Blocking polysynaptic inhibition via opioid receptor activation isolates excitatory synaptic currents without triggering epileptiform activity in organotypic hippocampal slices

The abundance of synaptic connectivity in the cultured hippocampal slice preparation allows measurements of the unitary excitatory connection between pairs of pyramidal neurons using simultaneous presynaptic and postysynaptic intracellular recordings. However, the useful yield of these recordings can be greatly reduced by the presence of polysynaptic inhibition that occludes the measurement of the monosynaptic excitatory postsynaptic current (EPSC). We have found that the traditional method of eliminating contaminating synaptic inhibition with GABA receptor antagonists is of limited usefulness because the recurrent excitatory connections in organotypic slices cause epileptiform bursting in the absence of inhibitory function. This bursting obscures EPSCs to an even greater extent than the normally occurring polysynaptic inhibitory transmission. Here, we report a new method for isolating monosynaptic EPSCs using the mu-opioid agonist peptide DAMGO to reduce polysynaptic inhibition during these recordings. Activation of mu-opioid receptors is known to hyperpolarize inhibitory neurons. We found that DAMGO application reduces the amplitude and frequency of polysynaptic inhibition, allowing isolation of the excitatory connection between the two neurons being recorded. Furthermore, because inhibitory function is not completely eliminated by DAMGO application, epileptiform bursting very rarely develops. Therefore, the use of DAMGO to prevent polysynaptic inhibition without causing epileptiform bursting provides a useful tool to substantially increase the yield of experiments measuring the unitary excitatory connection between pyramidal neurons in the cultured hippocampal slice preparation.