Monocytic cells secrete factors that evoke excitatory synaptic activity inrat hippocampal cultures

Microglia- and macrophage-induced neuronal death may underlie a number of n eurodegenerative diseases. The effects of factors secreted by monocytic cel ls were studied on glutamatergic synaptic transmission between cultured rat hippocampal neurons. Conditioned media from differentiated human U937 cell s was collected after 24 h and applied to neurons (0.5%-30% dilution). Unac tivated U937 cells spontaneously released factors that when applied to neur onal cultures evoked bursts of action potentials and elicited neuronal deat h (29 +/- 4%). Conditioned media collected from U937 cells evoked intracell ular calcium ([Ca2+](i)) spiking (0.5%-2% dilution) and at higher concentra tions evoked sustained increases in intracellular calcium (3%-30% dilution) , as measured by indo-1-based photometry in single neurons. Activation of t he U937 cells with zymosin A (500 mu g/ml) enhanced the potency of the cond itioned media to increase intraneuronal [Ca2+](i) as indicated by a leftwar d shift in the concentration-response curve. Selective antagonists to volta ge-gated Na+ and Ca2+ channels and NMDA-gated channels (tetrodotoxin, nimod ipine, and (+/-)-2-amino-5-phosphonopentanoic acid, respectively) blocked t he calcium transients elicited by unactivated and zymosin-A-treated conditi oned media. This pharmacologic profile is consistent with U937-released fac tors that excite the synaptic network that forms between cultured hippocamp al neurons. (C) 1999 Elsevier Science B.V. All rights reserved.