Spontaneous synchronous rhythmic activities are a common feature of immature neuronal networks. Although the mechanisms underlying such activities have been studied extensively, whether they might be controlled by modulatory information remains questionable. Here, we investigated the role of descending serotonergic (5-HT) inputs from the medulla to the spinal cord in the maturation of rhythmic activity. We found that in spinal cords maintained, as a whole, in organotypic culture without the medulla, the maturation of spontaneous activity is similar to that found in spinal cords developed in utero. Interestingly, in organotypic cultures without the medulla (i.e., devoid of descending inputs), numerous intraspinal neurons expressed 5-HT, unlike in spinal cords cultivated in the presence of the medulla or matured in utero. We demonstrated that this 5-HT expression was specifically dependent on the absence of 5-HT fibers and was repressed by 5-HT itself via activation of 5-HT 1A receptors. Finally, to verify whether the expression of 5-HT intraspinal neurons could compensate for the lack of descending 5-HT fibers and play a role in the development of spontaneous activity, we blocked the 5-HT synthesis using p-chlorophenylalanine methyl ester in cultures devoid of the medulla. Surprisingly, we found that this pharmacological treatment did not prevent the development of spontaneous activity but accelerated the maturation of intraspinal inhibition at the studied stages. Together, our data indicate that descending 5-HT raphe inputs (1) repress the expression of spinal serotonergic neurons and (2) slow the maturation of inhibitory systems in mouse spinal cord. Key words: neuronal phenotype; development; modulatory neurons; serotonin; disinhibition; GABA; glycine; neural networksEarly in development, various parts of the CNS express spontaneous synchronous rhythmic activity involving large ensembles of neurons (O'Donovan, 1999). For example, such activity has been found in the retina (Wong et al., 1995), brainstem (Fortin et al., 1999), hippocampus (Garaschuk et al., 1998), cochlear ganglion (Jones et al., 2001), auditory cortex (Lippe, 1994), thalamus (Itaya et al., 1995), and spinal cord (Hamburger and Balaban, 1963;Bekoff, 1976;O'Donovan and Landmesser, 1987;Nishimaru et al., 1996). In the rat spinal cord, spontaneous rhythmic activity has been found from embryonic day 13.5 (E13.5) to E18.5 (Nakayama et al., 1999) as mediated by glutamatergic and GABA-glycinergic synaptic transmission (Nishimaru et al., 1996). In the chick embryonic spinal cord, cholinergic, glutamatergic, and GABAergic synaptic transmission has also been described as mediating the generation of spontaneous bursts of activity (Milner and Landmesser, 1999). More recently, a large body of studies has focused on mechanisms by which spontaneous activity is generated (Tabak et al., 2000;Chub and O'Donovan, 2001). In contrast, how these spontaneous activities are modulated by extrinsic modulatory inputs and how these modulatory inputs contribute to the ontog...

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Rat myotubes have a resting [Ca2+]i of about 82 nM. Myotubes 3-5 days old (quiescent myotubes) display electrically induced and spontaneous transients in the intracellular concentration of free Ca2+ ions ([Ca2+]i) uncoupled to any detectable contraction. By contrast, 1- to 2-day-old myotubes are insensitive to electrical stimuli and, after 6 days in culture, stimulated myotubes always show [Ca2+]i transients and twitch contractions. The spatial distribution of [Ca2+]i variations in quiescent myotubes is heterogeneous, local increases in [Ca2+]i being mainly observed near the periphery of the cell. The small effect of different external Ca2+ concentrations and of Cd2+ on the amplitude of the [Ca2+]i oscillation indicates that the main source of Ca2+ may be the sarcoplasmic reticulum. This conclusion is supported by the close similarity between electrically induced and caffeine-induced [Ca2+]i maps. These findings suggest that, at an early stage of myotube ontogenesis, a part of the excitation/contraction coupling, as membrane ionic channels, voltage sensors and Ca2+ release and reuptake mechanisms, is functional but, apparently, still uncoupled to the contractile machinery.