Spinal circuits formation: a study of developmentally regulated markers in organotypic cultures of embryonic mouse spinal cord

Substantia Gelatinosa neurons in defined-medium organotypic slice culture are similar to those in acute slices from young adult rats

Peripheral nerve injury promotes an enduring increase in the excitability of the spinal dorsal horn. This change, that likely underlies the development of chronic pain, may be a consequence of prolonged exposure of dorsal horn neurons to mediators such as neurotrophins, cytokines, and neurotransmitters. The long-term effects of such mediators can be analyzed by applying them to spinal neurons in organotypic slice culture. To assess the validity of this approach, we established serum-free, defined-medium organotypic cultures (DMOTC) from E13-14 prenatal rats. Whole-cell recordings were made from neurons maintained in DMOTC for up to 42 days. These were compared with recordings from neurons of similar age in acute spinal cord slices from 15- to 45-day-old rats. Five cell types were defined in acute slices as 'Tonic', 'Irregular', 'Delay', 'Transient' or 'Phasic' according to their discharge patterns in response to depolarizing current. Although fewer 'Phasic' cells were found in cultures, the proportions of 'Tonic', 'Irregular', 'Delay', and 'Transient' were similar to those found in acute slices. GABAergic, glycinergic, and 'mixed' inhibition were observed in neurons in acute slices and DMOTC. Pure glycinergic inhibition was absent in 7d cultures but became more pronounced as cultures aged. This parallels the development of glycinergic inhibition in vivo. These and other findings suggest that fundamental developmental processes related to neurotransmitter phenotype and neuronal firing properties are preserved in DMOTC. This validates their use in evaluating the cellular mechanisms that may contribute to the development of chronic pain.

Early signs of motoneuron vulnerability in a disease model system: Characterization of transverse slice cultures of spinal cord isolated from embryonic ALS mice

Mutations in the SOD1 gene are associated with familial amyotrophic lateral sclerosis. The mechanisms by which these mutations lead to cell loss within the spinal cord ventral horns are unknown. In the present report we used the G93A transgenic mouse model of amyotrophic lateral sclerosis to develop and characterize an in vitro tool for the investigation of subtle alterations of spinal tissue prior to frank neuronal degeneration. To this aim, we developed organotypic slice cultures from wild type and G93A embryonic spinal cords. We combined immunocytochemistry and electron microscopy techniques to compare wild type and G93A spinal cord tissues after 14 days of growth under standard in vitro conditions. By SMI32 and choline acetyl transferase immunostaining, the distribution and morphology of motoneurons were compared in the two culture groups. Wild type and mutant cultures displayed no differences in the analyzed parameters as well as in the number of motoneurons. Similar results were observed when glial fibrillary acidic protein and myelin basic protein-positive cells were examined. Cell types within the G93A slice underwent maturation and slices could be maintained in culture for at least 3 weeks when prepared from embryos. Electron microscopy investigation confirmed the absence of early signs of mitochondria vacuolization or protein aggregate formation in G93A ventral horns. However, a significantly different ratio between inhibitory and excitatory synapses was present in G93A cultures, when compared with wild type ones, suggesting the expression of subtle synaptic dysfunction in G93A cultured tissue. When compared with controls, G93A motoneurons exhibited increased vulnerability to AMPA glutamate receptor-mediated excitotoxic stress prior to clear disease appearance. This in vitro disease model may thus represent a valuable tool to test early mechanisms contributing to motoneuron degeneration and potential therapeutic molecular interventions.

Interneurons transiently express the ERG K+ channels during development of mouse spinal networks in vitro

During spinal cord maturation neuronal excitability gradually differentiates to meet different functional demands. Spontaneous activity, appearing early during spinal development, is regulated by the expression pattern of ion channels in individual neurons. While emerging excitability of embryonic motoneurons has been widely investigated, little is known about that of spinal interneurons. Voltage-dependent K+ channels are a heterogeneous class of ion channels that accomplish several functions. Recently voltage-dependent K+ channels encoded by erg subfamily genes (ERG channels) were shown to modulate excitability in immature neurons of mouse and quail. We investigated the expression of ERG channels in immature spinal interneurons, using organotypic embryonic cultures of mouse spinal cord after 1 and 2 weeks of development in vitro. We report here that all the genes of the erg family known so far (erg1a, erg1b, erg2, erg3) are expressed in embryonic spinal cultures. We demonstrate for the first time that three ERG proteins (ERG1A, ERG2 and ERG3) are co-expressed in the same neuronal population, and display a spatio-temporal distribution in the spinal slices. ERG immuno-positive cells, representing mainly GABAergic interneurons, were present in large numbers at early stages of development, while declining later, with a ventral to dorsal gradient. Patch clamp recordings confirmed these data, showing that ventral interneurons expressed functional ERG currents only transiently. Similar expression of the erg genes was observed at comparable ages in vivo. The role of ERG currents in regulating neuronal excitability during the earliest phases of spinal circuitry development will be examined in future studies.

Lysolecithin induces demyelination in vitro in a cerebellar slice culture system

Demyelination is a hallmark of several human diseases, including multiple sclerosis. To understand better the process of demyelination and remyelination, we explored the use of an in vitro organotypic cerebellar slice culture system. Parasagittal slices of postnatal Day 10 (P10) rat cerebella cultured in vitro demonstrated significant myelination after 1 week in culture. Treatment of the cultures at 7 days in vitro (DIV) with the bioactive lipid lysolecithin (lysophosphatidylcholine) for 15-17 hr in vitro produced marked demyelination. This demyelination was observed by immunostaining for the myelin components myelin basic protein (MBP), myelin oligodendrocyte glycoprotein (MOG), and 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNPase). After a transient demyelinating insult with lysolecithin in vitro, the cultures recovered with oligodendrocyte differentiation recapitulating a normal time course; there was initially re-expression of CNPase and MBP during this recovery, and this was followed by MOG. In addition, there seemed to be some limited remyelination during the recovery phase. Lysolecithin thus induces demyelination in an in vitro organotypic cerebellar slice culture system, providing a model system for studying myelination, demyelination, and remyelination in vitro.

Interneurone bursts are spontaneously associated with muscle contractions only during early phases of mouse spinal network development: a study in organotypic cultures

For a short time during development immature circuits in the spinal cord and other parts of the central nervous system spontaneously generate synchronous patterns of rhythmic activity. In the case of the spinal cord, it is still unclear how strongly synchronized bursts generated by interneurones are associated with motoneurone firing and whether the progressive decline in spontaneous bursting during circuit maturation proceeds in parallel for motoneurone and interneurone networks. We used organotypic cocultures of spinal cord and skeletal muscle in order to investigate the ontogenic evolution of endogenous spinal network activity associated with the generation of coordinate muscle fibre contractions. A combination of multiunit electrophysiological recordings, videomicroscopy and optical flow computation allowed us to measure the correlation between interneurone firing and motoneurone outputs after 1, 2 and 3 weeks of in vitro development. We found that, in spinal organotypic slices, there is a developmental switch of spontaneous activity from stable bursting to random patterns after the first week in culture. Conversely, bursting recorded in the presence of strychnine and bicuculline became increasingly regular with time in vitro. The time course of spontaneous activity maturation in organotypic slices is similar to that previously reported for the spinal cord developing in utero. We also demonstrated that spontaneous bursts of interneurone action potentials strongly correlate with muscular contractions only during the first week in vitro and that this is due to the activation of motoneurones via AMPA-type glutamate receptors. These results indicate the occurrence in vitro of motor network development regulating bursting inputs from interneurones to motoneurones.