Descending 5-Hydroxytryptamine Raphe Inputs Repress the Expression of Serotonergic Neurons and Slow the Maturation of Inhibitory Systems in Mouse Embryonic Spinal Cord

Abstract

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...

ERG Conductance Expression Modulates the Excitability of Ventral Horn GABAergic Interneurons That Control Rhythmic Oscillations in the Developing Mouse Spinal Cord

“…3). This observation is supported by previous studies of the developmental evolution of spinal network activity (Branchereau et al, 2002;Whelan 2003).…”

“…Among them, an essential role is played by GABA-releasing interneurons that control wiring and network excitability within a restricted spatiotemporal window preceding birth (Caserta and Barker, 1994;Somogyi et al, 1995;Nishimaru et al, 1996;Gao et al, 2001;Nakayama et al, 2002;Allain et al, 2004Allain et al, , 2006. During embryogenesis, the role played by GABAergic neurons is finely tuned by activity-dependent programs that control GABA phenotype expression and repression in accordance with network operation (Feldblum et al, 1998;Branchereau et al, 2002). The action of GABA during development is often excitatory until shortly before birth, when locomotor networks acquire the ability to generate alternating motor commands between flexor and extensor motoneurons.…”

“…The action of GABA during development is often excitatory until shortly before birth, when locomotor networks acquire the ability to generate alternating motor commands between flexor and extensor motoneurons. At this late stage of prenatal development, GABA-mediated excitation is replaced by synaptic inhibition mediated by glycine and/or GABA (Barbeau et al, 1999;Gao et al, 2001;Branchereau et al, 2002;Hanson and Landmesser, 2003), with surviving GABAergic neurons predominantly located in the dorsal horn (Phelps et al, 1999;Tran et al, 2003). Given the importance of GABA-dependent depolarizations as developmental signals (Ben-Ari, 2002;Owens and Kriegstein, 2002;Alvarez et al, 2005), it is necessary to clarify the processes regulating the embryonic GABAergic phenotype.…”

Serotonin decreases generation of dopaminergic neurons from mesencephalic precursors via serotonin type 7 and type 4 receptors

“…Serotonin (5-HT) is involved in early embryogenesis, even before neurons appear (Loric et al, 1995;Walther and Bader, 1999). 5-HT is one of the first transmitters that appears during development, and is involved in proliferation and differentiation of 5-HT neurons, as well as many growth events in their target areas Buznikov et al, 2001;Branchereau et al, 2002;Suppress Pflieger et al, 2002). Data on the role of 5-HT in the differentiation and growth of dopamine (DA) neurons are contradictory (e.g., both stimulatory and inhibitory effects have been reported; Lauder et al, 1985;Liu and Lauder, 1992a,b;Witaker-Azmitia, 2001).…”

“…However, the mechanisms involved in the inhibitory effects of serotonin on generation of DA neurons from proliferating neurospheres of mesencephalic precursors have not been clarified. Serotonin interacts with seven families of receptors with different and sometimes opposing functions in a variety of cellular and developmental processes (Mouillet-Richard et al, 2000;Branchereau et al, 2002;Rumajogee et al, 2004). It is known that 5-HT receptors are functional and capable of playing a role in neuronal development from very early stages of ontogenesis (Walther and Bader, 1999;Nguyen et al, 2001), although little is known about the receptors and mechanisms involved.…”