I heard that a group did a 3D EM reconstruction showing that some neurons could form both excitatory and inhibitory synapses, but I have been unable to find it.
Just in case this is a matter of confusion, there are 2 parts to a synapse: presynaptic and postsynaptic. The (excitatory or inhibitory) effect any synapse may have, depends on the neurotransmitter released from the presynaptic cell and the state of the postsynaptic cell.
Typically cells receive both excitatory and inhibitory inputs through different synapses, but it is still possible for a single synapse to turn from excitatory to inhibitory, or vice versa, depending on the Nernst potentials of the receiving (postsynaptic) cell.
Yes, there are a number of papers that have come out showing that the same neuron can release both GABA (inhibitory synapse) and glutamate (excitatory synapse). Marisela Morales's group has shown this for VTA to Lateral Habenula projections (Root, et al. Cell Reports 2018 v23 p3465) and it's also been shown for Hippocampus (?) and one or two other areas as well I believe.
What makes a synapse excitatory or inhibitory is determined by the transmitter(s) being released AND the action of the transmitter in the postsynaptic cell, which is primarily a function of the types of transmitter receptors being expressed. For example, acetylcholine being released from neurons in the basal forebrain might act excitatory on one group of neurons (by activating postsynaptic nicotinic receptors) and inhibitory on other cells (by activating muscarinic receptors).
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