Monocytic cells secrete factors that evoke excitatory synaptic activity in rat hippocampal cultures.

Microglia- and macrophage-induced neuronal death may underlie a number of neurodegenerative diseases. The effects of factors secreted by monocytic cells were studied on glutamatergic synaptic transmission between cultured rat hippocampal neurons. Conditioned media from differentiated human U937 cells was collected after 24 h and applied to neurons (0.5%-30% dilution). Unactivated U937 cells spontaneously released factors that when applied to neuronal cultures evoked bursts of action potentials and elicited neuronal death (29+/-4%). Conditioned media collected from U937 cells evoked intracellular calcium ([Ca(2+)](i)) spiking (0.5%-2% dilution) and at higher concentrations evoked sustained increases in intracellular calcium (3%-30% dilution), as measured by indo-1-based photometry in single neurons. Activation of the U937 cells with zymosin A (500 microg/ml) enhanced the potency of the conditioned media to increase intraneuronal [Ca(2+)](i) as indicated by a leftward shift in the concentration-response curve. Selective antagonists to voltage-gated Na(+) and Ca(2+) channels and NMDA-gated channels (tetrodotoxin, nimodipine, and (+/-)-2-amino-5-phosphonopentanoic acid, respectively) blocked the calcium transients elicited by unactivated and zymosin -A-treated conditioned media. This pharmacologic profile is consistent with U937-released factors that excite the synaptic network that forms between cultured hippocampal neurons.