Optogenetic stimulation of long-range inputs and functional characterization of connectivity in patch-clamp recordings in mouse brain slices

Knowledge of cell type specific synaptic connectivity is a crucial prerequisite for understanding brain wide neuronal circuits. The functional investigation of long-range connections requires targeted recordings of single neurons combined with the specific stimulation of identified distant inputs. This is often difficult to achieve with conventional, electrical stimulation techniques, because axons from converging upstream brain areas may intermingle in the target region. The stereotaxic targeting of a specific brain region for virus-mediated expression of light sensitive ion channels allows to selectively stimulate axons coming from that region with light. Intracerebral stereotaxic injections can be used in well-delimited structures, such as the anterodorsal thalamic nuclei, and also in other subcortical or cortical areas throughout the brain. Here we describe a set of techniques for precise stereotaxic injection of viral vectors expressing channelrhodopsin in the anterodorsal thalamus, followed by photostimulation of their axon terminals in hippocampal slices. In combination with whole-cell patch clamp recording from a postsynaptically connected presubicular neuron, photostimulation of thalamic axons allows the detection of functional synaptic connections, their pharmacological characterization, and the evaluation of their strength in the brain slice preparation. We demonstrate that axons originating in the anterodorsal thalamus ramify densely in presubicular layers 1 and 3. The photostimulation of Chronos expressing thalamic axon terminals in presubiculum initiates short latency postsynaptic responses in a presubicular layer3 neuron, indicating a monosynaptic connection. In addition, biocytin filling of the recorded neuron and posthoc revelation confirms the layer localization and pyramidal morphology of the postsynaptic neuron. Taken together, the optogenetic stimulation of long-range inputs in ex vivo brain slices is a useful method to determine the cell-type specific functional connectivity from distant brain regions.