Patch-clamp recording of synaptic currents in acute and organotypic rat brain slices has been commonly used for studying synaptic physiology over the last decade. We have thus compared the features of spontaneous synaptic activity and the morphology of neurones in the hippocampal CA1 region in these two preparations, over the first 3 weeks of development.

We found that the number of spines and the frequency of spontaneous activity increased dramatically from P14 (postnatal day 14) to P21 in acute slices. Moreover, changes in morphology and spontaneous synaptic activity in acute slices, over this period, are fundamentally similar to those in organotypic slices DIV7 (7 days in vitro) to DIV21. (Acute slices, n = 12, t test, P < 0.05; organotypic slices, n = 21, t test, P < 0.05.)

Thus network development follows similar steps, whether the connections develop in vivo or in vitro, though over a slightly longer time scale in cultured tissue. This makes the organotypic slice a good model for synaptic development but underlines the importance of the time window chosen. Thus perhaps surprisingly, development is independent of experience, at least after P5 when the organotypic slices are made.

In both preparations the density of spines increases about 3-fold over the period studied. Similarly, for both acute and cultured slices the frequency of miniature excitatory postsynaptic currents (mEPSCs) also rises 3.7 and 3.4 times, respectively. (Acute slices, n = 8, t test, P < 0.05; organotypic slices, n = 21, t test, P < 0.05.) This suggests a linear relationship between the spine number and the mEPSCs of these neurones.

The one clear difference, as observed previously, is that hippocampal neurones in organotypic slices become hyperconnected (Debanne et al. 1995). This is reflected in an increased complexity of the dendritic tree. We used a fractal method to quantify this parameter and found that in organotypic slices CA1 neurones become richer in dendrites of the secondary and tertiary order. As the spine density is similar in both groups this results in a higher total synapse number in the organotypic slices. This increase is reflected in a higher frequency of spontaneous synaptic activity, while other parameters, such as amplitude and kinetics remain similar during development in both preparations. Overall this suggests that connectivity is greater in organotypic slices but the dendritic electrical properties are relatively unchanged.

We also studied changes in spine shape over development using confocal microscopy. The primary changes seen are a decrease in stubby spines from nearly 50 % down to about 25 %, and an increase from 25 % to 50 % in thin spines in both acute and cultured slices. The filopodia also decrease from around 15 % to 4 % in acute slices and to only 1 % in organotypics.

All the experiments conformed to current UK legislation. The animals were killed humanely.

All procedures accord with current UK legislation