Mouse spinal cord in cell culture. II. Synaptic activity and circuit behavior

Neurons in cell cultures of fetal mouse spinal cord (SC) and dorsal root ganglia (DRG) develop extensive synaptic interconnections. No spontaneous synaptic activity was detectable in the presence of tetrodotoxin or an elevated magnesium ion concentration, but statistical analysis of evoked excitatory postsynaptic potentials (EPSPs) indicated that the quantal size was 200-250 μV, which was below the noise level of the recording system used. In a sample of eight DRG-SC and seven SC-SC cell pairs linked by EPSPs, the quantal content of the SC-SC EPSPs was about 3.5-fold larger than for the DRG-SC EPSPs. The extrapolated equilibrium potential for the SC-SC EPSP was about 20 mV positive. The IPSP reversed at a membrane potential of 60-80 mV negative. Some examples of the types of synaptic circuits commonly encountered are given. Only one case of electrical coupling between neurons was found.