Activity‐dependent modulation of GABAergic synapses in developing rat spinal networks in vitro

Abstract

The role of activity-dependent plasticity in modulating inhibitory synapses was investigated in embryonic rat spinal cord slice cultures, by chronic exposure to non-NMDA receptor blockers. GABAergic synaptic efficacy in control and chronic-treated cultures was investigated by patch-recordings from visually identified spinal interneurons. In both culture groups proximal stimulation induced the appearance of postsynaptic currents (PSCs), which were fully antagonized by 20 microM bicuculline application and reverse polarity at potential values close to those reported for spontaneous GABAergic PSCs. In chronically treated cells GABAergic evoked PSCs displayed a larger failure rate and a smaller coefficient of variation of mean PSC amplitude, when compared to controls. As opposed to controls, chronic GABAergic evoked PSCs did not facilitate upon paired-pulse stimulation. Facilitation at chronic synapses was observed when extracellular calcium levels were decreased below physiological values (< 2 mM). Kainate was used to disclose any functional differences between control and treated slices. In accordance with the presynaptic action of kainate, the application of this drug along with GYKI, an AMPA receptor selective antagonist, changed, with analogous potency, short-term plasticity of GABAergic synapses from control and treated cultures. Nevertheless, in chronic cultures, the downstream effects of such activation unmasked short-term depression. Ultrastructural analysis of synapses in chronically treated cultures showed a reduction both in symmetric synapses and in the number of vesicles at symmetric terminals. Thus, based on electrophysiological and ultrastructural data, it could be suggested that during the development of spinal circuits, GABAergic synapses are modulated by glutamatergic transmission, and thus implying that excitatory transmission regulates the strength of GABAergic synapses.

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

“…The pH was adjusted to 7.4 with NaOH (osmolarity, 305 mOsm).Current-clamp recordings in the whole-cell configuration (patch-clamp) were obtained at room temperature (RT) (22 Ϯ 2°C) from visually identified ventral spinal neurons with pipettes (4 -6 M⍀) filled with a solution of the following composition (in mM): 120 K-gluconate, 20 KCl, 10 HEPES, 10 EGTA, 2 MgCl 2 , and 2 Na 2 ATP; pH 7.35 with KOH. In accordance with previous investigations (Streit et al, 1991;Ballerini and Galante, 1998;Ballerini et al, 1999;Galante et al, 2000;Rosato-Siri et al, 2002;Avossa et al, 2003;Furlan et al, 2005), neurons were empirically termed interneurons when their cell body diameter was between 10 and 20 m, thus making them clearly distinguishable from cells morphologically resembling motoneurons (30 -60 m diameter) and immunoreactive to the motoneuron marker SMI32 (supplemental Fig. 2, available at www.jneurosci.org as supplemental material).Unlike interneurons, motoneurons generated long processes that often terminated on the muscle fibers distributed laterally to the slice (Ballerini et al, 1999;Avossa et al, 2003).…”

Regulation of Gephyrin Cluster Size and Inhibitory Synaptic Currents on Renshaw Cells by Motor Axon Excitatory Inputs

“…Conversely, decreased activity in neocortical cultures reduced postsynaptic mIPSC amplitudes and GABA A receptor immunofluorescence (Kilman et al, 2002). Similarly, alterations of network activity in embryonic or neonatal spinal cords homeostatically regulated the strength of GABAergic synapses (Chub and O'Donovan, 1998;Rosato-Siri et al, 2002). At Renshaw cell mixed inhibitory synapses, the action of excitatory activity was more pronounced on the glycinergic component compared with GABAergic currents, and it was accompanied by increased gephyrin clustering.…”

Bidirectional homeostatic plasticity induced by interneuron cell death and transplantation in vivo

“…Recent studies have begun to reveal the underlying molecular mechanisms of homeostatic synaptic changes, including the AMPA receptor subunits, synapse-associated calcium-binding proteins, and intracellular signaling cascades involved (14,17,18). Changes to activity also trigger homeostatic plasticity of inhibitory synaptic transmission (19)(20)(21)(22)(23). Homozygous deletion of glutamate decarboxylase 1 (Gad1), the rate-limiting enzyme in the synthesis of GABA, reduced miniature inhibitory postsynaptic current (mIPSC) amplitudes in cultured hippocampal neurons but also blocked further homeostatic changes to mIPSCs.…”