Astrocytic contribution to functioning synapse formation estimated by spontaneous neuronal intracellular Ca²⁺ oscillations

Glial contribution to in vitro synaptic function was investigated in a neuron-glia co-culture system by monitoring spontaneous oscillations of intracellular Ca²⁺ in neurons. Rat cortical neurons, grown stably on a cortical astrocyte monolayer, extended neurites resulting in marked functional synapse formation. Little synapse formation was observed in neuronal co-culture with meaningeal fibroblasts or endothelial cells. Aged astrocytes in vitro (C35) were found to attenuate synaptic development, while young astrocytes (C5) markedly promoted synaptic function. C5 and C35 astrocyte media conditioned yielded no significant synaptogenic effect, indicating diffusible factor(s) are not responsible for our observation. Modulation of astrocytic proliferation and differentiation by gliostatin, a glial growth inhibitor, or dibutyryl cAMP affected neuronal synaptic function on the co-cultures. Site-specific analysis in homologous and heterologous neuron-astrocyte co-cultures among cortex, hippocampus, septum, and striatum revealed that homologous combinations of neurons and astrocytes derived from identical brain regions elicited the largest number of synchronizing neur̀ons. Thee results suggest that in vivo neuronal synaptic function essentially requires the participation of adjacent astrocytes, which is site-specific and age-dependent.