Development of spontaneous synaptic activity in culture of the chick spinal cord neurons

Changes on the properties of glycine receptors during neuronal development

Glycine receptors (GlyRs) play a major role in the excitability of spinal cord and brain stem neurons. During development, several properties of these receptors undergo significant changes resulting in major modifications of their physiological functions. For example, the receptor structure switches from a monomeric alpha or heteromeric alpha 2 beta in immature neurons to an alpha 1 beta receptor type in mature neurons. Together with these changes in receptor subunits, the postsynaptic cluster size increases with development. Parallel to these modifications, the apparent receptor affinity to glycine and strychnine, as well as that of Zn(2+) and ethanol increases with time. The mature receptor is characterized by a slow desensitizing current and high sensitivity to modulation by protein kinase C. Also, the high level of glycinergic transmission in immature spinal neurons modulates neuronal excitability causing membrane depolarization and changes in intracellular calcium. Due to these properties, chronic inhibition of glycinergic transmission affects neurite outgrowth and produces changes in the level of synaptic transmission induced by GABA(A) and AMPA receptors. Finally, the high level of plasticity found in immature GlyRs is likely associated to changes in cytoskeleton dynamics.

Early expression of glycine and GABA(A) receptors in developing spinal cord neurons. Effects on neurite outgrowth

Using fluorometric and immunocytochemical techniques, we found that high glycine concentrations or blockade of glycine receptors increases neurite outgrowth in developing mouse spinal cord neurons. Glycine- and GABA(A)-activated currents were demonstrated during applications of glycine and GABA (50-100 microM) in 5 days in vitro (DIV) neurons. Long application (> or =10 min) of 100 microM glycine desensitized the membrane response by more than 95%. Application of glutamate in the absence of external Mg(2+), at several membrane potentials, did not produce any detectable membrane response in these cells. Immunocytochemical studies with NR1 and GluR1 antibodies showed a delayed appearance of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors respectively. Spontaneous synaptic activity was readily observed in 5 DIV neurons. The use of various receptor antagonists (strychnine, bicuculline, DL-2-amino-5-phosphonovalerate [APV], 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX]) revealed that this activity was predominantly glycinergic, and to a smaller extent, GABAergic. In the presence of bicuculline, APV and CNQX, we detected abundant spontaneous depolarizing potentials which often reached the action potential threshold. Further evidence for functional synaptic activity was provided by the detection of co-localization of gephyrin and synaptophysin at 5 DIV using confocal microscopy. Fluorometric studies with Fluo-3, a Ca(2+) indicator, in 5 DIV cultures showed the presence of spontaneous fluctuations associated with tetrodotoxin-sensitive synaptic events. The number of neurons displaying these fluctuations was significantly increased (>100%) when the cells were bathed in a strychnine-containing solution. On the other hand, these synaptically mediated Ca(2+) events were blocked by the co-application of strychnine and bicuculline. This suggests that glycine and GABA(A) receptors provide a fundamental regulation of both neuronal excitability and intracellular Ca(2+) at this early time of development.The neurotrophic effects of agonists and antagonists for glycine, GABA(A) and glutamate receptors were examined in neurons cultured for 2 or 5 DIV. From all the agonists used, only high concentrations of glycine increased neurite outgrowth in 5 DIV neurons. We found that strychnine also increased neurite outgrowth, whereas tetrodotoxin (1 microM), nimodipine (4 microM) and bicuculline (20 microM) completely blocked it. On the other hand, APV (50 microM) and CNQX (20 microM) were unable to affect neurite outgrowth. These data suggest that spinal glycine receptors depress neurite outgrowth by shunting neuronal excitability. Outgrowth induction possibly results from the enhanced activity found after the inhibition of glycinergic activity. We postulate that this resets the intracellular calcium at a concentration that favors neurite outgrowth.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Spontaneous motoneuronal activity mediated by glycine and GABA in the spinal cord of rat fetuses in vitro

1. Spontaneous motoneuronal activity was monitored from the lumbar ventral roots in an isolated spinal cord preparation from rat fetuses at embryonic days (E) 13.5-18.5. 2. Spontaneous bursts that were synchronized in both left and right ventral roots were observed periodically (mean interval, 1.5-2.6 min) from E14.5 to 17.5. This activity was abolished in Ca(2+)-free saline or by application of tetrodotoxin (1 microM), indicating that it was synaptically mediated. 3. The glutamate receptor blocker kynurenate (4 mM) failed to block spontaneous bursts at E14.5-15.5, though it completely abolished them at E17.5. The glycine receptor antagonist strychnine (10 microM) completely blocked spontaneous bursts at E14.5-15.5. Bicuculline, a GABAA receptor antagonist, reduced the amplitude of the spontaneous bursts. 4. At E15.5, a brief application of glycine (250 microM to 2 mM) evoked excitatory responses resembling the spontaneous bursts in both time course and amplitude. Such glycine-induced responses were not observed under Ca(2+)-free conditions, suggesting that they were synaptically evoked. These synaptic responses were not blocked by kynurenate (4 mM), but they were abolished by strychnine (10 microM). 5. It is concluded that glycine and GABA generate the earliest spontaneous motor activity of the fetus and function transiently as excitatory transmitters in the embryonic spinal cord.