The presence of glia stimulates the appearance of glycinergic synaptic transmission in spinal cord neurons

Previous studies, using electrophysiological and fluorimetric analysis with a calcium sensitive dye, have shown that 5-7 DIV developing spinal cord neurons displayed high levels of glycinergic transmission. GABAergic and AMPAergic neurotransmission contributed much less to the overall transmission. Here, we show that culturing neurons in absence of a glia cell monolayer reduced the frequency of glycinergic spontaneous IPSCs (0.1 ± 0.01 Hz), without altering the level of overall transmission (3 ± 1.1 Hz). The predominant transmission was mediated by GABAA receptors (72% of total synaptic events). In addition, combination of bicuculline and CNQX blocked synaptically mediated calcium transients recorded with fluo-3. Furthermore, application of glycine revealed the presence of extrasynaptic receptors in these neurons (25 ± 6 pA/pF). Culturing neurons on a glial cell monolayer increased the frequency of glycinergic currents (0.4 ± 0.02 Hz), without changing the amplitude of the current (20 ± 4 pA). The use of a glia-conditioned media reversed the effect of growing the neurons in a glia-deprived condition. These results indicate that the establishment of glycinergic transmission is dependent on the presence of a glia derived soluble factor. However, functional GlyRs were still able to insert in the neuronal membrane in a glia-independent manner.