Two papers are using the method.
One attached and the other cited here: "Single rodent mesohabenular axons release glutamate and GABA" Root et al 2014
Tetrodotoxin blocks voltage-dependent sodium channels thus preventing the generation or propagation of action potentials.
As such, we can distinguish monosynaptic connections from multisynaptic ones by adding TTX to the bath. This is often accompanied by 4AP to block delayed rectifier potassium currents. Together, TTX and 4AP are often referred to as "network effect block".
In other words, you can administer TTX to block any potential "relays" there may be between your two cells.
Adding TTX to the bath will block sodium voltage-dependent current and therefore prevent generation of spike. Meaning that any neurotransmitter release dependent on spike activity is absent. In that condition, you can only record miniature monosynaptic synaptic currents, which are not triggered by spike.
Supplemental figure 3 and supplemental text from "Cho JH, Deisseroth K, Bolshakov VY. Synaptic encoding of fear extinction in mPFC-amygdala circuits. Neuron. 2013 Dec 18;80(6):1491-507." have a very nice explanation of the logic behind using TTX and 4-AP isolating monosynaptic inputs. They also show a positive control in which disynaptic inputs are not rescued by 4-AP. Here is a link to their supplemental materials:
http://www.cell.com/cms/attachment/2024817714/2044471575/mmc1.pdf
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