantly with age. Their latency decreased whereas the slope and the area, measured over the first 15 ms, increased. The increase of the ventral funiculus input to motoneurons was slightly more pronounced than that of the monosynaptic dorsal root-evoked potentials from day 0 to day 4. These data suggest a gradual arrival of ventral descending axons in the lumbar enlargement which may be responsible for the gradual acquisition of postural control that takes place during the first days after birth. This is a prerequisite for the development of the adult pattern of quadrupedal locomotion, with elevated trunk.

The in vitro brain stem–spinal cord preparation of neonatal (0- to five-day-old) rats was used to establish whether pathways descending from the brain stem are capable of modulating synaptic transmission from primary afferents to lumbar motoneurons within the first few days after birth. We stimulated the ventral funiculus of the spinal cord at the cervical (C1–C2) level. Single-pulse stimulations evoked both excitatory and inhibitory postsynaptic potentials in ipsilateral lumbar (L2–L5) motoneurons which were recorded intracellularly. Twin-pulse stimulations evoked bursts of action potentials in ventral roots. The amplitude of the monosynaptic dorsal root-evoked excitatory postsynaptic potential decreased when a conditioning stimulation was applied to the ventral funiculus 50–300 ms prior to the stimulation of the ipsilateral dorsal root. A decreased input resistance of the motoneurons during the early part (25–100 ms after the artifact) of the ventral funiculus-evoked postsynaptic potentials could account, at least partly, for the decreased amplitude of the dorsal root-evoked response. However, the duration of the inhibition of the dorsal root-evoked excitatory postsynaptic potential was longer than that of the decrease in input resistance. Ventral funiculus stimulation evoked antidromic discharges in dorsal roots. Recordings of dorsal root potentials showed that these discharges were generated by the underlying afferent terminal depolarizations reaching firing threshold. The dorsal root discharge overlapped with most of the time-course of the ventral funiculus-evoked inhibition of the response to dorsal root stimulation, suggesting that part of this inhibition may be exerted at a presynaptic level. The number of antidromic action potentials evoked in dorsal roots by ventral funiculus stimulation increased significantly in saline solution with chloride concentration reduced to 50% of control. Bursts of action potentials disappeared when chloride was removed completely. Antidromic discharges were therefore due to chloride conductance. The number of action potentials evoked in ventral roots was increased in low-chloride saline solutions. Removing chloride from the bathing solution resulted in an unstable ventral root activity. Bath application of the GABA A receptor antagonist, bicuculline (5–10 μ M), blocked the ventral funiculus-evoked antidromic discharges in the dorsal roots. The increase in chloride conductance which generated the depolarizations underlying the dorsal root discharges was therefore mediated by an activation of GABA A receptors. In contrast, bursts of action potentials in the ventral roots were increased in both amplitude and duration under bicuculline. Our data demonstrate that pathways running in the ventral funiculus of the spinal cord exert a control on interneurons mediating presynaptic inhibition at birth.

Charcot–Marie–Tooth disease type 1A is the most frequent hereditary neuropathy affecting the peripheral nervous system. A partial duplication of chromosome 17 (17p11.2) involving the PMP22 gene is responsible for dysmyelination–demyelination processes leading to motor and sensory impairments. Murine models of this disease are now widely used to investigate the mechanisms occurring at the behavioural and physiological levels. In this study, adult transgenic mice (6 months old) having integrated 7 copies of the human PMP22 gene were used to compare the motor performance, evaluated by using a complex locomotor test (the rotarod test), with both the number of functional motoneurons innervating the soleus muscle and the level of myelination in the sciatic nerve. Two levels of motor deficits were detected and led us to divide the population into two subgroups. In both impaired groups, the level of motor deficit was strongly correlated with the number of functional motoneurons evaluated by retrograde labeling from the muscle, but not with the number of myelinated fibers or the thickness of the myelin sheath (g-ratio). It therefore appears that the number of motor units may be a key element in motor impairments observed in Charcot–Marie–Tooth disease type 1A disease. These findings may have implications for therapeutic procedures, which should focus on the survival of the motoneuronal pool and/or the maintenance of functional neuro-muscular connexions to reduce motor impairments in humans.