PASSIVE AND SYNAPTIC PROPERTIES OF HIPPOCAMPAL-NEURONS GROWN IN MICROCULTURES AND IN MASS-CULTURES

1. We used whole cell recordings to compare passive membrane propertie s and synaptic properties of postnatal rat hippocampal neurons grown f or 7-15 days in either conventional mass cultures or on physically res tricted microisland cultures. Despite matching microisland and mass cu lture cells across several variables, there were significant differenc es between neurons in the two groups regarding passive membrane charac teristics and synaptic proper ties. 2. Microisland neurons displayed s ignificantly faster charging of the membrane capacitance than mass cul ture counterparts matched with microisland neurons for age, somal diam eter, and transmitter phenotype. When we used a two-compartment equiva lent circuit model to quantify this result, microisland neurons displa yed approximately half the distal capacitance of mass culture neurons. These data suggest that microisland neurons elaborate less extensive neuritic arborizations than mass culture neurons. 3. Evoked synaptic r esponses were enhanced on microislands compared with mass cultures. Ex citatory and inhibitory autaptic currents were more frequent and displ ayed larger amplitudes on single-neuron microislands than in matched m ass culture neurons. 4. In recordings from pairs of neurons in the two environments. we observed a significantly higher probability of obtai ning a monosynaptic response on two-neuron microislands than in matche d mass culture pairs (85% vs. 42%). Evoked excitatory postsynaptic cur rents were also significantly larger in the microisland environment. w ith evoked excitatory synaptic currents from two-neuron microislands e xhibiting a mean amplitude 20-fold larger than mass culture monosynapt ic responses. 5. The differences in evoked synaptic responses were not reflected in differences in the amplitude or frequency of spontaneous miniature excitatory postsynaptic currents (mEPSCs). Analysis of mEPS C rise times, decay times, and peak amplitudes within individual cells suggests that electrotonic filtering is not an important contributor to the variability of peak amplitudes and decay times of synaptic curr ents in cells of either culture environment. However, composite data a cross neurons in both cultures reveal a significant correlation betwee n mEPSC rise and decay times. 6. Our results suggest that the microisl and preparation may be a useful tool for exploring factors that influe nce synapse formation and development. Additionally, the preparation i s a particularly convenient model for the study of single-neuron-media ted synaptic events.