28Although we know a great deal about monosynaptic connectivity, transmission and integration 29 in the mammalian nervous system from in vitro studies, very little is known in vivo. This is partly 30 because it is technically difficult to evoke action potentials and simultaneously record small 31 amplitude subthreshold responses in closely (< 150 µm) located pairs of neurons. To address 32 this, we have developed in vivo two-photon targeted multiple (2 -4) whole-cell patch clamp 33 recordings of nearby neurons in superficial layers 1 to 3 of primary somatosensory cortex.
34Here we describe a step-by-step guide to this approach in the anesthetised mouse, including: 35 the design of the setup, surgery, preparation of pipettes, targeting and acquisition of multiple 36 whole-cell recordings, as well as in vivo and post-hoc histology. The procedure takes ~ 4 hours 37 from start of surgery to end of recording and allows examinations both into the 38 electrophysiological features of unitary excitatory and inhibitory monosynaptic inputs and the 39 synaptic mechanisms of correlated neuronal activity.45 properties of synapses. In the intact brain, however, synapses function during different 46 behavioural and network states as well fluctuating levels of neuromodulators and far less is 47 known about monosynaptic transmission and the activity of connected neurons in vivo.
48In vivo approaches to identify connected pairs of neurons in the mammalian nervous system 49 have typically performed electrophysiological recordings of multiple single neurons and 50 examined the average response of one neuron to spontaneously occurring action potentials in 51 another neuron. "Spike triggered averaging" of cortical neurons has been performed both with 52 multiple extracellular recordings (Bartho, 2004; Csicsvari et al., 1998; English et al., 2017; 53 Fujisawa et al., 2008; Reid and Alonso, 1995; Swadlow and Gusev, 2002) and a combination 54 of extracellular and intracellular membrane potential recordings (Bruno, 2006; Crochet et al., 55 2005; London et al., 2010; Matsumura et al., 1996; Yu and Ferster, 2013). However, because 56 it is not yet possible to record the activity of all neurons presynaptic to the cells of interest and 57 cortical neurons can fire simultaneously, it is difficult to confirm whether correlated activity is 58 the result of a direct synaptic connection between the two recorded neurons or input from a 59 third, unrecorded, neuron with similar firing dynamics. One approach to get around the problem 60 of synchronous input is to have experimental control of action potential timing using single cell 61 stimulation, while simultaneously recording the evoked membrane potential response from a 62 second neuron. While, care has to be taken in concluding that any synaptic response is the 63 result of a monosynaptic rather than polysynaptic input (Berry and Pentreath, 1976; Parker, 64 2010), this approach has been used in vivo to characterise the wiring and functional properties