If you have a setup with two or more manipulators, and if you are careful enough, you can obtain whole-cell recording from multiple neurons in brain slices. Then you can examine connectivity between neurons by stimulating them one by one.

Using this method, we found that cortical neurons communicate with each other not only in digital mode (mediated by all-or-none action potentials) but also in analog mode (synaptic strength is dependent of presynaptic membrane potential level), that is, action potential is not absolutely 1, it could be less or more than 1 (Shu et al., 2006 Nature).

During recording, the recorded cells can be loaded with biocytin, and thus be fixed and stained later. The DAB staining could show the location and morphology of the cells recorded. Neurolucida system can be used to perform 3D reconstruction of the cells.